Global climate change and the effect of conservation practices in US agriculture

Global Environmental Change 10 (2000) 197}209

Global climate change and the e!ect of conservation practices in US agriculture Noel D. Uri *, Herby Bloodworth
Natural Resources Conservation Service NRCS/RID, U.S. Department of Agriculture, Rm 1-2118A, 5601 Sunnyside Avenue, Beltsville, MD 20705, USA
Natural Resources Conservation Service, U.S. Department of Agriculture, Washington DC, USA Received 6 May 1999

Abstract The use of conservation practices by agriculture in the United States will enhance soil organic carbon and potentially increase carbon sequestration. This, in turn, will decrease the net emission of carbon dioxide. A number of studies exist that calibrate the contribution of various individual, site-speci"c conservation practices on changes in soil organic carbon. There is a general absence, however, of a comprehensive e!ort to measure objectively the contribution of these practices including conservation tillage, the Conservation Reserve Program, and conservation bu!er strips to an change in soil organic carbon. This paper "lls that void. After recounting the evolution of the use of the various conservation practices, it is estimated that organic carbon in the soil in 1998 in the United States attributable to these practices was about 12.2 million Mt. By 2008, there will be an increase of about 25%. Given that there is a signi"cant potential for conservation practices to lead to an increase in carbon sequestration, there are a number of policy options that can be pursued. These include education and technical assistance, "nancial assistance, research and development, land retirement, and regulation and taxes.  2000 Published by Elsevier Science Ltd.

1. Introduction Organic carbon is soils plays a key role in the carbon cycle and has a potentially large impact on the greenhouse e!ect (Lal et al., 1998). Soils in the United States contain an estimated 1.5;10 g of carbon, or twice as much as the atmosphere and three times the level held in terrestrial vegetation (Post et al., 1990). The annual net release of carbon from agriculture has been estimated at 0.8;10 g, or about 14% of current fossil fuel emissions globally (Schlesinger, 1995). In addition to the in#uence that soil carbon has on global warming, it also plays a key role in determining long-term soil quality. The ability to sequester carbon in soils by proper tillage and erosion management provides long-term justi"cation for soil conservation programs (Hunt, 1996; Weinhold and Halvorson, 1998). There is, however, a paucity of information on the changes in soil organic carbon (SOC) that accrue from key soil conservation programs and policies (Donigian et al., 1994). After recounting the trend in the use of conservation practices in agriculture in the

* Corresponding author.

United States, this paper provides some objective estimates of the impact that the increase in the use of three conservation practices have on changes in soil organic carbon and hence the potential to sequester carbon. The three conservation practices considered are conservation tillage, the Conservation Reserve Program (CRP), and conservation bu!er strips.

2. The trend in the use of conservation practices and their potential impact on carbon dynamics Carbon levels in the soil are determined by the balance of inputs, as crop residues and organic amendments, and carbon losses through organic matter decomposition. Thus, management to increase soil organic carbon and to enhance the potential to sequester carbon requires increasing carbon inputs, decreasing decomposition, or both. This is the aim of the conservation practices considered here. Additionally, in the context of managing soils to sequester carbon, erosion is a transport process rather than a loss or gain of soil organic carbon. It is not clear whether erosion, at the regional level, increases or decreases carbon stocks in the soil and sediments

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Fig. 1. Percent of planted hectares on which conservation tillage is used in the United States: 1963}1998.

(Van Noordwijk et al., 1997; Paustien et al., 1997). Hence, this is not explicitly addressed. 2.1. Conservation tillage Conservation tillage is probably the best-known conservation practice. Conservation tillage evolved from practices that range from reducing the number of trips over the "eld to raising crops without primary or secondary tillage. Current emphasis is on leaving crop residues on the surface after planting rather than merely reducing the number of trips across the "eld, although the two are closely related. In 1963, the Soil Conservation Service of the US Department of Agriculture began recording the area of cropland planted by minimum tillage, which was than used on about 1.6 million planted hectares (about 1% of the total). By 1967, the area had doubled (Mannering et al., 1987). One of the di$culties in following the trends has been the absence of any consistent de"nition of conservation tillage. Before 1977, `minimum tillagea was used which aimed at reducing the number of tillage trips over a "eld. Although a large portion of the area on which minimum tillage was used would have had considerable amounts of residue after planting, a signi"cant portion would not have met the current de"nition of conservation tillage (Schertz, 1986). In 1977, the Soil Conservation Service changed the term minimum tillage to conservation tillage and de"ned it as a form of noninversion tillage that retains protective amounts of residue on the surface throughout the year.

Conservation tillage included no tillage, strip tillage, stubble mulching, and other types of noninversion tillage. In 1984, the Soil Conservation Service changed the de"nition to the one currently used (Conservation Technology Information Center, 1998): Any tillage and planting system that maintains at least 30% of the soil surface covered by residue after planting to reduce soil erosion by water. Where soil erosion by wind is the primary concern, any system that maintains at least 184 kg (per ha) of #at, small grain residue equivalent on the surface during the critical wind erosion period. Two key factors in#uencing crop residue are (1) the type of crop, which establishes the initial resiude amount and determines its persistence, and (2) the type of tillage operations prior to and including planting. The use of conservation tillage in the United States had an identi"able upward trend until the last few years, which show no discernible change. A longer-term perspective can be obtained from Fig. 1. The use of conservation tillage increased from 1% of planted hectareage in 1963 to approximately 37% of planted hectareage in 1998 (Schertz, 1988; CTIC, 1998). A disaggregated view of the use of conservation tillage can be obtained by considering speci"c crops and states/regions. The

 Schertz estimates conservation tillage for the period 1968 through 1981. His data was taken from a variety of sources and adjustments are made for changes in the de"nition of conservation tillage.

