Review of the Impact of Crop Residue Management on Soil Organic Carbon in Europe

WORKING PAPER 2017-15

Review of the impact of crop residue management on soil organic carbon in Europe

Authors: Stephanie Searle and Kristine Bitnere Date: December 15, 2017 Keywords: cellulosic biofuel; policy; sustainable agriculture

Introduction removal have focused on corn stover the EU, and one of the requirements for in the United States, however, the situ- receiving this aid is that farmers must Crop residues, including wheat straw ation in the EU may be different. In the comply with the Good Agriculture and and corn stover, are a feedstock for next section, we review existing EU Environment Conditions (GAEC) stan- the production of cellulosic biofuel legislation relevant to crop residues dards. Biofuels used for compliance that could contribute to meeting and discuss the effectiveness of these with the current Renewable Energy advanced biofuel targets and decar- measures in preventing overharvesting Directive (RED) must also meet these bonization goals for the transport of residues for biofuel. The following standards, because the current RED sector. Crop residues are an eligible sections review literature on the role that applies in the year 2020 requires pathway to compliance with the trans- of crop residues in sustainable agricul- cross-compliance with CAP (Council port target and advanced biofuel sub- tural management, analyze available of the EU, 2009). In 2015, the Council target laid out in the recast Renewable data on the impact of crop residue Regulation 73/2009 was replaced Energy Directive 2009/30/EC for the harvesting on soil organic carbon with Council Regulation No 1307/2013, period 2021–2030 (REDII) (European (SOC), and present guidelines for which includes revised environmental Commission, 2016). Biofuel made policy design to allow the use of crop standards (Council of the EU, 2013). from crop residues theoretically residues for advanced biofuel produc- However, the current RED was not could contribute substantial volumes tion while ensuring sustainable man- amended to reflect these changes. toward these goals (Searle & Malins, agement practices. In this study, we 2016b). However, the potential large- focus on sustainable management in CAP and GAEC do not address crop scale use of crop residues for biofuel the EU context, which may differ from residues directly. Instead, GAEC sets raises concerns about environmen- the United States and other regions. minimum standards for the protec- tal impacts. In particular, incentiv- tion of broad environmental param- izing the collection and use of crop eters, including wildlife habitat, water residues that otherwise would have EU legislation on crop quality, and soil quality. The specific been retained in fields can affect residues GAEC standards relating directly to soil carbon and soil quality. Policy soil quality include: measures regulating the use of crop The main legislation relevant to crop residues could mitigate this risk. residues that applies at the EU level is • Minimum soil cover and land the Common Agriculture Policy’s (CAP) management practices to prevent This study reviews the evidence on rules on cross-compliance according soil erosion the environmental impacts of crop to Council Regulation 73/2009, Article residue harvest in the European Union 6 (1) (Council of the EU, 2009). CAP • Maintaining soil organic matter (EU). Many studies on crop residue provides direct payments to farmers in • Maintaining soil structure

Acknowledgements: This work was generously supported by the European Climate Foundation. We thank the Biofrontiers group for constructive dialogue and input, and Chelsea Petrenko and Nic Lutsey for helpful reviews.

Member states may choose how to & Buckwell, 2016). The European Court reducing evaporation from the soil implement these standards by setting of Auditors (2016) reported that the surface, improving soil structure, sup- more specific rules depending on the information available from cross-com- porting living organisms, contributing environmental context and agricul- pliance did not allow the European nutrients to the soil, and providing tural practices of each country. Setting Commission to adequately assess the water filtration and retention capacity standards for crop residue manage- systems’ effectiveness. (Powlson, Glendining, Coleman, ment is one implementation option & Whitmore, 2011; Lal, 2014; SoCo member states can use to meet the While cross-compliance with CAP/ Project Team , 2009; Nicholson et al., above GAEC standards. All member GAEC is one of the required sustain- 2014; Johnston et al., 2009). Whether states prohibit the burning of stubble ability criteria with which economic and how much crop residue can be (the plant stalk below where the stalk operators must comply in the current harvested without significant negative is cut by harvesting equipment) as a RED, economic operators do not have impacts on these ecosystem services measure to ensure soil organic matter to show proof of compliance in order is a critical question in understanding maintenance. While all member states for biofuel produced from their crops the potential for producing sustain- have some requirements on soil cover, to be eligible to contribute to the RED able, low-carbon biofuel from this type in most countries’ restrictions only target. The RED implicitly assumes of feedstock. This section describes apply during the winter or on agricul- that farmers will comply with CAP/ the environmental role of crop residues tural land with significant slope. In most GAEC to receive direct payments, in the field, focusing on what is already cases, member states allow leaving and therefore that checking compli- understood on SOC impacts of residue stubble, ploughing crop residues into ance again in the context of RED is retention versus removal. The follow- the soil, planting winter crops or cover unnecessary. The majority of large ing section presents a new quantitative crops, or applying mulch as eligible scale crop producers receive these assessment of SOC impacts of residue direct payments and thus should methods of establishing soil cover. The retention compared to removal. comply (Matthews, 2016). However, retention of crop residues in the field is the European Court of Auditors (2015) thus an option for GAEC compliance in SOC is important both for carbon noted a 27% noncompliance rate with some member states, but is not strictly storage to mitigate climate change cross-compliance. Member states are required in any. No member state has and for contributing to healthy soils. responsible for enforcing compliance rules on a minimum amount of residue SOC increases the water retention through random checks. that should be left in the field. Further capacity of sandy soils (Rawls et al., 2003) and improves soil structure details on implementation of relevant Cross-compliance with CAP/GAEC is (Smith, 2016), and can thus theoreti- GAEC standards by member state are not included in the commission’s RED cally improve crop yields (Nicholson et provided in Appendix A (European II proposal. There is thus little assur- al., 2014). SOC is formed from biomass Commission, 2017). ance from existing requirements that input to the soil from plant roots harvesting crop residues for the pro- Since 2015, farmers have been able and dead above-ground biomass. It duction of advanced biofuel will not to apply for greening payments that is distinct from soil inorganic carbon have negative impacts on soil carbon require specific measures to maintain (SIC), which occurs in mineral forms, and soil quality. Even if CAP/GAEC permanent grassland (including a such as limestone, and does not have compliance had been included in this ban on ploughing and conversion of the same water retention properties proposal, it is not strictly required for environmentally sensitive permanent nor support biota in the same way as all crop residue suppliers, is not ade- grassland), crop diversification, and SOC. In annual cropping systems, the quately enforced, and is too general to maintaining an “ecological focus area” main biomass inputs to the soil are ensure residue harvesting is sustainable of at least 5% of the arable area of decaying roots and crop residues. SOC (European Commission, 2006). the holding. However, the greening is lost from soils through decomposi- requirement is voluntary for farmers tion by microorganisms and erosion. who apply for direct payments. Environmental role of crop The maintenance or change in SOC residues in the field over time is the net balance between The CAP/GAEC requirements are these inputs and outputs. general, and some assessments indicate Crop residues provide a number of that member states’ implementing pro- environmental services when left in It is generally accepted that the reten- visions fail to provide sufficient envi- the field, including contributing to the tion of crop residues in the field con- ronmental protection (Hart, Baldock, formation of SOC, preventing erosion, tributes to greater SOC formation

