ASPEN GLOBAL CHANGE INSTITUTE ENERGY PROJECT

ASPEN GLOBAL CHANGE INSTITUTE ENERGY PROJECT
                 March 2021 Quarterly Research Review

UNDERSTANDING SOIL CARBON SCIENCE TO IDENTIFY STRATEGIES FOR CLIMATE MITIGATION AND
ADAPTATION
By Mark A. Bradford (Yale University), Chelsea J. Carey (Point Blue Conservation Science), Daniel
A. Kane (Yale University), Emily E. Oldfield (Environmental Defense Fund), Darya Watnick (Yale
University), Stephen A. Wood (The Nature Conservancy)

Disagreements drive science forward but can generate uncertainty for policy and practice. Soil
scientists strongly disagree about whether soil carbon sequestration (SCS) can effectively help
to decarbonize the atmosphere. The debate has spread beyond academia with some
organizations making the case that meaningful drawdown of carbon through SCS in agricultural
soils, achieved through adoption of regenerative agricultural practices, is not viable. Other
organizations, by contrast, promote the same practices as necessary for natural climate
solutions.

Our recently published article in Nature Sustainability outlines our concerns that this conflicting
messaging undermines efforts to restore soils to safeguard human and environmental well-
being. We laid out technical research and knowledge supporting soil-based initiatives for
climate adaptation of agriculture, provision of clean water, protection of biodiversity, and the
sustainability of agricultural communities. We concluded the SCS debate does not undermine
the substantive body of science supporting immediate, widespread adoption of practices
protecting and restoring soil carbon (Figure 1).

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Figure 1. Pathways through which knowledge in soil science can flow to inform soil restoration by rebuilding soil
organic carbon (SOC). Debate is critical for addressing uncertainties related to building soil carbon. Yet the way
debate is conducted – in particular, with regards to soils as a climate mitigation solution – can undermine the flow
of credible and agreed soil science to inform soil restoration. The authors suggest that appropriate
contextualization of the debate leads to a set of recommended scientific actions that will advance policies and
practices to restore soils on working lands. Source: Bradford et al. 2019.

Consensus encouragingly exists across academic, business, nonprofit, and policy circles as to
the importance of protecting and restoring agricultural soils to minimize soil erosion, rebuild
fertility, improve rural livelihoods, and provide climate-secure food production. Yet the
underlying tension remains, centered on soil carbon.

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Soil health and SCS both rely on rebuilding soil organic carbon (SOC; Figure 1) to achieve the
broad goals of environmental and human well-being. This reliance on soil carbon creates a
logical incompatibility: If SCS’ potential to provide meaningful carbon drawdown is debated,
how can improved soil health—which also relies on building soil carbon—be realistically
achieved through agricultural practices?

The answer lies in the strict definition of carbon sequestration, which requires net removal of
atmospheric carbon dioxide (CO2). Fertility can be restored by rebuilding organic carbon
concentrations in surface soils, yet rebuilding soil carbon in top soils may not equate with
sequestration because external inputs like compost may originate outside the farm system,
thus representing organic carbon relocation from one place to another. Similarly, in some
systems, surface soil carbon gains may be offset by losses deeper in the soil. Yet the benefits of
higher carbon in surface soils occur irrespective of net atmospheric drawdown.

Higher carbon in surface soils promises both climate-smart and less-intensive agriculture. Soil
carbon binds together mineral soil particles into aggregates, minimizes erosion, provides
aeration and freer movement for roots, creates a pore structure allowing rainwater infiltration,
and increases thermal stability. A recent global synthesis found that increases in soil organic
carbon up to around 2 percent translate, on average, to yield gains for major staple crops (see
Oldfield et al. 2019). The work suggests that building soil carbon can achieve the same yields in
many regions, but with reduced nitrogen fertilization and irrigation.

A new synthesis (Kane et al. 2021) shows the potential for soil carbon to reduce yield and
income losses under drought. Using spatial data for rain-fed corn agriculture in the United
States, we find that higher concentrations of surface soil carbon help sustain yields and
minimize crop insurance claims in drought years, with the effect of soil carbon most
pronounced when droughts are most severe (Figure 2).

                                                                                       Figure 2. Relationships between
                                                                                       corn yield and soil organic
                                                                                       matter (SOM) concentration, at
                                                                                       the county-level, for rain-fed
                                                                                       U.S. agriculture for common
                                                                                       soil orders (e.g., mollisols).
                                                                                       SOM is used to represent soil
                                                                                       organic carbon, where
                                                                                       approximately half of SOM is
                                                                                       carbon. The increasing
                                                                                       steepness of the slope of the
                                                                                       relationship between SOM and
                                                                                       drought, as you move from
                                                                                       moderate to very severe
drought (left to right across the panels), underscores the increasing adaptation value of higher SOM as drought
severity intensifies. Drought severity is relative to non-drought years for the time period of the analysis (2000–
2016). Source: Kane et al. 2021.