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percentage of US cropland that was conservation tilled increased from 26 to 37% over the 10-year period (1989}1998) but the increase di!ers by crop. That is, conservation tillage is used mostly on soybeans, corn, and small grains. More than 45% of total corn and soybean planted hectareage in 1998 was conservation tilled. Where double-cropping was used, nearly 74% of soybean hectareage, 45% of corn hectareage, and 49% of sorghum hectareage employed conservation tillage systems. Full-season corn is the most extensively grown crop in the United States accounting for 27% of total planted hectareage in 1998. Currently, nearly 40% of planted hectareage on which corn is grown is conservation tilled. A decline in the use of conservation tillage on corn between 1995 and 1998 re#ects reduced use of conservation tillage in Indiana which produces relatively large amounts of corn producing state, and to a lesser degree Ohio. Both States encountered unusually wet weather conditions at the time of planting in the spring. Because of that, more than 260,00 ha in Indiana alone (over 5% of total planted hectareage) that were conservation tilled in 1995 reverted to conventional tillage by 1998. Cotton has the lowest proportion of conservation tillage, increasing from 3 to 12% between 1989 and 1998. Other important crops, like peanuts, potatoes, beets, tobacco, and vegetables have also improved residue management and erosion control, even though their cultural practices preclude the use of conservation tillage. Kentucky leads the United States in conservation tillage, with an adoption rate of 71%. Delaware, Iowa, Maryland, Missouri, Nebraska, and Tennessee all have between 50 and 63% of cropland conservation-tilled. States with adoption rates less than 10% include Arizona, Florida, Massachusetts, Rhode Island, and Vermont. The Appalachian and Corn Belt regions lead in conservation tillage adoption with 49 and 48%, respectively, while the Delta, Southeast, Paci"c and Southern Plains have only between 20 and 24%. The substantial fall in the use of conservation tillage in some of the Midwestern States between 1993 and 1994 re#ects the heavy rains and #ooding in 1993 that destroyed crop hectareage. Nearly 2 million fewer ha were planted in 1993 than in 1992 and 1994. Most of this land was returned to production in 1994, but rills and gullies on the surface and sand and soil deposits on the bottomlands forced farmers to till the soil more. Much of the decline in mulch tillage in 1994 is attributed to this. Thus, the use of conservation tillage in Illinois fell 16.3% between 1993 and 1994 while Kansas, Minnesota, Ohio,  CTIC (1998) is the source of the data. CTIC data collection at the crop/state level based on a consistent de"nition of conservation tillage began only in 1989.  Nearly 48% of Indiana's planted hectareage is in corn. Additionally, more than 7% of total corn planted hectareage in the United States is in Indiana.

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and Wisconsin experienced similar, although less precipitous, declines. Another important factor leading to the decline in conservation tillage in the Corn Belt in 1994 was the absence of a government set-aside program. Previously idled hectares that were returned to production were tilled using conventional practices (Environmental Protection Agency, 1996). Mulch tillage continues to be the dominant type of conservation tillage although there is increased use of no tillage. (Note that de"nitions of mulch tillage and other types of conservation tillage are provided in Appendix A.) Mulch tillage accounted for 53% of conservation tillage and was used on 20% of the cropland in the United States in 1998. No tillage accounted for 44% of conservation tillage, being used on 16% of the cropland in the United States, while ridge tillage was used on only about 1% of the cropland in 1998. Over the past few years, there has been little change in the use of no tillage relative to much tillage. Tillage a!ects decomposition processes through the physical disturbance and mixing of soil, by exposing soil aggregates to disruptive forces, and through the distribution of crop residue in the soil (Beare et al., 1994). Tillage also a!ects soil temperature, aeration and water relations by its impact on surface residue cover and soil structure (Paustien et al., 1999). By increasing the e!ective soil surface area and continually exposing new soil to wetting/drying and freeze/thaw cycles at the surface, tillage makes aggregates more susceptible to disruption and physically protected inter-aggregate organic material becomes more available for decomposition (Elliott and Coleman, 1988). Thus, long-term use of conservation tillage leads to an increase in soil organic carbon, an enhancement of soil quality, and an improvement in soil resilience (Grant, 1997; Black and Tanaka, 1997). Conservation tillage can improve soil aggregation and change the vertical distribution and retention of soil organic carbon (Beare et al., 1994). Edwards et al. (1992), for example, found that conversion from conventional tillage to conservation tillage increased soil organic carbon by 56% over a 10-yr period. A number of other studies have also shown more soil organic carbon content associated with conservation tillage than with conventional tillage (e.g., Tracy et al., 1990; Wood et al., 1991; Power, 1994; Lafond et al., 1994). An increase in soil organic carbon associated with conservation tillage also leads to an improvement in soil structure and aggregation compared with conventional tillage (Bruce et al., 1990).

 This is a general statement. There are instances where conservation tillage (no tillage) will result in an increase in soil organic carbon for one depth of soil for a given soil type and a decrease for a greater depth for the same soil type (Wander et al., 1998).

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A survey of the literature indicates that the carbon sequestration potential of associated with the adoption of conservation tillage rages from 0.10 to 0.50 Mt/ha/yr for humid temperate regions and from 0.05 to 0.20 MT/ha/yr for semiarid and tropical regions (Lal, 1995). The long run increase in soil organic carbon with time, however, may follow a sigmoid response. Consequently, the increase in soil organic carbon might be modest in the "rst 5 yr after conservation tillage adoption. Over the subsequeny 5 yr, however, it will potentially be large. It has been suggested that the increase in soil organic carbon associated with the adoption of conservation tillage will continue for a period of 25}50 yr depending on climatic conditions, soil characteristics, and production management practices (Franzluebbers, 1997; Hunt, 1996; Wood et al., 1991; Zobeck et al., 1995). Still, there are a number of unresolved questions with respect to modifying the processes that promote carbon loss from the soil. One is the relative importance of the di!erent processes a!ected by tillage such as soil disturbance and the way tillage impacts changing carbon inputs and losses. An answer to this question is necessary before an accurate assessment can be made of the long}run e!ect of di!erent tillage systems on the carbon level in the soil (Barker et al., 1996). The bene"cial e!ects of conservation tillage will be lost if the soil is tilled even after a relatively long period under conservation tillage (Gilley et al., 1997). Hence, it is generally conceded that while there may be an increase in soil organic carbon for soils managed with mulch tillage or ridge tillage where minimum tillage occurs, there is an absence of data to test this hypothesis (Franzluebbers, 1996). Consequently, for conservation tillage, only no tillage results in a signi"cant increase in soil organic carbon (Kern and Johnson, 1993; Paustien et al., 1999). Moreover, when the conversion from conventional tillage to no tillage occurs, the realized bene"ts of an increase in soil organic carbon occurs very quickly (McCarty et al., 1998). Finally, compounding the issue of measuring the bene"cial e!ects of the adoption of conservation tillage on an increase in soil organic carbon is the fact that this increase is highly variable even for the same crop and soil type (Donigian et al., 1994). This is due to the fact that rates of carbon emissions from the soil are signi"cantly correlated with temperature and precipitation and have a pronounced seasonal pattern (Kern and Johnson, 1993).

 Kern and Johnson use a number of disparate studies to estimate the change in soil organic carbon. The studies relied upon come from a number of di!erent countries using di!erent production technologies. Additionally very di!erent soil types and crops are aggregated together. They "nd that converting about 70% of cropland from conventional tillage to conservation tillage will o!set 0.7%}1.1% of the US fossil fuel emissions in the United States between 1990 and 2020.