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and higher SOC levels compared 3 to complete residue removal (e.g., Colby site Hugton site Ottawa site 2.5 Powlson et al., 2011; Nicholson et al., 2014), but the degree of SOC benefit 2 as well as the amount of residue nec- 1.5 essary to provide SOC benefits have 1 been unclear. In a review of long-term experiments from various regions 0.5 around the world, Powlson et al. (2011) 0 found that most studies reported SOC dierence (tC/ha) greater SOC with residue application -0.5 or retention compared to complete -1 residue removal, but that this differ- 0% 20% 40% 60% 80% 100% 120% ence was statistically significant in a Percent residue retention minority of studies. Figure 1. Difference in SOC between residue retention and removal by percentage of The amount of residue needed to residue retained after 2 years. (Kenney, 2011) prevent additional SOC loss is not well understood or agreed upon. Earlier relative paucity of studies reporting with small particle size, may theoreti- studies in the United States suggested on SOC impacts of varying amounts cally support greater SOC accumu- 30% of corn stover could be sus- of residue application or retention lation than soils with larger particle tainably removed (Andrews, 2006), at the same experimental site, so size. Clay allows greater aggregation while it has generally been believed the relationship between amount or bonding in soils, reducing erosion. in the EU that two-thirds of straw of residue applied and SOC is not In addition, clay can encase organic could be removed (Joint Research clearly understood. particles and prevent or slow decom- Center [JRC], 2009; Kretschmer, position (von Lutzow et al., 2006). Kenney (2011) and other studies report Allen, Kieve, & Smith, 2013). In a life- The starting SOC content of soil may the amount of stover removed as a cycle assessment of biofuel produced also affect further SOC accumulation percentage of total stover production. from corn stover, Liska, Yang, Milner, if soils begin to reach SOC saturation However, yields of stover and other Goddard, and Blanco-Canqui (2014) (Six, Elliott, & Paustian, 1999). crop residues can vary considerably by concluded that any residue removal location, so the percentage removed is would result in SOC loss compared Tillage or ploughing practices also may not entirely informative. For example, influence the impact of residue reten- to complete residue retention in removing 50% of stover from a field fields, although this study did not tion on SOC. Tillage mixes air into the yielding 10 t/ha stover leaves twice soil and reduces soil aggregation, accel- specifically evaluate evidence of SOC as much stover on the ground as erating the decomposition of residues impacts with varying levels of residue removing 50% from a field with a 5 and organic matter (Stubbs, Kennedy, retention. Some studies have explic- t/ha stover yield. The latter depletes & Schillinger, 2004). Tillage could thus itly studied how varying amounts SOC to a much greater extent than reduce the effectiveness of residue of residue affect SOC levels. In an the former. Understanding how crop retention in building SOC. This effect experiment in Denmark, Thomsen residue removal or retention affects is apparent in U.S. data on land used and Christensen (2004) found a soil carbon and quality according to to grow continuous corn. Using data linear relationship between SOC and the absolute amount, rather than the from Searle and Malins (2016a), we find straw input with four varying levels percentage, is thus likely to be more statistically significantly greater SOC of straw input ranging from zero to meaningful. It may be possible to accumulation in no-till plots compared 12 t/ha/yr. Kenney (2011) also found remove a larger fraction of very high- to conventional-till plots in the United increasing SOC with increasing stover yielding residue crops without adverse input compared to complete stover environmental impacts. States, with full stover retention on all removal in an experiment in Illinois, plots (Figure 2; p < 0.05). At least in the United States, but it is not clear from Other agricultural practices may United States, practicing no-till does this study whether this relationship influence the relationship between seem to enhance the positive effect of is linear. The results from Kenney are residues and SOC. Soil texture may residue retention on SOC. In the EU, shown in Figure 1. Overall, there is a matter. Clays, which are mineral soils however, no-till is far less common than

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in the United States (Horowitz, Ebel, & 1.2 Ueda, 2010; Eurostat, 2015). Evidence is thus lacking on the combined effects of 1 no-till or reduced-till and residue retention in that context. There is disagree- 0.8 ment in the literature on the extent to which no-till can improve SOC globally 0.6 (Luo, Wang, & Sun, 2010; Powlson et al., 2015). 0.4

There may be reason to expect that 0.2 lower amounts of residue are necessary to achieve any particular level of 0

