Soil Science Society of America Journal 67:928-936 (2003)
© 2003 Soil Science Society of America
DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS
Crop Residue Returns and Equilibrium Soil Organic Carbon in England and Wales
J. Webb*,a,
P. Bellamyb,
P. J. Lovelandb and
G. Goodlassa
a ADAS Research, Wergs Road, Wolverhampton WV6 8TQ, UK
b National Soil Resources Institute, Cranfield University, Silsoe, Bedfordshire MK45 4DT, UK
* Corresponding author(m99102{at}adas.co.uk)
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ABSTRACT
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Increased temperatures caused by climatic change may increase the turnover of soil organic matter (SOM) and hence reduce soil organic C (SOC). This effect may be exacerbated if crop yields decrease in consequence of policies that limit fertilizer-N applications to reduce N pollution from agriculture. Model simulations were made of changes in SOC over 140 yr under three fertilizer-N regimes to examine the effects of changes in fertilizer-N use on SOC in arable soils in England and Wales (E&W). The RothC model was used in preference to CENTURY as the input fertilizer-N could be changed in RothC and could not in CENTURY. Results indicate that decreasing annual fertilizer-N use to 50 or 100 kg ha-1 less than is currently applied to cereals in E&W will have a negligible impact on SOC in arable soils over the next 140 yr. Soils with >180 g kg-1 clay with 16 to 27 g kg-1 SOC at the beginning of the model runs, were predicted to have about 21 to 23 g kg-1 SOC after 140 yr, while soils with <180 g kg-1 clay and about 12 g kg-1 SOC would change little over 140 yr. Increases in temperature because of climate change were predicted to reduce SOC concentrations to about 18 to 20 and 11 g kg-1 respectively.
Abbreviations: DM, dry matter • DPM, decomposable plant material • E&W, England and Wales • Nopt, the amount of fertilizer-N required to obtain economic optimum yield • NSI, National Soils Inventory • OM, organic matter • RPM, resistant plant material • SFP, UK Survey of Fertilizer Practice • SOC, soil organic C • SOM, soil organic matter
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Effects of Crop Residues on Soil Organic Carbon in England and Wales
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Soil organic matter stabilizes soil aggregates, makes soil easier to cultivate, increases soil water holding and buffering capacities and SOM breakdown releases plant nutrients (Carter and Stewart, 1996). Although no critical level of SOM has been identified below which soil quality decreases markedly or irreversibly, decreasing SOM is still of concern since it might adversely affect some or all of the above properties. The SOM content depends on soil type (Schimel et al., 1994), frequency and type of cultivation (Heenan et al., 1995), cropping and residue management (Grace et al., 1995), and fertilizer-N input (Bhogal et al., 1997). The equilibrium SOM concentration will reflect these factors, and the smaller it is, the less satisfactory the soil may be for continued crop production, albeit because of the importance of other factors, including husbandry and climate, there is no simple relationship between SOM and crop production (Sojka and Upchurch, 1999). The smallest amounts of SOM are usually found in arable soils (Haynes et al., 1991), in which the majority of organic matter additions are as crop residues.
To reduce N pollution from agriculture, measures have been introduced in Europe to limit N inputs to crops (e.g., Archer, 1992). Usually there is no requirement to apply less N than is needed for optimum yields. However, in some countries, for example, Denmark and Sweden, further limits on N applications have been imposed (Birkmose, 1999; Jakobsson, 1999). Where such limits are applied, crop yields and residue returns will be reduced and hence in arable systems there may be a reduction in the potential equilibrium SOC. There is also concern that increased temperatures caused by climate change will further reduce SOC in arable soils in E&W because of the increased rate of SOM decomposition (Addiscot, 1983).
The object of this study was to model C changes to estimate the effects of reducing fertilizer-N applications by 50 and 100 kg ha-1 less than current usage, on the equilibrium SOC in a range of soil types in arable cropping in E&W. Some modeled comparisons were also made of SOC under grass.