More studies of the sort by Gilley et al. (1997) that assess the relative carbon content of di!erent tillage practices on comparable soils over space (i.e., geographically disperse) and for di!erent climatic conditions are needed. 2.2. Conservation reserve program The policy to take land out of production and place it into conservation uses was "rst used in the Soil Bank Program of the 1950s, and has been signi"cantly increased in the current Conservation Reserve Program (CRP). This program was initiated by Congress in Title XII of the Food Security Act of 1985. As a voluntary long-term program, the CRP provides participants (farm owner and operators) with an annual per-hectare rent and half the cost of establishing a permanent land cover (usually grass or trees) in exchange for retiring highly erodible and/or environmentally sensitive cropland from production for 10}15 yrs. Although the enrollment mandate established in the 1985 Act was 16.2}18.2 million ha by the end of the 1990 crop year, by that point 13.7 million ha had been enrolled. The primary goal of the CRP during 1986}1989 was to reduce soil erosion on highly erodible cropland. Secondary objectives included protecting the long-run capability of the United States to produce food and "ber, reducing sedimentation, improving water quality, fostering wildlife habitat, curbing production of surplus commodities, and providing income support for farmers (Osborn, 1996). Only recently has the increase in soil organic carbon and the carbon sequestration potential of land in the CRP been discussed (Gebhart et al., 1994). The Food, Agriculture, Conservation, and Trade Act of 1990 extended the CRP enrollment period through 1995, and redirected the goals of the CRP toward improving water quality and other environmental concerns. Under the 1990 Act, an additional 1 million ha were enrolled, bringing total enrollment to 14.7 million ha by 1993. Subsequent appropriations legislation capped CRP enrollment at 15.4 million ha. The most recent farm legislation as manifest in the Federal Agricultural Improvement and Reform Act of 1996 continues the CRP through 2002. Under this legislation the US Department of Agriculture was given authority to re-enroll existing CRP contracts, as well as enroll new hectares, subject to a maximum annual enrollment of 15.4 million ha. While the establishment of perennial grass cover was an original objective the CRP, these new grasslands have the potential to reduce concomitantly atmospheric carbon emission levels due to accumulation and incorporation of litter into surface soils and the relatively large amounts of net primary production allocated toward root growth in grasslands (CAST, 1992). The CRP provides an opportunity to mitigate radiative forcing by sequestering atmospheric carbon emissions as carbon in the soil and trees (CAST,

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1992). Gebhart et al. (1994) assessed the impact of the CRP on soil organic carbon in Texas, Kansas, and Nebraska. The carbon sequestration rate of CRP was 1.1 million Mt/ha/yr. They suggest that the principal cause of the increase in soil organic carbon was erosion control. Follett (1993) estimates that about 14.6 million ha of CRP land could sequester between 3 and 10 million Mt of carbon equivalent as soil organic carbon over a 10-yr period. Paustien et al. (1995) estimated that CRP hectareage would sequester about 25 million mt over a 10-yr period. The rate of soil organic carbon accumulation ranged from less than 10 g/m/yr to more than 40 g/m /yr, with the highest rates in more humid regions.  Paustien et al. (1999), in a di!erent study looking at annually cultivated land placed into the CRP with perennial grass cover, estimates the sequestration potential of CRP land at 16 million Mt/yr. Hence, there is a considerable range in the estimate of the increase in soil organic carbon attributable to land in the CRP planted with grass. McConnell and Quinn (1988) have shown that soil organic carbon contents of the surface 0}25 cm of cropland abandoned and/or reseeded to perennial grasses were similar to adjacent native rangeland following about 50 yr of recovery. Conversely, surface soil organic carbon of cropland continuously cultivated was substantially lower than that of native rangeland and abandoned and reseeded cropland. Dormaar and Smoliak (1985) observed similar trends for abandoned cropland and native rangeland in Alberta, Canada. Based on these and other studies, it is clear that land use change from crop production to perennial grass cover associated with the CRP may sequester atmospheric carbon back into the soil carbon pool, thereby changing soil from sources to sinks for atmospheric carbon. Most of the higly erodible land (HEL) contracted into the CRP had su!ered much erosion, organic matter loss, and structural deterioration while it was in cultivated crop production. When lands are returned to grass, their structure and organic matter improve and tend to approach the structure and organic matter content of the original grassland soils (Gebhart et al., 1994). The degree of soil improvement from 10 yr of grass is a function of site-speci"c factors. As a general rule, the greater the amount of soil structure deterioration from past cultural practices, the more likely that grass management will improve the soil's characteristics. Rasiah and Kay (1994) found that if soils had higher levels of organic matter and other stabilizing materials at the time of grass introduc-

 There are other estimates as well. The interested reader is referred to Paustien et al. (1998).  Highly erodible land is cropland that has an erodibility index greater than or equal to eight. The technical de"nition of the erodibility index as well as other relevant terms is presented in Appendix B.

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tion, the time required for soil structure regeneration was reduced. Soils in the CRP typically "t into the category of degraded soils whose organic matter is lower than that of surrounding soils, because they were primarily allowed into the program based on their higly erodible classi"cation (Lindstrom et al., 1992; Barker et al., 1996). If CRP land is planted with trees, the potential carbon sequestration is ostensibly considerably greater than for grass (Moulton and Richards, 1990). Credible estimates of this potential, however, are still being developed (Lee, 1995; McCarl and Callaway, 1993). CRP contracts are beginning to expire, so farmers have the option to return the land to crop production. For land that would be returned to production, the improvements in soil quality and erosion reduction gained during the CRP contracts will be rapidly lost if conventional tillage is used. Coincidentally, most of the carbon accumulated during the CRP contract period is likely to be lost within a few years. Thus, only those areas which are maintained in perennial vegetation and not reverted to annual cropping will result in an increase in soil organic carbon and provide a signi"cant long-term sink for carbon. 2.3. Conservation buwer strips Conservation bu!ers strips are small areas or strips of land in permanent vegetation, desinged to intercept pollutants and manage other environmental concerns. As with CRP hectares planted to grass, conservation bu!er strips have the potential to reduce atmospheric carbon emission levels by the accumulation and incorporation of litter into surface soils and the relatively large amounts of net primary production dispersed to root growth in grasslands (CAST, 1992). Bu!ers include riparian bu!ers which are established along steams to absorb sediment and agricultural chemicals before the runo! enters a stream, "lter strips designed to absorb run-on water and sediments from upstream land, grassed waterways which are designed for safe disposal of excess water from cropland, shelterbelts, windbreaks, living snow fences, contour grass strips, cross-wind trap strips, shallow water areas for wildlife, "eld borders, alley cropping, herbaceous wind barriers, and vegetative barriers. Strategically placed bu!er strips in the agricultural landscape can e!ectively mitigate the movement of sediment, nutrients, and pesticides with farm "elds and from farm "elds. Filter strips are especially e!ective in retaining surface-applied swine manure and other constituents because they decrease the runo! rate and encourage sedimentation (Chaubey et al., 1994). When coupled with

 In 1998, 6.5% of the hectares enrolled in the CRP was planted with trees (Economic Research Service, 1998).