SOC benefit in the EU compared to the SOC sequestration (tC/ha/yr) United States, as conditions and man- -0.2 agement practices vary. For example, the EU overall tends to have lower -0.4 erosion rates (e.g., Nearing, Xie, Liu, & Conventional tillage No till Ye, 2017), which is one factor affect- ing soil carbon and quality. There is a Figure 2. Efect of tillage on SOC sequestration on continuous corn plots with residue fair amount of literature in the United retention in the United States. States on SOC impacts of corn stover residue retention increase SOC over (and conversely, does complete retention versus removal, but the time?” because many studies included removal lead to SOC loss com- findings of this body of research may in our analysis did not measure SOC pared to retention)? not be fully applicable to the EU. Less changes over time. Instead, they research and analysis have been done • How does the amount of retained measure and compare SOC levels in on this topic in the EU context specifi- crop residue affect SOC, and is plots that have experienced residue cally. In the next section, we conduct a there a threshold level above retention over several years compared thorough review of relevant experi- which additional residue retention to those with residue removal. ments that have been conducted in EU does not offer greater benefits? Therefore, similarly to Powlson et al. countries and present a meta-analysis (2011), we answer the question, “Does This study addresses these ques- of findings across these studies. residue retention result in greater SOC tions by assessing the difference in levels compared to complete residue SOC between fields on which residue Meta-analysis of SOC removal?” There is an important dis- is retained and those on which it is removed. In addition, we examine impacts with residue tinction between these questions, and we argue that the latter is more impacts on SOC in relation to other management practices relevant to biofuel policy. The concern factors commonly reported in the In this section, we present a meta- with incentivizing crop residue har- underlying studies, including tillage analysis on SOC change with residue vesting through a biofuel policy is practices, soil type, fertilizer treat- application or retention using data that the policy may result in greater ment, and length of experiment. from experimental studies in the removal of residues, and thus greater Although there is high variability among geographic locations included EU. Our goal is to understand how loss of SOC, than would otherwise in this meta-analysis that cannot be SOC changes with residue retention have occurred. and whether and how this relation- fully captured through our consider- ship depends on other management This assessment thus aims to answer ation of these additional parameters, factors through a comprehensive two key questions: we believe that a holistic view of the analysis of the available data. science reported to date is necessary • Does residue retention lead to to either confirm or challenge our It is important to note that our analysis greater SOC levels compared understanding of sustainable residue does not answer the question, “Does to complete residue removal management practices.

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METHODOLOGY the experimental plots. Soil sampling relationships between variables in this depth varied from 10 to 30 cm. Study meta-analysis. We use a threshold of p We include all relevant studies we duration ranged from 7 to 54 years. A < 0.05 to indicate statistically signifi- could find that were performed in selection of these details is listed by cant relationships for all regressions. the EU, identified using an online study in Appendix B. Probability values are a measure of the search engine. Our analysis includes probability that a trend seen between 14 studies performed in Belgium, the For the meta-analysis of SOC changes two variables is actually due to the UK, Denmark, Ireland, Sweden and due to different residue treatment, we variation in the x-variable, rather than France. These studies are listed in considered each paired-plot observa- a spurious correlation resulting from Appendix B. All of the studies we tion as an individual sample point. For the random scatter of data. assessed used a paired-plot design, example, suppose a study measured which directly compares SOC in SOC on four plots with varying agri- identical plots that are given differ- cultural management techniques: (a) RESULTS ent treatments, such as complete zero residue, no-till; (b) zero residue, Residue application rate residue removal versus varying levels plough; (c) 5 t/ha residue, no-till; of residue application. Many of these and (d) 5 t/ha residue, plough. We Overall, our results strongly indicate studies did not measure or report SOC would list two observations for this that residue application results levels at the beginning of the experi- study in our analysis, one comparing in greater SOC levels compared to ment, before the treatments were plots a and c to infer the effect of complete residue removal. In most applied, and instead only report SOC residue application with no-till, and comparisons included in our analysis, levels after a certain number of years the other comparing plots b and d to plots on which residues were applied of applying the residue treatments. infer the effect of residue application had higher soil carbon stocks than Here we assess the difference in SOC with ploughing. The combination of paired plots with no residue applica- between plots that applied residues bulk density and SOC concentration tion. In reality, some natural variation versus plots with complete residue measurements is necessary to cal- occurs in soil carbon stocks from plot removal, and do not assess changes culate total SOC stocks for any plot to plot, contributing to the error in in SOC over time with and without of land, but not all studies reviewed our analysis. The lowest amount of residue application or retention. here reported bulk density. For these residue left in fields in the studies studies, we estimated bulk density included in our analysis is 4 t/ha. It The studies included in this assess- based on SOC concentration follow- thus appears that, in the contexts of ment employed various other agri- ing a formula reported in Guo and these EU studies, leaving 4 t/ha or cultural management practices, in Gifford (2002). To compare studies more residue in fields increases soil some cases to measure the effects of that measured SOC with different carbon levels compared to complete practices such as ploughing or fertil- sampling depths, we estimated SOC removal. The available data for the izer application. In most of the cases, at 30 cm for all studies following the EU do not allow us to investigate the soil was ploughed. In a few cases, the depth profile given in Jobbágy and effects of applying smaller amounts soil was ploughed using a tine plough, Jackson (2000). We acknowledge of residue. which lightly turns the soil but does that using these data standardization not thoroughly mix it, which can be techniques introduces error into our While it is clear from our data that considered a type of “conservation observations; however, restricting our applying residue results in greater SOC tillage,” or no tillage was performed. analysis to studies that report bulk compared to complete residue removal, The experiments applied various fer- density and SOC at 30 cm sampling one key question is whether applying tilizer types (e.g., inorganic fertilizer depth would result in too few studies higher amounts of residue increases the versus manure) and fertilization rates. to make meaningful comparisons. rate of SOC accumulation. To compare Residues were sourced from wheat, We argue that the error introduced rates of SOC accumulation across spring barley, maize, and unspecified by these data standardization tech- studies, we divide the difference in SOC mixed cereals. The reported residue niques is a reasonable trade-off for between plots with residue application yields varied from 3 to 10 t/ha. The the ability to learn from a larger and paired plots with removal by the reported residue retention rate varied number of studies. number of years of the experiment. This from 4 to 20 t/ha. In some cases, metric is similar to reporting the rate additional residues from other fields We conduct simple and multiple of soil carbon accumulation over time were added to those produced on linear regression analyses to infer with residue application (t/ha/year),

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but again, we note that we are only able 1.5 to assess differences between paired plots, not SOC changes over time. Overall, the difference in SOC stocks 1 per year between plots with residue application compared to plots without 0.5 residue application is positively corre- lated with the amount of residue applied (p < 0.05). This relationship is shown in 0 Figure 3.

