Potential environmental effects of corn (Zea mays L.) stover removal with emphasis on soil organic matter and erosion

Agriculture, Ecosystems and Environment 89 (2002) 149–166

Review

Potential environmental effects of corn (Zea mays L.) stover removal with emphasis on soil organic matter and erosion Linda Mann∗ , Virginia Tolbert, Janet Cushman Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6038, USA Received 29 March 2000; received in revised form 21 December 2000; accepted 22 January 2001

Abstract Recent concerns about CO2 emissions and global warming have prompted renewed interest in using corn stover for energy production. Lack of markets, concerns about sustained soil productivity, and lack of commercial conversion technologies have precluded the widespread harvest of corn residues for this purpose. This paper reviews existing literature to evaluate the major environmental impacts potentially associated with stover harvest from reduced tillage corn production sites. Issues of greatest concern are erosion and soil organic carbon (SOC) dynamics, the latter both for its role in soil quality and yield and for global carbon cycle implications. About half of the literature examined concerned research in the United States, many publications described research activities conducted for decades, and major soil types in corn producing regions were well represented. Regional differences were primarily temperature and rainfall effects on stand establishment and yield, with potential feedback effects on SOC. Several research papers discussing the effects of residue harvest were found, but few field studies were found that explicitly studied the effects of corn stover harvest and most discussions acknowledge potential tradeoffs among beneficial and adverse effects. It was concluded that more information is needed on several topics to determine potential long-term effects of residue harvest, including (1) erosion and water quality, especially pesticides and nitrate, (2) rates of transformation of different forms of SOC, (3) effects on soil biota, and (4) SOC dynamics in the subsoil. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Corn; Residue; Harvest; Erosion; Soil quality; Review

1. Introduction Over the last few decades, concerns about CO2 emissions and global warming have increased at the same time that reduced tillage methods have been increasingly used in agricultural production. These topics are related and have resulted in a multitude ∗ Corresponding author. Tel.: +1-865-376-9934, fax: +1-865-576-8646. E-mail addresses: [email protected], [email protected] (L. Mann), [email protected] (V. Tolbert).

of long- and short-term research projects to evaluate the effects of different tillage practices on carbon and nitrogen dynamics, erosion, and other aspects of soil quality (Paul et al., 1997a; Doran et al., 1994; Paustian et al., 1998b). In 1970s and early 1980s, interest in United States energy self-sufficiency prompted research on the potential of using farm crop residues for energy production. Residues, including unharvested leafy stalks also known as stover, can be used for energy production, either through direct combustion of residues or through microbial conversion of residues to ethanol fuel. Although corn

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(Zea mays) grain has been used for ethanol production, technologies to convert corn residues to ethanol fuels have been developed only recently. If concerns about environmental sustainability, especially soil quality, are understood and appropriate measures to ensure sustainable harvest can be implemented, corn residues may provide an additional, significant source of corn-based renewable fuels. Currently, there is renewed interest in using some stover production to partially offset energy production from fossil fuel (Lal et al., 1998). This interest is primarily centered on highly productive corn lands where erosion control might not require leaving all of the corn stover in the field (Paustian et al., 1998a; Paine et al., 1996). In some locations, continuous corn production with reduced-tillage or no-till continuous corn has resulted in dense ground cover of corn stover. Such dense cover potentially contributes to planting difficulties, which in turn can lead to lower yields, and subsequently lower soil organic carbon (SOC) sequestration rates (Cannell and Hawes, 1994; Swan et al., 1996). This review synthesizes relevant literature to evaluate the potential environmental effects of removing corn stover. Issues of greatest concern are erosion and SOC dynamics, the latter both for its role in soil quality and yield and for global carbon cycle implications. Although fundamental to actual production and harvest of corn stover as an energy crop, economic concerns are beyond the scope of this review. This review was initially prepared to advise the United States Department of Energy concerning potential adverse environmental effects of stover removal for energy production and was provided as background material for participants at the USDA-ARS and DOE Biofuels Workshop, 15–16 February 2000, Kansas City, MO, USA. Although a brief synopsis of potentially importance influences of stover removal on SOC dynamics is included in order to provide context for potential effects, a complete synthesis of research on SOC dynamics is well beyond the scope of this review.

