sign in sign up
<<
Home , Soil carbon, Soil science

comment science for policy and practice Soil-based initiatives to mitigate change and restore both rely on rebuilding soil organic carbon. Controversy about the role might play in 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 , losses via 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 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 and overgrazing. Losses vary spatially by type and duration observations, most current soil sampling is Adoption of practices such as cover crops of , as as biophysical conditions too limited to reliably quantify management and silvopasture can protect and rebuild such as , 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 verification challenges, most soil scientists water availability and nutrient provision and agroforestry can protect soil carbon agree with the basis for of soils1, benefits of soil restoration can and recoup some losses, by minimizing soil initiatives. That is, that rebuilding soil include improved fertility, reduced disturbance and maximizing root inputs3. carbon will translate to outcomes such as and 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 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 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 -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 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, 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 , 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 credible agreements within soil science paradigm means there are multiple technical • Highlight we have sufficient knowledge to recommend principles to rebuild SOC explanations as to how practices might • Set expectations and explain uncertainties for achievable SOC accrual rates translate to accrual of persistent soil carbon. • Improve verification of SOC change within landscapes Representation of this emerging 17 • Build SOC knowledge through research in working landscapes understanding in soil models is underway . • Communicate debates in ways that maintain credibility of the agreed science Nevertheless, the more than 40-year history of soil biogeochemical modelling in agricultural systems is based primarily on the long-held paradigm of biochemical resistance18. Confidence in the accuracy of projections of soil carbon responses to Well-informed policy Difficulty in achieving effective combined management and environmental and practice to protect policies and practices to and restore soils protect and restore soils change will increase as new modelling efforts represent — often with new data Fig. 1 | Pathways through which knowledge in soil science can flow to inform soil restoration by science approaches — the emerging suite rebuilding soil organic carbon (SOC). Debate within and beyond the discipline of soil science is critical of new ideas about controls on soil carbon 11 for addressing uncertainties related to building SOC. However, the way the debate is being conducted — persistence . In addition, assuming high- in particular with regards to soils as a climate mitigation solution — is undermining the flow of credible resolution field measurement technologies 19 and agreed soil science to inform soil restoration. We suggest that appropriate contextualization of the are broadly adopted , uncertainty will be debates leads to a set of recommended scientific actions that will advance policies and practices to reduced as datasets emerge to benchmark restore soils on working lands. GHG, . Credit: graphic by Luminant Design. predictions and refine parameterizations. Given that these modelling and measurement efforts are relatively nascent9, in the near term it will remain challenging exacerbating tensions about whether Notably, much of the debate about soils to state with high certainty the biophysical enough carbon can be rebuilt and retained as a climate solution extends beyond the feasibility of annual-to-decadal target rates in soils at a rate that meaningfully traditional expertise of soil science into for rebuilding soil carbon. mitigates climate change. The uncertainty policy and human behaviour sciences. For is conflated in the public sphere with example, there are concerns that a focus Moving forward the plausibility of soil health initiatives on soil carbon distracts resources from Despite uncertainties, it is important to because they similarly rely on rebuilding emission reduction efforts in energy and communicate that a credible scientific basis soil carbon. transportation sectors1. Such arguments exists for restoring agricultural soils by

Nature Sustainability | VOL 2 | December 2019 | 1070–1072 | www.nature.com/natsustain 1071 comment