This result is heavily influenced by

SOC dierence (tC/ha/yr) -0.5 observations with residue application rates above 10 t/ha. Furthermore, this 2 relationship has a low R -value (0.12), -1 indicating that the regression has very 0510 15 20 25 low predictive power; in other words, Residue application (t/ha/yr) this regression is unlikely to accurately predict actual net SOC accumulation Figure 3. Annualized SOC difference between residue application and removal by residue that would be achieved with any given application rate. level of residue application. Figure 0.8 4 omits observations with residue application rates above 10 t/ha, and 0.6 with this restriction, no relationship is observed between residue application 0.4 rate and the difference in SOC stocks per year between plots with residue 0.2 application versus removal. A strict 0 interpretation of these data is that: -0.2 1. Residue application increases SOC compared to complete -0.4

residue removal. SOC dierence (tC/ha/yr) -0.6 2. Within the range of 4–10 t/ha residue application, the applica- -0.8 024681012 tion rate does not matter (e.g., applying 10 t/ha residue will have Residue application (t/ha/yr) the same result on SOC change as Figure 4. Annualized SOC difference between residue application and removal by residue applying 4 t/ha). application rate for rates ranging from 0–10 t/ha. 3. Applying more than 10 t/ha We thus consider our results to be would thus naturally expect a greater residue does result in greater inconclusive on the question of whether difference in SOC between plots with SOC accumulation compared to adding residue amounts greater than 4 residue application compared to applying less than 10 t/ha. t/ha results in higher SOC accumula- those with removal the longer these However, there is no logical reason why tion. This is unsurprising given the high experimental treatments occur. This a threshold rate of residue application, variability amongst our data in experi- idea is supported by our data. Figure 5 over which additional input results in ment location, time period, crop, and shows that overall, greater SOC gains were found with residue application greater SOC gain, should exist. The lack other management practices. of a relationship between residue appli- compared to residue removal plots cation rate and SOC difference per year the longer an experiment had been Duration of residue treatments for application rates 10 t/ha and lower ongoing. We perform a multiple regres- calls into question the validity of the Any effect of residue application on sion analysis for total SOC difference correlation found in Figure 3. SOC should compound over time. One between plots (residue application vs.

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removal, in t/ha) versus the number of 20 years of the experiment, with residue application rate as a covariate; this regression is significant with p < 0.05 15 for both variables and R2 = 0.37, sug- gesting time has a significant effect on SOC accumulation. 10

There may also be reason to believe that such changes are rapid in the early 5 years of an experiment and that the annual change decreases over time. Previous meta-analyses have found 0 such a temporal pattern for SOC changes with land use change. For and no residue application (t/ha) example, when a forest is converted -5 to cropland, SOC is initially lost rapidly, Total dierence in SOC between residue and after a number of years the SOC stocks stabilize at a lower level than -10 the initial stocks (Murty, Kirschbaum, 0102030405060 Mcmurtie, & Mcgilvary, 2002). With Years of experiment residue application, one might expect to observe a slowing of SOC accumula- Figure 5. Difference in SOC between residue application and removal by number of years tion with time if soils begin to become of experimental treatment. saturated with carbon (Six et al., 1999; 2 discussed further below). However, it is not clear from our data that SOC changes over time with residue treat- 1.5 ments are necessarily nonlinear or stabilize at a certain point. Performing 1 the multiple regression analysis on log-transformed data, a technique that 0.5 should better fit a trend of slowing SOC accumulation with time, actually resulted in a poorer fit. In some cases 0 in practice, farmers may begin retaining residues on fields on which they -0.5 had previously harvested all collect- able residues, and for these cases it is -1 important to understand how long SOC recovery may take; however, we are Dierence in C stock per year (tC/ha/yr) unable to answer this question with the -1.5 available data in the EU context. 0% 10% 20% 30% 40% Percent clay in soil Soil type and texture Figure 6. Difference in SOC between residue application and removal by clay content in soil. There is also reason to expect that soil texture can affect how quickly SOC increased soil aggregation (Bronick Within our dataset, however, there is accumulates with residue applica- & Lal, 2005; von Lutzow et al., 2006). no apparent relationship between clay tion. SOC has been found to accumu- Conversely, sandy soils are prone to content and the annualized difference late faster in clay versus silt fractions (Houot, Molina, Clapp, & Chassod, leaching, which may reduce carbon in SOC between residue application 1989), and clay may slow SOC loss from retention capacity (SoCo Project and removal (Figure 6). Similarly, we erosion and decomposition through Team, 2009; Powlson et al., 2011). found no difference in the annualized