2. Background As noted in recent reviews (Tiessen et al., 1994; Buol, 1995; Paul et al., 1997a; Doran et al., 1994; Paustian et al., 1998b), the complex interactions of factors affecting production of crops including corn

have been the subject of research for centuries. Corn production is affected by interactions of climate, soil properties, and management activities. However, the way yield and soil quality are affected by interactions of these three factors is not always clear. Erosion, SOC and nutrient dynamics, water availability, and physical qualities of the root zone are strongly affected by interactions of climate, inherent soil characteristics, and current and past management strategies of tillage type, fertilization, and crop rotations. Most research on corn yields, SOC and nitrogen dynamics, and erosion are from the Central United States corn belt (Fig. 1, Table 1). Of more than 200 readily available recent research publications on the subject of corn production, erosion, organic matter and nutrient dynamics, ∼50% concerned research in the United States, ∼25% were from the United States corn belt, and ∼25% were from Canadian research sites. Several of the research projects in corn-growing regions have been conducted for many years (Table 1). Most of the older, long-term projects were originally established to evaluate effects of manure additions and crop rotations on yields and sustainability (Mitchell et al., 1991; Paul et al., 1997a). In subsequent years, the effects of erosion, mineral fertilizers, lime, and tillage have been added to research objectives. Although most of these long-term studies do not evaluate the effects of residue removal per se, comparisons of environmental effects of tillage and no-till corn production provide important data for evaluating the potential effects of corn stover removal on erosion and SOC. As can be seen from Table 1, research on corn has been reported for only a few soil types: primarily Borolls (e.g. Argialbolls, Haploborolls) and Boralfs in the northern United States corn belt and Canada, Udolls (e.g. Argiudolls, Hapludolls) and Alfisols (e.g. Hapludalfs, Fragiudalfs) throughout the United States corn belt, and Alfisols and Ultisols (e.g. Paleudults) in the southeastern and northeastern corn-growing regions. However, these soil types are typical of most corn producing regions and provide a basis for evaluating potential regional differences of effects of corn stover harvest. This is important to ensure that implications for harvest removal are not derived exclusively from data from corn-producing regions such as the United States corn belt where the majority of research on SOC and erosion in relation to corn production has been conducted.

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Fig. 1. Map of the corn belt region of the United States.

Few long-term empirical studies were found that dealt directly with the effects of corn stover removal, and results from several of these studies have been reported only recently (Linden et al., 2000; Clapp et al., 2000). Because of complex interactions among corn crop management activities, erosion, SOC, nutrients, and yield, simulation models are usually necessary to evaluate potential effects of changes in crop management. Although no single simulation model incorporates all aspects of these complex interactions, several widely used simulation models have been

developed to deal with erosion, SOC dynamics, fertility, and yield. Models range in complexity from simple equations used in calculating erosion to complicated process models that involve daily time steps, or more generalized ‘box’ models for evaluating long-term effects (see review by Parton et al. (1996), Powlson et al. (1995), Paul et al. (1997a)). Several studies have developed, tested, and applied simulation models to corn production and management questions in the corn belt (Table 2). Although none of these model applications examined the potential effects of

Table 2 Examples of recent modeling studies of various factors in corn production Location

Model

Factors considered Yield

Hundred locations in the United States corn belt United States corn belt United States eastern corn belt Illinois

EPIC

N

×

EPIC N tillage residue management model EPIC, Gleams

C

×

Author Erosion

Tillage

×

×

Herbicides Lee et al. (1993)

×

×

×

×

×

×

Shaffer et al. (1995) × ×

Lindstrom et al. (1981), Foltz et al. (1993) Phillips et al. (1993)

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residue removal, some studies (Phillips et al., 1993; Parton et al., 1996) provide insights into potential effects through comparisons of till with no-till corn production and are discussed in the following sections.

3. Potential effects of corn stover removal on erosion Wind and water erosion have long been a concern in sustaining crop production in the United States and attempts to quantify erosion losses began in the 1940s for the corn belt. Over subsequent years, the effects of slope, rainfall, soil properties, crop type, management systems, residue cover, and their effects on surface roughness and particulate organic matter content were incorporated in estimates of erosion losses. These efforts culminated in a series of workshops during 1960s that resulted in the development of estimates of soil loss tolerances or T-values in Mg/ha per year thought at the time to be generally adequate for sustaining high productivity levels indefinitely for crop production (Wischmeier and Smith, 1978). These T-values were not, however, intended to be used to determine soil loss limits acceptable for management of water quality (Wischmeier and Smith, 1978; Hall et al., 1985; Larson et al., 1983). Recent publications have reiterated the importance of erosion control, calling into question the suitability of using T-values as a acceptable upper limit for tolerable soil losses (Lal, 1998; Pimentel et al., 1995; Buol et al., 1997; USDA-NRCS, 1998; Pimentel and Kounang, 1999; Troeh et al., 1999). Several authors have expressed concerns about erosion losses in the corn belt in excess of the average rate of natural soil formation which ranges from 0.5 to 1 t/ha per year, well below established T-values. However, in current agricultural production, practical and economic considerations often preclude total elimination of erosion, and T-values remain widely and inappropriately used to estimate permissible levels of erosion. Potential erosion losses threaten declines in productivity of corn and other crops through losses of surface soil particles that contain the highest concentrations of SOC, and nutrients. Changes in soil physical characteristics resulting from erosion of surface layers which expose residuum or clay subsoil can be as important to long-term changes in productivity/yield potential as loss of nutrient rich surface soil (Lal, 1998). Modeling