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Fig. 2. Conservation bu!er strips installed * 1997, 1998.

appropriate upland treatments, including crop residue management, nutrient management, integrated pest management, winter cover crops, and similar management practices and technologies, bu!er strips allow farmers to achieve a measure of economic and environmental sustainability in their operations. Bu!er strips can also enhance wildlife habitat and protect biodiversity (Lant et al., 1995). Vegetated strips, ranging in width from 5 to 50 m, usually are installed along streams and on cropland. These bu!er strips slow water runo!, trap sediment, and enhance in"ltration within the bu!er. Bu!ers also trap fertilizers, pesticides, pathogens, and heavy metals, and they help trap snow and cut down on blowing soil in areas with strong winds. In addition, they protect livestock and wildlife from harsh weather and buildings from wind damage. If properly installed and maintained, they the capacity to remove up to 50% or more of nutrients and pesticides, remove up to 60% or more of certain pathogens, and remove up to 75% or more of sediment (Natural Resources Conservation Service, 1998). Conservation bu!ers also help stabilize a stream and reduce its water temperature as well as o!er a setback distance for agricultural chemical use from water sources (Martin, 1998). Given that the type of vegetation used for bu!er strips is very similar to that used for land enrolled in the CRP, the expected increase in soil organic carbon associated with the use of bu!er strips will be comparable to that for CRP hectareage. There are very few studies that objectively assess the increase in soil organic carbon and other bene"ts associated with bu!er strips. One extant study assessed the e!ectiveness of a double row of tall wheat grass established with 15 m between rows (Aase and Pikul, 1995). A gradual increase in soil organic content

was observed from upslope to down slope from 11 to 12 g/kg for 0}5 cm depth and 8}10 g/kg for 15}20 cm depth. Robinson et al. (1996) found that the initial 3 m of a vegetative "lter strip removed more than 70% of the sediments from runo!. The US Department of Agriculture has a voluntary program to install 3.2 million ha of conservation bu!er strips by 2002 (Natural Resources Conservation Service, 1998). Conservation bu!er strips have di!erent widths ranging from 6 to 30 m for "lter strips, 15}45 m for riparian bu!ers, and 24}30 m for grassed waterways. An average width of all types of conservation bu!er strips is about 10 m. Obective data on the installation of conservation buffer strips are scarce. The National Resources Inventory, however, is one available source that is based on a scienti"cally selected random sample (Soil Conservation Service, 1992). Fig. 2 gives information on the number of hectares of installed conservation bu!er strips in 1997 and 1998. These data were taken from the 1998 Special National Resources Inventory and represent the most current data available. The number of hectares of

 The Natural Resources Conservation Service (NRCS) conducted a Special National Resources Inventory in 1998 to identify changes in cropland use, vulnerabilty of land to soil erosion, erosion on land cultivated for crops, and soil conservation practices on cropland. The study was designed to provide timely information on such things as recent changes in land in production and its susceptibility to potential degradation in response to market conditions and the Federal Agriculture Improvement and Reform (FAIR) Act of 1996. The data were collected for 6000 Primary Sampling Units (PSUs) in the Spring of 1998. Consequently, the impact of adverse weather conditions such as the drought in the Southwest United States in the Summer of 1998 and other changes such as adjustments in production practices or crops grown are not re#ected in the results.

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grassed waterways is signi"cantly greater that the other conservation practices.

3. Conservation practices and the potential to sequester carbon There are di!erent interpretations as to what constitutes carbon sequestration in the context of carbon emissions mitigation polices. Among the alternatives is (1) soil organic matter (excluding litter), (2) soil organic matter plus soil litter and roots (referred to as total below ground carbon), and (3) below ground carbon plus the minimum standing stocks of above ground litter and biomass (referred to as the ecosystem carbon level). While di!ering in magnitude, the patterns of each of these di!erent quantities are similar (Paustien et al., 1999). The fact, however, that di!erent measures exist leads to some uncertainty in de"ning precisely what the carbon sequestration potential of the various conservation practices is. This problem is compounded when comparison across studies in undertaken. Results of di!erent studies using di!erent interpretations of what constitutes sequestered carbon are really not comparable. One other issue of concern that needs to be noted before turning to the estimation of the potential of cropland in the United States to sequester carbon is the impact of the inherent uncertainty in the short-term estimates of the potential to sequester carbon on the long-term carbon storage potential. The accuracy in the prediction of soil organic carbon stores depends greatly on the source of errors. Thus, errors in approximating the interaction between soil organic carbon and soil texture as well as the measure of the temporal rate of change of soil organic carbon are especially important (Hyvonen et al., 1998). The objective in what follows is to estimate the likely amount of soil organic carbon that cropland in the United States on which conservation practices are used contain. This will provide an estimate of the carbon sequestration potential of these practices. Two separate time periods will be considered * 1998, the period just past for which data on the extent of conservation practices are available, and 2008. For the latter period, forecasts of conservation tillage, CRP hectares, and conservation bu!er strip hectares will be needed. (a) The year 1998. (i) Conservation tillage. In 1998 37.2% of cropland in the United States was conservation tilled. Of this total, 43.8% was not tilled. That is, no tillage was practiced on 19.3 million ha in 1998 (CTIC, 1998). As noted previously, based on climatic conditions, soil organic carbon will be between 0.05 and 0.50 Mt/ha/yr depending on the climatic conditions, crops grown, soil characteristics, ad so on. Thus, in cropland on which no tillage was used in 1998, there was

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between 0.9 million Mt of carbon and 9.6 million Mt of carbon. This value is consistent with the results of Kern and Johnson (1993) but somwhat less than the value obtained by Lal et al. (1998). This is to be expected since Lal et al. (1998) use a di!erent set of assumptions in making their computation. Using the midpoint of this range, cropland under no tillage sequesters an amount of carbon equal to 1.0% of total carbon emissions in the United States from burning fossil fuels (Energy Information Administration, 1995). This relative amount is consistent with the value reported by Kern and Johnson (1993). 3.1. Conservation reserve program In 1998 12.5 million ha were enrolled in the Conservation Reserve Program. Most of the hectareage, as noted previously, was planted in grass. With CRP hectareage in grass, the amount of soil organic carbon that will be contained in the soil ranges between 0.3 and 0.7 Mt/ha/yr (Paustien et al., 1999). Thus, the amount of soil organic carbon in CRP hectareage was between 3.8 and 8.8 million Mt. This range is consistent with the results of, e.g., Follett (1993) but signi"cantly less than the value reported by Paustien et al. (1999). Using the midpoint of the range, the carbon in the soil associated with the CRP equals about 1.2% of carbon emissions in the United States from burning fossil fuels (Energy Information Administration, 1995). 3.2. Conservation buwer strips In 1998 there were 1.24 million ha of conservation buffer strips installed in the United States. Most of the hectareage was planted in grass. With the hectareage in grass, the amount of soil organic carbon that will be contained in the soil will be coomparable to what one "nds for CRP hectareage in grass. This range is between 0.3 and 0.7 Mt/ha/yr. Thus, the amount of soil organic carbon in conservation bu!er strip hectareage was between 0.4 and 0.9 million Mts. Using the midpoint of the range, the carbon in the soil associated with the CRP equals about 0.1% of carbon emissions in the United States from burning fossil fuels (Energy Information Administration, 1995). 3.2.1. The Year 2008 The year 2008 was not capriciously chosen for which to calculate changes in the soil organic carbon associated with an increase in the use of conservation practices in the United States. Rather, 2008 is the "nal year of the forecast horizon of the current US Department of Agriculture Baseline forecasts (World Agricultural Outlook Board, 1999). Forecasts provided include total planted cropland hectareage and CRP hectareage. What is not provided is a forecast of hectareage

