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The role of soil carbon in natural climate solutions

DOI:10.1038/s41893-020-0491-z
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D. A. Bossio
D. A. Bossio
D. A. Bossio
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S. C. Cook-Patton
S. C. Cook-Patton
S. C. Cook-Patton
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Peter Woods Ellis at The Nature Conservancy
Peter Woods Ellis
J. Fargione
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J. Fargione
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Abstract and Figures

Mitigating climate change requires clean energy and the removal of atmospheric carbon. Building soil carbon is an appealing way to increase carbon sinks and reduce emissions owing to the associated benefits to agriculture. However, the practical implementation of soil carbon climate strategies lags behind the potential, partly because we lack clarity around the magnitude of opportunity and how to capitalize on it. Here we quantify the role of soil carbon in natural (land-based) climate solutions and review some of the project design mechanisms available to tap into the potential. We show that soil carbon represents 25% of the potential of natural climate solutions (total potential, 23.8 Gt of CO2-equivalent per year), of which 40% is protection of existing soil carbon and 60% is rebuilding depleted stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72% for wetlands and 47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent carbon emissions, remove atmospheric carbon dioxide and deliver ecosystem services in addition to climate mitigation. Diverse strategies are needed to mitigate climate change. This study finds that storing carbon in soils represents 25% of land-based potential, of which 60% must come from rebuilding depleted carbon stores.
Additional SOC storage potential for 12 natural pathways to climate mitigation We estimate the annual maximum potential of climate mitigation with safeguards for the reference year 2030. The light-grey portions of the bars represent cost-effective mitigation levels assuming a global ambition to hold warming below 2 °C (<US$100 (MgCO2e)⁻¹ yr⁻¹). The dark-grey portions of the bars indicate low-cost mitigation levels (<US$10 (MgCO2e)⁻¹ yr⁻¹). Ecosystem service benefits linked with each pathway are indicated by coloured bars for biodiversity, water (filtration and flood control), food and air filtration. Most pathways also contribute biomass carbon (see Fig. 2), with the exception of pathways that are entirely SOC: biochar, cover cropping, both grazing options and avoided grassland conversion. More than half of the pathways (reforestation, cover cropping, biochar, trees in croplands, grazing, improved pasture options and coastal wetland restoration) represent enhanced SOC sinks, while the others are avoided SOC losses. The remaining 8 of the 20 pathways from Griscom et al.³² are not expected to have an impact on SOC and therefore have not been included in this figure. Icon credit: The Nature Conservancy.
Additional SOC storage potential for 12 natural pathways to climate mitigation We estimate the annual maximum potential of climate mitigation with safeguards for the reference year 2030. The light-grey portions of the bars represent cost-effective mitigation levels assuming a global ambition to hold warming below 2 °C (<US$100 (MgCO2e)⁻¹ yr⁻¹). The dark-grey portions of the bars indicate low-cost mitigation levels (<US$10 (MgCO2e)⁻¹ yr⁻¹). Ecosystem service benefits linked with each pathway are indicated by coloured bars for biodiversity, water (filtration and flood control), food and air filtration. Most pathways also contribute biomass carbon (see Fig. 2), with the exception of pathways that are entirely SOC: biochar, cover cropping, both grazing options and avoided grassland conversion. More than half of the pathways (reforestation, cover cropping, biochar, trees in croplands, grazing, improved pasture options and coastal wetland restoration) represent enhanced SOC sinks, while the others are avoided SOC losses. The remaining 8 of the 20 pathways from Griscom et al.³² are not expected to have an impact on SOC and therefore have not been included in this figure. Icon credit: The Nature Conservancy.
… 
Maximum climate mitigation potential of soil in 2030 across forest, agriculture and grassland, and wetland biome pathways with safeguards The bars to the left indicate the magnitudes of potential sinks, whereas the bars to the right indicate the magnitudes of avoided emissions. The dark portions of the bars represent SOC; the white portions represent vegetative biomass; and the dotted portion represents avoided CH4 and N2O through improved nutrient, rice and animal management. Note that owing to the strong likelihood of near-term increased CH4 emissions arising from increased SOC in peatlands⁷³, we do not include increased SOC sinks in freshwater peatlands on rewetting for restoration. Icon credit: The Nature Conservancy.