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studies and measured yield records from Mollisols and Alfisols in the corn belt indicate long-term yield reductions attributable to soil losses by erosion (Shaffer et al., 1995; Foltz et al., 1993; Lee et al., 1993). For example, 21- to 89-year measured average regional corn yields were reduced by ∼10% for severely eroded versus control plots. Yield reductions resulted from less plant-available soil water-holding capacity, less available rooting depth (shallower soils), and greater soil bulk density (Shaffer et al., 1995). Field studies in Indiana found up to 34% corn yield declines attributed to erosion in 3–10 year studies (USDA-NRCS, 1998). Plant available water also declined by as much as 75% on severely eroded sites. Similar reductions in yield in response to erosion in other corn-growing regions of the country were also noted in a recent review by Lal (1998) and the Soil Quality Institute (USDA-NRCS, 1998) and considerable loss in productivity is projected to occur on most soils if erosion occurs at present T-values. These studies highlight the importance of evaluating potential effects of corn stover harvest on erosion and subsequent yields. In addition to reducing corn yields, as erosion rates increase, sediment can also affect offsite environmental values, such as water quality (Cannell and Hawes, 1994; Phillips et al., 1993; Lal, 1998). As Lal (1998) concluded, there is a need for better data on rates of soil formation, parameters relating to soil quality, and offsite environmental concerns to determine soil loss tolerances that improve or minimize degradation of soil quality and yield and minimize offsite environmental effects. Because of these concerns, there is support for the view that any soil erosion is too much (USDA-NRCS, 1998; Lal, 1998; Troeh et al., 1999). In the early 1980s, the potential for accelerated wind and water erosion resulting from loss of the protective effect of corn stover was a major issue raised concerning the use of stover for energy production. The potential impacts to soil loss resulting from corn-stover removal were reviewed in detail for the corn belt (Fig. 1) by Lindstrom et al. (1981). Using the Universal Soil Loss Equation (USLE), they estimated potential soil loss for different tillage and crop rotation scenarios and concluded that more than 1.3 × 106 Mg of corn stover could be removed from more than half of the acreage used for corn production without adversely affecting soil erosion. However, their conclusion of no adverse effect was based on predicted levels of erosion

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Fig. 2. Predicted soil loss estimates with about 3000 or 1500 kg/ha of corn residue by USDA-NRCS major land resource areas (MLRAs) (adapted from data in Lindstrom et al. (1981)).

being less than T-values; soils in this region were estimated to lose from 3 to more than 20 Mg/ha per year under residue levels of ∼3000 kg/ha per year and were estimated to lose more than double that if residue left on site were reduced to ∼1500 kg/ha per year (Fig. 2). Several factors affect the amount of residue cover present in corn fields throughout the year. Although about 30% residue cover is still generally considered by the United State Department of Agriculture, National Resources Conservation Service (USDA-NRCS) as adequate to keep erosion below established T-values, as discussed above, T-values are not currently accepted by the scientific community as an adequate level of protection. Tillage operations have a cumulative effect on residue cover, with some types of tillage burying large proportions of residue, while others leave most residue on the surface (Table 3). Crop types in rotation with corn also affect the amount of residue present, but studies of predicted erosion losses showed no significant effect of the presence of winter cover crops and crop rotations on overall erosion losses associated with corn production (Shaffer et al., 1995). Because of the importance of effects of erosion on corn yields, leaving adequate residue cover in place

to prevent erosion losses is critical. In general, higher levels of residue cover result in lower erosion rates (Frye et al., 1985). As can be seen from Lindstrom’s predictions (Fig. 2), partial stover harvest in the corn belt could result in increased erosion losses well above rates of natural soil formation with potential implications for long-term changes in soil quality, site productivity, and yield. Combining data from Lindstrom et al. (1981) and data from Table 3, removing half the stover produced from a corn field under no-till was approximately comparable for erosion control to chisel or disc chisel plowing in terms of the amount of stover left on the surface. In one of the few long-term studies that evaluated potential effects of stover removal on erosion and yield, Karlen et al. (1994) found that doubling the corn stover on a site in Iowa for 10 years resulted in long-term increases in aggregate stability. However, due to complex interactions among many factors including nutrients, water supply, and stand establishment, effects of erosion on productivity with and without corn residue removal are sometimes difficult to determine. For example, in a 4-year study in Nebraska where 0–150% (0–15 Mg/ha) corn stover was added to no-till soil corn plots, Power et al.

