Agricultural site productivity: principles derived from long-term experiments and their implications for intensively managed forests

Forest Ecology and Management 138 (2000) 369±396

Agricultural site productivity: principles derived from long-term experiments and their implications for intensively managed forests Eric D. Vance* National Council for Air and Stream Improvement (NCASI), P.O. Box 13318, Research Triangle Park, NC 27709-3318, USA

Abstract Long-term agricultural ®eld experiments provide relevant information for questions being asked about the sustainability of managed forests, particularly as management regimes become more intensive. In this paper, concepts and criteria related to sustainable site productivity are reviewed and ®ndings from a range of long-term agricultural ®eld experiments are evaluated. Based on this evaluation, the following site productivity principles are identi®ed: (1) soil organic matter is the link between most management systems and sustainable site productivity; (2) nutrient de®ciencies can be corrected; (3) soil texture is a key variable affecting soil organic matter and site productivity; (4) return of crop residues enhances soil organic matter and site productivity; and (5) productive cropping systems have environmental bene®ts. While technological advances have proven to be highly successful for enhancing long-term crop yields, they also have the potential to mask underlying declines in site productivity. This hypothesis needs to be rigorously tested. Agricultural ®eld experiments show that crop productivity can be sustained for long time periods when appropriate management approaches are applied. Management requirements to achieve sustainability differ, however, depending on sitespeci®c edaphic and climatic characteristics and the needs of the crop. Site productivity principles derived from agricultural ®eld studies are also highly relevant to sustaining site productivity of managed forests. Several characteristics of managed forests should be considered when extrapolating conclusions from agricultural experiments. These include harvest removals of biomass and nutrients, residue type and distribution, management and system characteristics, and soil types used. Many characteristics associated with even intensively managed forests are sought as goals of conservation cropping systems and should contribute toward sustaining long-term site productivity. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Sustainable site productivity; Intensive forest management; Soil quality; Organic matter; Forest residues; Whole-tree harvesting; Fertilizer; Organic amendments

1. Introduction Questions have been raised about the sustainable productivity of intensively managed forests (National Research Council, 1990; Powers et al., 1990; Dyck and Cole, 1994; Johnson, 1994; Nambiar, 1996). * Tel.: ‡1-919-558-1979; fax: ‡1-919-558-1998 E-mail address: [email protected] (E.D. Vance).

Concerns over the sustainability of soil organic matter and fertility, in particular, are likely to increase as greater proportions of biomass are removed from forested sites over increasingly short rotation intervals. While a major barrier to analyzing forest site productivity questions is the scarcity of long-term data sets, agricultural systems have been intensively studied over long time periods and should provide information that is highly relevant to forest site productivity

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 4 2 5 - 4

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questions. One reason is that agricultural studies have addressed management impacts on crop productivity and soil fertility across a wide range of soil types and climatic regimes. They thus provide both a conceptual framework for evaluating the sustainability of intensively managed systems and a strong, mechanistic database to analyze relationships among management practices, soil properties and productivity. Secondly, because agricultural crops are commonly harvested annually, a much larger number of harvests and rotations can be examined than for forested systems. Finally, as forest management regimes become more intensive, they more closely resemble agricultural systems, with faster growth rates, high biomass removal, and increased use of fertilizers and pesticides. Findings from agricultural research may thus help project the long-term effects of current forest management regimes. Because agricultural research is highly relevant to questions being asked about the sustainability of managed forests, this paper reviews concepts and criteria used in assessments of agricultural site productivity and highlights ®ndings from long-term experiments. Sustainable site productivity principles are then summarized and their implications for sustainable forest management assessed. 2. Sustainable site productivity 2.1. Sustainability concepts Civilizations have risen and fallen on their ability to sustain agricultural productivity. Salt accumulation in irrigated soils and soil erosion have been major factors leading to productivity losses in early civilizations (Miller et al., 1985; Pesek, 1994). More recent examples of severe soil erosion, site degradation, and reduced crop productivity can be found in the US, particularly in the South and Great Plains (Larson et al., 1983; Healy and Sojka, 1985; Miller et al., 1985; Peterson et al., 1993; Doran et al., 1996). Although technological advances implemented in the 1950s increased crop yields, concerns over continued soil erosion and water quality degradation have led to implementation of conservation cropping practices and management systems. These include reduced-till or no-till systems, multiple cropping methods, cover

crops, increased return of crop residues, and conversion of highly erodible croplands to forest or pasture (Healy and Sojka, 1985; Francis and Youngberg, 1990; Peterson et al., 1993). There is considerable evidence that these new practices can signi®cantly improve site productivity and reduce environmental problems across a range of sites (Baker, 1985; Arshad et al., 1990; Dick et al., 1991; Peterson et al., 1993; Karlen et al., 1994; Tyler et al., 1994; Cassel et al., 1995). Philosophical perspectives on what constitutes a sustainable system differ. Most commonly, key system characteristics are evaluated based on output trends or relative to some baseline condition or ideal state. The baseline or ideal state is often de®ned either as (1) the natural, unmanaged state; (2) some preexisting state or selected point in history; or (3) the state of the system under some low intensity management regime. Although `snapshot' evaluations of agricultural systems relative to a baseline or ideal state can be revealing, the direction and rate of change of key system components provide the context needed to evaluate sustainability. For example, cultivation of virgin prairies for crop production in the midwestern US has often resulted in a 20±50% losses in soil organic matter, with some losses exceeding 70% (Healy and Sojka, 1985; Mann, 1986; Rasmussen and Collins, 1991). After crop management regimes that include appropriate nutrient inputs and residue management have been implemented, soil organic matter commonly increases until reaching some equilibrium level. Using soil organic matter as a surrogate for sustainable site productivity, two scenarios for cropping systems following cultivation can thus be depicted (Fig. 1). In one scenario, soil organic matter declines dramatically following cultivation but eventually rises to a new equilibrium level (Fig. 1A). If Point a is de®ned as the baseline and perceived as the goal to be met, the system would likely be considered unsustainable. However, the same system would probably be considered sustainable if evaluated based on trends at Points c and d, even though organic matter levels are well below the baseline. By contrast, if soil organic matter exhibited a more moderate, but downward trend following cultivation (Point b in Fig. 1B), it would likely be considered unsustainable even though the organic matter pool may be greater than at Points c and d in the ®rst example (Fig. 1A). This

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Fig. 1. Conceptual trends in soil organic matter following cultivation.

illustrates the importance of de®ning the context in which the state of the system is evaluated. Concepts of sustainability often emphasize the ability of a system to meet human needs. While this may be considered an external, anthropocentric perspective, it is also generally consistent with maintaining the integrity of the system itself because characteristics important for sustaining site productivity strongly overlap with those associated with high environmental quality. For example, soils exhibiting structural characteristics bene®cial for plant growth are also generally resistant to erosion. It is also true that high environmental quality itself is increasingly considered a part of the human needs equation. 2.2. General criteria used in evaluating agricultural sustainability Criteria used to quantify agricultural sustainability include (1) long-term crop yield; (2) long-term

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Fig. 2. Long-term yields and soil organic trends under different amendment treatments in the Broadbalk Winter Wheat Experiment, UK, 1852 to 1986 (adapted from Jenkinson, 1991). Inorganic fertilizer amendment rates were 144 kg N, 35 kg P, 90 kg K, and 12 kg Mg haÿ1 per year. The farmyard manure amendment rate was 35 Mg haÿ1 per year, containing about 3000 kg C, 225 kg N, 40 kg P, and 210 kg K. No amendment was used in the control treatment.

changes in soil properties; (3) resource use ef®ciency and conservation of mass principles; and (4) environmental quality. Probably the most relied upon and reasonable ®rst approximation of a sustainable system is long-term (i.e. >20 years) crop yield. The positive yield trend exhibited in the Broadbalk Continuous Wheat Experiment at Rothamsted, for example, is indicative of a sustainable system due to its 150-year record (Fig. 2). Agricultural sustainability is also evaluated based on resource use ef®ciency and conservation of mass principles, which are linked to both crop productivity and environmental quality. These criteria follow the shift in emphasis in recent years from maximum crop yield to optimal crop yield (Lockeretz, 1990), which has resulted largely from economic factors but also from changing public values concerning resource use and environmental quality. Because changes in crop yield do not indicate whether resources are used ef®ciently or high environmental quality standards are maintained, quantita-

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tive indices have been developed to measure system performance and ef®ciency. Total factor productivity (TFP) is a commonly applied resource-use ef®ciency index that utilizes the ratio of aggregate outputs to aggregate inputs using costs and prices as the common currency (Herdt and Steiner, 1995; Rayner and Welham, 1995). Although this index may appear to be solely an economic evaluation, it can also provide a measure of biological sustainability if costs and prices are held constant. For example, even though increasing crop yields may indicate that a system is sustainable, a decline in yield per unit fertilizer input could reveal underlying resource de®ciencies. With nearconstant inputs and management regimes associated with long-term ®eld experiments, the TFP index is, however, essentially analogous to crop yield rather than resource use ef®ciency (Cassman et al., 1995). Long-term changes in soil properties, particularly organic matter, is probably the second most commonly used indicator of agricultural sustainability. Soil physical, chemical, biological properties are all affected by crop management, both directly from soil disturbance (e.g. by compacting soil, adding or removing organic matter) or indirectly from manipulating vegetation (harvesting, planting, or changing residue quantities or distribution). Common concerns about sustainable site productivity include (1) reducing or re-distributing soil organic matter (which affects soil physical, chemical, and biological properties); (2) depleting soil nutrients in harvested biomass; (3) disrupting soil physical properties; and (4) altering soil organic matter, nutrients, and physical properties by increasing erosion. Another sustainability criterion is conservation of mass, with soil erosion the mass balance measure most often cited as the cause of declining agricultural sustainability (Larson et al., 1983; McCracken et al., 1985; Olson, 1994). Erosion can affect virtually all soil properties (Frye et al., 1985) and has been responsible for crop productivity losses of 30±50% or more on some soil types (Crosson, 1985; Langdale et al., 1985; McDaniel and Hajek, 1985; Reid, 1985; White et al., 1985). Although erosion can be measured directly, it is most often estimated using data on rainfall, soil properties, crop management practices, and topographic factors. Nutrient budgets can also be considered mass balance approaches. An unsustainable system with

respect to nutrients may exhibit reduced productivity or increased susceptibility to insect or disease infestations. Conversely, a system may also be considered unsustainable when nutrients are in excess supply because water quality is degraded. Using nutrient amendments to balance harvest removals is thus consistent with the goals of sustainable agriculture. Nutrient budgets can also be used to gain understanding of the mechanisms underlying system function and may allow declining sustainability trends to be detected early (Jenkinson and Parry, 1989; Paustian et al., 1990). In the Broadbalk Continuous Wheat Experiment, nutrient budgets revealed that less than 2% of inorganic N (whether applied as fertilizer at 48 or 192 kg haÿ1 per year) remained in the soil inorganic fraction at the time of harvest (Jenkinson, 1991). Most of the nitrate leached in late autumn and winter came from organic matter buildup and mineralization rather than from inorganic fertilizer. The investigator concluded that reducing fertilizer use on the plots would likely have only minor effects on nutrient leaching losses compared to those obtained by improving the timing of fertilizer application with respect to crop needs and by using cover crops (Jenkinson, 1991). Water balance calculations have also been used to determine whether arid-region management regimes are sustainable (Sadler and Turner, 1994). As indicated above, environmental quality is another general agricultural sustainability criterion. Water quality, with respect to sediments, fertilizers, and pesticides, is probably the environmental attribute most widely considered when agricultural sustainability is evaluated. Regulatory standards and best management practices (i.e. BMPs) have been developed to improve water quality, but there is continued debate over whether any impairment of water quality implies that a system is not sustainable. Environmental externalities such as water quality have also been included as part of the TFP index, resulting in what has been termed the total social factor productivity (TSFP) index (Herdt and Steiner, 1995). This approach assigns environmental costs that are deducted from output prices. A productive cropping system, for example, may have a negative TSFP index if it is degrading the environment. The challenge with the TSFP index has been assigning appropriate weighting and cost factors to subjective environmental concerns.