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MATERIALS AND METHODS
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Data Sources
Soil organic matter is equated to SOC, and most models calculate turnover of C rather than SOM. Changes in SOC may be expressed as:
In arable soils, annual 
SOC will usually be negative since most of the fresh OM is mineralized within the season of application (Matus and Rodríguez, 1994) and the amount of OM stabilized is likely to be smaller than the amount of SOC mineralized. The exponential decrease of SOC may be expressed as:
where Ce equals equilibrium SOC, C equals current SOC, Co equals initial SOC, a equals calculated constant, k equals decomposition constant, and t equals time.
Thus, providing there is no significant change in climate, as the decrease in SOC becomes smaller over time, the stabilized additions of C from crop residues will eventually balance the annual loss of SOC. In arable rotations the majority of OM returns are as crop residues, that is, the unharvested parts of the crops, roots, root exudates, straw, leaves, stubble etc., the amounts of which will be proportional to harvested crop yield. The effect of changes in fertilizer-N additions can therefore be estimated using measured crop responses to fertilizer-N additions. We modeled the effect of changing fertilizer-N use on equilibrium SOC in an all-cereal rotation using crop residue response data to fertilizer N from the ADAS database now named NITRIC, the cereal component of which is described in Goodlass et al. (1996), which includes cereal N response data from 261 sites. Residue return was calculated from: 1. yield response parameters, 2. N concentration in crop response parameters, 3. yields calculated for specified fertilizer-N applications, 4. N concentration in grain calculated for specified fertilizer-N applications, 5. grain N off take calculated from 3. and 4., 6. crop residue return calculated from 5. and N harvest indices of 0.73 for sandy soils (Webb et al., 1998) and 0.80 for other soil types (Bloom et al., 1988).
These estimates of N returns were converted to C using a C/N ratio of 45 (Palm and Rowland, 1997). We estimated the effects of reductions in fertilizer N on residue C returns at current fertilizer-N applications as reported by the UK survey of fertilizer practice (SFP) (Burnhill et al., 1996) and SFP-50 and SFP-100 kg N ha-1. Because of the large effect of soil type on crop yields (Goodlass et al., 1996) we made separate estimates of crop residue-C returns, based on crop yields on the main soil groups used in UK fertilizer recommendation systems (Anon, 1995), that is, clay, silt, sand, shallow-over chalk, and other mineral soils with default clay concentrations as given in Table 1.
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Table 1. Definition of the five soil types used to assess changes in soil organic C (SOC) by RothC when cereals are grown at three N inputs.
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Choice of Model
Nine SOC models were evaluated by Smith et al. (1997), using twelve datasets from seven long-term experiments. RothC was found to produce small errors for most datasets. Where RothC did not provide a good fit to the data, it was because RothC had been initialized to start with the same conditions for all treatments being modeled at a particular site. When run with slightly different initial C contents, the fits were improved. Overall, no difference was found between the performance of RothC and a group of five other soil C models. We further considered two models of SOC changes for this study, CENTURY (Parton, 1996) and RothC (Smith et al., 1996). Many of the inputs required are common to both models, some supplied as default values, others input by the user. Without returns of crop residues both models predicted continued decreases in SOC, with no equilibrium (Fig. 1)
. For all soil types RothC and CENTURY predicted decreases from the original SOC of 77 and 67% respectively.
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Fig. 1. Changes in soil organic C (SOC) modeled using CENTURY and RothC for the major soil groups under fallow. Roth C(——); CENTURY (- - - - - -) x axis = years from start of simulation; y axis = SOC g kg-1.
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To test the appropriateness of the two models for this study, we used data from a number of UK sites of contrasting soil types (Table 1). The CENTURY model was run for 100 yr (the maximum run possible). RothC was first run iteratively over 10 000 yr by adjusting the input from permanent pasture to produce an initial SOC the same as that originally measured, as found to improve the model's precision by Smith et al. (1997) (Tables 2 and 3). This gave estimates of the values of decomposable plant material (DPM), resistant plant material (RPM) etc., needed as input for the simulation runs over shorter timescales. Once this equilibrium was reached the RothC model was run for each soil type for 100 yr. Figure 1 shows the changes in SOC over 100 yr estimated using the two models. For the first 60 yr, the two models produced similar estimates. Thereafter, CENTURY always predicted greater values of SOC, most noticeably for the clay and chalk soils. The reasons for this difference are not clear, but may be a consequence of the way CENTURY utilizes the greater water capacities of these soil types. We chose the RothC model for the remainder of the study for two reasons. It was not possible to adjust the annual input of plant residues (and hence different N application rates) in the CENTURY model, and the RothC model could be run for up to 140 yr possibly allowing SOC to reach an equilibrium.