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(1986) found increased grain and stover production of 120 and 270 kg/ha per year for each Mg/ha of stover added but these increases were accompanied by decreases in soil temperatures and increases in soil water storage and nitrogen availability. Similarly, in a 6-year study in Wisconsin, yields increased but stand density decreased when corn stover was added to no-till corn (Swan et al., 1994). In contrast, in a 3-year study in Quebec, planting through corn residues left on site resulted in both lower stand density and lower yield (Burgess et al., 1996). In a 3-year study of irrigated corn in Nebraska, complex interactions were found between nitrogen fertilization, corn stover removal, and yield (Sims et al., 1998). The latter study suggests that when spring soil temperatures were cool and slowed to warm, yields were reduced when residue is not removed unless high nitrogen fertilization is used. However, with warmer spring temperatures, yields were higher with residue left in place. In contrast, a 3-year study in Minnesota found that residue removal resulted in colder soil in winter which resulted in later spring thaw (Sharratt et al., 1998). In a 13-year study in Minnesota, although there were no differences in yield in excessively dry years or

for the long-term average, leaving residue on site resulted in higher yields of corn in moderately dry years (Linden et al., 2000). None of these studies reported on erosion differences, but another study in Illinois used simulated rainfall on long-term no-till sites with and without residue removed and found an increase of sediment concentrations from 0.0045 to 0.0067 g/g with no-till, residue removed (King et al., 1995).

4. Potential effects of corn stover removal on SOC 4.1. SOC dynamics SOC is a major component in the global carbon cycle; therefore, it is important to evaluate corn stover harvest in relation to soil carbon sequestration (Paul et al., 1997a; Paustian et al., 1998a; Paustian et al., 1998b). Climate, inherent soil characteristics, and current management activities have complex interactions with corn stover production and SOC dynamics (Fig. 3) (Buol, 1995; Parton, 1996; Paustian et al., 1997; Lal et al., 1998). SOC in the surface layer

Fig. 3. Simplified conceptual model of interactions and feedbacks between tillage and soil factors affecting soil organic matter content (adapted from Fig. 2 in Paustian et al. (1997)).

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(∼0–30 cm depth) has the biggest effect on tilth, or soil workability, nitrogen dynamics, water retention, and other factors important to corn production and soil resistance to erosion. The SOC content of this surface layer also responds fairly quickly to changes in management, such as stover removal. Many longand short-term studies of corn production have shown an increase in soil carbon in the surface layers with no-till management (see references cited in Table 1 and reviews in Paul et al. (1997a), Paustian et al. (1998b)). Some components of soil quality, such as infiltration and aggregate stability are strongly regulated by changes in the surface by few centimeters. However, there is a pronounced stratification of soil carbon, with lower amounts below the surface by few centimeters with no-till. Results of these studies indicate that increases in SOC in the surface layers is a function of C inputs. Thus, additions of stover would result in greater increases in SOC than occurs if stover is removed. For example, Clapp et al. (2000) recently examined some of the complex interactions between stover harvest, N fertilization, and SOC dynamics in a 13-year experiment in Minnesota. They found that there were substantial changes in SOC in both the 0–15 and 15–30 cm depth in response to treatments. Where corn stover was removed from continuous no-till corn plots, SOC remained nearly unchanged over time, but increased about 14% in plots where stover was returned. Perhaps more important for long-term soil quality and carbon sequestration, they also found that the half-life for original or relic SOC was substantially lengthened when stover was not harvested and N fertilization was partially mixed with the stover. Similarly, Karlen et al. (1994) found that doubling the corn stover on a site in Iowa for 10 years resulted in long-term increases in aggregate stability and total SOC. Long-term research on corn production has emphasized the complex role of SOC in maintaining or enhancing crop production (see references in Table 1 relating to production, Buol, 1995; Pimentel and Kounang, 1999; Paul et al., 1997a). Research cited in Table 1 and reviews (Doran et al., 1994; Lal, 1998) demonstrate a clear relationship between corn crop growth, grain yield, soil quality, and SOC and its linkages with soil nutrients, especially nitrogen and phosphorus; soil organisms; and physical characteristics, especially aggregate structure and water holding