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2.3. Soil properties as indicators of sustainability Maintaining soil quality is inherent to most concepts of sustainability, and changes in soil properties over time and across management regimes are widely used as site productivity assessment tools. Soil indicators can be grouped into (1) organic matter; (2) fertility; (3) physical properties and tilth; and (4) microbial processes. Doran et al. (1996) stated that soil quality indicators should (a) encompass ecosystem processes and be relevant as process modeling inputs; (b) integrate soil physical, chemical, and biological properties and processes; (c) be sensitive to the long-term effects of management and climate; and (d) be components of existing data bases. Some scientists have suggested that indicators should be prioritized based on their in¯uence over quantity and stability of crop yield (Cassman et al., 1995). Because of its control over physical, chemical and biological properties, soil organic matter exerts an overriding in¯uence on crop productivity and environmental quality and is probably the single most widely used soil indicator of site productivity and sustainability. It is also a factor that is generally sensitive to crop management. As previously noted, soil organic matter and N typically decline exponentially to 20±50% of their original level following cultivation of virgin pasture (Healy and Sojka, 1985; Mann, 1986; Rasmussen and Collins, 1991; Grace et al., 1995). After virgin soils are cultivated, soil cropping and management practices in¯uence how rapid and to what extent soil organic matter increases to new equilibrium levels. Management alters soil organic matter by affecting the balance between residue return (by in¯uencing crop productivity and residue management) and decomposition rates (by in¯uencing soil temperature, moisture, and the physical distribution of residues). Each management practice has a unique set of direct and indirect effects on soil organic matter. This is evident from the fact that management-induced variability in organic matter after 50 years of crop management is of the same magnitude as organic matter loss following cultivation of virgin soils (Rasmussen and Collins, 1991). Soil fertility, de®ned as the `status of the soil with respect to the amount and availability to plants of elements necessary for plant growth' (Brady, 1974), is

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another widely used measure of sustainable site productivity. While soil nutrient content, particularly N, is a commonly used indicator of soil fertility, most fertility assessments emphasize extractable nutrient indices more closely related to that fraction available for crop uptake. Soil physical properties such as water holding capacity, water-®lled pore space, and bulk density are also valuable soil assessment tools, although they have not been as successfully related to crop productivity. Measures of the physical suitability of a soil for crop growth are sometimes integrated into the concept of soil tilth, de®ned by the Soil Science Society of America as `the physical condition of soil relating to tillage, ®tness as a seedbed, and its impedance to seedling emergence and root penetration' (Karlen et al., 1990). Soil tilth is a more subjective concept than single measures such as organic matter or extractable nutrients, however, and is not normally applied quantitatively. Finally, soil microbial populations provide an integral link between soil organic matter and nutrient availability, and play an important role in soil structural development and in buffering soils against management-induced environmental impacts. Three aspects of the soil microbial population can be distinguished in these roles: (1) the quantity of soil microbial biomass; (2) the composition of the soil microbial community; and (3) processes carried out by soil microorganisms. Because soil fertility and physical properties are both highly in¯uenced by soil organic matter, their relative in¯uence can be dif®cult to separate when organic matter-productivity relationships are evaluated. This was achieved to some degree in Rothamsted experiments on potatoes, sugar beets, and spring barley, where soil organic matter and soluble P were manipulated over a period of 12 years while soil N and K were maintained at adequate levels (Powlson and Johnston, 1994). The experiment showed that at the same level of soluble P, soils with higher organic matter produced higher yields. This was attributed to improvements in soil structure and root growth and supported by ®ndings from more controlled greenhouse experiments. By contrast, the positive in¯uence of soil organic matter on productivity of a North Dakota wheat system was attributed to its contribution to N mineralization over the growing season, since adequate soil water levels were maintained by trickle irrigation (Bauer and Black, 1994).

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2.4. Effects of tillage on soil physical properties Because tillage is a ubiquitous management practice that has substantial impacts on agricultural site productivity, it will be considered separately here. By design, tillage can improve crop productivity by enhancing soil aeration, water availability, and other soil physical properties. Tillage can also have negative, longer-term effects on soil structural characteristics, however, which can feed back to affect soil organic matter, nutrient availability, and soil biological properties. The most immediate effect of tillage is to mix and aerate the soil, which stimulates microbial decomposers and accounts for much of the organic matter loss that follows. Repeated tillage also disperses soil aggregates which can negatively affect soil structural characteristics and increase the potential for erosion (Rasmussen and Collins, 1991; Curtin et al., 1994; Drees et al., 1994; Robinson et al., 1994). Tillage can also indirectly reduce soil aggregation by reducing surface organic residues, which protect against raindrop impacts, and labile soil organic matter, which is the primary agent responsible for binding soil particles into aggregates (Curtin et al., 1994; Papendick, 1994; Robinson et al., 1994). Tillage of wet soils can cause soil structural breakdown and clod formation, and tillage of very dry soils can break down aggregates and increase wind erosion (Robinson et al., 1994). No-till or reduced-till systems that increase surface residues also generally increase aggregate stability, reduce surface crusting, increase in®ltration, increase earthworm and microbial activity, reduce soil erosion, and increase crop yield (Bruce et al., 1990; Drees et al., 1994; Karlen et al., 1994; Pikul and Zuzel, 1994; Cassel et al., 1995; Salinas-Garcia et al., 1997). However, relative effects of reduced-till versus conventional till systems on soil bulk density and soil strength vary with soil type (Hammel, 1989; Larney and Kladivko, 1989; Hill, 1990; Lal et al., 1994). Soils low in organic matter, for example, may bene®t from reduced tillage more than higher organic matter soils with more favorable soil structural characteristics (Larney and Kladivko, 1989). Vehicular traf®c associated with other agricultural practices can compact soil and reduce hydraulic conductivity and aeration, restrict root growth and soil organism activity, decrease soil nutrient supply,

increase erosion potential, and increase fertilizer requirements (Hill and Meza-Montalvo, 1990; Horn et al., 1995; Lipiec and Stepniewski, 1995; Soane and van Ouwerkerk, 1995; Whalley et al., 1995). Compaction and tillage can also make soils more susceptible to subsequent compaction under lesser loading levels (Robinson et al., 1994). Although no-till management can increase vehicular traf®c and cause soil compaction, no-till soils may have a more favorable structure than tilled soils at the same bulk density due to higher organic matter levels in the former (Soane and van Ouwerkerk, 1995). Also, traf®c-induced increases in soil strength are often less in no-tilled than in conventionally-tilled systems (Hill and Meza-Montalvo, 1990; Pierce et al., 1994). 2.5. Quantitative soil assessments Quantitative soil assessments combine soil properties into measures of soil quality, a concept that integrates chemical, physical, and biological properties in assessing a soil's ®tness as a medium for plant growth and an environmental buffer (Parr et al., 1992; Larson and Pierce, 1994). Some suggest that sustainability is achieved only when soil quality is maintained or improved (Larson and Pierce, 1994). A key component of almost all soil quality assessments is organic matter, and various indices of nutrient availability and soil structural integrity are also common components. Because `soil quality' can be highly subjective term, quantitative assessments have been developed to more explicitly quantify and de®ne it. A weakness of many such assessments is their lack of independent testing in the ®eld against actual sustainability criteria such as crop productivity or soil erosion. The `soil productivity calculator' is one quantitative assessment developed for tropical systems that has been used to predict crop productivity and has been tested across several sites (Aune and Lal, 1995). Crop yield is calculated as a function of potential yield at a given site, based on critical limits for selected soil properties. Soil properties considered include soil organic C, acidity, N, available P, exchangeable K, bulk density, rooting depth, and weed infestation, all of which are key factors affecting productivity of Oxisols, Ultisols, and Al®sols. Management and site factors considered include fertilizer use (N, P, and K),

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liming, tillage, vehicular traf®c, residue management, and soil erosion. Regression equations in the model were developed empirically from liming, fertilization, and compaction experiments across a range of sites. The model was tested using independent data sets derived from three long-term experiments. Correlation coef®cients (r2) for measured versus actual yields were 0.91 for maize and 0.97 for soybeans. Except for fertilizers, which in¯uenced crop yields at all sites, relationships between most soil properties and yields were site-dependent. The calculator is proposed for (1) predicting short-term effects of fertilization, liming, tillage and subsoiling on yields; (2) predicting longerterm, management-induced yield trends; (3) evaluating land classi®cation systems; and (4) assessing effects of accelerated erosion by estimating changes rooting depth. Soil quality has also been assessed with respect to erosion potential using a sensitivity analysis of soil functions coupled with an erosion prediction model (Karlen and Stott, 1994). Critical soil functions were related to the soil's ability to (a) facilitate water entry; (b) facilitate water transfer and absorption, (c) resist physical degradation; and (c) sustain plant growth. Potential physical and chemical indicators for each

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function were identi®ed, prioritized and weighted. Functions and indicators were combined into four conceptual scoring functions (more is better, less is better, an optimum range, and an undesirable range) to convert numerical or subjective ratings into unit-less values ranging from 0 to 1 (Fig. 3). Databases, general knowledge, and computer simulation models were all used to assign scoring relationships to the functions. The validity of this approach has not been independently con®rmed. Another soil quality assessment was developed based on what are termed `minimum data sets' and `pedotransfer functions' (Larson and Pierce, 1994). Minimum data sets are sets of soil properties that can be easily measured, are reproducible and standardized, and represent key soil variables relevant to management effects. Pedotransfer functions are statistical or empirical functions that relate soil characteristics with one another and are used to estimate soil properties that are dif®cult to measure directly. Interestingly, there was little difference in a soil propertybased productivity index for conventionally tilled wheat and native grassland systems in North Dakota based on this assessment, even though they differed substantially in soil organic C and bulk density.

Fig. 3. Soil quality scoring relationships used to relate soil indicators to their functions (Karlen and Stott, 1994, after Wymore, 1993). Functions are described as (A) more is better, (B) less is better, (C) an optimum range, and (D) an undesirable range. The letters L, B, and U refer to the lower threshold, baseline, and upper threshold, respectively.

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Fig. 4. Soil quality assessment control charts (from Larson and Pierce, 1994; after Pierce and Larson, 1993; Montgomery, 1985; Ryan, 1989).

Soil quality has also been evaluated using timedependent statistical procedures such as `control charts', where upper and lower limits for different soil variables are set from known tolerances or past variance (Larson and Pierce, 1994) (Fig. 4). When the process or system being considered is outside of the upper or lower limits, soil quality may be changing. This assessment thus (1) quanti®es the magnitude and dynamics of soil quality and its response to management; (2) differentiates normal variation in a parameter with variation that indicates a system change; and (3) helps de®ne processes by which management in¯uences soil quality. Again, the example used for this approach was a soil property, aggregation, rather than a direct measure of long-term site productivity. 3. Site productivity findings from long-term experiments Long-term experiments provide invaluable information for assessing soil productivity questions. Sites subjected to annual harvests and other management practices that span many decades also reveal site productivity factors not apparent in forested plots where harvests and site manipulation occur much more infrequently. Below are brief descriptions of approaches, major ®ndings, and interpretations of long-term agricultural experiments located on a range of sites and climatic regimes that may have particular relevance to forest site productivity questions.