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difference in SOC between residue 1.5 application and removal in sandy soils (with >33% sand content or classified as “sandy”) compared to not sandy 1 soils. However, few studies included in our analysis were performed on soils with clay content greater than 20%, so 0.5 we are unable to draw conclusions on the effect of clay content on SOC accumulation in high-clay soils. It is possible 0 that a greater effect on SOC accumulation would be seen with additional data using soils with higher clay content. SOC dierence (tC/ha/yr) -0.5 Some authors have indicated that SOC accumulation depends on the initial SOC; if it is very high, then residue -1 retention might have a smaller effect 30 40 50 60 70 80 due to SOC saturation (Six et al., 1999). Ending SOC stock on residue removal plots (tC/ha) We might thus expect to see lesser Figure 7. Annualized difference in SOC between residue application and removal by SOC SOC accumulation on plots with high stock at removal sites. initial SOC, and greater SOC accumulation on plots with lower initial SOC. We do not have the data that would 0.7 Conventional tillage Conservational tillage No-till be necessary to answer this question, 0.6 as the initial SOC stocks were not 0.5 reported in many studies. Instead, we can use the ending SOC stock reported 0.4 in the studies as an indicator of initial 0.3 SOC stocks. It is likely that SOC stocks 0.2 would have gradually decreased on 0.1 at least some of these plots over the course of the experiments, but this 0 measure still likely reflects large dif- -0.1 Di erence in SOC (tC/ha/yr) ferences in initial SOC stocks across -0.2 different geographic locations. We see Site 1 Site 2 no relationship between SOC differ- Johnston et al., 2009 van Groenigen Hazarika et al., et al 2011 2009 ence (i.e., the difference between plots with residue retention and those with Figure 8. Difference in SOC between residue application and removal by tillage practices. removal, annualized over the course of the experiment) and ending SOC stock over the duration of the experiments 3%. This suggests that SOC saturation on plots with residue removal (Figure 7; included in our analysis. Again, natural is unlikely to occur in most arable soils p > 0.05). The result is the same when variability amongst experiments could in the EU, and thus that residue appli- we add the residue application rate as mask such an effect. Furthermore, it is cation should generally have a positive a covariate to the regression (p > 0.05). possible that a reduced SOC accumula- effect on SOC regardless of soil type. Our interpretation is that, within our tion effect could be seen with residue dataset, residue application has the application on soils with higher SOC Effect of tillage same effect on improving SOC levels levels than those included in our dataset. in soils that already had relatively high Six et al. (1999) suggest that SOC satu- In our review, only three studies initial SOC as is does in soils with low ration occurs at concentrations greater specifically tested the combined initial SOC. This result is consistent with than 5% in soil, and SOC concentration effects of tillage and residue appli- our previously noted finding that SOC within our dataset among studies that cation at four sites; these findings gains do not appear to have slowed reported this parameter did not exceed are presented in Figure 8. In each of

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these studies, conventional tillage 1.4 was compared with conservational Mineral fertilizer No fertilizer or reduced tillage (e.g., turning soil 1.2 with tines rather than, e.g., a mold- 1 board plough), and the same amount 0.8 of residue was applied across tillage 0.6 treatments for each site. Only one 0.4 study also included a no-till plot. In 0.2 contrast to the U.S. data shown in 0 Figure 2, with our EU dataset there is -0.2

no clear interaction between tillage Di erence in SOC (tC/ha/yr) -0.4 and residue application on SOC. As Site 1 Site 2 with other parameters, the lack of Katterer et Houot et al., Perrson and Nicholson et al., 1997 al., 2011 1991 Kirchmann, a clear result on tillage is likely due 1994 to the paucity of data, in particular a lack of results on no-till plots with Figure 9. Difference in SOC between residue application and removal with fertilizer residue application. versus no fertilizer application.

Fertilizer treatment 0.6

The decomposition of residues by 0.4 microbes, and in particular the release of nitrogen contained in biomass 0.2 residue, can be slowed if insufficient nitrogen is available in the soil. The 0 addition of mineral nitrogen may thus -0.2 be necessary to increase the speed of microbial activity in order to achieve the -0.4 full fertilization benefits of the residue itself (Houot et al., 1989; Nicholson Dierence in SOC (tC/ha/yr) -0.6 et al., 2014; Schjønning, Heckrath, & Site 1a Site 1b Site 2 Christensen, 2009). However, it is not -0.8 clear whether the effect of nitrogen 050 100 150 200 250 availability on residue decomposition Nitrogen fertilizer (kg/ha/yr) affects SOC levels in the long term. Figure 10. Difference in SOC between residue application and removal by level of Our data show that the annualized dif- nitrogen fertilizer application in Nicholson et al. (1997). ference in SOC with residue application versus removal is not consistently may aid in soil nitrogen levels and slope, as erosion rates could be very affected by the addition of mineral could thus affect crop yields in future high on slopes with complete residue fertilizer in studies that compared years, it does not appear that fertil- removal. Slope was generally not fertilizer and no fertilizer treatments izer addition affects SOC directly with reported among the studies included (Figure 9). residue application. in our analysis, so we were not able Nicholson, Chambers, Mills, and to take this into account. We believe Slope Strachan (1997) measured SOC on it probable that none of the studies plots with varying levels of nitrogen Lastly, we note that residue effects included here reported results from fertilizer addition and with or without on SOC could be influenced by slope. land with high slope. straw. In this case we see no consis- Erosion generally increases with slope. tent interaction between fertilizer The extent to which residue reten- SUMMARY OF RESULTS and residue application on SOC levels tion reduces erosion compared to (Figure 10). While there is reason to complete residue removal may thus As with any meta-analysis, it is dif- believe that mineral fertilizer addition be greater on fields with significant ficult to draw precise conclusions