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that is conservation tilled. That will be estimated here. (i) Conservation Tillage: There is some uncertainty about the extent to which conservation tillage * no tillage * will be used in 2008. Consequently, the forecast of its adoption and use must be provided in the context of a con"dence interval. To forecast the use of no tillage in 2008, a forecast of the proportion of total planted hectareage that is conservation tilled (recall that the historical data are pictured in Fig. 1) is combined with the Baseline forecast of total planted hectareage to yield the number of hectares that is conservation tilled. Assuming that the historical relationship between the proportion of hectares that are mulch tilled, ridge tilled, and no tilled holds, the number of hectares that will not be tilled in 2008 will be obtained. To estimate the proportion of total planted hectares that will be conservation tilled in 2008, a simple logistic model is "t to the historical data. The model is estimated as ct "(1/(1#a exp(!bt)))#u , (1)  R where ct denotes the proportion of hectares conservation tilled, t denotes the time period, u is the stochastic term, and a and b are coe$cients to be estimated. Using nonlinear least-squares with an adjustment for "rst-order serial correlation (which is present and which is the highest order empirically observed), the estimated model is ct"(1/(1#5.5571 exp(!0.1295 t ))     with

(2)

tilled hectareage with its attendant 95% con"dence interval is combined with the estimate of the organic carbon content of soil that is conservation tilled, then between 0.9 and 11.6 million Mt of carbon will be contained in the soil that is conservation tilled in 2008. This represents about a 19% increase over 1998. (ii) Conservation reservation program: The Baseline forecast predicts that CRP hectareage will increase to its mandated limit of 14.7 million ha by 2002 and remain constant thereafter. Assume that all of the CRP hectareage is planted in grass. The CRP hectareage will contain between 4.4 and 10.3 million Mt of carbon in 2008. This is a 17% increase in the carbon in CRP hectareage over 1998. (iii) Conservation buwer strips: For conservation bu!ers strips, it is assumed that the target of 3.2 million ha will be reached by 2008 if not by the year 2002 which is the original US Department of Agriculture target year. These conservation bu!er strips will contain between 1 and 2.4 million Mt of carbon. This translates into a 162% increase in soil organic carbon over the level in 1998. Combining the forecasts for each of the di!erent conservation practices and absent any further policy initiatives, soil organic carbon associated with the use of these practices is expected to increase by approximately 25% between 1998 and 2008. Clearly, the conservation practices considered here contribute greatly to organic carbon in the soil. An increase in their use will further increase soil organic carbon and hence enhance the potential to sequester carbon in the United States. There are a number of policy options available to promote the adoption of conservation practices. These are discussed in the next section.

SSE"0.5881 log of the likelihood function"21.7547 and the standard errors of the estimates in parentheses. Using this functional relationship, the proportion of total planted hectareage that is expected to be conservation tilled in 2008 is 0.54. The associated 95% con"dence interval is 0.46}0.62. Combining this with the Baseline forecast and the historical relationship between the three di!erent conservation tillage types means that between 17.7 and 23.1 million ha will be no tilled in 2008. For the next step in the computation, assume that none of the previously no tilled hectareage is tilled. That is, the net increase in no tilled hectareage comes just from cropland that was previously conventionally tilled. Also assume, consistent with the literature, that the increase in the soil organic carbon of cropland converted from conventional tillage to no tillage is relatively rapid. Then, if the forecast of no

 This denotes the sum of squared errors of the regression.

4. The use of public policies to promote the use of conservation practices Several public policies can be used to a!ect farmers' adoption of conservation practices: education and technical assistance, "nancial assistance, research and development, land retirement, and regulation and taxes. Each policy has implications about agricultural pro"ts and the allocation of public funds. 4.1. Education and technical assistance If a preferred conservation practice would be pro"table for a farmer but the farmer is unaware of its bene"ts, education e!orts can lead to voluntary use of the conservation practice. Educational activities generally take the form of demonstration projects and information campaigns in print and electronic media, newsletters, and meetings. Demonstration projects provide more direct and detailed information about farming practices and

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production systems, and how these systems are advantageous to the producer (Bosch et al., 1995). Information assumes an especially signi"cant role in the case of new or emerging technologies (Saha et al., 1994). When adoption of a conservation practice would lead to an increase in long-term pro"ts, but either new skills are needed or farming operations must be adapted for the conservation practice to produce the highest net bene"ts, technical assistance can be provided to those who choose to adopt. Technical assistance is the direct, one-on-one contact provided by an assisting agency or private company for the purpose of providing a farmer with the planning and knowledge necessary to implement a particular conservation practice on the individual farm. Requirements for successful implementation vary between individual farms because of resource conditions, operation structure, and owner/operator managerial skill. Testing a conservation practice on part of the farm enhances its potential for adoption (O$ce of Technology Assessment, 1990; Nowak and O'Keefe, 1995). Technical assistance is often critical, especially for conservation practices that require a greater level of management than the farmer currently uses (Dobbs et al., 1995). Both education and technical assistance can be provided by either public or private sources, and both will induce adoption by farmers for whom the conservation practice would be more pro"table than the one they had been using.

specify a uniform subsidy rate across resource conditions. Uniform rates, however, invariably introduce production distortions. Because resource and production characteristics can vary widely, di!erent farms may need di!erent sets of conservation practices to achieve the same environmental goal. A production system that is appropriate for one farm may be inappropriate for another. The e!ectiveness of a conservation system in controlling erosion depends on several factors, including the frequency, timing, and/or severity of wind and precipitation, the exposure of land forms to weather, the ability of exposed soil to withstand erosive forces, the plant material available to shelter soils, and the propensity of conservation practices to reduce or extenuate erosive forces. An e$cient "nancial assistance program would have a list of eligible conservation practices that included all alternatives appropriate for each farm. Cost-share and incentive payment policies are based on the fact that targeted farmers would not voluntarily adopt the preferred conservation practice but the public interest calls for the conservation practices to be used more widely. Financial assistance is not a substitute for education and technical assistance. Even with "nancial assistance, a farmer will not adopt a technology if he or she is unfamiliar with it.

4.2. Financial assistance

Research and development policies can be used to enhance the bene"ts of a given conservation practice. The objective of the research would be to either improve the performance or to reduce the costs of the conservation practice. Data gathering and analysis, as well as monitoring also contribute to R&D by providing information necessary to assess the determinants of adoption and the e!ectiveness of conservation practices in achieving public goals. In addition, R&D funds could be allocated to ensure that the conservation practice is adaptable for more circumstances. R&D is a long-term policy strategy with an uncertain probability of success, but it may also reap the greatest gains in encouraging the voluntary adoption of a preferred technology because it can increase the pro"tability of the conservation practice for a wider range of potential adopters.

