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Abstract

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.
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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. Oldfield, Elsa M. Ordway, Joseph Rudek, Jonathan Sanderman and Stephen A. Wood
We argue there is scientific
consensus on the need to
rebuild soil organic carbon
(hereafter, ‘soil carbon’) for sustainable land
stewardship. Soil carbon concentrations and
stocks have been reduced in agricultural
soils following long-term use of practices
such as intensive tillage and overgrazing.
Adoption of practices such as cover crops
and silvopasture can protect and rebuild
soil carbon. Given the positive effects of
soil carbon on erosion resistance, aeration,
water availability and nutrient provision
of soils1, benefits of soil restoration can
include improved fertility, reduced fertilizer
and irrigation use, and greater resilience to
stressors such as drought2. Rebuilding soil
carbon is thus the foundation for many soil
health initiatives15.
At the same time, there is disagreement
about the advisability and plausibility of
rebuilding soil carbon as part of climate
mitigation initiatives1,37. The urgency
to address climate change elevates these
disagreements to the public sphere, where
they are portrayed as strongly adversarial,
and indeed opinions on soils as a mitigation
strategy appear diametrically opposed
within the academic literature1,4,5,7. We
suggest that the debate about the role of
agricultural soils in climate mitigation is
eroding scientific credibility in the related
but distinct effort to protect and restore
these soils by rebuilding carbon (Fig. 1).
We synthesize the science supporting
actions to rebuild soil carbon for improved
fertility, highlight areas of uncertainty, and
suggest how to move forward to promote
confidence in the scientific credibility of soil
health initiatives.
Agreement in soil science
There are agreed foundations in soil science
that support intentions to protect and
rebuild soil carbon (Fig. 1). All soils — from
the most marginal to fertile — are vulnerable
to soil carbon losses and fertility decline2.
In agricultural landscapes, including
cropland, grazing land and plantation
forestry, soil carbon losses via erosion and
decomposition have generally exceeded
formation rates of soil carbon from plant
inputs. Losses associated with these land
uses are substantive globally, with a mean
estimate to 2-m depth of 133 Pg carbon8,
equivalent to ~63 ppm atmospheric CO2.
Losses vary spatially by type and duration
of land use, as well as biophysical conditions
such as soil texture, mineralogy, plant
species and climate8. Adopting regenerative
approaches such as conservation agriculture
and agroforestry can protect soil carbon
and recoup some losses, by minimizing soil
disturbance and maximizing root inputs3.
New soil forms at decadal-to-centurial
timescales, making soils effectively non-
renewable; yet fertility can be restored
by rebuilding the organic carbon
concentrations in the remaining topsoil2.
The rate and total amount of carbon that
can be rebuilt is dependent on biophysical
conditions, meaning that the effects of
management on soil carbon will differ from
place to place and are hard to predict
with high certainty for any one locale3,9.
However, the biophysical controls are
understood well enough to set realistic
bounds for soil carbon maxima and
accumulation rates, and to guide appropriate
actions to achieve them. The bounds for
accumulation rates do, however, remain
poorly constrained: the lower bound is
generally agreed to be above zero (that is,
there is potential to accrue carbon) and soil
scientists generally agree when the upper
bound is unrealistically high.
It is hard to narrow the bounds because
detection of change in soil carbon at
management-relevant time (for example,
<5 years) and within-field spatial scales is
logistically challenging9,10. This is because
approximately half of the organic carbon in
soil is relatively unaffected by management,
meaning that total stocks change slowly2.
Further, there are pronounced local-scale
differences in the amount of carbon stored
because biophysical conditions such as soil
moisture, that affect the amount of soil
carbon, vary markedly within a field. Even
within seemingly homogenous fields, a
high spatial density of soil observations is
therefore required to detect the incremental
‘signal’ of management effects on soil carbon
from the local ‘noise11. Given the time
and expense of acquiring a high density of
observations, most current soil sampling is
too limited to reliably quantify management
effects at field scales9,10.
