potential soil c sequestration on us agricultural soils

POTENTIAL SOIL C SEQUESTRATION ON U.S. AGRICULTURAL SOILS M. SPEROW 1 , M. EVE 2 and K. PAUSTIAN 2,3 1 Division of Resource Management, West Virginia University, P.O. Box 6108,

Morgantown, West Virginia 26506-6108, U.S.A. E-mail: [email protected] 2 Natural Resource Ecology Laboratory, Colorado State University, Ft. Collins, Colorado, U.S.A. 3 Department of Soil and Crop Science, Colorado State University, Ft. Collins, Colorado, U.S.A.

Abstract. Soil carbon sequestration has been suggested as a means to help mitigate atmospheric CO2 increases, however there is limited knowledge about the magnitude of the mitigation potential. Field studies across the U.S. provide information on soil C stock changes that result from changes in agricultural management. However, data from such studies are not readily extrapolated to changes at a national scale because soils, climate, and management regimes vary locally and regionally. We used a modified version of the Intergovernmental Panel on Climate Change (IPCC) soil organic C inventory method, together with the National Resources Inventory (NRI) and other data, to estimate agricultural soil C sequestration potential in the conterminous U.S. The IPCC method estimates soil C stock changes associated with changes in land use and/or land management practices. In the U.S., the NRI provides a detailed record of land use and management activities on agricultural land that can be used to implement the IPCC method. We analyzed potential soil C storage from increased adoption of no-till, decreased fallow operations, conversion of highly erodible land to grassland, and increased use of cover crops in annual cropping systems. The results represent potentials that do not explicitly consider the economic feasibility of proposed agricultural production changes, but provide an indication of the biophysical potential of soil C sequestration as a guide to policy makers. Our analysis suggests that U.S. cropland soils have the potential to increase sequestered soil C by an additional 60–70 Tg (1012 g) C yr−1 , over present rates of 17 Tg C yr−1 (estimated using the IPCC method), with widespread adoption of soil C sequestering management practices. Adoption of no-till on all currently annually cropped area (129 Mha) would increase soil C sequestration by 47 Tg C yr−1 . Alternatively, use of no-till on 50% of annual cropland, with reduced tillage practices on the other 50%, would sequester less – about 37 Tg C yr−1 . Elimination of summer fallow practices and conversion of highly erodible cropland to perennial grass cover could sequester around 20 and 28 Tg C yr−1 , respectively. The soil C sequestration potential from including a winter cover crop on annual cropping systems was estimated at 40 Tg C yr−1 . All rates were estimated for a fifteenyear projection period, and annual rates of soil C accumulations would be expected to decrease substantially over longer time periods. The total sequestration potential we have estimated for the projection period (83 Tg C yr−1 ) represents about 5% of 1999 total U.S. CO2 emissions or nearly double estimated CO2 emissions from agricultural production (43 Tg C yr−1 ). For purposes of stabilizing or reducing CO2 emissions, e.g., by 7% of 1990 levels as originally called for in the Kyoto Protocol, total potential soil C sequestration would represent 15% of that reduction level from projected 2008 emissions (2008 total greenhouse gas emissions less 93% of 1990 greenhouse gas emissions). Thus, our analysis suggests that agricultural soil C sequestration could play a meaningful, but not predominant, role in helping mitigate greenhouse gas increases.

Climatic Change 57: 319–339, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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1. Introduction Agricultural soils represent a potential sink for reducing atmospheric carbon dioxide (CO2 ) concentrations. Many facets of agricultural land management and land use change have been examined for their potential to increase soil C stocks (Bruce et al., 1999; Lal et al., 1998; Lal et al., 1999; Paustian et al., 1997a; Paustian et al., 1997b). Lal et al. (1998, 1999) estimated that potential soil C sequestration from improved management on U.S. cropland was 75 to 208 Tg C (Teragrams = 1012 g = million metric tonnes) per year for several decades. Bruce et al. (1999) estimated that U.S. agricultural soils have the potential of sequestering 75 Tg C per year over the next 20 years. While these earlier analyses have been useful in providing a first-order estimate of sequestration potential, they have been based on highly aggregated data. Lal et al. (1998) and Bruce et al. (1999) used nominal values of C change rates (i.e., Mg ha−1 yr−1 ) or %C stock increase, multiplied by an aggregate land area for the whole U.S., for various agricultural practices. Thus, these estimates do not incorporate interactions of climate, soil and management, which vary significantly across the cropland area of the U.S. and would be expected to influence C sequestration rates. Our objective was to estimate potential soil C changes on U.S. agricultural soils using geographically distributed data on soils, climate, and management practices. Soil C sequestration potentials were derived using a modified version of the C emission/sink calculations in the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 1997a–c). We applied the method to U.S. agricultural soils as described by Eve et al. (2001), combined with more recent data, to first estimate soil C change between 1982 and 1997. These rates represent a baseline against which potential soil C change from projected land use changes can be compared. We then analyzed potential soil C change from increased adoption of conservation tillage, elimination of bare summer fallow, inclusion of winter cover crops in annual crop rotations, and conversion of marginal cropland to perennial grass set-aside land. These land use and land management activities are among the current practices that provide the greatest increase in soil C on agricultural soils (Cole et al., 1993; Lal et al., 1998, 1999; Bruce et al., 1999; Paustian et al., 1997a,b). No-till agriculture, in which crops are sown directly into untilled soil, greatly reduces the degree of soil disturbance normally associated with annual cropping. Cropland under no-till systems have been shown to increase soil C compared to more intensive tillage operations (McCallister and Chein, 2000; Franzluebbers et al., 1998; Paustian et al., 1997a; Six et al., 1999). Reduced soil disturbance under no-till slows decomposition of SOM and stabilizes C in microaggregate structures (Six et al., 1999), which, combined with reducing erosion, leads to increased soil C over conventional tillage operations (Paustian et al., 1997a; Hussain, et al., 1999). Soil C gain from no-till was found to be 0.2–0.6 Mg ha−1 yr−1 greater than with plow tillage in the eastern Corn Belt of the U.S. (Dick et al., 1998). A thirteen-