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capacity. The amount of stored SOC, both in this surface layer and at depth, is the net result of the annual addition of stover, cobs, leaves, and roots, and organic matter amendments, such as manures, balanced against respiration and leaching losses. The total amount of organic matter produced and added to the soil depends largely on climatic conditions, soil water status, nutrient availability, the growth and allocation to roots, stalks, and grain, and harvest removals. Thus, stover removal results in less potential storage of SOC. This effect has recently been demonstrated in long-term research in no-till corn by Clapp et al. (2000). After years to decades under a particular soil-climate, corn or corn rotation, and fertilization regime, SOC content is thought to reach an equilibrium between decomposition and organic matter inputs (see references in Table 1 relating to C and reviews in Paul et al., 1997a; Paustian et al., 1998a; Paustian et al., 1998b). Agricultural soils with low to intermediate levels of SOC often increase in carbon storage in proportion to residue inputs, but soils initially high in SOC are less responsive to changes in carbon input and may not gain measurable amounts of carbon with higher inputs (Paustian et al., 1997). A few studies, nearly all for northern, wetter, colder, finer textured soils indicate total SOC may have ‘topped out’ under the management practices in use for growing corn (Campbell et al., 1991; Paustian et al., 1997; Izaurralde et al., 1996). However, stover removal would reduce carbon inputs thus eventually lowering SOC storage even in these systems. As annual inputs increase, SOC usually increases until a new equilibrium is reached. Some of the SOC is thought to be transformed into polycyclic humins linked in chemical bonds with multivalent cations (e.g. Al, Fe, and Ca) and clay particles (Tisdall and Oades, 1982; Parton et al., 1993). This passive fraction is most resistant to decomposition with turnover times measured in tens of thousands of years and is, therefore, the most sequestered of SOC, but is the slowest to accumulate (Parton et al., 1993; Paul et al., 1997b). The size of this resistant pool can be more than half of the SOC (Collins et al., 1997), but factors controlling the formation and turnover of this relatively passive fraction are not well understood (Angers et al., 1995; Parton et al., 1996; Paul et al., 1997b). Clapp et al. (2000) have recently demonstrated that stover harvest had

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a significant effect on reducing the half-life of the passive fraction in Minnesota. The magnitude of this effect in other climates and soils in corn producing areas is currently unknown. Just as increasing annual additions of organic carbon results in a higher equilibrium level of SOC, reduced inputs of organic matter resulting from stover removal will result in a new equilibrium being reached at a lower SOC content. Many factors affect the rate at which a new equilibrium would be reached including initial SOC content, changes in respiration rates, and changes in relative amounts of labile and more recalcitrant forms of SOC. Fertilizer inputs also have complicated interactions with organic matter. For example, nitrogen additions increase amounts of residue produced, but also tend to increase decomposition rates. Other important factors potentially affected by stover removal with implications for SOC storage include soil temperature and moisture, soil physical characteristics, and composition and abundance of soil organisms.

mineral and organic particles into larger aggregates. The amounts of nutrients, especially nitrogen, that can be retained in the soil as well as nutrient availability and resulting fertilizer requirements for corn production are also affected by the chemical composition of SOC. The effects of soil biota on SOC transformations have been studied extensively (Omay et al., 1997; Sharma et al., 1997; Edwards, 1997) and there is some indication that soil aggregates formed by earthworm activity may contain compounds that are more resistant to decomposition than aggregates formed by bacterial and fungal material alone (Marinissen and Hillenaar, 1997). Thus, stover harvest may have indirect effects on aggregate stability through effects on earthworm populations. Organic carbon in microbial populations may comprise up to 20% of total SOC. Collins et al. (1997) and Karlen et al. (1994) found that doubling the corn stover on a site in Iowa for 10 years resulted in long-term increases in microbial C as well as increases in aggregate stability and total SOC. 4.3. Simulation models

4.2. Soil biota Species of soil biota found in corn fields also affect SOC dynamics (Holland and Coleman, 1987; Edwards, 1997; Power and Doran, 1988) and have been shown to be affected by the presence of surface residues. For example, fungal populations are more prevalent with no-till corn than with conventional tillage and are more efficient at converting raw organic matter to stable soil aggregates. Furthermore, Karlen et al. (1994) found decreases in fungal biomass and earthworm populations following stover removal with no-till management. Earthworms are also usually more abundant with reduced tillage in corn fields, generally increasing in abundance and diversity over time without tillage (see articles in Edwards, 1997). Corn residues contain various amounts of polysaccharides, simple sugars, amino acids, proteins, phenols, waxes, etc. For example, a study of corn residues reported about 29% soluble organic compounds, 27% hemicellulose, 28% cellulose, 6% lignin, 9% ash, and 10% nitrogen (Buyanovsky et al., 1997). Some of these compounds decompose fairly quickly, others decompose more slowly, some are transformed into other compounds, and some become biological ‘glue’ from microbes or earthworm feces that combine