3.1. Rothamsted experiments (UK) Rothamsted contains some of the world's oldest continuous agricultural research plots. The Broadbalk Continuous Wheat Experiment, established in the 1840s, has demonstrated that productivity can be sustained on some sites for very long periods. This experiment shows that winter wheat yields on a silty clay loam soil (a Chromic Luvisol, or a Paleudalf or Hapludalf in the USDA soil taxonomy classi®cation system) are higher now than they were over 150 years ago, even without soil amendments (Jenkinson, 1991; Powlson and Johnston, 1994) (Fig. 2). In fact, a period of reduced productivity in the 1920s was traced to a shortage of hand-weeding labor, with subsequent weed control accomplished by using bare fallow years (1 in 5) and herbicide applications after 1957 (Powlson and Johnston, 1994). Much of the increased wheat yield at Broadbalk resulted from using improved wheat cultivars. The effect of the cultivars was enhanced on fertilizer and manure plots compared to control plots, and the new varieties also improved fertilizer N recovery rates from about 30 to 70% due to higher grain yields (Johnston and Powlson, 1994). An interesting aspect of this experiment is that roughly equivalent yields were obtained with annual additions of inorganic fertilizers and farmyard manure, despite the fact that manure increased soil organic matter by 2.5 times over levels in non-amended plots while the fertilizers increased organic matter by less than 20% (see

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Fig. 5. Total carbon in topsoil (0±23 cm) for (A) a silty clay loam soil at the Hoosfield Barley Experiment, Rothamsted (annual barley) and (B) a sandy loam soil at Woburn (UK) (annual cereals). For (A) annual treatments since 1852 are unmanured (open circle); P, K, Mg fertilizers‡48 kg N haÿ1 (solid circle); farmyard manure at 35 t haÿ1 (solid square); and farmyard manure at 35 t haÿ1, 1852±1871 only (open square). For (B) annual treatments since 1976 are unmanured (open circle); NPK fertilizer (solid circle); and farmyard manure rotation (open triangle) (from Powlson and Johnston, 1994).

Fig. 5A for very similar organic matter responses in a nearby experiment). Nutrient quantities supplied were 144 kg N, 33 kg P, and 90 kg K for the inorganic fertilizer treatment and 225 kg N, 40 kg P, and 210 kg K for the manure treatment (Powlson and Johnston, 1994). Nutrient additions are not directly comparable, however, since not all nutrients supplied by the manure treatment would be immediately available to the crop. Soil organic matter was also sustained at nearconstant levels in plots without amendments (Jenkinson, 1991). Since 1980, higher wheat yields were found where inorganic fertilizer was added to manure plots com-

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pared to either treatment alone (Johnston and Powlson, 1994). This ®nding has provided new insights on site productivity in the Broadbalk study, since yields on the plots have been traditionally uncoupled from soil organic matter levels. The boost in yields when inorganic fertilizer was added to the manure plots was attributed to the higher N needs of current cultivars not met by the manure treatment. The investigators offered two possible explanations for higher yields in the combined fertilizer-manure treatment than where suf®cient inorganic fertilizer alone was added. One explanation was that the manure improved soil structure and enhanced water-holding capacity, which may be particularly important for fast growing cultivars. Secondly, N mineralization from manure late in the growing season may have enhanced crop yields, whereas fertilizer N applied earlier in the year may not have been available to the crop late in the season (Johnston and Powlson, 1994). There thus appears to be recent evidence for the importance of soil organic matter in this experiment, a conclusion supported by similar results from Rothamsted's Hoos®eld Barley Experiment located on a similar soil type (Johnston and Powlson, 1994). Total soil N at Broadbalk followed patterns very similar to those for organic matter, with soil N sustained at constant levels in non-amended plots, at higher levels in plots receiving inorganic fertilizers, and at the highest levels in manure-amended plots. Nutrient budget calculations at Broadbalk revealed substantial N inputs (about 50 kg N haÿ1 per year) from unidenti®ed atmospheric sources which may have contributed to sustainable productivity and soil organic matter levels in non-amended plots. Nutrient budget experiments revealed that most of the N leakage from the system was derived from mineralization of soil organic matter N rather than from residual fertilizer N, although mineralization rates were higher on plots that had been fertilized. In contrast to the Broadbalk Continuous Wheat and Hoos®eld Barley Experiments, yields of barley and wheat at Rothamsted's Woburn site could not be sustained with inorganic fertilizers alone (Powlson and Johnston, 1994; Poulton, 1995). The difference in sustainable productivity between the sites appears to be primarily due to differences in soil type. In contrast to the silty clay loam soil at Hoos®eld (and Broadbalk), soil organic matter could not be sustained

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in the more coarsely textured soil (a sandy loam Cambric Arenosol, or a Udipsamment in the USDA soil taxonomy classi®cation system) at Woburn without organic amendments (Fig. 5). The yield decline may have resulted from changes in soil moisture relationships due to organic matter depletion or to increased soil acidity resulting from base cation leaching losses (Poulton, 1995). Not only would the more ®nely textured soil at Broadbalk retain more water, but it would also serve to `protect' organic matter from microbial decomposition (Jenkinson, 1991). It should be noted that the crop productivity in some other Rothamsted experiments was not sustainable due to problems with root fungal pathogens and stem nematodes (Powlson and Johnston, 1994). 3.2. Morrow Plots (University of Illinois) Established in 1876, the Morrow Plots are the oldest agronomic research plots in the US and include the oldest continuous corn plot in the world (Odell et al., 1984; Miller and Larson, 1990; Mitchell et al., 1991; Darmody and Peck, 1997). The Plots have been used to assess effects of crop rotation and soil amendments such as manure, lime, N, P, and K fertilizers on crop yields. The experiment, based on a highly fertile, silt loam soil (Aquic Argiudoll), demonstrated that crop yields could be increased continuously in plots where

soil amendments (manure‡lime‡P) were added. Much of the increase was due to the introduction of improved, hybrid corn. Corn yields were higher under crop rotations (corn±soybeans or corn±oats±clover) than under continuous cropping (Fig. 6), but were reduced in all cropping systems without soil amendments. Inorganic fertilizer treatments were initiated at the Morrow Plots in the 1950s to determine whether the loss of productive capacity due to exploitive cropping methods is temporary or permanent and whether productivity declines can be restored with chemical fertilizers (Darmody and Peck, 1997). The experiment demonstrated that yield declines in nonamended plots were due to soil nutrient losses that could be restored by applying fertilizers (NPK), which also increased yields on plots previously receiving the manure-lime-P treatments. A crop rotation plus fertilizer treatment resulted in both the highest corn yield and the highest soil organic matter and N levels. Cropping decreased soil organic matter levels when no amendments were added, although the decline was retarded somewhat with crop rotations compared with continuous corn. A continuous but more gradual decline in soil organic matter was observed even in rotation plots receiving manure, lime and P, but soil organic matter was increased dramatically by complete fertilization.

Fig. 6. Corn yields on the North subplots of the Morrow Plots. Three-year rotation is corn±oats±hay, and the 2-year rotation is corn±soybeans (from Darmody and Peck, 1997).

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3.3. Sanborn Field (University of Missouri) Established in 1888, Sanborn Field has been used to test the effects of various crop rotations (including wheat, corn, alfalfa, red clover, bromegrass, lespedeza, and soybean), tillage systems, and soil fertility treatments on crop yield (Mitchell et al., 1991; Brown et al., 1995; Buyanovsky et al., 1997). The experiment, based primarily on a silt loam soil (Udollic Ochraqualf), demonstrated that high yields could not be sustained with crop rotations unless nutrients removed in harvest, particularly N, were replaced (Fig. 7). Both productivity and soil organic matter were sustained over four decades using crop rotations and fertilizers (Brown et al., 1995). Legumes, crop rotations, or animal manures did not maintain productivity without additional nutrient supplements. Productivity of continuous wheat could not be sustained due to weed and disease infestations. Because of confounding effects of changing crop varieties and manure characteristics, crop yields over 100 years did not always correlate strongly with soil organic matter levels. The experiment also showed that high crop productivity reduced erosion by increasing organic residue inputs and that nutrient additions could not restore productivity of highly eroded soils with reduced water holding capacity (Mitchell et al., 1991). About 56% of

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the topsoil was lost in continuous corn plots with residues removed, and rotation plots with residues removed up to 1970 lost about 30% of topsoil relative to plots used for timothy production (Gantzer et al., 1991). Manure treatments on continuous corn plots increased topsoil thickness by about 20% compared to unfertilized plots, while manure or inorganic fertilizer treatments had little effect on topsoil thickness in crop rotation plots. Inorganic fertilizers and manures had similar, positive effects on soil physical properties, including reduced bulk density and increased hydraulic conductivity (Anderson et al., 1990). 3.4. Old Rotation Experiment (Auburn University) The Old Rotation Experiment, established in 1896, contains the oldest continuous cotton research plots in the US (Mitchell et al., 1991). The study was based on a sandy clay loam (Typic Hapludult) soil located at the juncture of the southern Piedmont Plateau and the Gulf Coastal Plain (Traxler et al., 1995). It showed that P and K de®ciencies limited productivity during early years (low applications of P and K were made across all treatments), and subsequently, N became limiting as the yields in cotton-winter legume plots began to exceed those in no-N plots. In the cotton-winter legume rotation, yields declined initially before

Fig. 7. Yields of winter wheat (5 year moving averages) grown continuously with no treatment on Sanborn Field. Treated plots either received 13.4 mg haÿ1 per year manure or full (NPK) fertilizer (from Buyanovsky et al., 1997).

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increasing ®ve-fold between 1920 and 1950 due to higher rates and improved timing of P and K fertilization, improved insect control, and improved crop varieties. When winter legumes were included in the rotation, cotton yields were sustained at levels almost equivalent to those produced with fertilizer N. Crop rotation systems and winter legumes had a positive effect on soil organic matter. Fertilizer N also had a smaller, positive effect on soil organic matter levels. In addition to directly measuring productivity, sustainability in the Old Rotation Experiment was estimated using TFP and TSFP indices, which included costs associated with soil erosion and pesticide use (Traxler et al., 1995). In all treatments, output per unit input was higher in 1991 than in 1896, even when externalities (i.e. environmental effects) were included in the assessment (Traxler et al., 1995). A positive productivity trend for all treatments and similar productivity gains from organic and inorganic N inputs were also found. 3.5. Residue management experiment (Columbia Basin Agricultural Research Center, Oregon) The residue management experiment originated in 1931 to investigate the effects of different management regimes on productivity and soil properties of a wheat-fallow system located on a silt loam soil (Typic Haploxeroll) in northeast Oregon (Duff et al., 1995; Rasmussen and Smiley, 1997). With added N, wheat yields increased at a near linear rate of about 50 kg haÿ1 per year between 1932 and 1992, with about 87% of the increased yield attributed to

improved wheat varieties and the remainder due to improved weed control and water use ef®ciency. Even in zero-N treatments, improved N use ef®ciency of new varieties increased yields 40%. Highest grain and straw yields were obtained in plots where manure was added. Yields in low fertility plots declined from 20% less than manure-treated plots at the beginning of the experiment to 43±57% less at the end. These trends appeared to be linked to soil organic matter; soil C and N declined in all treatments except where manure was added, particularly in treatments where crop residues were removed or burned. Organic matter loss was attributed primarily to reduced C inputs during fallow periods. `Biological sustainability' of the different treatments was also assessed using a TSFP yield index where costs and prices were held constant at 1992 levels (Duff et al., 1995). Without added manure, biological sustainability declined before improved varieties were introduced, even when N fertilizers were applied. After new varieties were adopted, there was a positive biological sustainability trend in amended plots. By contrast, the assessment indicated that the low-N input treatments were only marginally sustainable, particularly where residues were burned, due to declines in soil C and N. The investigators speculated that yields would not be sustainable in treatments exhibiting high soil organic matter losses. The TSFP calculation revealed the complexity of attempting to assess externality costs. For example, many of the externality costs and bene®ts considered for pesticide use appear speculative (Table 1), particularly since pesticides had not been detected in either ground or surface waters in dryland wheat sites in the

Table 1 Pesticide externality costs and benefits considered for a TSFP sustainability index in a semi-arid wheat/fallow systema,b Costs

Benefits

Regulation Monitoring Human health Environmental Reduction in natural pest enemies Secondary pest outbreaks Pest resistance Fish, wildlife, bees, domestic animals

Reduction in Soil erosion from improved crop establishment/straw production Weed-induced clogging of drainage culverts Spread of weeds/pathogens to uninfested regions Food/feed fungal contamination

a b

Analysis from Duff et al. (1995). Pesticides had not been detected in surface or ground waters associated with semi-arid wheat systems.