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year study in the southeastern U.S. found that total soil C accumulation in no-till systems were 18% higher than conventional tillage systems, despite similar residue input levels (Beare et al., 1994). Land in much of the semi-arid regions of the U.S. is commonly planted to crops every other year, leaving a bare fallow period between crops. Weeds are chemically or mechanically controlled and the land is left idle to accumulate moisture during the fallow year for use by the subsequent crop. However, by using more diversified crop rotations in conjunction with no-till practices, the frequency of summer fallow can be greatly reduced or eliminated entirely (Peterson et al., 1998). It has been widely observed that soil C is lower in systems employing summer fallow than in continuous cropping systems (Campbell et al., 2000; Janzen et al., 1998; Bremer et al., 1994; Campbell and Zentner, 1993; Peterson et al., 1998). Switching from annual crops to perennial vegetation increases residue production, plant roots and reduces soil disturbance, thus enhancing soil C sequestration (Paustian et al., 1997b). The Conservation Reserve Program (CRP) is a voluntary cropland retirement program established by the U.S. government that changes land use from annual to perennial vegetation. One of the objectives of the CRP is to reduce soil erosion by targeting cropland identified as highly erodible land (HEL) for enrollment. Areas enrolled in CRP must plant native or introduced grass species or trees and remove land from crop production for ten years. Follett et al. (2001a,b) estimated annual soil C sequestration rates (0–20 cm) of 9.5 Tg C yr−1 on 10.6 Mha of CRP land, based on paired sampling of 14 sites in the Great Plains and western Corn Belt. A similar total of 11 Tg C yr−1 , but for a deeper (0–300 cm) depth increment, was estimated earlier by Gebhart et al. (1994), based on sampling at five sites in the southern and central Great Plains. Paustian et al. (2001) estimated soil C sequestration rates of 6 Tg C yr−1 on 10 Mha of CRP land using a regional application of the Century model (Metherell et al., 1993; Parton et al., 1994). In some climatic regions, land dedicated to annual crops can be planted with a grass or legume cover crop after harvesting the cash crop to protect the soil over winter. Cover crops can reduce erosion and nitrate leaching by providing vegetative cover after the main crop harvest, as well as improve soil structure and provide a green manure for the subsequent cash crop (Troeh et al., 1991). Including a winter cover crop in annual crop rotations also increases residue inputs to the soil and hence soil C sequestration (Collins et al., 1992; Campbell et al., 1991; Hu et al., 1997). To estimate soil C sequestration potential, our analysis assumed widespread adoption of management practices that have been shown to be effective in increasing soil C stocks. Thus the analysis represents a ‘best-case’ scenario of the biophysical soil C potential from existing, well-established practices, which are technically feasible to implement. However, our analysis does not address the economic feasibility and likely adoption rate of these soil C sequestering practices, nor does it consider potential technologies that might be implemented in the future to increase soil C stocks.