Maximum climate mitigation potential of soil in 2030 across forest, agriculture and grassland, and wetland biome pathways with safeguards The bars to the left indicate the magnitudes of potential sinks, whereas the bars to the right indicate the magnitudes of avoided emissions. The dark portions of the bars represent SOC; the white portions represent vegetative biomass; and the dotted portion represents avoided CH4 and N2O through improved nutrient, rice and animal management. Note that owing to the strong likelihood of near-term increased CH4 emissions arising from increased SOC in peatlands⁷³, we do not include increased SOC sinks in freshwater peatlands on rewetting for restoration. Icon credit: The Nature Conservancy.
… 
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AnAlysis
https://doi.org/10.1038/s41893-020-0491-z
1The Nature Conservancy, Arlington, VA, USA. 2Woods Hole Research Center, Falmouth, MA, USA. 3Institute of Biological and Environmental Sciences,
University of Aberdeen, Aberdeen, UK. 4Yale School of Forestry and Environmental Studies, New Haven, CT, USA. 5Center for Mountain Futures,
Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, China. 6Silvestrum Climate Associates LLC, San Francisco, CA, USA. 7Conservation
International, Arlington, VA, USA. ✉e-mail: deborah.bossio@tnc.org
Protecting and restoring soil organic matter delivers many ben-
efits to people and nature1,2. Globally, soils hold three times
more carbon than the atmosphere3, and the role of soil organic
matter as a regulator of climate has been recognized by scientists
for decades4. Recent work has highlighted the historical loss of car-
bon from this pool3 and the threat of future accelerated loss under
warming scenarios4,5. Soil organic carbon (SOC) as a natural climate
solution (NCS) thus has a role through both restoring a carbon sink
and protecting against further CO2 emissions in response to pre-
dicted land-use change and climate change.
This dual role for soil in the global carbon budget suggests that
climate benefits can be achieved through strategies that both con-
serve existing SOC stocks (avoid loss) and restore stocks in carbon-
depleted soils6. There are important additional benefits. Protecting
and increasing SOC storage can (1) protect or increase soil fertil-
ity, (2) maintain or increase resilience to climate change, (3) reduce
soil erosion and (4) reduce habitat conversion (where implemented
through the conservation of natural ecosystems), all in line with the
United Nations Sustainable Development Goals (SDGs)7, the goals
of the United Nations Framework Convention on Climate Change
(UNFCCC) and the United Nations Convention on Combating
Desertification (UNCCD). As such, SOC is promoted as a common
denominator among a variety of global and national initiatives7.
Although recent academic comment and perspective pieces point
the way towards accelerated action on soils8,9, there remains much
uncertainty around actionable pathways for achieving the global
opportunity. Here we examine the scientific and policy context sur-
rounding SOC projects, to aid prioritization and decision-making.
Status of SOC as a climate solution
Despite the scientific consensus around its potential and multiple
benefits, the deployment of SOC storage and sequestration for
climate mitigation remains limited in practice. There is a grow-
ing interest in soil in international climate mitigation conversa-
tions, with the recognition of wetland drainage and rewetting as an
accounting option under the Kyoto Protocol (formalized in 2011),
the launch of the 4 per 1000 Initiative in Paris in 2015 and the formal
recognition of SOC sequestration in the UNFCCC process in 2017
(COP23 decision 4/CP.23). To date there are only a few dozen
projects that address SOC in registered compliance or voluntary
carbon markets. Fewer than 60 projects (half of them in Australia)
provided under 50 kt of CO2-equivalent (CO2e) removals by soil
in agriculture and grassland projects per year10. This is less than
0.0001% of the estimated mitigation potential11. As a comparison,
there are 1,500 carbon projects covering 12 Mha of land in the forest
sector12. The small soil-carbon numbers are due in part to the sec-
tor’s near exclusion from early carbon market mechanisms, notably
the Kyoto Protocol’s Clean Development Mechanism, which limited
potential SOC mitigation to afforestation and reforestation projects.
Nevertheless, the past two decades have witnessed the emergence of
a variety of robust methodological approaches for the calculation
of mitigation benefits and the issuance of carbon credits in a wide
range of project categories covering croplands, grasslands, savan-
nahs, peatlands and coastal wetlands. While still occupying no more
than a niche in the toolbox for international climate action, there is
experience on SOC projects to provide confidence and to support
the development of mitigation plans at larger scales10.