Even with the measurement and
verification challenges, most soil scientists
agree with the basis for soil health
initiatives. That is, that rebuilding soil
carbon will translate to outcomes such as
reduced erosion and yield stability2. Well-
demonstrated relationships between soil
carbon and desired soil properties (for
example, macroaggregation) support these
expectations. Further, emerging global
datasets support the notion that increasing
soil carbon in croplands will increase
yields12. It is unresolved as to whether these
spatial relationships adequately represent
outcomes of rebuilding soil carbon over
time. Additionally, without proper nitrogen
fertilizer management, greater soil carbon
can increase emissions of greenhouse gases
such as nitrous oxide from agricultural
soils13. Equally, the effects of soil health
practices such as no-till are mixed: while
losses of sediment-bound phosphorus to
waters may be reduced, dissolved reactive
phosphorus losses can increase14. Thus,
although there is agreement about needing
to rebuild soil carbon, quantification of the
benefits and potential undesired outcomes
is required to specify soil carbon targets that
reap the greatest net benefit.
Uncertainty in soil science
The measurement challenges for quantifying
change in soil carbon go hand-in-hand
with a paucity of large-scale verifiable
observations of management effects.
Together these challenges make it difficult
to adjudicate whether reasonable lower
or upper limits for soil carbon change are
more likely1,47. Such uncertainties are
NATURE SUSTAINABILITY | www.nature.com/natsustain
comment
exacerbating tensions about whether
enough carbon can be rebuilt and retained
in soils at a rate that meaningfully
mitigates climate change. The uncertainty
is conflated in the public sphere with
the plausibility of soil health initiatives
because they similarly rely on rebuilding
soil carbon.
Notably, much of the debate about soils
as a climate solution extends beyond the
traditional expertise of soil science into
policy and human behaviour sciences. For
example, there are concerns that a focus
on soil carbon distracts resources from
emission reduction efforts in energy and
transportation sectors1. Such arguments
do not apply to soil health initiatives where
the primary goal is to restore soil fertility.
The success of climate mitigation and soil
health initiatives may, however, both require
widespread change in grower practices to
rebuild soil carbon at scale1, necessitating
expertise and policy innovation from a
wide circle of disciplines. Yet uncertainty
about the likelihood of widespread adoption
of new practices does not challenge the
credibility of the soil science underpinning
initiatives to restore soil fertility by
rebuilding soil carbon (Fig. 1).
Theoretical advances within soil science
do, however, introduce uncertainty into
projections of how soil carbon will respond
to changing conditions. Specifically,
technologies permitting direct observation
of the chemistry, form and location of soil
carbon are overturning long-held beliefs
that the biochemical resistance to microbial
breakdown — of plant-carbon inputs and
of large macromolecules thought to form
through chemical reactions in soils — are
primary mechanisms through which soil
carbon persists15. Instead, the new paradigm
suggests that relatively simple molecules,
which are otherwise readily consumed by
microbes, persist in soil because of their
physical location and chemical attraction
to mineral surfaces15. The rapid generation
of fresh insights16 stimulated by this recent
paradigm means there are multiple technical
explanations as to how practices might
translate to accrual of persistent soil carbon.
Representation of this emerging
understanding in soil models is underway17.
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
combined management and environmental
change will increase as new modelling
efforts represent — often with new data
science approaches — the emerging suite
of new ideas about controls on soil carbon
persistence11. In addition, assuming high-
resolution field measurement technologies
are broadly adopted19, uncertainty will be
reduced as datasets emerge to benchmark
predictions and refine parameterizations.
Given that these modelling and
measurement efforts are relatively nascent9,
in the near term it will remain challenging
to state with high certainty the biophysical
feasibility of annual-to-decadal target rates
for rebuilding soil carbon.