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2. Materials and Methods Baseline and potential soil C stock changes were calculated using the IPCC inventory factors in conjunction with land use, management and soil information derived from 1997 National Resources Inventory (NRI) data (Nusser and Goebel, 1997) and a number of ancillary data sets. The NRI consists of about 1.3 million actual and imputed sample locations across the U.S. in which land use, land management, and other resource information has been collected every five years since 1982. While the IPCC inventory method is comprehensive in accounting for greenhouse gas (GHG) sinks and sources, we used it only to estimate soil organic C stocks and flows that result from land use and land management changes in the conterminous U.S. for the period 1982 to 1997. The standard inventory period used in the IPCC method is twenty years. Since our land use data from NRI span a 15-year period, estimates of the baseline C stock changes were adjusted proportionally. Organic soils (i.e., peat and muck soils) used for agricultural production were not included in our analysis of potential C sinks. 2.1. IPCC INVENTORY APPROACH The Intergovernmental Panel on Climate Change (IPCC) developed the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories to provide methods by which signatory countries to the United Nations Framework Convention on Climate Change (UNFCC) could estimate ‘emissions by sources and removals by sinks’ of greenhouse gases. As part of the Land Use, Land Use Change and Forestry section of the guidelines, a method for estimating net C emissions from soils was developed. The method estimates average annual C emissions and/or sinks from land use and management changes, based on computed soil C stock changes over a 20-year inventory period. Default values for baseline soil C stocks are provided along with a series of coefficients that determine carbon stock changes as a function of climate, soil type, disturbance history, tillage intensity, productivity, and residue management (IPCC, 1997b). Documentation related to the inventory methods for land use and management change are in the IPCC Workbook Module 5 (Land-Use Change and Forestry; IPCC, 1997b) and Reference Manual, Chapter 5 (Land-Use Change and Forestry; IPCC, 1997c). The IPCC Guidelines were developed for use by all member countries of the UNFCCC, including countries lacking detailed data on land use and management changes. Thus the data requirements represent a compromise between the level of detail required to conduct the most accurate inventory estimates for each country and the input data likely to be available in most countries (IPCC, 1997b). The uncertainty in the estimates of greenhouse gas emissions from agriculture and from land-use change and forestry may be as high as ±50% (IPCC, 1997b). However, because each input data set has an associated level of uncertainty that gets passed


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through the analysis, it is difficult to directly quantify the level of uncertainty in this type of analysis (Cannell et al., 1999; Houghton et al., 1999). The IPCC method involves the stratification of land area into major climatic regions and soil types to determine reference soil carbon stocks. Within each climate-soil stratum, the areas associated with different land use and land management systems are categorized at the beginning and end of the inventory period. Soil C stock changes are then computed based on changes in land use and management that occur within the inventory period, for the entire land area included in the inventory. 2.2. CLIMATE AND SOIL STRATIFICATION The IPCC identifies eight broad climatic regions based on average temperature and precipitation and length of dry season (in the tropics). Based on these criteria we delineated six climate regions (Figure 1) for the conterminous U.S.: cold temperate, dry (CTD), cold temperate, moist (CTM), warm temperate, dry (WTD), warm temperate, moist (WTM), sub-tropical dry (STD), and sub-tropical moist (STM) (Eve et al., 2001). As our basic spatial unit for the analyses we used Major Land Resource Areas (MLRA), which represent broad ‘ecoregions’ defined by similar soils, climate, water resources, and land uses (NRCS, 1981). Average daily temperature and precipitation were computed for each MLRA using the PRISM (Parameterelevation Regressions on Independent Slopes) climate mapping system (Daly et al., 1994; Daly et al., 1998) and then used to assign each of the 193 MLRA’s to one of the six IPCC climate regions. The IPCC method groups soils into six major types, based on physiochemical characteristics such as texture, mineralogy and drainage, which influence their inherent capabilities to stabilize soil C and their response to management. The soil groupings include (i) ‘high clay activity’ mineral soils (i.e., having a high charge density with predominantly 2:1 clay mineralogy); and (ii) ‘low clay activity’ mineral soils (i.e., having a low charge density with primarily 1:1 clays); the former being more common in moderately-weathered, temperate environments and the latter predominating in more highly-weathered, warm, humid environments. Other soil groups include (iii) sandy textured soils (i.e., >70% sand, 30% surface residue retention (CTIC, 1998), have a default value of 1.05. Default tillage factors are somewhat different for tropical soils (IPCC, 1997a).

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2.5. BASELINE SOIL C STOCK CALCULATIONS We first computed soil C stocks changes between 1982 and 1997 (the period of record for NRI) to provide an estimate of C changes that have already occurred, as a baseline from which potential soil C increases can be compared. We used the default values from the IPCC inventory documentation for carbon stocks under native vegetation and base, input and tillage factors (IPCC, 1997b). We computed baseline soil C change using the twenty-year IPCC inventory default factors (IPCC, 1997b), adjusted for our shorter 15-year inventory period (i.e., multiplied by 0.75). We estimated the change in soil C for each of the 1.3 million observed and imputed NRI points, each of which has statistically determined area expansion factors (Nusser and Goebel, 1997), to derive estimates of soil C stock change within each MLRA and for all U.S. cropland. The average change in soil C stock for each climate-soil-land use/management category were computed with the following equations: N T (SC1997 − SC1982) ∗ 0.75 (1) δC = NRI=1 t =1