Experience with implementation has not yet caught up with aspi-
rations in the political arena. While soil targets for mitigation are
included in only eight nationally determined contributions (NDCs)
to the UNFCCC9, the UNFCCC is now exploring agriculture and
soils—including with respect to “[improved] SOC, soil health and
soil fertility under grassland and cropland as well as integrated sys-
tems, including water management” as a more explicit part of their
agenda13. At the same time, nations are moving forward to invest
in solutions and set targets that address the food security and land-
use commitments of the SDGs. Beyond governments, a growing
number of companies are including SOC in their set of options to
build the resilience and long-term profitability of agricultural value
chains9. This enthusiasm arises because, in general, SOC enhance-
ment practices are considered to have positive cobenefits, do not
require additional land area, have minimal water footprints and are
readily deployable considering that they do not require changes in
land use11,14.
The role of soil carbon in natural climate solutions
D. A. Bossio 1 ✉ , S. C. Cook-Patton 1, P. W. Ellis1, J. Fargione 1, J. Sanderman 2, P. Smith 3,
S. Wood1,4, R. J. Zomer 5, M. von Unger6, I. M. Emmer6 and B. W. Griscom 7
Mitigating climate change requires clean energy and the removal of atmospheric carbon. Building soil carbon is an appealing
way to increase carbon sinks and reduce emissions owing to the associated benefits to agriculture. However, the practical
implementation of soil carbon climate strategies lags behind the potential, partly because we lack clarity around the magnitude
of opportunity and how to capitalize on it. Here we quantify the role of soil carbon in natural (land-based) climate solutions and
review some of the project design mechanisms available to tap into the potential. We show that soil carbon represents 25% of
the potential of natural climate solutions (total potential, 23.8 Gt of CO2-equivalent per year), of which 40% is protection of
existing soil carbon and 60% is rebuilding depleted stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72%
for wetlands and 47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent carbon emis-
sions, remove atmospheric carbon dioxide and deliver ecosystem services in addition to climate mitigation.
NATURE SUSTAINABILITY | VOL 3 | MAY 2020 | 391–398 | www.nature.com/natsustain 391
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... Sequestering carbon in soil is a potential response to mitigating climate change (Bossio et al. 2020;Frank et al. 2017) and agricultural soil carbon management (SCM) holds much promise in this regard (Amin et al. 2016;Sykes et al. 2019;Yang et al. 2019). Farmers' ability to sequester soil carbon will depend on various biophysical and socioeconomic factors in their management environment (Bossio et al. 2020). ...
... Sequestering carbon in soil is a potential response to mitigating climate change (Bossio et al. 2020;Frank et al. 2017) and agricultural soil carbon management (SCM) holds much promise in this regard (Amin et al. 2016;Sykes et al. 2019;Yang et al. 2019). Farmers' ability to sequester soil carbon will depend on various biophysical and socioeconomic factors in their management environment (Bossio et al. 2020). In addition to emissions reduction goals, SCM can provide a range of co-benefits, such as improved waterholding capacity, soil fertility (Frank et al. 2017;Zomer et al. 2017) and productivity (Branca et al. 2013). ...
... In addition to emissions reduction goals, SCM can provide a range of co-benefits, such as improved waterholding capacity, soil fertility (Frank et al. 2017;Zomer et al. 2017) and productivity (Branca et al. 2013). Landbased SCM could reduce carbon emissions by 23.8 Gt CO 2 -equivalent per year (25% contribution to total anthropogenic emissions reduction) (Bossio et al. 2020;Lal 2004). A number of international and national initiatives highlight the importance of SCM for mitigating climate change, including the COP21 initiative to increase soil organic carbon stocks by 0.4% per year (Minasny et al. 2017;Rumpel et al. 2018) and the Australian Government's Emission Reduction Fund (ERF) programme (Australian Government 2020; Verschuuren 2017). ...