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given the variation in the data and The necessary amount of crop residue to accurately measure SOC changes studies used. However, our data left in each field to ensure soil health (Garcia-Oliva and Masera, 2004). The clearly support two ideas: benefits depends on that field’s char- second reason is that SOC benefits can acteristics: slope, climate, erosion risk, be realized without maintenance of • In the EU, retaining residue and other agricultural management existing SOC levels as long as residue amounts of at least 4 t/ha/yr gen- practices. Ideally, sustainable residue harvest for biofuel does not result in erally results in greater SOC accu- retention rates will be determined on greater SOC losses than would have mulation or reduced SOC loss a highly local basis. Our assessment occurred in the absence of biofuel compared to complete residue does, however, support the idea that, demand. The role of biofuel policy removal from crop fields. in many cases, any level of residue in crop residue management should • This SOC benefit increases over retention 4 t/ha/yr or higher will likely simply be to ensure that incentivizing the years that residue is applied. result in SOC benefits compared to biofuel does not lead to adverse envi- complete residue removal. However, ronmental impacts. In this framework, Whether other factors, including soil this observation is a generalization regulation should focus on encourag- type, starting SOC levels, tillage, and of SOC impacts seen in EU studies, ing or requiring certain management fertilizer affect the SOC benefits of and does not mean that a minimum practices known to protect SOC and residue retention is not clear from our of 4 t/ha/yr of crop residue should soil health, rather than tying biofuel analysis. Our analysis suggests that, be applied in all cases. These findings production to the achievement or regardless of how these other man- should also not be extended to the maintenance of certain SOC levels. agement practices are performed, United States or other regions. retaining some residue in fields will To summarize, the findings in this There is the potential for other sus- likely have a significant SOC benefit. study support the following recom- tainable management practices to mendations for policies supporting support the role of crop residues in sustainable cellulosic biofuels: Policy recommendations ensuring soil health and soil carbon for the use of crop residues sequestration. While our analysis did • Focus on encouraging or requir- in advanced biofuel not show a clear effect of conserva- ing sustainable management production tion tillage practices on soil carbon practices. with residue retention, it is clear in Regulating the harvest of crop residues the U.S. context that switching from • Ensure leaving a minimum amount will likely be complex under any cir- conventional tillage to no-till, along of crop residue in the field each cumstance, as there exists high natural with leaving residue in the fields, can year, according to sustainable har- variability in agricultural systems. It is promote SOC sequestration. Planting vesting practices. impossible to predict the net effect cover crops reduces erosion and may • Determine sustainable harvesting that residue management decisions help protect soil health when some amounts based on local conditions. will have on soil health and carbon residue is harvested (United States • Encourage complementary sus- storage on any farm. However, there Department of Agriculture, 2015). is mounting evidence that certain Avoiding residue harvest on high risk tainable management practices agricultural practices can promote areas, such as slopes or riparian areas, such as no-till and cover crops. better soil health and sustainable crop also can help ensure soil health. • Avoid residue harvesting from yields. Our assessment of the available high risk areas. evidence demonstrates that residue The maintenance of existing SOC retention leads to greater SOC levels levels in agriculture is often cited as There is not necessarily a trade-off on EU farms compared to complete a concern or goal (e.g., Kemp, 2015). between cellulosic biofuel produc- residue removal. A key practice to However, we argue against the idea of tion and soil health; these goals can ensure sustainable agriculture and using SOC levels as a metric to deter- be complementary to one another. sustainable cellulosic biofuel produc- mine sustainable residue harvesting Following the guidelines outlined here tion should thus be the annual reten- for biofuel production for two reasons. can simultaneously support the devel- tion of a certain level of straw or other The first is that SOC is highly variable opment of a low carbon bioeconomy crop residues in fields. and long time periods are required and sustainable agriculture.

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Liska, A. J., Yang, H., Milner, M., Goddard, S., & Blanco-Canqui, Searle, S., & Malins, C. (2016a). A critique of soil carbon H. (2014). Biofuels from crop residue can reduce soil carbon assumptions used in ILUC modeling (Working paper 2016-

and increase CO2 emissions. Adam Liska Papers, 16. Retrieved 13). Retrieved from the International Council on Clean from http://digitalcommons.unl.edu/bseliska/16/?utm_ Transportation, http://www.theicct.org/sites/default/files/pub- source=digitalcommons.unl.edu%2Fbseliska%2F16&utm_ lications/ICCT_soil-carbon-assumptions-ILUC_20160613.pdf medium=PDF&utm_campaign=PDFCoverPages Searle, S., & Malins, C. (2016b). Waste and residue availability for Luo, Z., Wang, E., & Sun, O. J. (2010). Can no-tillage stimulate advanced biofuel production in EU member states. Biomass carbon sequestration in agricultural soil? A meta-analysis of and Bioenergy, 89, 2-10. Retrieved from https://www.sciencedi- paired experiments. Agriculture, Ecosystems and Environment, rect.com/science/article/pii/S0961953416300083 139, 224-231. doi:10.1016/j.agee.2010.08.006 Six, J., Elliott, E. T., & Paustian, K. (1999). Aggregate and soil Matthews, A. (2016). Research for AGRI committee – CAP organic matter dynamics under conventional and no-tillage reform post-2020 – Challenges in agriculture. Retrieved systems. Soil Science Society of America Journal, 63(5), 1350- from http://www.europarl.europa.eu/RegData/etudes/ 1358. doi:10.2136/sssaj1999.6351350x STUD/2016/585898/IPOL_STU(2016)585898_EN.pdf Smith, C. W., (2016). Effects of implementation of soil health Murty, D., Kirschbaum, M. U. F., Mcmurtrie, R. E., & Mcgilvary, management practices on infiltration, saturated hydraulic H., (2002). Does conversion of forest to agricul- conductivity (Ksat), and runoff. United States Department of tural land change soil carbon and nitrogen? A review Agriculture. Natural Resource Conservation Centre. Retrieved of the literature. Global Change Biology, 8, 105-123. from https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/ doi:10.1046/j.1354-1013.2001.00459.x download?cid...ext=pdf Nearing, M. A., Xie, Y., Liu, B., & Ye, Y. (2017). Natural and SoCo Project Team. (2009). Addressing soil degradation in EU anthropogenic rates of soil erosion. International Soil and agriculture: Relevant processes, practices and policies (Report Water Conservation Research, 5(2), 77-84. doi:10.1016/j. on the project “Sustainable Agriculture and Soil Conservation iswcr.2017.04.001 [SoCo]”). Retrieved from https://esdac.jrc.ec.europa.eu/ESDB_ Archive/eusoils_docs/other/EUR23767_Final.pdf Nicholson, F., Kindred, D., Bhogal, A., Roques, S., Kerley, J., Twining, S.,… Ellis, S. (2014). Straw incorporation review Stubbs, T. L., Kennedy, A. C., & Schillinger, W. (2004). Soil ecosys- (Research Review 81). Home-Grown Cereals Authority. tem changes during the transition to no-till cropping. Journal Retrieved from https://cereals.ahdb.org.uk/publications/2014/ of Crop Improvement, 11(1), 105–135. doi: 10.1300/J411v11n01_06 july/30/straw-incorporation-review.aspx Thomsen, I. K., & Christensen, B. T. (2004). Yields of wheat and Nicholson, F. A., Chambers, B. J., Mills, A. R., & Strachan, P. J. soil carbon and nitrogen contents following long-term incor- (1997). Effects of repeated straw incorporation on crop poration of barley straw and ryegrass catch crops. Soil Use fertilizer nitrogen requirements, soil mineral nitrogen and and Management, 20(4), 432-438. doi:10.1111/j.1475-2743.2004. nitrate leaching losses. Soil use and management, 13, 136-142. tb00393.x doi:10.1111/j.1475-2743.1997.tb00574.x United States Department of Agriculture, Natural Resources Powlson, D. S., Glendining, M. J., Coleman, K., & Whitmore, A. P. Conservation Service, National Soil Survey Center. (2015). (2011). Implications for soil properties of removing cereal straw: Soil health literature summary; Effects of conservation prac- Results from long-term studies. Agronomy Journal, 103, 1, tices on soil properties in areas of cropland. Retrieved from 279-287. doi:10.2134/agronj2010.0146s https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/ download?cid...ext=pdf Powlson, D. S., Stirling, C. M., Jat, M. L., Gerard, B. G., Palm, C. A., Sanchez, P. A., & Cassman, K. G. (2015). Limited potential of von Lutzow, M., Kogel-Knabner, I., Ekschmitt, K., Matzner, no-till agriculture for climate change mitigation. Nature climate E., Guggenberger, G., Marschner, B., & Flessa, H. (2006). change, 4, 678–683. doi:10.1038/NCLIMATE2292 Stabilization of organic matter in temperate soils: Mechanisms and their relevance under different soil conditions – A Rawls, W. J., Pachepsky, Y. A., Ritchie, J. C., Sobecki, T. M., & review. European Journal of Soil Science, 57, 426-445. Bloodworth, H. (2003). Effect of soil organic carbon on soil water retention. Geoderma 116 (2003), 61-76. doi:10.1016/ doi:10.1111/j.1365-2389.2006.00809.x S0016-7061(03)00094-6 Schjønning, P., Heckrath, G., & Christensen, B. T. (2009). Threats to soil quality in Denmark—A review of existing knowledge in the context of the EU soil thematic strategy (DJF Report Plant Science No. 143). Retrieved from http://pure.au.dk/portal/ files/2933167/djfma143.pdf.pdf