SC1997 = (Ha1997 × SCR × BF × TF1997 × IF1997)

(2)

SC1982 = (Ha1982 × SCR × BF × TF1982 × IF1982) ,

(3)

where: δC = the change in C stocks for that land use scenario over the 15-year period (expressed as Tg), N = the NRI points, T = conventional tillage, reduced tillage, and no-till (in the baseline, each NRI point contained a proportion of each tillage system based upon CTIC derived data), Ha1997 = the number of hectares in that land use (crop rotation, CRP, etc.) in 1997, SC1997 = soil carbon stock in 1997, SC1982 = soil carbon stock in 1982, SCR = the IPCC default estimate of soil C under native vegetation-reference level (varies by climatic zone and soil type), 0.75 = factor to adjust 20-year inventory to the 15 years of data used, BF = the IPCC base factor (in this analysis of soil C potentials, the base factor is the same for 1982 and 1997), TF1997 = the IPCC tillage factor based upon the tillage system in 1997, IF1997 = the IPCC input factor based upon residue inputs from cropping activities in 1997, Ha1982 = the number of hectares in that land use (crop rotation, CRP, etc.) in 1982, TF1982 = the IPCC tillage factor based upon the tillage system in 1982, IF1982 = the IPCC input factor based upon residue inputs from cropping activities in 1982. The total change in soil C stocks for the climatic region is the sum of soil C stock changes for each land use category within the region. Baseline changes in soil C stocks were then converted to annual average rates of change (Tg C yr−1 ) for the fifteen-year inventory period. Sample calculation results are included in Table I for different crop rotation and tillage changes between 1982 and 1997.

CRC-CT CRC-NT CHA CRC-NT RSF-CT CRP

1982 1997 1982 1997 1982 1997

Utisols Utisols

Utisols Utisols

Mollisols Mollisols

Soil order

13102 13102

27841 27841

40872 40872

Area (ha)

120

120

108

MLRA

70 70

70 70

80 80

Native carbon b (mg C/ha)

0.7 0.8

0.9 0.7

0.7 0.7

Base factor c

0.9 1.0

1.0 1.0

1.0 1.0

Input factor d

1.0 na

na 1.1

1.0 1.1

Tillage factor e

0.58 0.73

1.75 1.50

2.29 2.52

Soil C stock f (Tg C)

0.156

–0.25

0.228

Stock change g (Tg C)

0.010

–0.067

0.015

Annual change h (Tg C/yr)

Conservation Reserve Program; CT = Conventional Tillage; NT = No-Till. The first crop rotation pair in the table shows a crop rotation where tillage intensity is changed from conventional tillage (full soil inversion) to no-till on a continuous row crop rotation (e.g., maize). The second crop rotation pair identifies soil C changes that result from a change from perennial (continuous hay) to annual cropping (row crop) with no-till. The third crop rotation pair estimates soil C changes from a change from conventionally tilled small grain – fallow to CRP. b The native soil carbon stock has a default value in the IPCC method based upon soil type and climate. The native soil C stock for IPCC defined high activity mineral soils (e.g., Mollisols) in the cold temperate moist region (MLRA 108) is higher than the native soil C stock for IPCC defined low activity mineral soils (e.g., Utisols) in the warm temperate moist region (MLRA 120). c The base factor represents the fractional soil C stock remaining after long-term cultivation for temperate cultivated soils. d The input factor is based upon the residue inputs from the crop rotation. e The tillage factor represents the change in soil C from tillage activities. Use of no-till is assumed to increase the soil C by 10% over conventional tillage. na = not applicable. f Soil C stock is estimated as Area × native carbon × base factor × input factor × tillage factor. g Stock change = (Soil C Stock 1997 – Soil C stock1982 ) ∗ 0.75. h Annual change = Stock change/15 (years of available data).

a CRC = Continuous Row Crop (e.g., maize-soybean rotation); CHA = Continuous Hay; RSF = Row Crop-Small Grain-Fallow; CRP =

Rotation and tillage a

Year

Table I Example of how the IPCC method is implemented using three different MLRAs with different soil orders to estimate soil C changes between 1982 and 1997 POTENTIAL SOIL C SEQUESTRATION ON U.S. AGRICULTURAL SOILS