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region. The investigators concluded that there was no ®rm basis for assigning a net cost or bene®t for pesticide use. After going through a similar procedure for fertilizers, a combined externality cost (for pesticide‡fertilizer use) of US$ 2.50 haÿ1 was designated. 3.6. Permanent Rotation Trial (Waite Agricultural Research Institute, South Australia) This experiment was established in 1925 to evaluate effects of 11 wheat rotation sequences (with fallow, pasture and legume combinations) on wheat yield and soil organic matter on a ®ne sandy loam Rhodoxeralf (Grace et al., 1995). Most rotations exhibited a 69-year linear yield decline, whereas the yield for continuous wheat recovered during the last 20 years after signi®cant declines over the ®rst 40 years. Soil organic matter declined linearly with increasing fallow frequencies and decreasing pasture frequencies. In all rotation systems, soil organic matter declined when residues were removed. Much smaller declines in soil organic matter occurred on a nearby site with a more ®nely textured (clay) soil when similar rotations were used. The investigators speculated that yields in continuous wheat plots increased and are sustainable due to continual inputs of residues with high lignin and high C/N ratios. Such residues are more resistant to decomposition than those produced during legume phases, and improved soil structure and nutrient supply through the buildup of `light fraction' organic matter pools. 4. Sustainable site productivity principles Based on the ®ndings of long-term ®eld experiments and other agricultural research, principles governing sustainable site productivity can be derived. Most of these principles are directly relevant to questions being asked about intensive forestry. 4.1. Soil organic matter is the link between most management systems and sustainable site productivity There is clear evidence that organic matter is a key determinant of soil physical, chemical, and biological characteristics important to plant growth. In many

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agricultural systems, there is a strong, direct relationship between soil organic matter and agricultural crop productivity. This relationship is most obvious on sites with coarse-textured soils and in semi-arid regions, where there are naturally low organic matter levels and where increases brought about by management can have dramatic effects on water conservation, nutrient availability, root penetration resistance, and crop yield (Tester, 1990; Rasmussen and Collins, 1991). Site productivity in such systems can also rapidly decline if adequate soil organic matter levels are not maintained. Examples of direct relationships between soil organic matter and productivity include Rothamsted's Woburn site and the residue management experiment in Oregon. Long-term observations also revealed a strong relationship between soil organic matter and productivity on the Morrow Plots on a fertile, silt loam soil (Odell et al., 1984) and in a North Dakota wheat system (Bauer and Black, 1994). In the latter system, each metric ton of organic matter was calculated to contribute 35.2 kg haÿ1 of spring wheat aerial dry matter and 15.6 kg haÿ1 of grain due to increased N mineralization throughout the growing season. Studies of infertile soils in developing countries also demonstrate that many sites require a critical level of organic matter, below which sustainable yields are dif®cult to achieve with inorganic fertilizers alone (Greenland, 1994). In these cases, organic matter additions may be needed to improve cation exchange capacities on low activity clays and to supply nutrients not provided by fertilizers (Greenland, 1994). Even when there is a strong relationship between productivity and organic matter, agricultural systems must be understood well enough to properly attribute cause and effect. As discussed below, productive cropping systems also tend to enhance soil organic matter levels. Knowing the baseline soil organic matter status before cropping treatments are established can aid this interpretation, although this is sometimes not documented for the older studies. In other systems, the relationship between soil organic matter and productivity is more indirect and complex. In the Broadbalk Continuous Wheat Experiment, for example, soil organic matter and productivity appeared uncoupled for 130 years. Recent evidence of higher yields on plots receiving manure and inorganic fertilizer compared to plots where suf®cient

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inorganic fertilizer alone was added suggests additional bene®ts of the manure on soil structure or on the availability of N late in the growing season. The relationship between soil organic matter and productivity across many sites is also complicated by the reciprocal relationship between the two, with more productive systems tending to increase soil organic matter due to greater plant residue return to the soil. Consequently, management practices that increase soil organic matter may enhance crop growth, but may also re¯ect increased productivity from some other cause. It can also be dif®cult to make accurate such generalizations across different systems, soil types, and climatic regimes due to the changing the balance between vegetative productivity and soil microbial decomposition rates. Inverse relationships between soil organic matter and crop productivity can also occur when enhanced soil organic matter decomposition increases nutrient mineralization, which at least temporarily stimulates crop growth. This was demonstrated in the Permanent Rotation Trial in Australia (Grace et al., 1995). 4.2. Nutrient deficiencies can be corrected Fertilizers are generally effective for replacing nutrients removed in harvest and for sustaining long-term site productivity. Many long-term experiments (the Morrow Plots and the Old Rotation Experiment are good examples) have shown that nutrient de®ciencies caused by high harvest removals and not replaced with soil amendments can be easily corrected, with crop yields restored to original or higher levels. Exceptions occur on some eroded sites (e.g. Sanborn Field), where other factors such as low water holding capacity and soil organic matter limit crop productivity. As suggested from Rothamsted's Exhaustion Land Experiment (Johnston and Powlson, 1994), physical and chemical ®xation processes speci®c to particular soils can also limit the effectiveness of P and K fertilizers for restoring adequate levels and availability of those nutrients. 4.3. Soil texture is a key variable affecting soil organic matter and site productivity Agricultural research has demonstrated that soil texture is a key factor affecting soil organic matter

levels, crop response to management, and productivity potential. This is illustrated by the contrasting trends exhibited in Rothamsted's Hoos®eld and Woburn experiments described above (Powlson and Johnston, 1994; Poulton, 1995). In the Woburn experiment, which was based on a sandy loam soil, both soil organic matter and crop productivity declined precipitously without appropriate soil amendments. By contrast, organic matter was sustained over 150 years by crop residue inputs alone and could be increased substantially with either inorganic or organic amendments in the Hoos®eld Barley and Broadbalk Continuous Wheat Experiments, both of which were based on silty clay loam soil (Powlson and Johnston, 1994; Poulton, 1995). The importance of soil texture was also shown after many years of similar cropping sequences in the Permanent Rotation Trial and related experiments in Australia, where organic matter levels were higher in soils with higher clay contents (Grace et al., 1995). Also, in North American Great Plains grasslands, soil texture was found to play a major role in controlling organic matter dynamics and has been used as one of four predictors (with temperature, moisture, and plant lignin content) of regional trends in soil organic matter (Parton et al., 1987). A primary reason that soil texture is so signi®cant is its role in `protecting' organic matter in clay-sized organo-mineral complexes that are stabilized and not readily accessible to microbial decomposition (Jenkinson, 1988; Van Veen et al., 1989). Cultivation disrupts these complexes which, in part, explains the subsequent loss of soil organic matter (Cambardella and Elliott, 1992). Soil C and N thus tend to increase as soil particle size decreases and may shift from sand fractions to clay-size fractions over long time periods (Leinweber and Reuter, 1992). Since texture affects both soil organic matter and other properties related to water and oxygen availability, it also in¯uences soil microbial and chemical processes. In one study, soil textural differences had greater effects on these processes than the combined effects of residue addition, residue placement, and nutrient amendments (Huffman et al., 1996). The importance of texture is also demonstrated by its presence as a principal component of most soil organic matter models (Jenkinson and Rayner, 1977; Parton et al., 1987; Donigian et al., 1995). Because of their positive effects on organic matter retention and water-

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and nutrient-holding capacities, ®ne-textured soils tend to sustain crop productivity to a greater degree than do coarse-textured soils. This generalization does not hold, however, where physical characteristics of ®ne-textured soils, such as soil aeration and root penetration, limit productivity. 4.4. Return of crop residues enhances soil organic matter and site productivity The quantity of crop residues left on site following harvest is one of the most important management factors affecting soil organic matter levels and site productivity. Across a range of sites, a strong, positive relationship between crop residue return and soil organic matter has been found (Rasmussen and Collins, 1991; Duff et al., 1995; Grace et al., 1995). Effects of removing residues are most acute in semiarid environments where crop productivity is low (Rasmussen and Collins, 1991). The Broadbalk Continuous Wheat Experiment demonstrated that, depending on soil type and other factors, even crop residue inputs alone can sometimes sustain soil organic matter (Jenkinson, 1991). The strong relationship between the frequency of fallow (unplanted) periods and organic matter loss during cropping evident across a range of crop types and geographic regions (Persson and Mattsson, 1988; Rasmussen and Collins, 1991; Monreal and Janzen, 1993; Grace et al., 1995) also demonstrates the importance of plant residues. Accelerated wind and water

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erosion during fallow periods can also contribute to organic matter losses. The fraction of the year during which a crop is growing also in¯uences soil organic matter levels (Franzluebbers et al., 1994). 4.5. Productive cropping systems have environmental benefits Examples of soil erosion, degraded water quality, and other negative environmental outcomes resulting from agricultural production have been widely cited (Healy and Sojka, 1985; Soule et al., 1990; Pesek, 1994). However, there is considerable evidence that new technologies leading to increased crop yields can improve site productivity and have environmental bene®ts. A primary bene®t of productive cropping systems is an increased return of plant residues to the soil system. Management regimes that increase crop productivity generally increase residue inputs, resulting in increased soil organic matter levels and reduced erosion. In one New York study, soil organic C correlated strongly with a crop productivity index across seven crop production areas (Fig. 8) (Flach et al., 1997). Since the areas studied were previously uncultivated with similar climates and soil types (welldrained, medium-textured), the relationship showed that productive cropping systems reduced organic matter loss. Long-term addition of inorganic N fertilizers have also been shown to increase soil organic matter relative to non-amended controls due to increased plant productivity and greater residue inputs

Fig. 8. Effect of cropping index (i.e. yields) on soil C for seven New York production areas (from Flach and Cline (1954) and Flach et al. (1997).

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Fig. 9. Effect of fertilizer N on soil C and N in the tillage±fertility experiment, Oregon (from Rasmussen and Rohde, 1988).

(Odell et al., 1984; Rasmussen and Collins, 1991; Paustian et al., 1992; Ismail et al., 1994; Glendining and Powlson, 1995). The positive effect of N fertilization on soil C and N was evident in the tillage-fertility experiment established in 1940 in semiarid northeast Oregon (Fig. 9). The positive effect of fertilizer on organic matter is generally most apparent in humid environments (Rasmussen and Collins, 1991). Because of its effect on organic matter, inorganic fertilizers can also increase soil porosity and in®ltration and decrease bulk density (Pikul and Zuzel, 1994). Adequate nutrient supplies and balance resulting from fertilization can also improve crop vigor, and reduce incidence of disease and the need for fungicides (Davis, 1994). Pesticides themselves, when wisely applied, can also enhance residue return, reduce erosion, and provide other environmental bene®ts by increasing crop productivity (Table 1) (Duff et al., 1995). Productive cropping systems can also positively in¯uence subsoil chemical and physical properties. This was demonstrated in Argillic horizons from Ultisols subjected to long-term crop management in the southeastern US (Hardy et al., 1990). Subsoils in areas that had been cropped, fertilized, or limed over 15 or 30 years had higher cation exchange capacities, higher base saturation, and reduced exchangeable aluminum compared to soils from unmanaged woodland or sod sites. Productive cropping systems can thus enhance the role of soil as an environmental buffer against agricultural chemical leaching losses.