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Climate projections include increasingly frequent and severe droughts. The Kane et al. findings
support the intent of the “4 per 1000” initiative launched at the 2015 United Nations climate
change conference to protect and rebuild soil carbon. The initiative includes three pillars:
intensify climate adaptation of agriculture, improve food security, and mitigate carbon. The
third pillar of climate change mitigation through SCS, however, became a focal point of
scientific disagreement around the potential of SCS to meaningfully contribute to carbon
drawdown.

The 4 per 1000 initiative was named and originally communicated around the substance of this
third pillar, that an annual growth rate of 0.4 percent in the carbon stock of surface soils would
significantly reduce atmospheric CO2 concentrations. The target was interpreted literally, rather
than as an illustrative example of how much carbon soils hold and might sequester. Whatever
the original intention, the “4 per 1000” branding spurred polarizing and acerbic academic
debate on whether enough carbon can be sequestered in soils at a meaningful rate to mitigate
climate change.

The SCS debate is important and stems from measurement challenges, paucity of large-scale
verifiable observations of management effects, potential unintended consequences such as
increases in nitrous oxide (N2O) emissions, theoretical advances in our understanding of SCS
mechanisms, and feasibility of widespread producer adoption (for example, see Amundson and
Biardeau 2018). Soil carbon initiatives are wrestling with these real challenges. We note that
naysayers about SCS are on no firmer footing than those championing it for carbon drawdown.

Soil as a common good

Regardless of where you stand on the SCS debate, the need to protect and restore soils remains
an imperative. Over the last few years, notions of soil fertility and quality have developed into
ones of soil health and now soil security. In their recent perspective in Nature Reviews Earth &
Environment, Lehmann and colleagues document how the framings have evolved to expand the
scale, ecosystem services, and stakeholders dependent on securing soils as a natural resource
(Figure 3). Lehmann et al. clarify the need to view soil as a common good, just like freshwater
and air, to which all people have rights.

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Figure 3. Evolving framings of soils as a natural resource, from the most historical framing of fertility to the most
recent framing of security (left side of figure). The evolving concepts increase the spatial scale and number of
functions, ecosystem services, and stakeholders (right side of the figure) included in the framing. The concepts also
differ in the view of soil rights and assessments (bottom left of figure). Source: Lehmann et al. 2020.

Another article by Sanderman and colleagues published in the Proceedings of the National
Academy of Sciences, USA confirms that all soils, from the most marginal to fertile, are
vulnerable to soil carbon losses. 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 (Figure 4). Losses associated with these land
uses are substantive globally, with a mean estimate to 2-m depth of 133 Pg-carbon lost since
adoption of agriculture (~12,000 years before present), equivalent to ~63 parts per million of
atmospheric CO2. Just as with many other environment issues, the rate of loss has been much
greater over recent decades than it was when human land use was more diffuse and less
intense on both a per capita and cumulative basis. The need for action on soils cannot be
overstated.

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Figure 4. Losses of soil organic carbon (SOC) stocks associated with conversion of land to cropping and grazing
down to 30 cm (A) and 1 m (B). Points represent a paired observation of the native cover type (e.g., forest) and the
converted cover type (e.g., cropping). Points that fall below the line have lost SOC on agricultural conversion
compared to the native cover type. Source: Sanderman et al. 2017.

Measurement and verification

Now is the time to go beyond soil science to draw on knowledge and cutting-edge research in
ecosystem, climate, and carbon-cycle science. Those fields open up new thinking about how
measurement and verification (MRV) might support carbon markets based on adoption of
regenerative and climate-smart agricultural practices. Questions tackled by scientists in those
fields deal directly with how to scale from local impact to regional, continental, and global
consequences for carbon budgets.

Insights from such work suggest that whereas it might be hard to reliably quantify changing
carbon stocks at local scales, such as a farm or field, confidence in regional-scale estimates of
mean change can be achieved because the influence of “local uncertainty” is minimized. Major
businesses, for example, might then be able to build confidence that widespread adoption of
agricultural practices that rebuild soil carbon might help them reduce Scope 3 emissions (e.g.,
emissions from agriculture and land use change, which typically constitute the majority of
emissions from food and beverage companies), regardless of whether verification of SCS for any
one farm is feasible.