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Table 2. Average yield and soil organic C (SOC) returns based on average data from ADAS winter wheat, winter barley, and spring barley experiments.
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Table 3. Parameters used in the RothC model runs to determine the effect of changing fertilizer-N additions on equilibrium organic C concentration in five soil types.
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Application of RothC
We chose the longest possible run-time in RothC (140 yr) because the 100-yr runs (above) suggested that SOC in arable cropping would reach an equilibrium after around 100 yr. Climate data for Lincolnshire (eastern England) were used (Table 4, from Smith, 1976), this county was taken to represent eastern England, the main area of arable cropping in E&W. Open pan evaporation was taken as potential evapotranspiration (Smith, 1976) divided by 1.33 (Smith et al., 1996). The depth of topsoil was taken as 20 cm. Estimates of decomposability of crop residues were the default values given in the model documentation (a DPM/RPM ratio of 1.44). No additions of livestock manures were assumed, as these are not applied to the majority of arable crops in eastern England. The percentage crop cover for each month, and monthly input of crop residues were varied according to the model run. We calculated annual returns of SOC as crop residues for each soil type for different yields of winter wheat (Triticum aestivum), winter and spring barley (Hordeum vulgare) for each of the three fertilizer-N applications (SFP, SFP-50, and SFP-100 kg N ha-1) using the methodology outlined above. We ran the model with and without these returns to represent current practice and potential input-reduction scenarios. To further examine the effect of initial SOC concentration on SOC concentrations after 140 yr, the initial SOC for the sandy soil was increased by 10 g kg-1 (high-start sands).
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Table 4. Average climate data used for runs of ROTHC model to determine the effect of changing fertilizer-N additions on equilibrium organic C concentration in five soil types.
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RESULTS AND DISCUSSION
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Effects of Fertilizer Nitrogen
At current and reduced fertilizer-N applications, crop C returns were significantly (p < 0.05) greater for clay and silt soils than for the other soil groups (Table 2). After 140 yr, SOC concentrations differed according to soil type (Fig. 2)
. The significance of these differences could not be assessed, as the model does not give an estimate of uncertainty in its output.
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Fig. 2. Changes in soil organic C (SOC) in five soil types modeled using RothC growing cereals at three N inputs. Current fertilizer-N usage(——); Current fertilizer-N usage: -50 kg N ha-1(- - - -); Current fertilizer-N usage: -100 kg N ha-1 (......); High start sands (- · - · -); x axis = years from start of simulation; y axis = SOC g kg-1; (a) = silt, (b) = sand, (c) = clay, (d) = chalk, and (e) = light.
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With crop residue returns included in the simulations, the soils fell into two classes: those in which SOC decreased (soils with >20 g kg-1 OC); those in which SOC increased (soils with <20 g kg-1 OC). Neither of those groups of soils appeared to be close to equilibrium after 140 yr, but there were differences according to clay content. Soils with >180 g kg-1 clay were at about 21 to 23 g kg-1 SOC, but only about 13 g kg-1 for the low clay soils, except for the high-start sands at about 17 g kg-1 SOC (Fig. 2).
The changes in fertilizer-N inputs modeled here had little impact on SOC concentrations after 140 yr (Fig. 2), although soil type clearly influenced the final SOC concentration. None of the scenarios examined reached equilibrium. This is in contrast to the results obtained in studies such as that reported by Smith et al. (2000) who estimated, using the CENTURY model, that by 2010 agricultural soils in Canada will no longer be a net source of CO2. One of the explanations put forward by Smith et al. (2000) for their results was the increase in the proportion of agricultural soils on which crops are planted without cultivation, together with a decrease in summer fallow and an increase in fertilizer-N use leading to greater returns of crop residues and hence greater sequestration of CO2. In our study it was assumed that crops would be established following cultivation, and this is likely to remain so to incorporate the substantial amounts of straw (up to 6 Mg ha-1) produced by cereal crops in E&W that can no longer be burned, and for which there are limited opportunities to use as bedding for livestock in eastern areas.