Because of the complexity of organic matter dynamics, computer simulation models are usually necessary to evaluate potential effects of changes in corn management, such as stover removal. Most researchers studying rates of transformations in order to predict effects of different corn management activities divide the continuum of turnover times into several different categories measured in years or decades, sometimes with additional categories for turnover times measured in days, months, centuries or thousands of years (Balesdent et al., 1990; Parton et al., 1996; Collins et al., 1997; Post et al., 1999; Paul et al., 1997b). Transformations that are complete in less than one growing season affect what is often called the fast, active, or labile pool of SOC. Transformations that take years to centuries to occur affect what are often called intermediate, slow, or recalcitrant pools. The terms used to categorize turnover times are relative, depending on the research objectives or the particular study (Collins et al., 1997; Cambardella and Elliott, 1994; Paul et al., 1997b; Buyanovsky et al., 1997). For example, using the EPIC simulation model to predict effects of different corn management scenarios, Lee et al. (1993) used two major soil pools corresponding to fresh stover and SOC. Each of these major pools were subdivided into

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two or more pools. In a regional study of the United States corn belt and current practices of continuous corn or corn–soybean rotations, Lee et al. (1993) found that total SOC to a depth of 1 m decreased, even with no-till, unless a winter cover crop was included. The study emphasized the important role of SOC pools below the plow layer to carbon sequestration and indicated that about 60% of the loss of total SOC in the past 60 years of cultivation in this region occurred below the plow layer. Recent empirical work by Clapp et al. (2000) highlights the potential role of stover removal on labile and recalcitrant SOC transformations. Parton et al. (1996) and Parton (1996) reviewed applications of the CENTURY and EPIC simulation models and other data to evaluate the potential effects of reduced tillage on SOC storage in the corn belt. They concluded that there is considerable uncertainty about the impact of tillage and residue inputs on deep SOC and nutrient dynamics due to a lack of data. However, model results generally indicate that winter cover crops can increase SOC levels, especially in the south and southeastern United States. They also conclude that the major factor affecting SOC amounts is the relative amount of carbon added to the soil, although they did not look at potential effects of corn stover removal. Deep tillage can increase above- and below-ground production (Erbach et al., 1992; Evans et al., 1996), and could, therefore, potentially increase carbon inputs with minimal effects on surface stover through the use of strip or zone tillage (e.g. paraplow, Table 3).

5. Potential effects of corn stover removal on pests, pesticides, diseases, and stand establishment Few data were found on potential effects of corn stover removal on pest and disease management or stand establishment per se (Swan et al., 1994; Burgess et al., 1996). These results were in agreement with research on residue management in continuous no-till corn where stover contributed to planting difficulties (Cannell and Hawes, 1994). Pest and disease management and stand establishment in continuous no-till corn and no-till corn rotations are still being refined for different regions and soil types (Locke and Bryson, 1997; USDA-NRCS, 1999) and management recommendations and their implications for stover removal

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can vary from one region to another. Crop rotations often reduce some types of weeds, insect pests, and diseases by removing host-crops and increasing competition with weeds (Stinner et al., 1988; Liebman and Dyck, 1993). For example, no-till corn can enhance populations of southern corn rootworm, but also can enhance predatory arthropod populations so there is little net effect on corn yields (Brust and House, 1990). Stem and stalk rots and foliar diseases in corn tend to increase in frequency with continuous cropping and reduced tillage because these pathogens tend to survive in infested stover residue. Similarly, greater amounts of corn residues left on the soil surface can change the microclimate of emerging seedlings by providing a wetter, cooler environment than more intensive tillage systems. These changes in microclimate may increase, decrease or have no effect on plant diseases, depending on the disease (USDA-NRCS, 1998). Cooler temperatures can inhibit corn germination, but Hatfield and Prueger (1996) found corn residues in Iowa did not affect mean daily soil temperatures the following spring and in Minnesota, Sharratt et al. (1998) found that residue removal resulted in colder soil in winter and later spring thaw. However, several studies have reported poor corn crop establishment and early development with no-till and have attributed this to cooler, wetter conditions at planting or phytotoxic effects of corn residues (Kaspar et al., 1990). Direct effects of growth inhibitors in prior year corn residue have not been shown to have an adverse effect on corn germination or growth (Breakwell and Turco, 1989; Kaspar et al., 1990). Regardless of the underlying mechanism, clearing residues or a narrow band of tillage within a planting strip can improve germination and stand establishment under conditions of heavy stover, perhaps due to improved soil–seed contact (Kaspar et al., 1990; Cannell and Hawes, 1994; Vaughan and Evanylo, 1998; Perfect and McLaughlin, 1996). Therefore, stover removal may reduce some of the problems encountered with no-till corn production by reducing diseases such as stem and stalk rots and foliar diseases, improving germination and stand establishment. In particular, stover removal in areas with cool, wet conditions, especially in northern areas might reduce disease incidence. Removal of residue in planting strips in no-till corn in Iowa has been reported to result in enhanced seedling emergence and higher yields with increasing band widths cleared of residue