Improved crop varieties also provide environmental bene®ts related to both higher crop productivity and more ef®cient nutrient use. New cultivars used in the Broadbalk Continuous Wheat Experiment, for example, improved fertilizer N recovery from about 30 to 70% due to higher grain yields (Johnston and Powlson, 1994). One comparison of successive cultivars in Broadbalk showed that the greater N recovery occurred in the newer, more productive cultivar despite a lower concentration of N in the grain (Austin et al., 1993). This effectively reduced the N required for each unit of grain produced by 40±60%. In the Residue Management Experiment, improved N use ef®ciency of new crop varieties increased yield 40% in plots where no external N sources were added (Duff et al., 1995). Increased crop productivity brought about by new crop varieties can also increase residue inputs to soil, although new varieties also tend to have smaller proportions of residues per unit grain produced (Jenkinson, 1991). New crop varieties that are more competitive with weeds and have increased resistance to pests and disease may also reduce herbicide and pesticide use (Forcella and Burnside, 1994; Pesek, 1994). Future weed management practices will also be more scienti®cally based, relying more on safer and more selective herbicides and pesticides and site-speci®c information on weed ecology, crop physiology, and climate (Forcella and Burnside, 1994; Funderburk and Higley, 1994). These new practices, coupled

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with model-based systems for improving the ef®cient and timely use of agricultural chemicals, should reduce environmental impacts and help sustain site productivity. Productive cropping systems also increase potential soil C storage and reduce atmospheric CO2 accumulation, which has been linked to projected climate change. In one analysis, the effects of future crop management regimes on soil C levels over an area representing 60±70% of total US cropland in the Corn Belt, the Great Plains, and the Great Lakes regions were evaluated (Donigian et al., 1995). The analysis used data on climate, soil types, management regimes, and soil C storage, coupled with model projections and geographic information systems. Results suggested that if current agricultural practices were continued over the same land base, C sequestration would increase by 32% over 1990 levels by the year 2030. The projected 1 Gt increase in C sequestration resulted primarily from an assumed 1.5% annual increase in crop yield, based on USDA projections. The authors concluded, ``The study results provide a strong indication, even with the uncertainty associated with model-based projections, that agricultural trends are leading to generally improved fertility and increased soil organic carbon sequestration even without speci®c policies designed to promote these objectives.'' Crop management practices that increase productivity can also bene®t the environment by reducing the area of land required for intensive management, leaving more acres available to be managed for other uses such as wildlife habitat. For example, one analysis for Kansas and Iowa estimated that each Mg of fertilizer used increases crop productivity enough to replace up to 11 ha of farm land (Carlson, 1987). Enhancing productivity on the most productive lands also reduces the need to grow crops on marginal lands more susceptible to erosion, reducing losses of soil and agricultural chemicals and contamination of surface waters (Davis, 1994). 5. Potential for technological advances to mask underlying declines in site productivity Although technological advances can have a variety of environmental bene®ts, they also have the potential to mask underlying resource de®ciencies. High-yield-

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ing crop varieties, for example, may be more insensitive to changes in soil factors due to their improved nutrient and water use ef®ciencies. Technological masking effects may also delay detection of ¯at or declining yield trends in long-term studies where organic matter levels have declined (Cassman et al., 1995). In the Residue Management Experiment in northeast Oregon, improved N use ef®ciency, higher harvest indexes, and greater disease resistance of new wheat varieties increased wheat yields 40% despite reduced soil organic C (Fig. 10). Scientists associated with the experiment speculate that declines in soil organic C in treatments where no manure is added will eventually lead to a decline in yields (Duff et al., 1995). Throughout the Palouse region of the Paci®c Northwest, signi®cant erosion has caused a 10% loss of cropland topsoil, with an average soil loss of 31 Mg haÿ1 per year over 50 years of cropping (Young et al., 1985). Despite this loss, new wheat varieties and improved management practices have doubled average wheat yields over the period. It remains to be seen whether diminishing soil organic matter in these systems will reduce crop productivity, increase soil erosion, or have other negative environmental consequences in the future. The masking hypothesis is based, in part, on the premise that past relationships between crop yields, soil organic matter, and other soil properties are still valid after new technologies are implemented. Although yields and organic matter may appear uncoupled in the short-term, the question remains whether the coupling will re-surface when some critical level of soil organic matter or another soil factor is reached. It is possible that productivity of improved crop varieties supplied with adequate resources (e.g. nutrients and water) from outside sources might be sustainable. Where adequate external resources are not supplied, however, productivity could eventually decline with reduced soil water- and nutrient-holding capacities. Potential masking effects may be highest where soil changes (e.g. erosion) are slow but accumulate over time. Positive, synergistic relationships found between technological advances such as new crop varieties and soil depth or fertilizer treatments (Young et al., 1985; Jenkinson, 1991) also point to the potential signi®cance of such soil changes. To help elucidate trends in productivity and site resources, crop yield and soil quality assessments are needed

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Fig. 10. Wheat yields from the Residue Management Experiment (Oregon) as affected by soil organic carbon, soil amendments, and variety. Arrows indicate initial conditions in 1931±1933. The regression line is for traditional varieties during 1931±1966. Yields for the 1976±1978 and 1986±1988 periods reflect introduction of new, high yielding varieties. Yields are 3-year averages beginning with the year of soil C measurement in 1931, 1941, 1951, 1964, 1975 and 1986. Dashed lines connect data points for the same treatments for the 1964±1966, 1976± 1978, and 1986±1988 periods (from Cassman et al., 1995).

both before and after (and without) new technologies (e.g. new crop varieties) have been introduced (Cassman et al., 1995). 6. Implications for forest management Most ®ndings and principles highlighted in this report are relevant to questions being asked about the sustainability of managed forests, particularly where intensive management regimes are employed. Results from agricultural experiments certainly suggest that productivity of short-rotation, intensively managed forests can be sustained over long time periods. Just as in agricultural systems, however, the inputs and management practices required to sustain site productivity depend to a large extent on site factors. Site factors that limit productivity and their changes over time need to be understood to determine these requirements. Soil organic matter is undoubtedly important for sustaining long-term site productivity. Soil texture and harvest residues left on site are key factors affecting organic matter levels and should both be considered when developing management guidelines. These two

factors also interact, so that harvest residues may be particularly important on coarse-textured soils. Conversely, soil texture should be considered when locating sites that will be subjected to repeated whole-tree harvesting or burning. On sites that are impoverished with respect to soil organic matter, land application of organic amendments such as mill or municipal biosolids may be a particularly effective restoration tool. The direct relationship between soil organic matter and productivity likely does not hold in forested systems where wet or cool conditions limit soil organic matter decomposition. In the histosols of forested bogs or wetlands, the same conditions that limit decomposition and cause organic matter buildup (cold temperatures or low soil oxygen concentrations) also limit tree growth. Regarding soil nutrients, the good news is that fertilizer applications can easily restore many impoverished sites unless they are highly eroded. However, repeated P fertilizer applications may be needed to restore fertility to some soils that rapidly ®x P. Another lesson that can be applied to forests is that, on some sites, expanding or intensifying management can have environmental bene®ts related to factors such as soil organic matter retention, erosion, and water quality.

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Positive effects of fertilizer applications on organic matter levels in agricultural soils likely apply to some forested sites as well. Fertilizers can thus ameliorate site productivity both through effects on soil organic matter and nutrients. While increasing management intensity can improve soil quality, soil properties should be periodically monitored so that preemptive management measures designed to prevent future site productivity declines can be planned. Soil monitoring may be particularly important following dramatic increases in productivity resulting from new tree varieties or other technological changes. Although principles derived from this review are generally applicable to managed forests, several key factors should be considered when interpreting conclusions from agricultural experiments. These factors may differentiate managed forests with agricultural systems and could have substantial impacts on site productivity. They include (1) the proportion of crop biomass and nutrients removed during harvest; (2) the amount and type of harvest residues left on site; (3) system and management characteristics; and (4) site characteristics. 6.1. Biomass and nutrient harvest removal and residue return An important variable affecting how well agricultural site productivity principles can be applied to managed forests is the quantity of biomass and nutrients removed during harvest and the proportion of that biomass returned to the site as residue. Annual aboveground biomass accumulation and the fraction of biomass typically removed during harvest are compared for selected agricultural, forest and short-rotation woody crop systems in Fig. 11. These comparisons are not meant to depict the range of values encountered for the different systems, but do illustrate principles relevant to long-term site productivity. The comparison shows that intensively managed woody-cropping systems (i.e. hybrid poplar) have the potential to accumulate biomass much faster than do traditionally managed forests (Douglas-®r and loblolly pine) and can have similar or higher biomass accumulation rates as do some agricultural crops (i.e. corn and wheat). The comparisons also show that the fraction of total aboveground biomass removed from the site during forest harvest tends to be higher than in

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harvests of agricultural crops. This is particularly true for whole-tree harvesting, which is becoming increasingly common and is typically used for short-rotation woody crops. A somewhat different picture emerges when annual nutrient accumulation and removal are compared for the different systems (Fig. 11). Annual nutrient accumulation expressed per unit area tends to be higher for corn and wheat than for loblolly pine and Douglas-®r systems, although the relative difference depends on the nutrient being considered. For example, aboveground harvested biomass is about 1.2 fold higher for corn than for loblolly pine (bole-only harvest), while differences in N and P removed are about 21- and 29fold, respectively. Differences between corn and loblolly pine are roughly seven-fold, three-fold, and 12-fold for K, Ca, and Mg removals, respectively. Higher nutrient concentrations in the harvested agricultural product (grains) than in the harvested forest product (boles) explain some of these differences. Conversely, the low nutrient portion of the agricultural crop (e.g. stalks) is left as residue on site whereas the opposite is true for forest residues (i.e. branches and foliage in bole-only harvests). For loblolly pine and Douglas-®r, the proportion of each of the nutrients removed (relative to the aboveground total) is lower than the proportion of biomass removed. For corn and wheat, the proportions N, P, an Mg removed is higher than the proportion of biomass removed, while the proportions of K and Ca removed are lower than for biomass. Consequently, although higher proportions of biomass may be removed in forest and woody crop harvests compared to agricultural harvests, nutrient removals are offset to some extent by lower nutrient concentrations in the harvested product. The intensity of forest management also signi®cantly in¯uences biomass nutrient uptake and nutrient use ef®ciency. A comparison of nutrient (N, P, K, Ca, Mg) uptake as a function of biomass accumulation in stands of several deciduous tree species in France showed that, except for Mg, nutrient uptake per unit biomass increased dramatically with management intensity (i.e. genetic improvement, fertilization, rotation lengths <10 years) (Fig. 12). Another analysis using a number of short-rotation and natural poplar stands showed that N utilization ef®ciency (i.e. biomass production per unit N) increased rapidly up to 10

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Fig. 11. Aboveground biomass, N, P, K, Ca, and Mg, and potential harvest biomass removals of these constituents for agricultural and forest crops. Comparisons are for corn (C) (Buol, 1995); wheat (W) (Buol, 1995); loblolly pine, 16 years old (LP) (Wells and Jorgensen, 1975); Douglas-fir, low site, 42 years old (DFL) (Cole and Rapp, 1981); Douglas-fir, high site, 55 years old (DFH) (Mann et al., 1988); hybrid poplar, low productivity, 4 years old (HPL) (Hansen and Baker, 1979); and hybrid poplar, high productivity, 4 years old (HPH) (Heilman and Stettler, 1986). Bars represent total aboveground biomass, shaded area represents potential portion removed (percentage in parentheses) as product (i.e. grain for agricultural crops, stem for loblolly pine and Douglas-fir, and total aboveground woody biomass for hybrid poplar). Open area of bar represents portion left on site as residues.

years of age before leveling (Fig. 13). These studies show that several factors may work together to dramatically increase the quantity of nutrients required to sustain long-term site productivity as forest management regimes intensify. 6.2. Residue type and distribution Agricultural studies have shown that although the amount of crop residues returned is a very important

determinant of soil organic matter levels, residue type also plays a signi®cant role. In Australia's Permanent Rotation Trial, residues high in lignin and with high C/ N ratios were more resistant to decomposition and contributed more to sustaining soil structure and nutrient-supplying capacity than did residues with lower lignin concentrations and C/N ratios (Grace et al., 1995). Investigators linked the sustainability of continuous wheat to the return of the high lignin residues, because of their demonstrated positive

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Fig. 12. Effect of management intensity on nutrient immobilization and woody above-ground biomass production for deciduous coppice forest stands in France (from Ranger and Nys, 1996).

effects on soil structural characteristics and nutrient supply. Likewise, a 30-year-old Swedish ®eld amendment experiment comparing biannual additions of straw or sawdust (with and without fertilizer N), green manure, and farmyard manure with bare fallow found that the amendment type had substantial effects on soil C levels (Paustian et al., 1992) (Fig. 14). Although changes in soil C (ranging from a 30% increase to a 30% decrease relative to initial levels) were roughly related to annual C input, residue type also had a strong effect on soil C levels at about the same C input rate. Highest soil C accumulation per unit C input occurred in the sawdust‡N and manure amendments, both of which had lignin concentrations of about 30%. Lowest C accumulation occurred in the straw amendment, which had lignin concentrations of about 15%. Higher C accumulation with green manure (6% lignin) than with straw contrasts with ®ndings from the Australian study, and may have resulted from higher