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2.6. POTENTIAL STOCK CHANGE CALCULATIONS The C sequestration potential for U.S. agricultural soils was estimated by sequentially incorporating potential land use and land management changes, to avoid double counting of non-complementary soil C sequestering activities. For example, increased cropping intensity through inclusion of a winter cover crop and reduced tillage intensity are complementary activities, i.e., the application of one practice does not eliminate the use of the other practice. In contrast, accounting for reduced tillage on areas that have already been designated as perennial set-aside, which by definition has no tillage, would lead to double counting. Thus, we analyzed soil C sequestration of potential activities in the following order: conversion of HEL (highly erodible land) to set-aside, inclusion of winter cover crops, reduction in bare fallowing, and reduction in tillage intensity. Thus, for example, soil C in cropland set-aside cannot be further increased through reduced tillage, reduced fallow periods, or increased cropping intensity. To compute the sequestration potential on set-aside, we first maintained the present (as of 1997) 13.2 Mha of cropland enrolled in CRP. All additional annual cropland classified as HEL (and not already enrolled in CRP) was assumed converted into set-aside. Land with an erodibility index greater than eight is identified as HEL (United States Soil Conservation Service, Conservation Planning Division, 1994). Conversion of HEL to a set-aside program removes land from crop production, so it is the first potential activity addressed in the analysis. Winter cover crops include both annual and perennial species, usually grasses or legumes, and their use has been promoted for controlling nitrate leaching as well as providing green manure and improving soil quality. Typically they would be grown in combination with summer annuals (e.g., maize, soybean, spring cereals). While nearly all areas of the country are capable of using winter cover crops, they are generally not economically feasible in the semi-arid areas of the U.S., where soil moisture is the main limiting factor for crop growth. Thus we did not include annual cropped areas within the three dry climate zones, i.e., CTD, WTD or STD, in the estimates for winter cover crops. In the other climate regions, all continuous annual crop rotations were included in the area estimates for application of winter cover crops. However, rotations that already included hay were excluded. The IPCC inventory method base and tillage factors remain unchanged when a winter cover crop is included, but the input factor is adjusted upward (from 1 to 1.1) to account for the additional residue inputs. The practice of summer fallow every 2nd or 3rd year is common in much of the semi-arid areas of the Great Plains and Pacific Northwest used for wheat and other small grain production. The 1997 NRI data indicate that over 20 Mha or 15% of all U.S. cropland, mainly for wheat production, included fallow periods in the crop rotation. In the IPCC method, crop rotations with summer fallow are classified as low residue input systems with an input factor value of 0.9. In computing the potential C sequestration, all rotations including summer fallow were converted to continuous


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small grain rotations, which are defined as having a medium input factor of 1.0. However, summer-fallow area classified as HEL was not included, having been previously allocated to set-aside in the calculation of potential C sequestration. A variety of tillage practices exist that have less detrimental effects on soil C, due to less frequent, shallower and less intensive soil disturbance. Collectively, such practices are referred to as conservation tillage systems. The Conservation Technology Information Center (CTIC) defines conservation tillage to include notill, mulch till, strip till, and ridge till, all of which retain at least 30% residue on the soil surface (CTIC, 1998). Conservation tillage systems other than no-till are collectively referred to as reduced till. Based on CTIC data and other sources, we estimated that conventional tillage occurred on 64.9%, reduced tillage on 27.2%, and long term (greater than five years) no-till on 7.9% of the area under annual cropland in 1997 (Eve et al., 2001). We analyzed potential soil C response to greater adoption of conservation tillage by completely eliminating intensive tillage, assuming varying proportions of reduced (75, 50, 25, 0%) and no-till (0, 25, 75 and 100%), respectively. The increases in conservation tillage were applied to all annual cropland not previously allocated to set-aside or perennial crop systems.

3. Results and Discussion We estimate that changes in agriculture and land management occurring between 1982 and 1997 resulted in total soil C sequestration of 17.1 Tg C yr−1 for mineral soils (Table II). Of this total, 8.2 Tg C yr−1 resulted from decreased tillage intensity and 8.9 Tg C yr−1 from other management changes. Our 1997 baseline soil C sequestration rate is higher than the 1992 baseline (14 Tg C yr−1 ) estimated by Eve et al. (2001), which used data from the 1982 and 1992 NRI. Increases in cropland planted to forest and conservation tillage between 1992 and 1997, as well as decreased area in bare fallow, and changes in hay and pasture management account for most of the difference (Figure 2). Soil C increases of 4.5 Tg C yr−1 from existing CRP enrollments were estimated using the IPCC method. This is lower than estimates by Follet et al. (2001a; 7.6–11.5 Tg C yr−1 ), Paustian et al. (2001; 6 Tg C yr−1 (mean simulated rate of accumulation of below ground C)) and Lal et al. (1998; 5–11 Tg C yr−1 ) but within the range of Gebhart et al. (1994; 2–29 Tg C yr−1 ). Converting remaining HEL (HEL not removed from crop production by CRP) to perennial grass ‘set-aside’ was estimated to increase soil C accumulations by 10.5 Tg C yr−1 (Table III) over baseline conditions. Potential soil C increases from converting HEL to grassland were greatest in the cool and warm temperate moist (CTM and WTM) climate zones. Converting all HEL to a set-aside program such as CRP would remove an additional 25.8 Mha from crop production, over CRP enrollment of 13 Mha in 1997, bringing total land set aside from cropping activities to nearly 39 Mha