Moving forward
Despite uncertainties, it is important to
communicate that a credible scientific basis
exists for restoring agricultural soils by
Agricultural management has reduced SOC
Rebuilding SOC through management is fundamental for restoring soil fertility
SOC accrual rate and maximum depends on biophysical conditions
Challenge in detecting near-term management effects on SOC
Ongoing SOC data collection necessary to verify models and practice
Highlight we have sufficient knowledge to recommend principles to rebuild SOC
Set expectations and explain uncertainties for achievable SOC accrual rates
Improve verification of SOC change within landscapes
Build SOC knowledge through research in working landscapes
Communicate debates in ways that maintain credibility of the agreed science
Recommended actions given agreements within soil science
Following these actions increases chance of success by reinforcing
credible agreements within soil science
Agreement within soil science
Well-informed policy
and practice to protect
and restore soils
Difficulty in achieving effective
policies and practices to
protect and restore soils
Strong foundation of technical research and knowledge
Supports intentions to protect and rebuild SOC within reasonable bounds
1.
3.
Beyond soil science
Are targets to build SOC economically,
politically and socially achievable?
Does a focus on soil as a carbon solution
undermine other climate mitigation options?
Is verification of SOC change economically
feasible?
Should scientific uncertainty preclude action
to rebuild SOC?
Within soil science
Major processes of SOC formation and
persistence
Does building SOC equate to GHG
mitigation?
Are targets to build SOC biophysically
achievable?
Is process-based knowledge required to
reliably model SOC change?
How much does change in SOC influence
desired (for example, yield) and undesired
(for example, N2O) outcomes?
Active debates on rebuilding SOC
Engage in debate to address uncertainties about rebuilding SOC
2.
Contextualizing active debates
on rebuilding SOC supports a
set of effective reinforcement
actions
Failure to appropriately contextualize
debates undermines and obscures the
need for recommended actions
Fig. 1 | Pathways through which knowledge in soil science can flow to inform soil restoration by
rebuilding soil organic carbon (SOC). Debate within and beyond the discipline of soil science is critical
for addressing uncertainties related to building SOC. However, the way the debate is being conducted —
in particular with regards to soils as a climate mitigation solution — is undermining the flow of credible
and agreed soil science to inform soil restoration. We suggest that appropriate contextualization of the
debates leads to a set of recommended scientific actions that will advance policies and practices to
restore soils on working lands. GHG, greenhouse gas. Credit: graphic by Luminant Design.
NATURE SUSTAINABILITY | www.nature.com/natsustain
comment
rebuilding soil carbon that has been reduced
by management (Fig. 1). The message is
increasingly obscured by disagreements
about whether soil carbon should be
included in climate mitigation portfolios1,47.
The conflation of arguments relating to
climate mitigation and soil health is not
surprising, because many initiatives (for
example, ‘California’s Healthy Soils’ and
‘4 per 1000’) share carbon sequestration
and soil restoration goals4. The confluence
of these goals arises from their mutual
reliance on rebuilding soil carbon. Yet
regardless of one’s position on the potential
for soil carbon to contribute to mitigation,
we submit that rebuilding soil carbon in
agricultural soils should be treated as a
distinct objective that is well supported by
soil scientific knowledge (Fig. 1).
As with restoration initiatives for other
natural resources (for example, forests),
action can happen despite unanswered
scientific questions20. For example, neither
soil models nor data are yet sufficient for
reliably predicting the agricultural and
environmental net benefits of rebuilding soil
carbon across a broad range of contexts9,11.
However, soil science can provide technical
knowledge to establish expectations for
reasonable rates of carbon accrual (even if
the difference between the upper and lower
bounds is large) and to estimate uncertainties
and verify changes in soil carbon. The
logistic challenges of measurement at
scale will be reduced by development of
affordable, accurate, in-field measurement
technologies for soil carbon19. Raising
awareness of current and forthcoming
soil scientific knowledge and capabilities
should help scientists, policymakers
and practitioners alike navigate ongoing
debates about soil carbon, thereby ensuring
the uninterrupted flow of information
supporting soil health initiatives (Fig. 1).