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crop productivity and residue return due to higher N supplied by the green manure. Fertilizer N also increased C accumulation rates when added to the sawdust and straw amendments. Although the positive effect of added N on soil C and N accumulation was assumed to result from increased crop productivity and residue inputs, accumulation in high N treatments was also greater than predicted by model simulations that included crop productivity effects. Other factors that might explain greater organic matter accumulation in response to fertilizer are lower decomposition rates due to increased crop transpiration (i.e. lower soil moisture), condensation reactions where C and N are incorporated into stabilized products, or changes in microbial yield ef®ciencies (Paustian et al., 1992). In general, the Australian and Swedish studies suggest that high lignin woody residues remaining following forest harvest will likely sustain soil organic matter levels and bene®cial soil physical properties over a longer period of time than do typical agricultural crop residues. Residue lignin concentrations range from about 16 to 24% for cereals such as wheat and barley and from 11 to 16% for corn, while wood lignin concentrations typically range from 20 to 30% (Biermann, 1993; Paustian et al., 1997). The fact that animal manure, a traditional agricultural amendment, resulted in higher C retention per unit C added than any amendment other than sawdust‡N in the Swedish study is also a signi®cant ®nding. This comparison demonstrates that both lignin and N (and possibly other nutrients) in residues and amendments have an important in¯uence on soil organic matter retention, with the positive effects of N due both to increased crop productivity and other mechanisms. While many forest residues are high in lignin, many are more analogous to the sawdust (without N) amendment than to the high nutrient animal manure. Consequently, high lignin residues in conjunction with a nutrient source may have the potential to enhance soil organic matter retention and contribute to long-term site productivity to a greater extent than typical forest residues alone. In this respect, the potential effects of forest-applied municipal biosolids on long-term site productivity may be similar to those demonstrated for animal manure applied to agricultural sites. The distribution of harvest residues is another factor with signi®cant implications for long-term site productivity. Logging slash (i.e. forest harvest residues) is

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Fig. 13. Effect of stand age on N utilization efficiency for managed plantations and natural stands of Populus (from Hansen and Baker, 1979).

often left in piles or windrows, which concentrates organic matter and nutrients at particular points rather than throughout the site. In one loblolly pine harvest study, N and P displaced into windrows during site

preparation was estimated to exceed whole-tree harvest removals by 200% or more (Tew et al., 1986). This contrasts with agricultural crop residues, which tend to be more evenly distributed.

Fig. 14. Effect of amendment C input rate and type on soil C accumulation (0±20 cm) in a 30 year old Swedish field experiment (from Paustian et al., 1992).

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6.3. Management and system characteristics Several management and system characteristics that differentiate forested systems from agricultural systems have signi®cant implications for maintaining long-term site productivity. Even short-rotation forestry systems are much less intensively managed than most agricultural systems, and forest systems have many characteristics that are sought as crop management conservation goals. These include infrequent tillage, a year-round presence of a crop on site, and surface accumulation of organic residues. All of these factors have been shown to enhance soil physical characteristics and fertility, and reduce soil runoff and erosion in agricultural systems (Dick et al., 1991; Rasmussen and Collins, 1991; Drees et al., 1994; Franzluebbers et al., 1994; Ismail et al., 1994; Lal et al., 1994). Because of their longer growing season and more extensive rooting systems, managed forests also do not require high soil nutrient concentrations in close proximity to roots as do annual cropping systems, making them less susceptible to nutrient losses through leaching, denitri®cation, or erosion (Buol, 1995). This line of evidence indicates that characteristics of even short-rotation forest systems likely bene®t sustainable soil productivity and water quality to a greater degree than do traditional agricultural row crops. There have been few ®eld comparisons to test this assumption. One recent study showed that agricultural cropland converted to short-rotation woody crops reduced surface runoff and improved water quality (sediments and nutrient concentrations) during the initial year of establishment (Thornton et al., 1998). The investigators speculated that differences would likely become more pronounced as the woody crops become more established. Another study showed few differences in ®rst-year water quality between row crops and short-rotation woody crops but, again, investigators hypothesized that enhanced soil properties measured under woody crops would lead to reduced runoff and soil erosion, and improved water quality in subsequent years (Joslin and Schoenholtz, 1997). 6.4. Soil and site characteristics While many characteristics of forests and forest management systems contribute to sustainable site

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productivity, some forest soils and site types present a challenge to managers. Forested sites are traditionally less fertile and have greater topographic relief than agricultural systems, which require greater shortterm economic returns. In the US, a large portion of forest land is concentrated on low fertility soils in the Ultisol and Spodosol orders, compared with cropland which is largely based on high fertility soils of the Mollisol and Al®sol orders (McCracken et al., 1985). Also, soil erosion has the potential to easily exacerbate negative chemical and physical properties of many forest soils. Effects of erosion are particularly dramatic on shallow soils with infertile subsoils more commonly found on forest lands than on the deep soils with fertile subsoils often used for agricultural production (McCracken et al., 1985). Erosion-induced reductions in soil water holding capacity is the primary factor affecting productivity in shallow soils or soils with heavy clay subsoils, while declines in fertility may be most signi®cant for other soil types (Frye et al., 1985). One factor favoring intensively managed, short-rotation forest systems is that they are likely to be located on higher quality sites more closely resembling those used for agricultural production. 7. Conclusion Agricultural ®eld studies have demonstrated that with proper management, crop yields can be sustained over very long periods. Most long-term experiments show that maintaining adequate levels of soil organic matter and replacing nutrients removed in harvest are key elements of sustaining site productivity. Soil amendments and management practices required to sustain site productivity vary greatly, however, depending on soil type, site conditions, and climatic factors. Long-term studies have also shown that fertilizers can normally correct nutrient depletion or de®ciencies, except on eroded sites where soil physical properties have also been degraded. A number of examples point to soil texture is a key determinant of soil organic matter levels and system sustainability, due to its effects on soil water relationships and organic matter sequestration. Across a range of sites, the return of crop residues has been found to be a key management factor that signi®cantly in¯uences soil organic matter levels and site productivity.

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Although intensive agricultural practices (e.g. use of inorganic fertilizers, pesticides, and frequent tillage) have caused declines in site productivity and environmental degradation in many regions, conservation cropping systems have led to signi®cant improvements. There is also increasing evidence that productive cropping systems can increase soil organic matter and erosion-resistance, increase the ef®ciency of fertilizer and pesticide use, reduce the use of marginal lands and the land base required for agricultural production, and increase soil C sequestration. New technologies can thus contribute to system sustainability and provide environmental bene®ts. Technological advances that increase crop productivity also have the potential to mask underlying resource de®ciencies and declines in site productivity, however, and increased monitoring of soil properties may be needed to detect such changes. Principles derived from agricultural ®eld studies should be highly relevant to questions about the sustainability of intensively managed forests. However, characteristics associated with managed forests should in¯uence how agricultural research ®ndings are interpreted and applied. While greater proportions of biomass may be removed during forest harvests compared with harvests of agricultural crops, forest nutrient removals are offset to some extent due to lower nutrient concentrations in harvested forest crop. The more woody (i.e. higher lignin) forest harvest residues may also sustain soil organic matter and fertility to a greater degree than do agricultural residues, provided that they are distributed evenly across the site. However, this factor does not apply when whole-tree harvest systems are employed. Many characteristics associated with managed forest systems, such as the year-round presence of the crop on site, infrequent tillage, and accumulation of forest residues on the soil surface, are sought as goals in conservation cropping systems. Coupled with longer rotation intervals compared to annually harvested agricultural crops, these characteristics should enhance long-term site productivity. References Alban, D.H., Perala, D.A., Schlaegel, B.E., 1978. Biomass and nutrient distribution in aspen, pine and spruce stands on the

same soil type in Minnesota, pine and spruce stands on the same soil type in Minnesota. Can. J. For. Res. 8, 290±299. Anderson, S.H., Gantzer, C.J., Brown, J.R., 1990. Soil physical properties after 100 years of continuous cultivation. J. Soil Water Conserv., January±February, 117±121. Arshad, M.A., Schnitzer, M., Angers, D.A., Ripmeester, J.A., 1990. Effects of till vs. no-till on the quality of soil organic matter. Soil Biol. Biochem. 22, 595±599. Aune, J.B., Lal, R., 1995. The tropical soil productivity calculator Ð a model for assessing effects of soil management on productivity. In: Lal, R., Stewart, B.A. (Eds.), Soil Management: Experimental Basis for Sustainability and Environmental Quality. Lewis Publishers, Boca Raton, FL, pp. 499±520. Austin, R.B., Ford, M.A., Morgan, C.L., Yeoman, D., 1993. Old and modern wheat cultivars compared on the Broadbalk Wheat Experiment. Eur. J. Agron. 2, 141±147. Baker, J.L., 1985. Conservation tillage: water quality considerations. In: D'Itri, F.M. (Ed.), A Systems Approach to Conservation Tillage. Lewis Publishers, Ann Arbor, MI, pp. 217±238. Baker, J.B., Blackmon, B.G., 1977. Biomass and nutrient accumulation in a cottonwood plantation. Soil Sci. Soc. Am. J. 41, 632±636. Bauer, A., Black, A.L., 1994. Quantification of the effect of soil organic matter content on soil productivity. Soil Sci. Soc. Am. J. 58, 185±193. Biermann, C.J., 1993. Essentials of Pulp and Papermaking, Academic Press, New York. Boyle, J.R., Phillips, J.J., Ek, A.R., 1973. Whole tree harvesting: nutrient budget evaluation. J. For. 71, 760±762. Brady, N.C., 1974. The Nature and Properties of Soils, 8th Edition. Macmillan, New York. Brown, J.R., Osburn, D.D., Redhage, D., Gantzer, C.J., 1995. Multi-crop comparisons on Sanborn Fields, Missouri, USA. In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 111±132. Bruce, R.R., Langdale, G.W., Dillard, A.L., 1990. Tillage and crop rotation effect on characteristics of a sandy surface soil. Soil Sci. Soc. Am. J. 54, 1744±1747. Buol, S.W., 1995. Sustainability of soil use. Ann. Rev. Ecol. Syst. 26, 25±44. Buyanovsky, G.A., Brown, J.R., Wagner, G.H., 1997. Sanborn field: effect of 100 years of cropping on soil parameters influencing productivity. In: Paul, E.A., Paustian, K, Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL, pp. 205±225. Cambardella, C.A., Elliott, E.T., 1992. Particulate soil organicmatter changes across a grassland cultivation sequence. Soil Sci. Soc. Am. J. 56, 777±783. Carlson, C.W., 1987. Technology unlocks cropland productivity. In: Whyte, W. (Ed.), Our American Land: 1987 Yearbook of Agriculture. US Department of Agriculture, Washington, DC, pp. 309±314. Carter, M.C., White, E.H., 1971. Dry weight and nutrient accumulation in young stands of cottonwood (Populus deltoides Bartr.), Auburn University Agricultural Experiment Station Circulation, 190, 14 pp.