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Table II Baseline areas of conventional, reduced, no-tillage and CRP for the conterminous U.S. by climatic region with estimated baseline changes (1982–1997) in soil C from IPCC inventory analysis (annual average rate of change) Climatic region a

Conventional tillage

Reduced tillage

No tillage

CRP b

Thousand hectares

Total soil C δ c Soil C sequestration (Tg C yr−1 )

CTD CTM STD STM WTD WTM

5,500 30,600 900 1,500 10,900 31,400

3,200 16,000 140 400 2,100 12,000

1,100 4,000 2 20 200 4,500

2,300 4,100 60 6 2,400 4,300

1.2 3.7 0.2 –0.9 1.0 11.9

Total

80,800

33,800

9,800

13,200

17.1

a CTD = Cool Temperate Dry; CTM = Cool Temperate Moist; STD = Subtropical Dry;

STM = Subtropical Moist; WTD = Warm Temperate Moist; WTM = Warm Temperate Moist. b CRP = Conservation Reserve Program. c δ = annual average rate of change. d The change in soil C from activities other than reduced tillage intensity.

(Table IV). Over 16.6 Mha of cropland in the CTM and WTM climatic regions would be removed from crop production to attain the potential soil C gain. Our estimated potential of 10.5 Tg C yr−1 from an additional 26 Mha of setaside falls between other published estimates, but the annual rate of change, 0.4 Mg C per hectare, is smaller. Follett et al. (2001a) estimated potential soil C sequestration of 1.2–1.8 Tg C yr−1 (0.6–0.9 Mg C per hectare) from an increase in CRP enrollment of 2 Mha. Lal et al. (1998) estimated an additional 28.6 Mha of CRP could sequester 9–20 Tg yr−1 (0.3–0.7 Mg C per hectare). Bruce et al. (1999) estimated that an additional 8.7 Mha of grassland in set-aside had the potential of increasing soil C by 7.0 Tg C yr−1 (0.8 Mg C per hectare). The addition of a winter cover crop to annual crop rotations provides potential annual soil C increases of 22.8 Tg C yr−1 (Table III). The CTM and WTM climatic regions contribute all of the potential increases in annual soil C sequestration due to this practice, since the potential for winter cover crops were excluded from the dry cropping regions. Lal et al. (1998) estimated that the potential increase in soil C from inclusion of winter cover crops on 51 Mha was 5–15 Tg C yr−1 , which is lower than our estimated potential of 22.8 Tg C yr−1 . Potential soil C gains of 3.2 Tg C yr−1 were projected when all summer fallow operations are eliminated, which compares favorably with the Lal et al. (1998)


Table III Potential change in soil C resulting from elimination of summer fallow, conversion of highly erodible land (HEL) to set-aside (a program like CRP), or a winter cover crop is included in the crop rotation Climatic region a

Baseline

Potential land management changes Eliminate summer HEL b to set-aside c fallow

Addition of winter cover crop

Soil C sequestration (Tg C yr−1 ) CTD CTM STD STM WTD WTM

1.2 3.7 0.2 –0.9 1.0 11.9

0.4 0.9 0.0 0.1 0.6 1.2

1.8 3.1 0.3 0.1 1.5 3.8

0.0 11.3 0.0 0.0 0.0 11.5

Total

17.1

3.2

10.5

22.8

a CTD = Cool Temperate Dry; CTM = Cool Temperate Moist; STD = Subtropical Dry;

STM = Subtropical Moist; WTD = Warm Temperate Moist; WTM = Warm Temperate Moist. b HEL = Highly Erodible Land. c Set-aside is cropland removed from crop production and planted to perennial grass. Table IV Area of baseline CRP (hectares enrolled in 1997), potential total area of highly erodible land (HEL) converted to set-aside (planted to perennial grass), and total area set-aside from cropping activities when HEL is removed from crop production (HEL converted to set-aside plus CRP hectares) Climate a

Baseline CRP b

HELc to set-aside d

HEL to set-aside plus CRP

Thousand hectares CTD CTM STD STM WTD WTM

2,300 4,100 60 6 2,400 4,300

5,400 8,400 360 47 3,300 8,200

7,700 12,600 400 52 5,700 12,600

Total

13,200

25,700

39,000

a CTD = Cool Temperate Dry; CTM = Cool Temperate Moist; STD = Subtropical

Dry; STM = Subtropical Moist; WTD = Warm Temperate Moist; WTM = Warm Temperate Moist. b CRP = Conservation Reserve Program. c HEL = Highly Erodible Land. d Set-Aside is cropland removed from crop production and planted to perennial grass.