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(Kaspar et al., 1990). In contrast, residue removal may increase incidence of disease from splatter of soil borne organisms onto plants in continuous corn cropping. Relationships of pesticides with crop residues and SOC are complex and not well understood (Locke and Bryson, 1997; Myers et al., 1995). Changes in SOC resulting from corn stover removal can either enhance or retard retention and decomposition of pesticides used in corn production (Diehl et al., 1995). Carryover effects from persistent herbicides may be greater in colder, wetter conditions of no-till (Locke and Bryson, 1997) which might be lessened with residue removal. Leaching of pesticides is affected by (1) precipitation intensity, amount, and timing, (2) macropores, and (3) interception and retention of rainfall by residues. Plant residues often have a higher capacity for herbicide sorption than soil and may desorb for years after the last application in no-till (Locke and Bryson, 1997). The persistent earthworm tunnels and root channels that result from no-till can increase macropore flow, which in turn can increase flushing of pesticides into water after heavy rains (Jayachandran et al., 1994; Myers et al., 1995). Conversely, organic matter lining earthworm tunnels may increase adsorption and retention of pesticides or may increase degradation rates (Locke and Bryson, 1997). Results of these studies in residue management imply that corn stover removal could lower overall herbicide adsorption and carryover, but might also result in a decrease of earthworm populations near the surface, potentially resulting in a decrease of leaching through macropores, but an increase in surface runoff (Locke and Bryson, 1997).

6. Additional issues of concern Nutrient supply and nitrate and pesticide leaching are issues of concern related to no-till continuous corn that might be affected by stover removal. The analysis by Lindstrom et al. (1981) included an evaluation of the potential for nutrient depletion resulting from crop–stover removal in the corn belt. Nutrients can readily be replaced with mineral fertilizers. Plant nutrients, especially nitrogen, are more of an issue related to availability to plants and leaching into water, both of which are linked with SOC dynamics and

soil biota. In a no-till study in Iowa corn production, Karlen et al. (1998) found that about half of the applied nitrogen was present after harvest and available for leaching to ground water. If earthworm macropores decrease, leaching would be less, but surface runoff could increase with less residue on the surface. Soil moisture is also affected by surface residues, which can reduce evapotranspiration by cooling the soil surface and plant roots (Denton and Wagger, 1992). Warmer soil temperatures resulting from stover removal would increase decomposition rates and could increase water stress, especially during the summer and during drought years (Power et al., 1986; Cannell and Hawes, 1994; Linden et al., 2000).

7. Discussion Because of uncertainties identified through development and application of carbon cycle models, the long-term effects of stover removal can not be accurately predicted for all corn-growing regions. Although significant progress has been made during the last 30 years in quantifying the relative importance of the factors shown in Fig. 3, spatial and temporal aspects of all soil types and management choices, including stover removal, are not fully quantified (Parton et al., 1996; Paul et al., 1997a). Furthermore, the organic matter “box” in Fig. 3 contains many components with equally complex and incompletely quantified dynamics (Christensen, 1996; Paustian et al., 1997; Paul et al., 1997b). However, recent results from long-term stover removal studies in colder corngrowing regions clearly show large effects on carbon sequestration (Karlen et al., 1994; Clapp et al., 2000). Deep tillage and cultivar selection may offer opportunities to increase total SOC storage through enhanced root growth below 30 cm. Tillage restricted to the seed row, otherwise known as zone tillage, has little effect on residue cover, thus would not have much effect on erosion. However, the effect of zone tillage on turnover times of SOC pools is unknown. Although, several studies have shown that tillage following several years of no-till breaks up soil aggregates resulting in increased mineralization of SOC and nitrogen in the cultivated layer and subsequent losses of carbon that was gained from years of no-till (Pierce et al., 1994), no data were found to determine what effect periodic

