Soil Carbon Science for Policy and Practice Soil-Based Initiatives to Mitigate Climate Change and Restore Soil Fertility Both Rely on Rebuilding Soil Organic Carbon

comment Soil carbon science for policy and practice Soil-based initiatives to mitigate climate change and restore soil fertility both rely on rebuilding soil organic carbon. Controversy about the role soils might play in climate change mitigation is, consequently, undermining actions to restore soils for improved agricultural and environmental outcomes. Mark A. Bradford, Chelsea J. Carey, Lesley Atwood, Deborah Bossio, Eli P. Fenichel, Sasha Gennet, Joseph Fargione, Jonathan R. B. Fisher, Emma Fuller, Daniel A. Kane, Johannes Lehmann, Emily E. Oldfeld, Elsa M. Ordway, Joseph Rudek, Jonathan Sanderman and Stephen A. Wood e argue there is scientific forestry, soil carbon losses via erosion and carbon, vary markedly within a field. Even consensus on the need to decomposition have generally exceeded within seemingly homogenous fields, a Wrebuild soil organic carbon formation rates of soil carbon from plant high spatial density of soil observations is (hereafter, ‘soil carbon’) for sustainable land inputs. Losses associated with these land therefore required to detect the incremental stewardship. Soil carbon concentrations and uses are substantive globally, with a mean ‘signal’ of management effects on soil carbon stocks have been reduced in agricultural estimate to 2-m depth of 133 Pg carbon8, from the local ‘noise’11. Given the time soils following long-term use of practices equivalent to ~63 ppm atmospheric CO2. and expense of acquiring a high density of such as intensive tillage and overgrazing. Losses vary spatially by type and duration observations, most current soil sampling is Adoption of practices such as cover crops of land use, as well as biophysical conditions too limited to reliably quantify management and silvopasture can protect and rebuild such as soil texture, mineralogy, plant effects at field scales9,10. soil carbon. Given the positive effects of species and climate8. Adopting regenerative Even with the measurement and soil carbon on erosion resistance, aeration, approaches such as conservation agriculture verification challenges, most soil scientists water availability and nutrient provision and agroforestry can protect soil carbon agree with the basis for soil health of soils1, benefits of soil restoration can and recoup some losses, by minimizing soil initiatives. That is, that rebuilding soil include improved fertility, reduced fertilizer disturbance and maximizing root inputs3. carbon will translate to outcomes such as and irrigation use, and greater resilience to New soil forms at decadal-to-centurial reduced erosion and yield stability2. Well- stressors such as drought2. Rebuilding soil timescales, making soils effectively non- demonstrated relationships between soil carbon is thus the foundation for many soil renewable; yet fertility can be restored carbon and desired soil properties (for health initiatives1–5. by rebuilding the organic carbon example, macroaggregation) support these At the same time, there is disagreement concentrations in the remaining topsoil2. expectations. Further, emerging global about the advisability and plausibility of The rate and total amount of carbon that datasets support the notion that increasing rebuilding soil carbon as part of climate can be rebuilt is dependent on biophysical soil carbon in croplands will increase mitigation initiatives1,3–7. The urgency conditions, meaning that the effects of yields12. It is unresolved as to whether these to address climate change elevates these management on soil carbon will differ from spatial relationships adequately represent disagreements to the public sphere, where place to place and are hard to predict outcomes of rebuilding soil carbon over they are portrayed as strongly adversarial, with high certainty for any one locale3,9. time. Additionally, without proper nitrogen and indeed opinions on soils as a mitigation However, the biophysical controls are fertilizer management, greater soil carbon strategy appear diametrically opposed understood well enough to set realistic can increase emissions of greenhouse gases within the academic literature1,4,5,7. We bounds for soil carbon maxima and such as nitrous oxide from agricultural suggest that the debate about the role of accumulation rates, and to guide appropriate soils13. Equally, the effects of soil health agricultural soils in climate mitigation is actions to achieve them. The bounds for practices such as no-till are mixed: while eroding scientific credibility in the related accumulation rates do, however, remain losses of sediment-bound phosphorus to but distinct effort to protect and restore poorly constrained: the lower bound is waters may be reduced, dissolved reactive these soils by rebuilding carbon (Fig. 1). generally agreed to be above zero (that is, phosphorus losses can increase14. Thus, We synthesize the science supporting there is potential to accrue carbon) and soil although there is agreement about needing actions to rebuild soil carbon for improved scientists generally agree when the upper to rebuild soil carbon, quantification of the fertility, highlight areas of uncertainty, and bound is unrealistically high. benefits and potential undesired outcomes suggest how to move forward to promote It is hard to narrow the bounds because is required to specify soil carbon targets that confidence in the scientific credibility of soil detection of change in soil carbon at reap the greatest net benefit. health initiatives. management-relevant time (for example, <5 years) and within-field spatial scales is Uncertainty in soil science Agreement in soil science logistically challenging9,10. This is because The measurement challenges for quantifying There are agreed foundations in soil science approximately half of the organic carbon in change in soil carbon go hand-in-hand that support intentions to protect and soil is relatively unaffected by management, with a paucity of large-scale verifiable rebuild soil carbon (Fig. 1). All soils — from meaning that total stocks change slowly2. observations of management effects. the most marginal to fertile — are vulnerable Further, there are pronounced local-scale Together these challenges make it difficult to soil carbon losses and fertility decline2. differences in the amount of carbon stored to adjudicate whether reasonable lower In agricultural landscapes, including because biophysical conditions such as soil or upper limits for soil carbon change are cropland, grazing land and plantation moisture, that affect the amount of soil more likely1,4–7. Such uncertainties are 1070 NATURE SUSTAINABILITY | VOL 2 | DECEMBER 2019 | 1070–1072 | www.nature.com/natsustain comment 1. Strong foundation of technical research and knowledge do not apply to soil health initiatives where Supports intentions to protect and rebuild SOC within reasonable bounds the primary goal is to restore soil fertility. Agreement within soil science The success of climate mitigation and soil health initiatives may, however, both require Agricultural management has reduced SOC • widespread change in grower practices to • Rebuilding SOC through management is fundamental for restoring soil fertility rebuild soil carbon at scale1, necessitating • SOC accrual rate and maximum depends on biophysical conditions expertise and policy innovation from a • Challenge in detecting near-term management effects on SOC wide circle of disciplines. Yet uncertainty • Ongoing SOC data collection necessary to verify models and practice about the likelihood of widespread adoption 2. Active debates on rebuilding SOC of new practices does not challenge the Engage in debate to address uncertainties about rebuilding SOC credibility of the soil science underpinning initiatives to restore soil fertility by rebuilding soil carbon (Fig. 1). Within soil science Beyond soil science Theoretical advances within soil science • Major processes of SOC formation and • Are targets to build SOC economically, do, however, introduce uncertainty into persistence politically and socially achievable? projections of how soil carbon will respond • Does building SOC equate to GHG • Does a focus on soil as a carbon solution to changing conditions. Specifically, mitigation? undermine other climate mitigation options? technologies permitting direct observation • Are targets to build SOC biophysically • Is verification of SOC change economically of the chemistry, form and location of soil achievable? feasible? carbon are overturning long-held beliefs • Is process-based knowledge required to • Should scientific uncertainty preclude action that the biochemical resistance to microbial reliably model SOC change? to rebuild SOC? breakdown — of plant-carbon inputs and • How much does change in SOC influence desired (for example, yield) and undesired of large macromolecules thought to form through chemical reactions in soils — are (for example, N2O) outcomes? primary mechanisms through which soil Contextualizing active debates Failure to appropriately contextualize carbon persists15. Instead, the new paradigm on rebuilding SOC supports a debates undermines and obscures the suggests that relatively simple molecules, set of effective reinforcement need for recommended actions actions which are otherwise readily consumed by microbes, persist in soil because of their physical location and chemical attraction 15 3. Recommended actions given agreements within soil science to mineral surfaces . The rapid generation 16 Following these actions increases chance of success by reinforcing of fresh insights stimulated by this recent

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