The Social-Ecological System of Farmers' Current Soil Carbon Management in Australian Grazing Lands
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Soil carbon sequestration programmes are a way of offsetting GHG emissions, however, it requires agricultural landholders to be engaged in such initiatives for carbon offsets to occur. Farmer engagement is low in market-based programmes for soil carbon credits in Australia. We interviewed long-term practitioners (n = 25) of rotational grazing in high-rainfall lands of New South Wales, Australia to understand their current social-ecological system (SES) of soil carbon management (SCM). The aim was to identify those components of the SES that motivate them to manage soil carbon and also influence their potential engagement in soil carbon sequestration programmes. Utilising first-tier and second-tier concepts from Ostrom's SES framework, the interview data were coded and identified a total of 51 features that characterised the farmers' SES of SCM. Network analysis of farmer interview data revealed that the current SES of SCM has low connectivity among the SES features (30%). In four workshops with interviewed farmers (n = 2) and invited service providers (n = 2) the 51 features were reviewed and participants decided on the positioning and the interactions between features that were considered to influence SCM into a causal loop diagram. Post-workshop, 10 feedback loops were identified that revealed the different and common perspectives of farmers and service providers on SCM in a consolidated causal loop diagram. Defining the SES relationships for SCM can identify the challenges and needs of stakeholders, particularly farmers, which can then be addressed to achieve local, national and international objectives, such as SCM co-benefits, GHG reduction, carbon sequestration targets and SDGs.
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... (8) (Wiesmeier et al. 2020). (9) Own calculation based on Bossio et al. (2020) and FAOSTAT (n.d.). (10) Own calculation based on Lugato et al. (2015) and (OECD n.d.). ...
... The global SOC sequestration potential related to cropland and grassland is estimated at 930 Mt CO2e/year (Bossio et al. 2020). This includes cover cropping, avoided grassland conversion, and improved grazing (optimal intensity, legumes in pastures) and corresponds to a sequestration rate of 0.19 t CO2e/ha/year. ...
... Avoiding grassland conversion to cropland (1.7 Mha/year) can avoid emissions up to 0.12-0.23 Gt CO2e/year for temperate, tropical and subtropical grasslands depending on soil depth Bossio et al. 2020). ...
Role of soils in climate change mitigation
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Soils play a central role in climate mitigation. They are both as a carbon sink and a source of greenhouse gas emissions (GHG). This report outlines the mitigation potential for GHG emissions of climate friendly soil management options at global, EU and German level. It also discusses different types of climate-friendly soil management measures and key considerations for their implementation.
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... For Soil Carbon and Biota: Our planet's life depends heavily on its soil. In response to predicted global temperature increases, soil organic carbon is becoming increasingly recognized both as a potential source of CO2 emissions and a natural sink for carbon, which can reduce atmospheric CO2 [97]. Increasing urbanization has altered the global C cycle and triggered climate change due to human activity. ...
... However, agroforestry systems have also been shown to reduce greenhouse gas emissions when used in conjunction with other land-use systems. Agroforestry deployment on cropland and grassland is also estimated to sequester carbon in recent publications [155,97,156]. However, current government policies and programs, which focus on mitigating climate change, do not adequately consider agroforestry [157]. ...
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... To this end, natural climate solutions are being proposed as cost-effective and relatively safe ways to capture the excess atmospheric CO 2 and store it within terrestrial and aquatic ecosystems [5][6][7][8] . These solutions are inspired by the recognition that natural biogeochemical processes within terrestrial and marine ecosystems already remove up to 50% of all human-caused CO 2 emissions annually 9 . ...
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... Urban greenspaces, such as urban forests, parks, gardens and lawns, are a common feature of cities and represent important ecosystems that could help offset the carbon (C) footprint of urban areas by storing C in their soils 1 . Despite their importance at both local and global scales 2,3 , examples of natural solutions to changing climates are dominated by natural and agricultural ecosystems 4 and fail to account for the potential soil C in urban greenspaces. Management practices in urban greenspaces such as planting of horticultural plants, mowing and irrigation , may alter the balance between soil C outputs from microbial decomposition and soil C inputs from plant photosynthesis and litter entrance 5 . ...
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... While many agree that sequestering carbon in agricultural soils is a viable natural climate change mitigation strategy [10,64,65], there is continual disagreement about if and how soil carbon stocks can be measured on local and national scales [17,66]. It is clear, however, that regional, national, or even global incentives for agricultural carbon sequestration should be supported by the best-established practices for measuring landscape-scale soil carbon stocks [17]. ...