E.D. Vance / Forest Ecology and Management 138 (2000) 369±396 Cassel, D.K., Raczkowski, C.W., Denton, H.P., 1995. Tillage effects on corn production and soil physical conditions. Soil Sci. Soc. Am. J. 59, 1436±1443. Cassman, K.G., Steiner, R., Johnston, A.E., 1995. Long-term experiments and productivity indexes to evaluate the sustainability of cropping systems. In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 231± 255. Cole, D.W., Rapp, M., 1981. Elemental cycling in forest ecosystems. In: Reichle, D.E. (Ed.), Dynamic Properties of Forest Ecosystems. Cambridge University Press, New York, pp. 341±409. Crist, J., Dawson, D.H., 1976. Anatomy and dry weight yields of two Populus clones grown under intensive culture, USDA Forest Service Research Paper NC-113. Crosson, P., 1985. National costs of erosion effects on productivity. In: Proceedings of the Symposium on Erosion and Soil Productivity, New Orleans, LA, 10±11 December 1984. American Society of Agricultural Engineering Publication 885, St. Joseph, MI, pp. 254±265. Curtin, D., Campbell, C.A., Zentner, R.P., Lafond, G.P., 1994. Long-term management and clay dispersibility in two Haploborolls in Saskatchewan. Soil Sci. Soc. Am. J. 58, 962±967. Darmody, R.G., Peck, T.R., 1997. Soil organic carbon changes through time at the University of Illinois morrow plots. In: Paul, E.A., Paustian, K., Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL, pp. 161±169. Davis, J.G., 1994. Managing plant nutrients for optimum water use efficiency and water conservation. In: Sparks, D.L. (Eds.), Advanced Agronomics, Vol. 53. Academic Press, New York, pp. 85±120. Dick, W.A., McCoy, E.L., Edwards, W.M., Lal, R., 1991. Continuous application of no-tillage to Ohio soils. Agron. J. 83, 65±73. Donigian Jr., A.S., Patwardhan, A.S., Jackson, R.B., Barnwell Jr., T.O., Weinrich, K.B., Rowell, A.L., 1995. Modeling the impacts of agricultural management practices on soil carbon in the central US. In: Lal, R., Kimble, J., Levine, E., Stewart, B.A. (Eds.), Soil Management and Greenhouse Effect. Adv. Soil Sci., Lewis Publishers, Boca Raton, FL, pp. 121±135. Doran, J.W., Sarrantonio, M., Liebig, M.A., 1996. Soil health and sustainability. In: Sparks, D.L. (Ed.), Advanced Agronomics, Vol. 56, pp. 1±54. Drees, L.R., Karathanasis, A.D., Wilding, L.P., Blevins, R.L., 1994. Micromorphological characteristics of long-term no-till and conventionally tilled soils. Soil Sci. Soc. Am. J. 58, 508± 517. Duff, B., Rasmussen, P.E., Smiley, R.W., 1995. Wheat/fallow systems in semi-arid regions of the Pacific NW America. In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 85±109. Dyck, W.J., Cole, D.W., 1994. Strategies for determining consequences of harvesting and associated practices on longterm productivity. In: Dyck, W.J., Cole, D.W., Comerford, N.B.

393

(Eds.), Impacts of Forest Harvesting on Long-Term Site Productivity. Chapman & Hall, London, pp. 13±40. Flach, K.W., Cline, M.G., 1954. Does cropping affect soil organic matter? Farm Res. 20, 13. Flach, K.W., Barnwell, T.O., Crosson, P., 1997. Impacts of agriculture on atmospheric carbon dioxide. In: Paul, E.A., Paustian, K., Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL, pp. 3±13. Forcella, F., Burnside, O.C., 1994. Pest management Ð weeds. In: Hatfield, J.L., Karlen, D.L. (Eds.), Sustainable Agriculture Systems. Lewis Publishers, Boca Raton, FL, pp. 157±197. Francis, C.A., Youngberg, G., 1990. Sustainable agriculture Ð an overview. In: Francis, C.A., Flora, C.B., King, L.D. (Eds.), Sustainable Agriculture in Temperate Zones. Wiley, New York, pp. 1±23. Franzluebbers, A.J., Hons, F.M., Zuberer, D.A., 1994. Long-term changes in soil carbon and nitrogen pools in wheat management systems. Soil Sci. Soc. Am. J. 58, 1639±1645. Frison, G., 1969. Alouni aspetti della nutrizione minerale del pioppi in vivaio: produzione in sostanza secca ed assorbimento di sostanze nutritizie. Cellulosa E Carta 20, 28±34. Frye, W.W., Bennett, O.L., Buntley, G.J., 1985. Restoration of crop productivity on eroded or degraded soils. In: Follett, R.F., Stewart, B.A. (Eds.), Soil Erosion and Crop Productivity. Soil Science Society of America, Madison, WI, pp. 335±356. Funderburk, J.E., Higley, L.G., 1994. Management of arthropod pests. In: Hatfield, J.L., Karlen, D.L. (Eds.), Sustainable Agriculture Systems. Lewis Publishers, Boca Raton, FL, pp. 199±228. Gantzer, C.J., Anderson, S.H., Thompson, A.L., Brown, J.R., 1991. Evaluation of soil loss after 100 years of soil and crop management. Agron. J. 83, 74±77. Giulimondi, G., Duranti, G., 1974. Ritmo d'incremento in sostanza secca e di utilizzazione in elementi nutritivi del pioppo in vivaro durinti il secondo anno. Cellulosa E Carta 11, 3±20. Glendining, M.J., Powlson, D.S., 1995. The effects of long continued applications of inorganic nitrogen fertilizer on soil organic nitrogen Ð a review. In: Lal, R., Stewart, B.A. (Eds.), Soil Management Ð Experimental Basis for Sustainability and Environmental Quality. Lewis Publishers, Boca Raton, FL, pp. 385±446. Grace, P.R., Oades, J.M., Keith, H., Hancock, T.W., 1995. Trends in wheat yields and soil organic carbon in the Permanent Rotation Trial at the Waite Agricultural Research Institute, South Australia. Aust. J. Exp. Agric. 35, 857±864. Greenland, D.J., 1994. Long-term cropping experiments in developing countries: the need, the history and the future. In: Leigh, R.A., Johnston, A.E. (Eds.), Proceedings of Conference on Long-term Experiments in Agricultural and Ecological Sciences Rothamsted, 14±17 July. CAB International, Wallingford, UK, pp. 187±209. Hammel, J.E., 1989. Long-term tillage and crop rotation effects on bulk density and soil impedance in northern Idaho. Soil Sci. Soc. Am. J. 53, 1515±1519. Hansen, E.A., Baker, J.B., 1979. Biomass and nutrient removal in short rotation intensively cultured plantations. In: Proceedings

394

E.D. Vance / Forest Ecology and Management 138 (2000) 369±396

of a Symposium on Impact of Intensive Harvesting on Forest Nutrient Cycling. State University of New York, Syracuse, NY, pp. 130±151. Hardy, D.H., Raper Jr., C.D., Miner, G.S., 1990. Chemical restrictions of roots in Ultisol subsoils lessened by long-term management. Soil Sci. Soc. Am. J. 54, 1657±1660. Healy, R.G., Sojka, R.E., 1985. Agriculture in the South: conservation's challenge. J. Soil Water Conserv. 40, 189±194. Heilman, P.E., Stettler, R.F., 1986. Nutritional concerns in selection of black cottonwood and hybrid clones for short rotation. Can. J. For. Res. 16, 860±863. Herdt, R.W., Steiner, R.A., 1995. Agricultural sustainability: concepts and conundrums. In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 3±13. Hill, R.L., 1990. Long-term conventional and no-tillage effects on selected soil physical properties. Soil Sci. Soc. Am. J. 54, 161± 166. Hill, R.L., Meza-Montalvo, M., 1990. Long-term wheel traffic effects on soil physical properties under different tillage systems. Soil Sci. Soc. Am. J. 54, 865±870. Horn, R., Domzal, H., van Ouwerkerk, C., 1995. Soil compaction processes and their effects on the structure of arable soils and the environment. Soil Till. Res. 35, 23±36. Huffman, S.A., Cole, C.V., Scott, N.A., 1996. Soil texture and residue addition effects on soil properties and continuous corn yields. Soil Sci. Soc. Am. J. 60, 1095±1101. Ismail, I., Blevins, R.L., Frye, W.W., 1994. Long-term no-tillage effects on soil properties and continuous corn yields. Soil Sci. Soc. Am. J. 58, 193±198. Jenkinson, D.S., 1988. Soil organic matter and its dynamics. In: Wild, A. (Ed.), Russell's Soil Conditions and Plant Growth. Wiley, New York, pp. 564±607. Jenkinson, D.S., 1991. The Rothamsted long-term experiments: are they still of use? Agron. J. 83, 2±10. Jenkinson, D.S., Parry, L.C., 1989. The nitrogen cycle in the Broadbalk Wheat Experiment, a model for the turnover of nitrogen through the soil microbial biomass. Soil Biol. Biochem. 21, 535±541. Jenkinson, D.S., Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted Classical Experiments. Soil Sci. 123, 298±305. Johnson, D.W., 1994. Reasons for concern over impacts of harvesting. In: Dyck, W.J., Cole, D.W., Comerford, N.B. (Eds.), Impacts of Forest Harvesting on Long-Term Site Productivity. Chapman & Hall, London, pp. 1±12. Johnston, A.E., Powlson, D.S., 1994. The setting-up, conduct and applicability of long-term, continuing field experiments in agricultural research. In: Greenland, D.J., Szabolcs, I. (Eds.), Proceedings of the Symposium on Soil Resilience and Sustainable Land Use, Budapest, 28 September±2 October 1992. CAB International, Wallingford, UK, pp. 395±421. Joslin, J.D., Schoenholtz, S.H., 1997. Measuring the environmental effects of converting cropland to short-rotation woody crops: a research approach. Biomass Bioenergy 13, 301±311. Karlen, D.L., Stott, D.E., 1994. A framework for evaluating physical and chemical indicators of soil quality. In: Doran,

J.W., Coleman, D.C., Bezdicek, D.F., Stewart, B.A. (Eds.), Defining Soil Quality for a Sustainable Environment. Soil Science Society of America, Special Publication No. 35, Madison, WI, pp. 53±72. Karlen, D.L., Erbach, D.C., Kaspar, T.C., Colvin, T.S., Berry, E.C., Timmons, D.R., 1990. Soil tilth: a review of past perceptions and future needs. Soil Sci. Soc. Am. J. 54, 153±161. Karlen, D.L., Wollenhaupt, N.C., Erbach, D.C., Berry, E.C., Swan, J.B., Eash, N.S., Jordahl, J.L., 1994. Long-term tillage effects on soil quality. Soil Till. Res. 32, 313±327. Lal, R., Mahboubi, A.A., Fausey, N.R., 1994. Long-term tillage and rotation effects on properties of a Central Ohio soil. Soil Sci. Soc. Am. J. 58, 517±522. Langdale, G.W., Denton, H.P., White Jr., A.W., Gilliam, J.W., Frye, W.W., 1985. Effects of soil erosion on crop productivity of southern soils. In: Follett, R.F., Stewart, B.A. (Eds.), Soil Erosion and Crop Productivity. Soil Science Society of America, Madison, WI, pp. 251±270. Larney, F.J., Kladivko, E.J., 1989. Soil strength properties under four tillage systems at three long-term study sites in Indiana. Soil Sci. Soc. Am. J. 53, 1539±1545. Larson, W.E., Pierce, F.J., 1994. The dynamics of soil quality as a measure of sustainable management. In: Doran, J.W., Coleman, D.C., Bezdicek, D.F., Stewart, B.A. (Eds.), Defining Soil Quality for a Sustainable Environment. Soil Science Society of America, Special Publication No. 35, Madison, WI, pp. 37±51. Larson, W.E., Pierce, F.J., Dowdy, R.H., 1983. The threat of soil erosion to long-term crop production. Science 219, 458±469. Leinweber, P., Reuter, G., 1992. The influence of different fertilization practices on concentrations of organic carbon and total nitrogen in particle-size fractions during 34 years of a soil formation experiment in loamy marl. Biol. Fertil. Soils 13, 119±124. Lipiec, J., Stepniewski, W., 1995. Effects of soil compaction and tillage systems on uptake and losses of nutrients. Soil Till. Res. 35, 37±52. Lockeretz, W., 1990. Major issues confronting sustainable agriculture. In: Francis, C.A., Flora, C.B., King, L.D. (Eds.), Sustainable Agriculture in Temperate Zones. Wiley, New York, pp. 423±438. Mann, L.K., 1986. Changes in soil carbon storage after cultivation. Soil Sci. 142, 279±288. Mann, L.K., Johnson, D.W., West, D.C., Cole, D.W., Hornbeck, J.W., Martin, C.W., Riekerk, H., Smith, C.T., Swank, W.T., Tritton, L.M., Van Lear, D.H., 1988. Effects of whole-tree and stem-only clearcutting on post-harvest hydrologic losses, nutrient capital and regrowth. For. Sci. 34, 412±428. McCracken, R.J., Lee, J.S., Arnold, R.W., McCormack, D.E., 1985. An appraisal of soil resources in the USA. In: Follett, R.F., Stewart, B.A. (Eds.), Soil Erosion and Crop Productivity. Soil Science Society of America, Madison, WI, pp. 49±66. McDaniel, T.A., Hajek, B.F., 1985. Soil erosion effects on crop productivity and soil properties in Alabama. In: Proceedings of the Symposium on Erosion and Soil Productivity, New Orleans, Louisiana, 10±11 December 1984. American Society of Agricultural Engineering Publication 8-85, St. Joseph, MI, pp. 48±58.