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cultivation has on the pool of carbon with the longest turnover times. In fact, except for intensive characterization work from a few sites, very little research was found concerning the slowest pools, and only one study was found concerning effects of stover harvest on turnover of these pools (Clapp et al., 2000). Neither the effects of infrequent tillage on total carbon are well documented, nor are the potential combined effects of deep tillage, infrequent tillage, and intermittent stover removal. Even with the relatively rapid response of the plow layer to changes in management, such as corn stover removal, there are many uncertainties associated with those changes (Post et al., 1999). In Fig. 3, the rate of change from one box to another has uncertainty and variability associated with it. Uncertainty results from difficulties in measuring rates of change and heterogeneity of individual soils as well as preceding conditions, seasonality of some changes, and other factors (Garten and Wullschleger, 1999; Post et al., 1999). Several authors (Parton et al., 1996; McCarty et al., 1998) have reviewed applications of the several carbon cycle and crop yield models and concluded that there is considerable uncertainty about the impacts of different crop management because of a lack of field data for model components. For example, more data are needed to characterize SOC transformations below the surface layer, the effect of changes in organic matter inputs on aggregate dynamics, potential effects of changes in biota on SOC dynamics, and the effects of deep tillage on net SOC storage. The complexity of SOC chemistry and interactions with soil biota adds additional uncertainty. In particular, shifts from fungal to bacterial decomposer communities could affect organic matter decomposition rates, with unknown net effect on soil respiration,

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labile, and non-labile SOC. Changes in amounts of residue could also affect earthworm species composition and population densities with unknown effects on soil porosity, infiltration, and nutrient leaching. Net effects of these differences may vary in response to different climate–soil regimes throughout corn-growing regions. Potentially lower SOC and associated implications for long-term changes in soil quality must be weighed against carbon costs in production and harvest and ethanol production from stover to determine whether or not there is a net reduction in carbon emissions. Ongoing research by USDA-ARS is addressing both short- and long-term agronomic concerns and environmental issues as well as corn stover harvesting, storage, and economic issues.

8. Conclusions Although several studies discussing effects of corn stover harvest were found, none related stover harvest to long-term SOC dynamics or carbon sequestration. Published studies, however, can be used for qualitative evaluation of stover harvest. In the few field studies that explicitly studied the effects of corn stover harvest, most discussions acknowledge potential tradeoffs among beneficial and adverse effects (Table 4). In long-term research, reported issues of greatest concern in comparisons of tillage and no-till were (1) corn production and economic return, (2) erosion, (3) leaching of herbicides, pesticides, and nitrate (and other nutrients to lesser extent) into surface and ground water, (4) tilth and physical characteristics of soil (compaction, macropores, microbial activity and effects on herbicide and pesticide degradation,

Table 4 Comparisons of potential benefits and disadvantages of corn stover harvest on factors affecting site characteristics and corn yield Factors

Potential benefits

Carbon and nutrients Erosion Microclimate Pests and Diseases Planting and harvest

Warmer germination temperatures Increased ease of control of some disease and insect pests Increased ease of planting using traditional equipment and practices

Disadvantages Less soil carbon and nitrogen content without amendments Increased erosion; potential offsite effects Increased drought stress Less control of other diseases and pests

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(5) fertilizer requirements, and (6) rotation effects (nitrogen requirements and effects on yield). These will also be issues of concern where amount of stover harvested is an added factor. Corn growing regions reporting long-term (5 years or more) research were in the United States and Canadian corn belt, the southeastern and northeastern United States. There was good representation of long-term research throughout corn-growing regions. If stover is removed, erosion rates would probably increase for all sites. A few important regional differences were seen in rates of soil warming in spring with implications for stand establishment in the northern corn-producing regions. Similarly, in cold, wet conditions, carryover effects of persistent herbicides may be greater. In southern corn-growing areas, winter cover crops could potentially improve net SOC. In non-irrigated corn growing regions, sediment would presumably be lost from the field. However, on highly productive, irrigated fields, and with only partial stover removal, most sediment would probably be deposited within the same field. Overall potential changes in productivity and carbon sequestration for the entire field might be negligible, but are currently not known. If stover is removed, the equilibrium amount of carbon in the soil will be lower in all cases. How much lower is unknown, but could be estimated using model simulations. However, existing models have acknowledged short-comings regarding both carbon flux rates and carbon storage capacity (Paustian et al., 1998a) and may not be adequate. More information is needed on several topics to determine the potential long-term effects of stover harvest. These include (1) effects on erosion losses and water quality issues, (2) rates of transformations of labile and recalcitrant forms of SOC in different corn-producing regions, (3) improved documentation of population dynamics and effects of soil biota on SOC, net soil respiration, erosion, and water quality, (4) more information on the potential for carbon storage below the surface 30 cm, and (5) effects of interactions of crop rotations and winter cover crops on SOC and net carbon emissions. These data are needed to determine if there are changes in erosion, productivity, and soil carbon turnover times, storage, and feedbacks to crop productivity resulting from corn stover harvest that are relevant to long-term carbon sequestration.

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