Financial assistance can be o!ered to overcome either short- or long-term impediments or barriers to adoption. If the conservation practice would be pro"table once installed but involves initial investment or transition and adjustment expenses, a single cost-share payment can be used to encourage the switch to the preferred conservation practice. Transition and adjustment costs include lost production, increased risk, or increased management costs due to learning how to use the new conservation practice e$ciently. Financial and organizational characteristics of the whole operation also may be a hindrance to adoption (O$ce of Technology Assessment, 1990; Nowak, 1991). When the conservation practice would not be more pro"table to the farmer than the current conservation practice but the environmental or other o!-farm bene"ts are substantial, public funds could be allocated on an ongoing basis to defray the loss in pro"ts to the farmer. Another form of "nancial incentive could be the granting of a tax credit for investment in a particular conservation practice. From a public perspective, the optimal "nancial assistance rates are those that induce the adoption of desired conservation practices at the least cost. E$cient rates would have to be set individually since farm and farmer characteristics vary widely (Caswell and Shoemaker, 1993). Therefore, for ease of implementation, most large "nancial assistance programs

4.3. Research and development (R&D)

4.4. Land retirement The policy that has the largest impact on farmers' choice of conservation practices is land retirement. The underlying premise is that large public bene"ts can be gained by radically changing agricultural conservation practices on particular parcels of land and that changes in individual conservation practices would not provide su$cient social bene"ts. For an individual to voluntarily agree to put the land in conserving uses, he or she would expect compensation in an amount at least as great as the

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lost pro"ts from production. The payment mechanisms that can be used to implement land retirement strategies are lump sum payments or annual `rental feesa. The former are often referred to as easements whereby the farmer's right to engage in nonconserving uses is purchased by the public sector for a speci"c period. Payment to an individual to retire land would result in a voluntary change in conservation practices.

Given that there is a signi"cant potential for conservation practices to lead to an increase in carbon sequestration, there are a number of policy options that can be pursued. These include education and technical assistance, "nancial assistance, research and development, land retirement and taxes.

Acknowledgements 4.5. Regulation, taxes, and incentives If voluntary measures prove insu$cient to produce the changes in conservation practices necessary to achieve public goals, regulation is a policy that can be used. The use of certain conservation practices could be prohibited, taxed, or made a basis for withholding other bene"ts. Preferred conservation practices could be required or tax incentives o!ered to promote their use thereby o!setting some of the cost of the requisite new equipment. Point sources of pollution have been subject to commandand-control policies for many years. There are recognized ine$ciencies associated with technology-based regulations because the least-cost technology combination to meet an environmental goal for an individual may not be permitted. It has been assumed that such loss in e$ciency is made up for by ease of implementation.

5. Conclusion The use of conservation practices by agriculture in the United States will enhance soil organic carbon and potentially increase carbon sequestration. This, in turn, will decrease the net emission of carbon dioxide. A number of studies exist that calibrate the contribution of various individual, site-speci"c conservation practices on changes in soil organic carbon. There is a general absence, however, of a comprehensive e!ort to measure objectively the contribution of these practices including conservation tillage, the Conservation Reserve Program, and conservation bu!er strips to a change in soil organic carbon. This paper attempts to "ll that void. After recounting the evolution of the use of the various conservation practices, it is estimated that organic carbon in the soil in 1998 in the United States attributable to these practices was about 12.2 million Mt. By 2008, there will be an increase of about 25%.

 Economic theory shows that the e$cient solution (i.e., least cost for society to achieve a particular level of environmental quality) is when the marginal cost of pollution reduction is the same for all producers (Kneese and Bower, 1968). Each individual could have di!erent combinations of conservation practices and technologies. To implement such a policy, however, would have an extremely high cost for an industry as large diverse as agriculture (He!erman, 1984).

The views expressed are those of the authors and do not necessarily represent the policies of the U.S. Department of Agriculture or the views of other U.S. Department of Agriculture sta! members.

Appendix A. Tillage system de5nitions Crop residue management (CRM) * A year-round conservation system that usually involves a reduction in the number of passes over the "eld with tillage implements and/or in the intensity of tillage operations, including the elimination of plowing (inversion of the surface layer of soil). CRM begins with the selection of crops that produce su$cient quantities of residue to reduce wind and water erosion and may include the use of cover crops after low-residue-producing crops. CRM includes all "eld operations that a!ect residue amounts, orientation, and distribution throughout the period requiring protection. The amounts of residue cover needed at speci"c sites are usually expressed in percentage but my also be in kilograms. Tillage systems included under CRM are conservation tillage (no tillage, ridge tillage, and mulch tillage) and reduced tillage. Conservation tillage * Any tillage and planting system that maintains at least 30% of the soil surface covered by residue after planting to reduce soil erosion by water. Where soil erosion by wind is the primary concern, any system that maintains at least 454 kg/ha of #at, small grain residue equivalent on the surface during the critical wind erosion period. Two key factors in#uencing crop residue are (1) the type of crop, which establishes the initial residue amount and determines its fragility, and (2) the type of tillage operations prior to and including planting. Conservation tillage systems include: Mulch tillage: The soil is disturbed prior to planting. Tillage tools such as chisels, "eld cultivators, disks, sweeps, or blades are used. Weed control is accomplished with herbicides and/or cultivation. Ridge tillage: The soil is left undisturbed from harvest to planting except for nutrient injection. Planting is completed in a seedbed prepared on ridges with sweeps, disk openers, coulters, or row cleaners. Residue is left on the surface between ridges. Weed control is accomplished with herbicides and/or cultivation. Ridges are rebuilt during cultivation.

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No tillage: The soil is left undisturbed from harvest to planting except for nutrient injection. Planting or drilling is accomplished in a narrow seedbed or slot created by coulters, row cleaners, disk openers, inrow chisels, or roto-tillers. Weed control is accomplished primarily with herbicides. Cultivation may be used for emergency weed control. Reduced-till (15+30% residue) * Tillage types that leave 15}30% residue cover after planting, or 227}454 kg/ha of small grain residue equivalent throughout the critical wind erosion period. Weed control is accomplished with herbicides and/or cultivation. Conventional-till (less than 15% residue) * Tillage types that leave less than 15% residue cover after planting, or less than 227 kg/ha of small grain residue equivalent through the critical wind erosion period. Generally includes plowing or other intensive tillage. Weed control is accomplished with herbicides and/or cultivation. Source: Conservation Technology Information Center (1998).