E.D. Vance / Forest Ecology and Management 138 (2000) 369±396 Miller, F.P., Larson, W.E., 1990. Lower input effects on soil productivity and nutrient cycling. In: Edwards, C.A., Lal, R., Madden, P., Miller, R.H., House, G. (Eds.), Sustainable Agricultural Systems. St. Lucie Press, Delray Beach, FL, pp. 549±568. Miller, F.P., Rasmussen, W.D., Meyer, L.D., 1985. Historical perspective of soil erosion in the United States. In: Follett, R.F., Stewart, B.A. (Eds.), Soil Erosion and Crop Productivity. Soil Science Society of America, Madison, WI, pp. 23±48. Mitchell, C.C., Westerman, R.L., Brown, J.R., Peck, T.R., 1991. Overview of long-term agronomic research. Agron. J. 83, 24±29. Monreal, C.M., Janzen, H.H., 1993. Soil organic-carbon dynamics after 80 years of cropping a Dark Brown Chernozem. Can. J. Soil Sci. 73, 133±136. Montgomery, D.C., 1985. Introduction to Statistical Quality Control. Wiley, New York. Nambiar, E.K.S., 1996. Sustained productivity of forests is a continuing challenge to soil science. Soil Sci. Soc. Am. J. 60, 1629±1642. National Research Council, 1990. Forestry Research Ð A Mandate for Change. National Academy Press, Washington, DC. Odell, R.T., Melsted, S.W., Walker, W.M., 1984. Changes in organic carbon and nitrogen of Morrow Plot soils under different treatments, 1904±1973. Soil Sci. 137, 160±171. Olson, K.R., 1994. Effects of soil formation, erosion, and management on long-term productivity. In: McIsaac, G., Edwards, W.R. (Eds.), Sustainable Agriculture in the American Midwest: Lessons from the Past, Propects for the Future. University of Illinois, Urbana, IL, pp. 188±214. Papendick, R.I., 1994. Maintaining soil physical conditions. In: Greenland, D.J., Szabolcs, I. (Eds.), Soil Resilience and Sustainable Land Use. CAB International, Wallingford, UK, pp. 215±234. Parr, J.F., Papendick, R.I., Hornick, S.B., Meyer, R.E., 1992. Soil quality: attributes and relationship to alternative and sustainable agriculture. Am. J. Alt. Agric. 7, 5±11. Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Sci. Soc. Am. J. 51, 1173±1179. Paustian, K. Collins, H.P., Paul, E.A., 1997. Management controls on soil carbon. In: Paul, E.A., Paustian, K., Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL, pp. 15±49. Paustian, K., Parton, W.J., Persson, J., 1992. Modeling soil organic matter in organic-amended and nitrogen-fertilized long-term plots. Soil Sci. Am. J. 56, 476±488. Paustian, K., AndreÂn, O., Clarholm, M., Hansson, A.C., Johansson, G., LagerloÈf, J.K., Lindberg, T., Pettersson, R., Sohlenius, B., 1990. Carbon and nitrogen budgets of four agro-ecosystems with annual and perennial crops, with and without N fertilization. J. Appl. Ecol. 27, 60±84. Persson, J., Mattsson, L., 1988. Soil C changes and size estimates of different organic C fractions in a Swedish long-term small plot experiment. Swed. J. Agric. Res. 18, 9±12. Pesek, J., 1994. Historical perspective. In: Hatfield, J.L., Karlen, D.L. (Eds.), Sustainable Agriculture Systems. Lewis Publishers, Boca Raton, FL, pp. 1±19.

395

Peterson, G.A., Westfall, D.G., Cole, C.V., 1993. Agroecosystem approach to soil and crop management research. Soil Sci. Soc. Am. J. 57, 1354±1360. Pierce, F.J., Larson, W.E., 1993. Developing criteria to evaluate sustainable land management. In: Kimble, J.M. (Ed.), Proceedings of the 8th International Soil Management Workshop: Utilization of Soil Survey Information for Sustainable Land Use. USDA-SCS, Lincoln, NE, National Soil Survey, pp. 7±14. Pierce, F.J., Fortin, M.-C., Staton, M.J., 1994. Periodic plowing effects on soil properties in a no-till farming system. Soil Sci. Soc. Am. J. 58, 1782±1787. Pikul Jr., J.L., Zuzel, J.F., 1994. Soil crusting and water infiltration affected by long-term tillage and residue management. Soil Sci. Soc. Am. J. 58, 1524±1530. Poulton, P.R., 1995. The importance of long-term trials in understanding sustainable farming systems: the Rothamsted experience. Aust. J. Exp. Agric. 35, 825±834. Powers, R.F., Alban, D.H., Miller, R.E., Tiarks, A.E., Wells, C.G., Avers, P.E., Cline, R.G., Fitzgerald, R.O., Loftus Jr., N.S., 1990. Sustaining site productivity in North American forests: problems and prospects. In: Gessel, S.P., Lacate, D.S., Weetman, G.F., Powers, R.F. (Eds.), Sustained Productivity of Forest Soils. Proceedings of the 7th North American Forest Soils Conference. University of British Columbia, Faculty of Forestry Publication, Vancouver, BC, pp. 49±79. Powlson, D.S., Johnston, A.E., 1994. Long-term field experiments: their importance in understanding sustainable land use. In: Greenland, D.J., Szabolcs, I. (Eds.), Proceedings of the Symposium on Soil Resilience and Sustainable Land Use, Budapest, 28 September±2 October 1992. CAB International, Wallingford, UK, pp. 367±394. Ranger, J., Nys, C., 1996. Biomass and nutrient content of extensively and intensively managed coppice stands. Forestry 69, 91±110. Rasmussen, P.E., Collins, H.P., 1991. Long-term impacts of tillage, fertilizer, and crop residue on soil organic matter in temperature semiarid regions, and crop residue on soil organic matter in temperature semiarid regions. Adv. Agron. 45, 93±134. Rasmussen, P.E., Rohde, C.R., 1988. Long-term tillage and nitrogen fertilization effects on organic nitrogen and carbon in a semi-arid soil. Soil Sci. Soc. Am. J. 52, 1114±1117. Rasmussen, P.E., Smiley, R.W., 1997. Soil carbon and nitrogen change in long-term agricultural experiments at Pendleton, OR. In: Paul, E.A., Paustian, K., Elliott, E.T., Cole, C.V. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL, pp. 353±360. Rayner, A.I., Welham, S.J., 1995. Economic and statistical considerations in the measurement of total factor productivity (TFP). In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 23±38. Reid, W.S., 1985. Regional effects of soil erosion on crop productivity Ð northeast. In: Follett, R.F., Stewart, B.A. (Eds.), Soil Erosion and Crop Productivity. Soil Science Society of America, Madison, WI, pp. 235±250. Robinson, C.A., Cruse, R.M., Kohler, K.A., 1994. Soil management. In: Hatfield, J.L., Karlen, D.L. (Eds.), Sustainable

396

E.D. Vance / Forest Ecology and Management 138 (2000) 369±396

Agriculture Systems. Lewis Publishers, Boca Raton, FL, pp. 109±134. Ryan, T.P., 1989. Statistical Methods for Quality Control, Wiley, New York. Sadler, E.J., Turner, N.C., 1994. Water relationships in a sustainable agriculture system. In: Hatfield, J.L., Karlen, D.L. (Eds.), Sustainable Agriculture Systems. Lewis Publishers, Boca Raton, FL, pp. 21±46. Salinas-Garcia, J.R., Hons, F.M., Matocha, J.E., 1997. Long-term effects of tillage and fertilization on soil organic matter dynamics. Soil Sci. Soc. Am. J. 61, 152±159. Soane, B.D., van Ouwerkerk, C., 1995. Implications of soil compaction in crop production for the quality of the environment. Soil Till. Res. 35, 5±22. Soule, J., Carre', D., Jackson, W., 1990. Ecological impact of modern agriculture. In: Carroll, C.R., Vandermeer, J.H., Rosset, P. (Eds.), Agroecology. McGraw Hill, New York, pp. 165±188. Switzer, G.L., Nelson, L.E., Baker, J.B., 1976. Accumulation and distribution of dry matter and nutrients in Aigeiros poplar plantations. In: Proceedings of the Symposium on Eastern Cottonwood and Related Species. Greenville, MS, pp. 359± 369. Tester, C.F., 1990. Organic amendment effects on physical and chemical properties of a sandy soil. Soil Sci. Soc. Am. J. 54, 827±831. Tew, D.T., Morris, L.A., Allen, H.L., Wells, C.G., 1986. Estimates of nutrient removal, displacement and loss resulting from harvest and site preparation of a Pinus taeda plantation in the Piedmont of North Carolina. For. Ecol. Manage. 15, 257±267. Thornton, F.C., Joslin, J.D., Bock, B.R., Houston, A., Green, T.H., Schoenholtz, S., Pettry, D., Tyler, D.D., 1998. Environmental effects of growing woody crops on agricultural land: first year effects on erosion, and water quality, and water quality. Biomass Bioenergy 15, 57±69. Traxler, G., Novak, J., Mitchell, Jr., C.C., Runge, M., 1995. Longterm cotton productivity under organic, chemical, and no

nitrogen fertilizer treatments, 1896±1992. In: Barnett, V., Payne, R., Steiner, R. (Eds.), Agricultural Sustainability: Economic, Environmental and Statistical Considerations. Wiley, New York, pp. 42±61. Tyler, D.D., Wagger, M.G., McCracken, D.V., Hargrove, W.L., 1994. Role of conservation tillage in sustainable agriculture in the southern United States. In: Carter, M.R. (Ed.), Conservation Tillage in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL, pp. 209±229. Van Veen, J.A., Merckx, R., Van De Geijn, S.C., 1989. Plant- and soil-related controls of the flow of carbon from roots through the soil microbial biomass. Plant Soil 115, 179±188. Wells, C.G., Jorgensen, J.R., 1975. Nutrient cycling in loblolly pine plantation. In: Bernier, B., Winget, C.H. (Eds.), Proceedings of the 4th North American Forest Soils Conference on Forest Soils and Forest Land Management. Laval University, Quebec, August 1973. Les Presses De L'Universite Laval, Quebec, pp. 137±158. Whalley, W.R., Dumitru, E., Dexter, A.R., 1995. Biological effects of soil compaction. Soil Till. Res. 35, 53±68. White, Jr., A.W., Bruce, R.R., Thomas, A.W., Langdale, G.W., Perkins, H.F., 1985. Characterizing productivity of eroded soils in the Southern Piedmont. In: Proceedings of the Symposium on Erosion and Soil Productivity, New Orleans, LA, 10±11 December 1984. American Society of Agricultural Engineering Publication 8-85, St. Joseph, MI, pp. 83±95. Wymore, A.W., 1993. Model-based Systems Engineering: An Introduction to the Mathematical Theory of Discrete Systems and to the Tricotyledon Theory of System Design. CRC Press, Boca Raton, FL. Young, D.L., Taylor, D.B., Papendick, R.I., 1985. Separating erosion and technology impacts on winter wheat yields in the Palouse: A statistical approach. In: Proceedings of the Symposium on Erosion and Soil Productivity, New Orleans, LA, 10±11 December 1984. American Society of Agricultural Engineering Publication 8-85, St. Joseph, MI, pp. 130±142.