Although predicted decreases in SOC for sandy soils may cause concern, there is evidence that aggregate stability and nutrient mineralization are primarily a function of the young or light fraction SOC, and that this fraction will not decrease in linear proportion to the total SOC (Körschens et al., 1998). For example, Grace et al. (1995) noted an increase in wheat yields since the 1960s on continuous wheat plots in Australia despite an overall decrease in SOC from 27.5 to 15.6 g kg-1 since 1925. The yield increases were attributed to an increase in the light fraction SOC. Körschens et al. (1998) concluded from a series of long-term experiments in Germany (begun in 1902) that SOC had reached equilibrium at about 7 g kg-1 for a sandy soil (30 g kg-1 clay) and about 20 g kg-1 for a loess soil (21 g kg-1 clay). Thus, the modeled output reported here is consistent with the results of long-term field studies. As a result of their work, Körschens et al. (1998) proposed upper (as well as lower) limits to SOC to reduce emissions of N and C. These limits increased with increasing clay concentrations, being 10 to 15 g kg-1 and 35 to 44 g kg-1 for soils with 40 and 380 g kg -1 clay respectively. These ranges are equivalent to 2 to 6 g kg-1 light fraction C for both soil types.
There was little effect of different N applications on the modeled SOC concentrations. Studies of the cumulative effect of differential fertilizer-N additions on SOM (Bhogal et al., 1997) indicate that there is little effect of fertilizer N below crop-N requirement for optimum yield (Nopt) because much of the increased dry matter (DM) production is removed in the harvested crop. However, above Nopt, the harvest index may decrease leading to proportionately greater return of unharvested crop residues. In the scenarios examined here fertilizer N was at or below Nopt.
Effect of Climate Change
The simulations were repeated using the medium-high option, perceived as the most likely outcome, of the climate change scenarios for the UK (Hulme and Jenkins 1998). The revised climate data are given in Table 5. The results are shown in Fig. 3
for current fertilizer-N inputs. The output represents an extreme response to climate change in that the climate change impact was applied from the present day, since RothC cannot accommodate incremental changes in the climate input file. Thus, the sharp decrease in SOC concentrations after 60 yr and more were not true reflections of the consequences of gradual climate change and undoubtedly represent very much a worst-case scenario. Nevertheless, the computer simulation indicates that expectations of a warmer climate in the UK will further reduce stocks of SOC in arable soils.
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Table 5. Medium-to-high climate change scenario data used for runs of ROTHC model to determine the effect of changing fertilizer-N additions on equilibrium organic C concentration in five soil types.
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Fig. 3. Effects of predicted climate change on soil organic C (SOC) concentrations on five soil types in England and Wales. Current climate (——); warmer climate (-----) x axis = years from start of simulation; y axis = SOC g kg-1; (a) = silt, (b) = sand, (c) = clay, (d) = chalk, and (e) = light.
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This finding is based on the greater mineralization of SOC as temperatures increase (Addiscot, 1983). However, Liski et al. (1999) point out that such predictions rely on the assumption that the breakdown of all SOC increases with temperature by as much as the mineralization of young SOC, but their study found the breakdown of old SOC to be little affected by changes in temperature. However, since it is the young SOC that appears to exert the greatest benefits to soil structure and fertility, the maintenance of stabilized SOC under a warmer climate may be of little benefit.
Influence of Initial SOC Concentration
There was little effect of increasing the initial SOC concentration by 10 g kg-1 (high-start sands), on SOC concentrations after 140 yr. The SOC decreased to become similar to that in the sandy soils of the lesser initial SOC concentration. Figure 4
shows the relationship between clay concentration and SOC for 1261 UK agricultural soils from the NSI database (Loveland, 1990). It supports the view that a proportion of SOC is protected within micropores and by adhesion to clay particles, since there are few datapoints below a line, which may be fitted from the equation:
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Fig. 4. The relationship between clay content and soil organic C (SOC) concentration in 1261 agricultural soils in England and Wales. x axis = soil clay (<0.002 mm) concentration g kg-1; y axis = SOC g kg-1.