Soil science must also be positioned
as one of many fields required to develop
effective action to restore agricultural soils
through rebuilding carbon. Specifically,
soil carbon restoration will likely only be
practical through strategies that motivate
change in agricultural management and
that are consistent with other goals1,3. For
example, incentives will be necessary in
cases where the financial return to growers
of adopting practices to rebuild soil carbon
are delayed. Yet incentives are not a panacea
and there may be instances where calls
to build soil carbon may be incompatible
with other goals, such as in some native
rangelands used for cattle grazing where
naturally low soil carbon and hence fertility
is important for conserving high levels of
endemic plant diversity. A singular focus
on soil carbon, then, is unlikely to be
consistent with all political, economic, social
and environmental contexts under which
soil science is applied. By recognizing this
wider context of multiple and sometimes
competing demands for human and
environmental wellbeing, soil science can
meaningfully be applied to guide effective
policies and actions to protect and restore
carbon in agricultural lands.
Mark A. Bradford  1*,
Chelsea J. Carey2, Lesley Atwood3,
Deborah Bossio4, Eli P. Fenichel1,
Sasha Gennet4, Joseph Fargione  4,
Jonathan R. B. Fisher  4, Emma Fuller5,
Daniel A. Kane1, Johannes Lehmann  6,7,
Emily E. Oldfield1, Elsa M. Ordway  8,
Joseph Rudek9, Jonathan Sanderman  10
and Stephen A. Wood  1,4
1School of Forestry and Environmental Studies,
Yale University, New Haven, CT, USA. 2Point Blue
Conservation Science, Petaluma, CA, USA. 3Science
for Nature and People Partnership, National Center
for Ecological Analysis & Synthesis, University of
California - Santa Barbara, Santa Barbara, CA,
USA. 4e Nature Conservancy, Arlington, VA, USA.
5Granular Inc, San Francisco, CA, USA. 6Soil and
Crop Science, School of Integrative Plant Science,
Cornell University, Ithaca, NY, USA. 7Institute of
Advanced Studies, Technical University Munich,
Garching, Germany. 8Department of Organismic
and Evolutionary Biology, Harvard University,
Cambridge, MA, USA. 9Environmental Defense
Fund, New York City, NY, USA. 10Woods Hole
Research Center, Falmouth, MA, USA.
*e-mail: mark.bradford@yale.edu
Published: xx xx xxxx
https://doi.org/10.1038/s41893-019-0431-y
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Acknowledgements
This work was part of the ‘Managing Soil Carbon’ working
group for the Science for Nature and People Partnership
(SNAPP). We thank Luminant Design LLC for figure
development and production.
Author contributions
The manuscript emerged from the SNAPP working group,
of which the authors were participants. M.A.B. wrote
the initial draft, which was refined by C.J.C., E.E.O.,
D.A.K., S.A.W., D.B., J.S., J.F. and J.L., prior to further
development by all authors. C.J.C. developed the initial
draft of the figure.
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Maintaining and enhancing soil organic carbon storage can mitigate climate change while promoting forage growth. California has adopted incentive programs to promote rangeland practices that build soil organic carbon. However, there is no standard framework for assessing the baseline level of soil organic carbon at the ranch scale. Here, we use the Ecological Site Description — a land-type classification system — to help ranch managers set priorities about where to implement practices to increase soil organic carbon. We measured baseline carbon stocks at 0 to 15 and 15 to 30 centimeters' depth across three ecological sites and two vegetation states (shrubland and grassland) at Tejon Ranch, California. We discovered increased levels of soil carbon at ecological sites in higher elevations, and more soil carbon in shrublands as compared to grasslands. Slope, elevation, and soil texture, as well as plant litter and shrub cover, were significant predictors of soil carbon. The Ecological Site Description framework can serve as an important tool to help range managers keep carbon in the soil and out of the atmosphere.