Appendix B. Soil erosion de5nitions The Universal Soil Loss Equation (USLE) is A"RKf (¸, S)CP, where A is the computed soil loss per unit area, expressed in the units selected for K and for the period selected for R. In practice, these are usually so selected that they compute A in kilograms per hectare per year, R, the rainfall and runo! factor, is the number of the rainfall erodibility index units plus a factor for runo! from snow melt or applied water where such runo! is signi"cant, K, the soil erodibility factor is the soil loss rate per erodibility index unit for a speci"ed soil as measured on a unit plot, which is de"ned as a 22 m length of uniform 9% slope continuously in clean-tilled fallow, ¸, the slope length factor, is the ratio of soil loss from the "eld slope to that from a 22 m length of uniform 9% slope continuously in clean-tilled fallow, S, the slope steepness factor, is the ratio of soil loss from the "eld slope gradient to that from a 22 m length of uniform 9% slope continuously in clean-tilled follow, C, the cover and management factor, is the ratio of soil loss from an area with speci"ed cover and management to that from an identical area in tilled continuous cover; and P, the support practice factor, is the ratio of soil loss with a supporting practice like contouring, strip cropping, or terracing to that with straight-row farming up and down the slope. Note that f (¸, S) indicates a nonlinear relationship between ¸ and S. Highly erodible land (HEL) is land determined to have an inherent erosion potential of over 8 times its soil loss tolerance (T) level. Determination is made by calculating

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the erodibility index (EI) for both water and wind erosion. If the EI for either water or wind is greater than 8, then the soil is classi"ed as HEL. The erodibility index is a number showing how many times the inherent erosion potential is of the soil loss tolerance (T) level. For water (sheet and rill) erosion, the number is calculated as EI"RK¸S/¹, where R, the rainfall and runo! factor, is the number of the rainfall erodibility index units plus a factor for runo! from snow melt or applied water where such runo! is signi"cant, K, the soil erodibility factor is the soil loss rate per erodibility index unit for a speci"ed soil as measured on a unit plot, which is de"ned as a 22 m length of uniform 9% slope continuously in clean-tilled fallow, ¸, the slope length factor, is the ratio of soil loss from the "eld slope to that from a 22 m length of uniform 9% slope continuously in clean-tilled fallow; and S, the slope steepness factor, is the ratio of soil from the "eld slope gradient to that from a 22 m length of uniform 9% slope continuously in clean-tilled fallow. The soil loss tolerance level (T) is the maximum rate of annual soil erosion that may occur and still permit a high level of crop productivity to be obtained economically and inde"nitely. Most values for cropland in the United States are between 1.4 and 2.3 kg/Mt/yr.

References Aase, J., Pikul, J., 1995. Terrace formation in cropping strips protected by tall wheatgrass barriers. Journal of Soil and Water Conservation 50, 110}112. Barker, J., Baumgardner, G., Turner, D., Lee, J., 1996. Carbon dynamics of the conservation and wetland reserve programs. Journal of Soil and Water Conservation 51, 340}346. Beare, M., Hendrix, P., Coleman, D., 1994. Water-stable aggregates and organic matter fractions in conventional and no-tillage soils. Soil Science Society of American Journal 58, 777}786. Black, A., Tanaka, D., 1997. A conservation tillage cropping systems study in the northern great plains of the United States. In: Paul, E., Paustien K., Elliott, E., Cole, C. (Eds.), Soil Organic Matter in Temperate Agroecosystems. CRC Press, Boca Raton, FL. Bosch, D., Cook, Z., Fuglie, K., 1995. Voluntary versus mandatory agricultural policies to protect water quality: adoption of nitrogen testing in Nebraska. Review of Agricultural Economics. 17, 13}24. Bruce, R., Langdale, G., Dillard, A., 1990. Tillage and crop rotation e!ects on characteristics of a sandy surface soil. Soil Science Society of America Journal 54, 1744}1747. CAST, 1992. Preparing U.S. agriculture for global climate change. Council for Agriculture and Technology, Report 119, Washington, DC. Caswell, M., Shoemaker, R., 1993. Adoption of pest management strategies under varying environmental conditions. Technical Bulletin 1827, Economic Research Service, U.S. Department of Agriculture, Washington, DC, December. Chaubey, I., Edwards, D., Daniel, T., Moore, P., Nichols, D., 1994. E!ectiveness of vegetative "lter strips in retaining surface-applied swine manure constituents. Transcations of the American Society of Agricultural Engineers 37, 845}850.

208

N.D. Uri, H. Bloodworth / Global Environmental Change 10 (2000) 197}209

Conservation Technology Information Center (CTIC), 1998. National Crop Residue Management Survey, CTIC, West Lafayette, IN. Dobbs, T., Bischo!, J., Henning, L. P#ueger, B., 1995. Case study of the potential economic and environmental e!ects of the water quality incentive program and the integrated crop management program: preliminary "ndings. Paper presented at annual meeting of the Great Plains Economics Committee, Great Plains Agricultural Council, Kansas City, MO, April. Donigian, A., Barnwell, T., Jackson, R., Patwardhan, A., Weinrich, K., Rowell, A., Chinnaswamy, R., Cole, C., 1994. Alternative Management Practices A!ecting Soil Carbon in Agroecosystems of the Central U.S.. U.S. Environmental Protection Agency, Washington, DC. Dormaar, J., Smoliak, S., 1985. Recovery of vegetative cover and soil organic matter during revegetation of abandoned farmland in a semiarid climate. Journal of Range Management 38, 487}491. Edwards, J., Wood, C., Thurlow, D., Ruf, M., 1992. Tillage and crop rotation e!ects on fertility status of a hapludalt soil. Soil Science Society of American Journal 56, 1577}1582. Elliott, E., Coleman, D., 1988. Let the soil work for us. Ecological Bulletin 39, 23}32. Energy Information Administration, 1995. Emissions of Greenhouse gases in the United States 1987}1994. U.S. Department of Energy, Washington, DC. Follett, R., 1993. Global climate change. Journal of Production Agriculture 6, 181}190. Franzluebbers, A., 1996. Water-soluble aggregation and organic matter in four soils under conventional and zero tillage. Canadian Journal of Soil Science 76, 387}393. Franzluebbers, A., 1997. Soil microbial biomass and mineralizble carbon of water-stable aggregates. Soil Science Society of America Journal 61, 1090}1097. Gebhart, D., Johnson, H., Mayeux, H., Polley, H., 1994. The CRP increases soil organic carbon. Journal of Soil and Water Conservation 49, 488}492. Gilley, J., Doran, J., Karlen, D., Kaspar, T., 1997. Runo!, erosion, and soil quality characteristics of a former conservation reserve program site. Journal of Soil and Water Conservation 52, 189}193. Grant, F., 1997. Changes in soil organic matter under di!erent tillage and rotations: mathematical modeling in ecosystems. Soil Science Society of America Journal 61, 1159}1175. He!erman, W., 1984. Assumptions of the adoption/di!usion model and soil conservation. In: English, B., Maetzold, J., Holding, B., Heady, E. (Eds.), Agricultural Technology and Resource Conservation. Iowa State University Press, Ames, IA. Hunt, P.G., 1996. Changes in carbon content of a Norfolk Loamy sand after 14 years of conservation or conventional tillage. Journal of Soil and Water Conservation 51, 255}258. Hyvonen, R., Agren, G., Bosatta, E., 1998. Predicting long-term soil organic carbon storage from short-term information. Soil Science Society of America Journal 62, 1000}1005. Kern, J., Johnson, M., 1993. Conservation tillage impacts on national soil and atmospheric carbon levels. Soil Science Society of America Journal 57, 200}210. Kneese, A., Bower, B., 1968. Standards, charges, and equity. In: Managing Water Quality: Economics, Technology, Institutions. John Hoopkins University Press, Baltimore, MD. Lafond, G., Derksen, D., Loeppky, H., Struthers, D., 1994. An agronomic evaluation of conservation tillage systems and continuous cropping in east central Saskatchewan. Journal of Soil Water Conservation 49, 387}393. Lal, R., 1995. Conservation tillage for carbon sequestration. Paper presented at the International Symposium on Soil-Source and Sink of Greenhouse Gases, Nanjing, China, September. Lal, R., Kimble, J., Follett, R., Cole, C., 1998. The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effects. Ann Arbor Press, Chelsea, MI.