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Figure 2. Areas affected by change in land use and land management activities between 1992 and 1997 that account for most of the difference in baseline soil C sequestration rates between Eve et al. (2001) and the analysis presented in this paper. Values in parentheses are the increased soil C sequestration rates resulting from the land use and land management changes.

estimated potential of 1–3 Tg C yr−1 from elimination of bare summer fallow. The temperate, moist (CTM and WTM) climatic regions provide the highest increases at 2 Tg C yr−1 , followed by the WTD region at nearly 0.6 Tg C yr−1 (Table III). Estimated soil C losses from baseline land-use change activities in the STM climatic region are –0.9 Tg C yr−1 . Eliminating fallow in the STM climatic region results in less soil C loss to –0.8 Tg C yr−1 . Adoption of less intensive tillage practices is one the largest contributors to potential soil C increase. The cold temperate moist (CTM) and warm temperate moist (WTM) climatic regions contain the most cultivated cropland and no-till is also most prevalent in those regions at present (Table II). The low baseline adoption of no-till in other climate zones (e.g., in STD, STM, and WTD, Table II) suggests that there are greater barriers to adoption of conservation tillage in those regions that need further analysis. We analyzed a range of potential changes in tillage practices and estimated soil C gains by climatic zones (Table V). Replacing conventional tillage operations with 75% reduced till and 25% no-till increases net soil C sequestration by 15 Tg C yr−1 over the present baseline condition. Elimination of conventional and reduced tillage with full adoption of no-till increases soil C sequestration by nearly 30 Tg


333

Table V Proportion of U.S. cropland under conventional, reduced, and no-tillage with estimated potential soil C change from increased adoption of conservation tillage on row crops, small grains and low residue crops – by climatic region. Baseline soil C sequestration rates result from 64.9% conventional tillage, 27.2% reduced tillage, and 7.9% no-till % CT, RT and NT a

Climatic region b CTD CTM STD

STM

WTD

WTM

Total

Soil C sequestration (Tg C yr−1 ) Baseline 0, 75, 25 0, 50, 50 0, 25, 75 0, 0, 100

1.2 1.5 1.6 1.7 1.8

3.7 9.6 11.6 13.5 15.5

0.2 0.3 0.4 0.4 0.4

–0.8 –0.1 0.0 0.2 0.3

1.0 2.3 2.6 2.9 3.2

11.9 18.9 21.2 23.4 25.7

17.1 32.5 37.3 42.1 46.8

a CT = conventional tillage, RT = reduced tillage, and NT = no-till. b CTD = Cool Temperate Dry; CTM = Cool Temperate Moist; STD = Subtropical

Dry; STM = Subtropical Moist; WTD = Warm Temperate Moist; WTM = Warm Temperate Moist.

C yr−1 . This estimate is within the range of the potential of 24–40 Tg C yr−1 from increased conservation tillage estimated by Lal et al. (1998). The net gain (over baseline conditions) of soil C sequestration from increased adoption of reduced tillage and no till operations is largest in the temperate, moist (CTM and WTM) climatic zones. Complete adoption of no-till management increases soil C by 12 Tg C yr−1 in the CTM and 14 Tg C yr−1 in the WTM climatic regions (Table V). These climatic regions account for nearly 86% of the net increase in soil C when no-till is adopted for all crop systems. The CTM and WTM climatic regions account for nearly 84% of increased soil C when conventional tillage is eliminated and 25% no-till adoption is assumed in all cropping systems. When all activities are implemented simultaneously, U.S. agricultural soils were estimated to have the potential to increase soil C by 66 Tg C yr−1 over the current baseline estimates of 17.1 Tg C yr−1 . Every MLRA that contains cultivated cropland and each IPCC climatic region show a positive net increase in soil C (Figure 3). The MLRAs with no change in soil C from potential land use and land management change contain either zero or very little cropland area (0.003% of all conterminous U.S. cropland) and represent a small area relative to the remainder of the country (3%). These MLRAs are dominated by sandy and low activity mineral soils (nearly 80% of the area) and are in cool climatic regions (over 90% of the area). The largest potential soil C increases (17 Tg C yr−1 total) occur in MLRAs 73, 103, 108, 111, and 131 (darkest areas in Figure 3) which are dominated by Udoll and Udalf soils and most of the land area is in farms and cropland (NRCS,

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Figure 3. Soil C increase estimated using the IPCC inventory method when all cultivated cropland is converted to no-till production, highly erodible land is converted to grassland, winter cover crops are included in the crop rotation, and bare summer fallow operations are eliminated, delineated by Major Land Resource Area (MLRA).