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There is increasing interest in using biostimulant products, such as microbial inoculants and alkali-extracted “humic” substances to help manage rangelands regeneratively and rebuild soil health. Understanding how plant and soil communities on rangelands respond to these products is therefore important. In this 3-year study, we examined the combined effects of a commercial inoculant and alkali-extracted “humic” product that are currently on the market (Earthfort Inc. Soil Provide and Revive®) and asked whether they influenced rangeland forage productivity and quality, soil microbial biomass and community composition, and abiotic soil parameters in Central Coastal California. Treatments were established in February 2018 and the products were applied two to three times a year during the growing season (approximately November—May). Sampling of plant and soil samples also began in February 2018 and continued in the fall and spring for three consecutive growing seasons. We found that forage productivity responded positively to the foliar application of these commercial products, with forage production on average 58% percent higher in treated compared to control sites. Some metrics of forage quality (acid detergent fiber, calcium, and fat content) also responded in a desirable way, but these benefits were not mirrored by changes belowground in the microbial community or abiotic parameters. While our study derives from one ranch and therefore requires confirmation of its ubiquity prior to broadscale adoption, our results provide new insights into the usefulness of this approach for managing rangeland productivity in California's Central Coast—and suggest biostimulants could warrant attention as a potential tool for regenerative stewardship of rangelands more broadly.
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Soil health has gained widespread attention in agronomic and conservation communities due to its many purported benefits, including claims that implementation of core soil health practices (e.g., conservation tillage, cover crops) will improve water quality by curtailing runoff losses of nutrients such as phosphorus (P). However, a review of the existing literature points to well-established findings regarding trade-offs in water quality outcomes following the implementation of core soil health practices. In fact, both conservation tillage and cover crops can exacerbate dissolved P losses, undermining other benefits such as reductions in particulate P (sediment-bound P) losses. Soil health management must be pursued in a manner that considers the complex interaction of nutrient cycling processes and produces realistic expectations. Achieving water quality goals through soil health practices will require adaptive management and continued, applied research to support evidence-based farm management decisions.
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Increasing the potential of soil to store carbon (C) is an acknowledged and emphasized strategy for capturing atmospheric CO2. Well-recognized approaches for soil C accretion include reducing soil disturbance, increasing plant biomass inputs, and enhancing plant diversity. Yet experimental evidence often fails to support anticipated C gains, suggesting that our integrated understanding of soil C accretion remains insufficient. Here we use a unique combination of X-ray micro-tomography and micro-scale enzyme mapping to demonstrate for the first time that plant-stimulated soil pore formation appears to be a major, hitherto unrecognized, determinant of whether new C inputs are stored or lost to the atmosphere. Unlike monocultures, diverse plant communities favor the development of 30-150 µm pores. Such pores are the micro-environments associated with higher enzyme activities, and greater abundance of such pores translates into a greater spatial footprint that microorganisms make on the soil and consequently soil C storage capacity.
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Resilient, productive soils are necessary to sustainably intensify agriculture to increase yields while minimizing environmental harm. To conserve and regenerate productive soils, the need to maintain and build soil organic matter (SOM) has received considerable attention. Although SOM is considered key to soil health, its relationship with yield is contested because of local-scale differences in soils, climate, and farming systems. There is a need to quantify this relationship to set a general framework for how soil management could potentially contribute to the goals of sustainable intensification. We developed a quantitative model exploring how SOM relates to crop yield potential of maize and wheat in light of co-varying factors of management, soil type, and climate. We found that yields of these two crops are on average greater with higher concentrations of SOC (soil organic carbon). However, yield increases level off at ∼2% SOC. Nevertheless, approximately two-thirds of the world's cultivated maize and wheat lands currently have SOC contents of less than 2%. Using this regression relationship developed from published empirical data, we then estimated how an increase in SOC concentrations up to regionally specific targets could potentially help reduce reliance on nitrogen (N) fertilizer and help close global yield gaps. Potential N fertilizer reductions associated with increasing SOC amount to 7% and 5% of global N fertilizer inputs across maize and wheat fields, respectively. Potential yield increases of 10±11% (mean±SD) for maize and 23±37% for wheat amount to 32% of the projected yield gap for maize and 60% of that for wheat. Our analysis provides a global-level prediction for relating SOC to crop yields. Further work employing similar approaches to regional and local data, coupled with experimental work to disentangle causative effects of SOC on yield and vice versa, is needed to provide practical prescriptions to incentivize soil management for sustainable intensification.