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Appendix A GAEC 4-6 implementation by the member states in 2016 (European Commission, personal communication)

GAEC 5 site specific GAEC 6 Maintenance of Country GAEC 4 minimum soil cover conditions to limit soil erosion soil organic matter All arable land must have green cover No working land when it is frozen, Austria throughout the off season between waterlogged, flooded or covered in Burning of stubble forbidden harvest and replanting snow Region segregated in parcels Winter crops, conservational tillage, or Burning of stubble forbidden; reporting depending on erosion rate. Different Belgium other measures required depending on of soil acidity and carbon content measures apply depending on erosion erosion rate required rate, e.g., conservational tillage 30% of specially classified areas should Tillage must be perpendicular to slopes Bulgaria Burning of stubble forbidden have unbroken/unseparated surfaces

Crops or crop residue cover required Tillage must be perpendicular to slopes Croatia Burning of stubble forbidden during growing season Burning of stubble forbidden; residues Green cover required on slopes in Cyprus Tillage must be perpendicular to slopes should be used for grazing, soil cover, winter or incorporated into soil Burning of stubble forbidden; manure, Czech Crop residue, manure, or catch crops Conservational agricultural technologies residue, or nitrogen fixing crops Republic required on slopes required on land with high erosion risk required on at least 20% of land At least 50% of arable land must be Ploughing prohibited on slopes in Denmark Burning of stubble forbidden covered by plants winter Cover crops, cultivation of grasses, Crop residues or green cover required Burning of stubble forbidden; reduced tillage, and/or tillage Estonia for at least 30% of agricultural land in successive cropping or crop rotation perpendicular to slope required on winter plans required slopes Crops or green cover required on arable Vegetation or stubble required on land during growing season; tillage Finland managed uncultivated arable land and Burning of stubble forbidden prohibited on 1m buffer strips along in groundwater areas waterways Crops, cover crops, or stubble required Burning of stubble forbidden for most France Ploughing prohibited on flooded land on fallow land crops Unmanaged green cover required Ground cover required on areas with Germany on fallow land and agricultural land Burning of stubble forbidden high erosion risk designated as Ecological Focus Areas Crop residues must be grazed or Vegetation or stubble required on incorporated into the soil; permits Greece Tillage must be perpendicular to slopes slopes during rainy season required for stubble burning

Stubble, cover crops or winter crops Certain crops prohibited on slopes Requirement for crop rotation for some Hungary required during winter crops Plants or crop residue required in Activities that increase erosion risk Ireland Burning of stubble forbidden autumn prohibited Minimum soil cover or minimum tillage Burning of stubble forbidden; fertilizer with residue retention required on Furrows required on slopes to collect Italy application required when incorporating unused arable land and high erosion runoff stubble into soil areas Vegetation or stubble required on Maintenance of drainage systems Burning of stubble or dry grass Latvia slopes during winter required prohibited Arable land must be planted with Agricultural crops prohibited in areas Burning of stubble or dry grass Lithuania agricultural plants with high erosion risk prohibited

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GAEC 5 site specific GAEC 6 Maintenance of Country GAEC 4 minimum soil cover conditions to limit soil erosion soil organic matter Burning of stubble prohibited; Luxembourg Ploughing prohibited on slopes Requirement to prevent ravine erosion requirements for crop diversity Tillage and planting must be Vegetation, stubble or mulch required Malta perpendicular to slopes Burning of stubble prohibited on unterraced land

Green manure crops required during Netherlands Tillage required to remove tire tracks Burning of stubble prohibited winter Vegetation, crop residues, or mulch required on high erosion areas; soil Crops requiring furrows prohibited and Poland Burning of stubble prohibited cover required and furrows banned on soil cover required on slopes high slopes Restrictions on cropping in high risk Portugal Vegetation required on high risk land areas Winter crops required or ploughing Tillage and planting must be Burning of stubble or pastures Romania prohibited on at least 20% of arable perpendicular to slopes prohibited land during winter Winter crops or stubble required on at Burning of stubble prohibited; crop Slovakia least 40% of sloped arable land during Requirement to prevent gully erosion rotations must be maintained winter Ploughing perpendicular to slopes, Burning of stubble prohibited; crop Slovenia Soil cover required stubble retention, or green cover rotations must be maintained required on steep slopes Inter-row soil cover required for permanent crops on slopes; tillage Spain Tillage must be perpendicular to slopes Burning of stubble prohibited restrictions on non-irrigated land sown with winter crops Green cover required on at least 50% Vegetation required on sloped land Sweden of arable land during autumn and/or Restrictions on burning of stubble near water courses during winter winter in the southern part of Sweden Farmers must take reasonable steps Measures to limit soil and bankside UK to establish soil cover, including crops, Burning of stubble prohibited erosion required cover crops, residues, etc.