Lant, C., Kraft, S., Gillman, K., 1995. Enrollment of "lter strips and recharge areas in the CRP and USDA easement programs. Journal of Soil and Water Conservation 50, 193}200. Lee, J., 1995. Potential E!ects of A!orestation on the Carbon Budget of the South Central United States. U.S. Environmental Protection Agency, Corvallis, OR. Lindstrom, M., Schumacher, T., Jones, A., Gantzer, C., 1992. Productivity index model for selected soils in north central united states. Journal of Soil and Water Conservation 47, 491}494. Mannering, J., Schertz, D., Julian, B., 1987. Overview of conservation tillage. In: E!ects of Conservation Tillage on Groundwater Quality. Lewis Publishers, Chelsea, MI. Martin, R., 1998. Conservation Bu!ers in Best Interest of All of Agriculture. Novartis Crop Protection, Inc, Spring"eld, MO. McCarl, B., Callaway, J., 1993. Carbon sequestration through tree planting on agricultural lands. Texas Agricultural Experiment Station, College Station, TX. McCarty, G., Lyssenko, N., Starr, J., 1998. Short-term changes in soil carbon and nitrogen pools during tillage management transition. Soil Science Society of America Journal 62, 1564}1571. McConnell, S., Quinn, M., 1988. Soil productivity of four land use systems in southeastern Montana. Soil Science Society of America Journal 52, 500}506. Nowak, P., 1991. Why farmers adopt production technology. In: Crop Residue Management for Conservation, Porceedings of a Nation Conference, Soil and Water Conservation Society, Ankeny, IA. Nowak, P., O'Keefe, G., 1995. Farmers and water quality: local answers to local issues. Draft Report submitted to the U.S. Department of Agriculture, Washington, DC, September. Osborn, T., 1996. Conservation. Provisions of the Federal Agricultural Improvement Act of 1996, Economic Research Service. U.S. Department of Agriculture, Washington, DC. Paustien, K., Andren, A., Janzen, H., Lal, R., Smith, P., Tian, G., Tiessen, H., Van Noordwijk, M., Woomer, P., 1997. Agricultural soils as a sink to mitigate CO emissions. Soil Use and Management 13, 230}244.  Paustien, K., Cipra, J., Cole, V., Elliot, E., Killian, K., Bluhm, G., 1999. The contribution of grassland CRP to C sequestration and CO  mitigation, unpublished manuscript, Natural Resource Ecology Laboratory. Colorado State University, Ft. Collins, CO. Paustien, K., Cole, V., Sauerbeck, D., Sampson, N., 1998. CO mitiga tion by agriculture. In: Paustien, K. (Ed.), Climatic Change. CRC Press, Inc., Boca Raton, FL. Post, W., Peng, T., Emanuel, W., King, A., Dale, V., DeAngelis, D., 1990. The global carbon cycle. American Scientist 78, 310}326. Power, J., 1994. Understanding the nutrient cycling process. Journal of Soil and Water Conservation 49, 16}23. Rasiah, R., Kay, B., 1994. Characterizing the changes in aggregate stability subsequent to introduction of forages. Soil Science Society of America Journal 58, 935}942. Saha, A., Love, H.A., Schwart, R., 1994. Adoption of emerging technologies under output uncertainty. American Journal of Agricultural Economics 76, 836}846. Schertz, D., 1986. Conservation tillage: an analysis of acreage projections in the United States. Journal of Soil and Water Conservation 33, 256}258. Schlesinger, W., 1995. An overview of the carbon cycle. In: Lal, R., Kimble, J., Levine, E., Stewart, B. (Eds.), Soils and Global Change. CRC Press, Boca Raton, FL. Tracy, P., Westfall, D., Elliot, E., Peterson, G., Cole, C., 1990. Carbon, nitrogen, phosphorous and sulfur mineralization in plow and no-till cultivation. Soil Science Society of America Journal 54, 457}461. U.S. Department of Agriculture, Economic Research Service, 1998. Agricultural Resources and Environmental Indicators. U.S. Department of Agriculture, Washington, DC. U.S. Department of Agriculture, Natural Resources Conservation Service, 1998. Bu!er Strips: Common Sense Conservation. U.S. Department of Agriculture, Washington, DC.

N.D. Uri, H. Bloodworth / Global Environmental Change 10 (2000) 197}209 U.S. Department of Agriculture, Soil Conservation Service, 1992. Instructions for Collecting 1992 National Resources Inventory Sample Data. Washington, DC. U.S. Congress, O$ce of Technology Assessment, 1990. Beneath the Bottom Line: Agricultural Approaches to Reduce Agrichemical Contamination of Groundwater. OTA-F-418, U.S. Government Printing O$ce, Washington, DC, November. U.S. Environmental Protection Agency, 1996. Notes on Agriculture. Environmental Protection Agency, Washington, DC. Van Noordwijk, M., Cerri, C., Woomer, P., Nughroho, K., Bernoux, M., 1997. Soil carbon dynamics in the humid tropical forest zone. In: Elliot, E., Kimble, J., Swift, M., (Eds.), The management of Carbon in Tropical Soils under Global Change: Science, Practice, and Policy. Geoderm.

209

Wander, M., Bidart, M., Aref, S., 1998. Tillage impacts on Depth distribution of total and partculate organic matter i three Illinois soils. Soil Science Society of America Journal 62, 1704}1711. Weinhold, B., Halvorson, A., 1998. Cropping system in#uences on several soil quality attributes in the northern great plains. Journal of Soil and Water Conservation 53, 254}258. Wood, C., Edwards, J., Cummins, C., 1991. Tillage and crop rotation e!ects on soil organic matter in a typic hapludalt of northern Alabaman. Journal of Sustainable Agriculture 2, 31}41. World Agricultural Outlook Board, 1999. USDA Agricultural Baseline Projections to 2008. U.S. Department of Agriculture, Washington, DC. Zobeck, T., Rolong, N., Fryear, D., Bilbro, J., Allen, B., 1995. Properties of recently tilled sod, 70-year cultivated soil. Journal of Soil and Water Conservation 50, 210}215.