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Take these eight steps to make soils more resilient to drought, produce more food and store emissions, urge Cornelia Rumpel and colleagues. Take these eight steps to make soils more resilient to drought, produce more food and store emissions, urge Cornelia Rumpel and colleagues.
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Soils contain more carbon than plants or the atmosphere, and sensitivities of soil organic carbon (SOC) stocks to changing climate and plant productivity are a major uncertainty in global carbon cycle projections. Despite a consensus that microbial degradation and mineral stabilization processes control SOC cycling, no systematic synthesis of long-term warming and litter addition experiments has been used to test process-based microbe-mineral SOC models. We explored SOC responses to warming and increased carbon inputs using a synthesis of 147 field manipulation experiments and five SOC models with different representations of microbial and mineral processes. Model projections diverged but encompassed a similar range of variability as the experimental results. Experimental measurements were insufficient to eliminate or validate individual model outcomes. While all models projected that CO2 efflux would increase and SOC stocks would decline under warming, nearly one-third of experiments observed decreases in CO2 flux and nearly half of experiments observed increases in SOC stocks under warming. Long-term measurements of C inputs to soil and their changes under warming are needed to reconcile modeled and observed patterns. Measurements separating the responses of mineral-protected and unprotected SOC fractions in manipulation experiments are needed to address key uncertainties in microbial degradation and mineral stabilization mechanisms. Integrating models with experimental design will allow targeting of these uncertainties and help to reconcile divergence among models to produce more confident projections of SOC responses to global changes.
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International initiatives such as the ‘4 per 1000’ are promoting enhanced carbon (C) sequestration in agricultural soils as a way to mitigate greenhouse gas emissions¹. However, changes in soil organic C turnover feed back into the nitrogen (N) cycle², meaning that variation in soil nitrous oxide (N2O) emissions may offset or enhance C sequestration actions³. Here we use a biogeochemistry model on approximately 8,000 soil sampling locations in the European Union⁴ to quantify the net CO2 equivalent (CO2e) fluxes associated with representative C-mitigating agricultural practices. Practices based on integrated crop residue retention and lower soil disturbance are found to not increase N2O emissions as long as C accumulation continues (until around 2040), thereafter leading to a moderate C sequestration offset mostly below 47% by 2100. The introduction of N-fixing cover crops allowed higher C accumulation over the initial 20 years, but this gain was progressively offset by higher N2O emissions over time. By 2060, around half of the sites became a net source of greenhouse gases. We conclude that significant CO2 mitigation can be achieved in the initial 20–30 years of any C management scheme, but after that N inputs should be controlled through appropriate management.
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Sampling and analysis or visual examination of soil to assess its status and use potential is widely practiced from plot to national scales. However, the choice of relevant soil attributes and interpretation of measurements are not straightforward, because of the complexity and site-specificity of soils, legacy effects of previous land use, and trade-offs between ecosystem services. Here we review soil quality and related concepts, in terms of definition, assessment approaches, and indicator selection and interpretation. We identify the most frequently used soil quality indicators under agricultural land use. We find that explicit evaluation of soil quality with respect to specific soil threats, soil functions and ecosystem services has rarely been implemented, and few approaches provide clear interpretation schemes of measured indicator values. This limits their adoption by land managers as well as policy. We also consider novel indicators that address currently neglected though important soil properties and processes, and we list the crucial steps in the development of a soil quality assessment procedure that is scientifically sound and supports management and policy decisions that account for the multi-functionality of soil. This requires the involvement of the pertinent actors, stakeholders and end-users to a much larger degree than practiced to date.
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Policymakers and investors have perceived securing soil organic carbon as too difficult, with uncertain returns. But new technical, policy and financial opportunities offer hope for rapid progress.