Adoption of lessons learned in these “macrosystem science” fields would expand the repertoire
of approaches currently being considered in soil carbon MRV protocols. Current protocols rely
on a single biogeochemical model and/or technological advances directly measuring soil
carbon. Confidence in regional-scale projections can be generated through convergence of
predictions from multiple biogeochemical models, given recognition that model projections at

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local scales of time and space are subject to many uncertainties. Because uncertainties differ
model to model, “multi-model” approaches capture a wide range of knowns and unknowns,
bolstering confidence by including them in the aggregate prediction. If MRV protocols expand
to consider metrics representative of broader soil health goals, such as yield resilience under
drought (Figure 2), broader environmental and societal aims of soil security might be verified
even if SCS could not be reliably measured at the farm scale.

Moving forward

In their 2019 Geoderma paper, Gonzalez Lago and colleagues argued to re-politicize soils. They
described an entrenched soil policy vacuum where, despite international treaties and
conventions, soil degradation concerns had not translated to national or sub-national
environmental policies that facilitate effective action. They argued an immediate need exists for
a narrative aligned with policymakers’ perceptions of the importance of soils to society, one
which would be effective at generating societal attention and traction.

It seems their call has been answered. Major food businesses such as General Mills are
advancing the adoption of regenerative practices in an effort to protect and build healthy soils
to strengthen agricultural ecosystems and rural communities, and several major nonprofits
have initiated or bolstered efforts related to soil conservation. Simultaneously, rapid
development and buy-in from major business and policy is underway, with the possibility that
the U.S. Department of Agriculture will establish a Carbon Bank under the Biden administration,
as part of efforts to generate climate-smart agriculture.

Twenty years ago, the conversation centered on soil quality (as opposed to health or security),
and we were on the verge of adopting major soil carbon bank initiatives. Those of us who
remember those discussions are uneasy. Government- and business-driven action on soils fell
away when markets for SCS failed to establish. To avoid repeating history while still harnessing
significant, new momentum, public and private sector support are needed to prioritize healthy
soils as the foundation for climate adaptation of agriculture and food security, even if
ultimately divorced from SCS.

Many soil scientists draw on the words of President Franklin D. Roosevelt when he signed a
1936 act to conserve the land’s natural resources: “The history of every Nation is eventually
written in the way in which it cares for its soil.” With globalization, these words must be
updated from nation to planet. The impacts of accelerating rates of soil degradation will be
compounded by the increasing severity and frequency of extreme weather events. Without
concerted action to protect and rebuild agricultural soil carbon, decreased food security will be
followed by land degradation, economic migration, and instability in national security. Paying
farmers only for the carbon they sequester therefore seems naive. Protecting and restoring
agricultural soil carbon is about food security and soil conservation writ large, and this should
be a primary goal of agricultural production and supply chains. Soil scientists should continue to
think about how to most appropriately contextualize active debates and uncertainties, so we
can clearly communicate recommendations based on agreements within soil science (Figure 1).

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Given the momentum in the soil carbon space, the need for clear communication is more
important than ever and will help ensure policy, markets, and management actions are well-
informed and successful.

Authors’ note: The views expressed in this piece are those of the authors and do not necessarily
reflect those held by their institutional affiliations and/or AGCI.

Featured research

Amundson, R. & Biardeau, L. Soil carbon sequestration is an elusive climate mitigation tool.
        Proceedings of the National Academy of Sciences, USA, 115, 11652-11656 (2018).
Bradford, M.A., Carey, C.J., Atwood, L., Bossio, D., Fenichel, E.P., Gennet, S., Fargione, J., Fisher,
        J.R.B., Fuller, E., Kane, D.A., Lehmann, J., Oldfield, E.E., Ordway, E.M., Rudek, J.,
        Sanderman, J., Wood, S.A. (2019) Soil carbon science for policy and practice. Nature
        Sustainability, 2, 1070-1072.
Gonzalez Lago, M., Plant, R., Jacobs, B. (2019) Re-politicising soils: What is the role of soil
        framings in setting the agenda? Geoderma, 349, 97-106.
Kane, D.A., Bradford, M.A., Fuller, E., Oldfield, E.E., Wood, S.A. (2021) Soil organic matter
        protects US maize yields and lowers crop insurance payouts under drought.
        Environmental Research Letters, https://doi.org/10.1088/1748-9326/abe492.
Lehmann, J., Bossio, D.A., Kögel-Knabner, I., Rillig, M.C. (2020) The concept and future
        prospects of soil health. Nature Reviews Earth & Environment, 1, 544-553.
Oldfield, E.E, Bradford, M.A., Wood, S.A. (2019) Global meta-analysis of the relationship
        between soil organic matter and crop yields. SOIL, 5, 15-32.
Sanderman, J., Hengl, T. & Fiske, G.J. Soil carbon debt of 12,000 years of human land use.
        Proceedings of the National Academy of Sciences, USA, 114, 9575-9580 (2017).

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