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Appendix B Studies included in the meta-analysis on residue management and soil carbon

Period Study Place (Years) Crop type Hazarika et al., 2009a Devon, UK 23 Winter wheat Houot et al., 1991b Grignon, Yvelines, France 28 Fallow Johnston et al., 2009c Woburn, UK 17 Wheat and oilseed rape Katterer et al., 2011d Ultuna, Sweden 53 Cereal rotation (barley, oats, wheat and maize) Gleadthorpe, UK 9 Arable rotation Nicholson et al., 1997e Morley, UK 7 Arable rotation Perrson and Kirchmann, 1994f Uppsala, Sweden 35 Cereals Powlson et al., 2011g Rothamsted and Woburn, UK 22 Winter wheat and oilseed rape Schjønning et al., 2004h Ronhave, Denmark 36 Spring barley Schjønning, 1986i Ronhave, Askov, Jyndevad, Hojer, Denmark 10 Spring barley Smith et al., 1997j Rothamsted, UK 7 Continuous wheat, barley Thomsen & Christensen, 2004k Askov, Denmark 18 Continuous spring barley Thomsen., 1993i Denmark, Askov 10 Continuous spring barley Trigalet et al., 2014m Gembloux, Belgium 53 Cereal rotation (barley, oats, wheat and maize) van Groenigen et al., 2011n Carlow, Ireland 8 Winter wheat

a Hazarika, S., Parkinson, R., Bol, R., Dixon, L., Russel, P. J., & Donovan, S. (2009). Effect of tillage system and straw management on organic matter dynamics. Agronomy for Sustainable Development, 29, 525-533. doi:10.1051/agro/2009024 b Houot, S., Molina, J. A. E., Clapp, C. E., & Chassod, R. (1989). Simulation by NCSOIL of net mineralization in soils from the Deherain and 36 Parcelles fields at Grignon. Soil Science Society of America Journal, 53, 451-455. doi:10.2136/sssaj1989.03615995005300020023x c Johnston, E., Poulton, P. R., & Coleman, K. (2009). Soil organic matter: Its importance in sustainable agriculture and carbon dioxide fluxes. In Advances in Agronomy, Vol. 101. doi:10.1016/S0065-2113(08)00801-8 d Katterer, T., Bolinder, M. A., Andren, O., Kirchmann, H., & Menichetti, L. (2011). Roots contribute more to refractory soil organic matter than above-ground crop residues, as revealed by a long-term field experiment. Agriculture, Ecosystems and Environment, 141 (1-2), 184-192. doi:10.1016/j.agee.2011.02.029 e Nicholson, F. A., Chambers, B. J., Mills, A. R., & Strachan, P. J. (1997). Effects of repeated straw incorporation on crop fertilizer nitrogen requirements, soil mineral nitrogen and nitrate leaching losses. Soil use and management, 13, 136-142. doi:10.1111/j.1475-2743.1997.tb00574.x f Persson, J., & Kirchmann, H. (1994). Carbon and nitrogen in arable soils as affected by supply of N fertilizers and organic manures. Agriculture, Ecosystems and Environment, 51 (1-2), 249-255. https://doi.org/10.1016/0167-8809(94)90048-5 g Powlson, D. S., Glendining, M. J., Coleman, K., & Whitmore, A. P. (2011). Implications for soil properties of removing cereal straw: Results from long-term studies. Agronomy Journal, 103, 1, 279-287. doi:10.2134/agronj2010.0146s h Schjønning, P. (2004). Grøn Viden: Langtidseffekter af halmnedmuldning. Markbrug nr 295. 8pp. Retrieved from https://pure.au.dk/ws/files/455853/ gvma295.pdf i Schjønning, P. (1986). Nedmuldning af halm ved ensidig dyrkning af vårbyg. II. Indflydelse af halm og stubbearbejdning på jordens indhold af kulstof, kvælstof, kalium og fosfor. Tidsskrift for Planteavl, 90, 141-149. Retrieved from http://docplayer.dk/33706973-Nedmuldning-af-halm-ved-ensidig- dyrkning-af-vaarbyg.html j Smith, P., Powlson, D. S., Glendining, M., Smith, J. (1997). Potential for carbon sequestration in European soils: Preliminary estimates for five scenarios using results from long-term experiments. Global Change Biology, 3, 67-79. doi:10.1046/j.1365-2486.1997.00055.x k Thomsen, I. K., & Christensen, B. T. (2004). Yields of wheat and soil carbon and nitrogen contents following long-term incorporation of barley straw and ryegrass catch crops. Soil Use and Management, 20(4), 432-438. doi:10.1111/j.1475-2743.2004.tb00393.x

15 l Thomsen, I. K. (1993). Turnover of N-straw and NH4NO3 in a sandy loam soil: Effects of straw disposal and N fertilization. Soil Biology and Biochemistry, 25(11), 1561-1566. doi:10.1016/0038-0717(93)90011-Y m Trigalet, S., Van Oost, K., Roisin, C., & van Wesemael, B. (2014). Carbon associated with clay and fine silt as an indicator for SOC decadal evolution under different residue management practices. Agriculture, Ecosystems and Environment, 196, 1-9. Retrieved from doi:10.1016/j.agee.2014.06.011 n van Groenigen, K. J., Hastings, A., Forristal, D., Roth, B., Jones, M., Smith, P. (2011). Soil C storage as affected by tillage and straw management: An assessment using field measurements and model predictions. Agriculture, Ecosystems and Environment, 140, 218-225. doi:10.1016/j.agee.2010.12.008

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