1981). The average annual precipitation ranges from 500 mm yr−1 in MLRA 73 (the rolling plains and breaks of Kansas and Nebraska) to 1650 mm yr−1 in MLRA 131 (Southern Mississippi Valley, Alluvium Arkansas, Mississippi, Missouri, and Tennessee) (NRCS, 1981). Potential land use and land management changes result in the inclusion of winter cover crops in 65% of the area involved in crop production. In addition, more than 2.9 Mha were converted from crop production to set-aside, a substantial increase over the 0.6 Mha enrolled in CRP in the baseline in these regions.

4. Conclusions Using the IPCC inventory approach allowed us to incorporate the interaction of climate, soils, and crop production management to estimate soil C sequestration potential in U.S. agricultural soils at a spatial resolution not available from previous research. Soil C sequestration rates for each crop rotation on each major soil type within each MLRA were estimated using spatially distributed data on soils, climate, historic and potential crop rotations, and management activities.


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This analysis provides insight about the geographic regions where different carbon sequestration strategies may be most effectively implemented. The IPCC approach contains some inherent limitations that should be considered. For example, soil C changes are assumed to occur only in response to changes in land use or management, and thus land under long-term constant management regimes are assumed to be in equilibrium, neither gaining nor losing soil C. It is well documented that the increases in crop productivity (due to increased fertilization, pesticides, genetic improvements) in recent decades have led to increased return of C in crop residues (Flach et al., 1997; Reilly and Fuglie, 1998). These productivity increases are likely contributing to increased C sequestration (Cole et al., 1993; Flach et al., 1997) independent of the management changes we have analyzed here. Thus, the baseline and potential soil C changes presented in this manuscript may be underestimated. The IPCC inventory also does not account for the effects of erosion on net soil C changes. However, the net effects of erosion on soil carbon balance at the regional level are as yet unclear, in that erosion processes can contribute both to accelerated losses of carbon (Lal et al., 1998) as well as sequestration of carbon in bottomlands and aquatic sediments (Stallard, 1998). Finally, uncertainties about the potential for carbon sequestration exist both regarding rates of carbon sequestration as a function of management practices, and more importantly, the extent of adoption of these practices by farmers facing economic and other constraints. Efforts to incorporate socio-economic factors limiting adoption, through integrated economic and ecosystem modeling, are being pursued using several approaches (Sperow et al., 2002; Schneider, 2000; McDowell et al., 1999, Antle et al., 2002), which will help to provide a more refined assessment of carbon sequestration potential. The IPCC is also developing methods to estimate the uncertainty associated with GHG emissions and sinks using empirical data and expert opinion (IPCC, 2000) and activities are ongoing to identify and analyze uncertainty (e.g., Ogle et al., in preparation). Overall, the IPCC inventory method indicates that U.S. agricultural soils have the potential of increasing soil C by 66 Tg C yr−1 (total of 83 Tg C yr−1 ) through increased adoption of no-till, inclusion of winter cover crops, elimination of fallow, and conversion of all HEL to grassland. From these same activities, Lal et al. (1998) estimated potential soil C increases in the range of 35–69 Tg C yr−1 . Agricultural policy activities designed to increase soil C should be directed at these activities to attain the greatest gain. Total U.S. GHG emissions, including CO2 , CH4 , N2 O, HFCs, PFCs and SF6 , from all sources in 1999 were estimated at 1,840 Tg CE (carbon equivalent; USEPA, 2001). Potential soil C sequestration estimated in this paper (83 Tg C yr−1 ) represents a potential sink of over 4% of these emissions and more than 5% of the estimated CO2 emissions from all sources in 1999 (1,516 Tg CE; USEPA, 2001). A comparison of potential soil C sequestration to GHG reduction targets for international agreements provides additional insight. The Kyoto Protocol, signed in 1998 but never ratified by the U.S., set an emission reduction target for the U.S. of 7%

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below the 1990 GHG emission levels. Using the latest five-year average increase in GHG emissions of 1.3% per year (USEPA, 2001) and assuming business as usual activities continue, 2008 emissions are estimated to be 2072 Tg CE. To achieve the GHG emission reduction goal of 7% below the 1990 level would require 2008 GHG emissions to be reduced by 540 Tg CE. The estimated potential C sink provided by agricultural soils represents 15% of the estimated required reduction. Therefore, our analysis suggests that agricultural soil C sequestration could play a meaningful, but not predominant, role in helping mitigate GHG increases. Lal et al. (1998) estimated CO2 emissions from agricultural cropland in the U.S. at 43 Tg C from (fossil) energy use, manufacture and distribution of fertilizer and pesticide and emissions from soil erosion (due to enhanced organic matter oxidation). Thus, the potential C sequestration calculated here is nearly twice the level of current gross CO2 emissions estimated for agriculture.

Acknowledgements This research was funded through grants from the United States Department of Agriculture (USDA) for the Fund for Rural America and the United States Environmental Protection Agency (USEPA) for the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS).

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