rebuilding soil carbon that has been reduced effective action to restore agricultural soils Cornell University, Ithaca, NY, USA. 7Institute of by management (Fig. 1). The message is through rebuilding carbon. Specifically, Advanced Studies, Technical University Munich, increasingly obscured by disagreements soil carbon restoration will likely only be Garching, Germany. 8Department of Organismic about whether soil carbon should be practical through strategies that motivate and Evolutionary Biology, Harvard University, included in climate mitigation portfolios1,4–7. change in agricultural management and Cambridge, MA, USA. 9Environmental Defense The conflation of arguments relating to that are consistent with other goals1,3. For Fund, New York City, NY, USA. 10Woods Hole climate mitigation and soil health is not example, incentives will be necessary in Research Center, Falmouth, MA, USA. surprising, because many initiatives (for cases where the financial return to growers *e-mail: [email protected] example, ‘California’s Healthy Soils’ and of adopting practices to rebuild soil carbon ‘4 per 1000’) share are delayed. Yet incentives are not a panacea Published online: 11 November 2019 and soil restoration goals4. The confluence and there may be instances where calls https://doi.org/10.1038/s41893-019-0431-y of these goals arises from their mutual to build soil carbon may be incompatible reliance on rebuilding soil carbon. Yet with other goals, such as in some native References regardless of one’s position on the potential rangelands used for cattle grazing where 1. Amundson, R. & Biardeau, L. Proc. Natl Acad. Sci. USA 115, 11652–11656 (2018). for soil carbon to contribute to mitigation, naturally low soil carbon and hence fertility 2. Bünemann, E. K. et al. Soil Biol. Biochem. 120, 105–125 (2018). we submit that rebuilding soil carbon in is important for conserving high levels of 3. Poulton, P., Johnston, J., Macdonald, A., White, R. & Powlson, D. agricultural soils should be treated as a endemic plant diversity. A singular focus Glob. Change Biol. 24, 2563–2584 (2018). distinct objective that is well supported by on soil carbon, then, is unlikely to be 4. Rumpel, C. et al. Nature 564, 32–34 (2018). 5. Vermeulen, S. et al. Nat. Sustain. 2, 2–4 (2019). soil scientific knowledge (Fig. 1). consistent with all political, economic, social 6. Minasny, B. et al. Geoderma 292, 59–86 (2017). As with restoration initiatives for other and environmental contexts under which 7. Baveye, P. C., Berthelin, J., Tessier, D. & Lemaire, G. Geoderma natural resources (for example, ), soil science is applied. By recognizing this 309, 118–123 (2018). 8. Sanderman, J., Hengl, T. & Fiske, G. J. Proc. Natl Acad. Sci. USA action can happen despite unanswered wider context of multiple and sometimes 114, 9575–9580 (2017). scientific questions20. For example, neither competing demands for human and 9. Harden, J. W. et al. Glob. Change Biol. 24, e705–e718 (2018). soil models nor data are yet sufficient for environmental wellbeing, soil science can 10. Saby, N. P. A. et al. Glob. Change Biol. 14, 2432–2442 (2008). 11. Bradford, M. A. et al. Nat. Clim. Change 6, 751–758 (2016). reliably predicting the agricultural and meaningfully be applied to guide effective 12. Oldfeld, E. E., Bradford, M. A. & Wood, S. A. SOIL 5, environmental net benefits of rebuilding soil policies and actions to protect and restore 15–32 (2019). carbon across a broad range of contexts9,11. carbon in agricultural lands. ❐ 13. Lugato, E., Leip, A. & Jones, A. Nat. Clim. Change 8, 219–223 (2018). However, soil science can provide technical 14. Duncan, E. W. et al. Agric. Environ. Lett. 4, 190014 (2019). 1 knowledge to establish expectations for Mark A. Bradford *, 15. Lehmann, J. & Kleber, M. Nature 528, 60–68 (2015). reasonable rates of carbon accrual (even if Chelsea J. Carey2, Lesley Atwood3, 16. Kravchenko, A. N. et al. Nat. Commun. 10, 3121 (2019). Deborah Bossio4, Eli P. Fenichel1, 17. Sulman, B. N. et al. 141, 109–123 (2018). the difference between the upper and lower 18. Smith, P. et al. Geoderma 81, 153–225 (1997). 4 4 bounds is large) and to estimate uncertainties Sasha Gennet , Joseph Fargione , 19. Viscarra Rossel, R. A. & Brus, D. J. Land Degrad. Dev. 29, and verify changes in soil carbon. The Jonathan R. B. Fisher 4, Emma Fuller5, 506–520 (2018). logistic challenges of measurement at Daniel A. Kane1, Johannes Lehmann 6,7, 20. Chazdon, R. & Brancalion, P. Science 365, 24–25 (2019). scale will be reduced by development of Emily E. Oldfeld1, Elsa M. Ordway 8, affordable, accurate, in-field measurement Joseph Rudek9, Jonathan Sanderman 10 Acknowledgements 19 1,4 This work was part of the ‘Managing Soil Carbon’ working technologies for soil carbon . Raising and Stephen A. Wood group for the Science for Nature and People Partnership 1 awareness of current and forthcoming School of Forestry and Environmental Studies, (SNAPP). We thank Luminant Design LLC for figure soil scientific knowledge and capabilities Yale University, New Haven, CT, USA. 2Point Blue development and production. should help scientists, policymakers Conservation Science, Petaluma, CA, USA. 3Science and practitioners alike navigate ongoing for Nature and People Partnership, National Center Author contributions debates about soil carbon, thereby ensuring for Ecological Analysis & Synthesis, University of The manuscript emerged from the SNAPP working group, the uninterrupted flow of information California - Santa Barbara, Santa Barbara, CA, of which the authors were participants. M.A.B. wrote 4 the initial draft, which was refined by C.J.C., E.E.O., supporting soil health initiatives (Fig. 1). USA. Te Nature Conservancy, Arlington, VA, USA. D.A.K., S.A.W., D.B., J.S., J.F. and J.L., prior to further 5 6 Soil science must also be positioned Granular Inc, San Francisco, CA, USA. Soil and development by all authors. C.J.C. developed the initial as one of many fields required to develop Crop Science, School of Integrative Plant Science, draft of the figure.

1072 Nature Sustainability | VOL 2 | December 2019 | 1070–1072 | www.nature.com/natsustain