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We decided to examine whether RothC would predict the SOC concentration for a given clay content, whatever the initial SOC concentration. To provide the most sensitive test, we chose an initial SOC concentration of 110 g kg-1, at the upper end of our range of measurements, and clay contents of 150, 300, and 450 g kg-1. Following initial equilibration of the model, it was run as fallow (no inputs) and also for wheat and barley with current applications of fertilizer-N, which gave a return of 2.37 Mg C ha-1 yr-1. Figure 5
shows the modeled SOC concentrations for these soils with no crop residues. From Figure 4 we would expect soils of 150, 300, and 450 g kg-1 clay to have SOC concentrations of no less than 65, 125, and 185 g kg-1 respectively. However, in the modeled output, soils behaved in a similar manner regardless of clay content. After 100 yr of fallow they all contain about 20 g kg-1 SOC and this decreased to about 10 g kg-1 after 140 yr. With standard crop residue returns, SOC concentrations were about 60 and 50 g kg-1 after 100 and 140 yr, respectively. This divergence from the relationship expected from data presented in Fig. 4 is because RothC is not parametized to have basal SOC concentration related to clay concentration, but clay concentration is used to calculate the ratio of C from crop residues lost as CO2 to that remaining in the soil.
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Fig. 5. Predicted change in soil organic C (SOC) for soils well supplied with SOC, as affected by clay content. 150 g kg-1 clay with current fertilizer-N usage (—); 300 g kg-1 clay with current fertilizer-N usage (...); 450 g kg-1 clay with current fertilizer-N usage(- - -); 150 g kg-1 clay fallow (—); 300 g kg-1 clay fallow (...); 450 g kg-1 clay fallow(---); (a) fallow, (b) standard crop residue returns, x axis = years from start of simulation, y axis = SOC g kg-1.
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Addition of Grass to the Scenario
Grass crops are generally considered to maintain or increase SOC concentrations (Dalal et al., 1995). We therefore reran the model for the basic soils types (Table 1) using initial SOC concentrations for grassland. While RothC was developed and usually validated for nonwaterlogged soils in arable rotations, it was considered suitable for use with permanent managed grass (P. Smith, personal communication, 2000). We set the annual C input to 2.76 Mg ha-1 based on results obtained from IACR Rothamstead Park Grass experiments (details of this work, including the 31 species growing on the experimental area, can be found in Thurston et al., 1976). The results are shown in Fig. 6
from which it can be seen that SOC increased for all soils. This is in contrast to the findings of the NSI in which resampled permanent grass sites showed a small decrease in SOC after 15 yr. However, experimentation with RothC showed that the equilibrium value for C additions, that is, the annual input below which there is a long-term decrease in SOC under grass was c 2.6 Mg ha-1 yr-1 that is, just below the Park Grass value. This suggests that inputs of C derived from the Park Grass experiment are greater than for most commercial grass crops in the UK. It also illustrates that for extended model runs small differences in input parameters significantly affected the modeled outcome.
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Fig. 6. Changes in soil organic C (SOC) under grassland predicted by RothC on five types in England and Wales. x axis = years from start of simulation, y axis = g kg-1 SOC; (a) = silt, (b) = sand, (c) = clay, (d) = chalk, and (e) = light.
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CONCLUSIONS
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Reductions in fertilizer-N use on arable crops in E&W, because of either legislation to protect the environment or falling commodity prices, are unlikely to greatly affect trends in SOC over the next 100 yr. Mineral soils with >180 g kg-1 clay were predicted to have SOC concentrations of about 22 g kg-1. Other mineral soils were predicted to have concentrations of about 13 g kg-1 SOC. The amounts forecast for sandy soils were within guidelines proposed for soil stability by Körschens et al. (1998), but for other soil types the modeled SOC concentrations were less than the proposed guidelines. Under grass, there is evidence that a relatively small change in C returns will mean that large areas of managed grassland in E&W may not continue to increase their storage of C.
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ACKNOWLEDGMENTS
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Funding by the UK Ministry of Agriculture, Fisheries and Food is gratefully acknowledged.
Received for publication April 5, 2001.
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ABSTRACT
Effects of Crop Residues...