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... Given the vast size of the global carbon (C) pool (Scharlemann et al., 2014), and the desire to promote additional soil C storage, the factors that control the persistence of soil C are of critical concern (Amelung et al., 2020;Bossio et al., 2020;Lal, 2004;Paustian et al., 2019). However, not all components of soil organic matter (SOM) have an equal potential to influence carbon-climate feedbacks, as certain soil pools are often more vulnerable to decay and removal than others (Heckman et al., 2022;Torn et al., 1997). ...
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... Soils are the largest carbon (C) pool in terrestrial ecosystems holding more than three times more C than the atmosphere, and the role of soil organic carbon (SOC) in soil fertility, food production, climate regulation, and ecosystem stability has been widely recognized (Zhao et al., 2018;Bossio et al., 2020). Insights into processes influencing the dynamics of SOC can help protect and increase the SOC stock, improve estimates of C-climate feedback, and design agricultural policies for improving soil quality . ...
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... Soil organic C (SOC), total nitrogen (TN), and total phosphorus (TP) are important for the functioning of natural and managed ecosystems Vitousek et al., 2010;Reeves, 1997). The benefits of SOC include maintaining soil structure, improving water infiltration rate and water-holding capacity, regulating nutrient supply, stabilizing the soil against erosion (Lorenz et al., 2019;Rice, 2005;Loveland and Webb, 2003), and retaining C that would otherwise be in the atmosphere (Georgiou et al., 2022;Bossio et al., 2020). Either TN or TP, or both, limit net primary production in most terrestrial ecosystems LeBauer and Treseder, 2008), and any additional SOC accumulation depends on sufficient available TN and TP, because soil organic matter contains these elements (Spohn, 2020;Du et al., 2020;Vitousek et al., 2010). ...
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Achieving the 1.5–2.0 °C temperature targets of the Paris climate agreement requires not only reducing emissions of greenhouse gases (GHGs) but also increasing removals of GHGs from the atmosphere [1,2]. Reforestation is a potentially large-scale method for removing CO2 and storing it in the biomass and soils of ecosystems [3,4,5,6,7,8], yet its cost per tonne remains uncertain [6,9]. Here, we produce spatially disaggregated marginal abatement cost curves for tropical reforestation by simulating the effects of payments for increased CO2 removals on land-cover change in 90 countries. We estimate that removals from tropical reforestation between 2020–2050 could be increased by 5.7 GtCO2 (5.6%) at a carbon price of US $20 CO2–1, or by 15.1 GtCO2 (14.8%) at US$50 tCO2–1. Ten countries comprise 55% of potential low-cost abatement from tropical reforestation. Avoided deforestation offers 7.2–9.6 times as much potential low-cost abatement as reforestation overall (55.1 GtCO2 at US$20 tCO2–1 or 108.3 GtCO2 at US$50 tCO2–1), but reforestation offers more potential low-cost abatement than avoided deforestation at US$20 tCO2–1 in 21 countries, 17 of which are in Africa.
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We need both natural and energy solutions to stabilize our climate
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  • Mar 2019
We respond to concerns raised by Baldocchi and Penuelas who question the potential for ecosystems to provide carbon sinks and storage, and conclude that we should focus on decarbonizing our energy systems. While we agree with many of their concerns, we arrive at a different conclusion: we need strong action to advance both clean energy solutions and natural climate solutions (NCS) if we are to stabilize warming well below 2°C. Cost‐effective NCS can deliver 11.3 PgCO2e yr‐1 or ~30% of near‐term climate mitigation needs through protection, improved management, and restoration of ecosystems, as we increase overall ambition.
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Organic amendment additions to rangelands: A meta‐analysis of multiple ecosystem outcomes
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  • Jan 2019
Interest in land application of organic amendments—such as biosolids, composts, and manures—is growing due to their potential to increase soil carbon and help mitigate climate change, as well as to support soil health and regenerative agriculture. While organic amendments are predominantly applied to croplands, their application is increasingly proposed on relatively arid rangelands that do not typically receive fertilizers or other inputs, creating unique concerns for outcomes such as native plant diversity and water quality. To maximize environmental benefits and minimize potential harms, we must understand how soil, water, and plant communities respond to particular amendments and site conditions. We conducted a global meta‐analysis of 92 studies in which organic amendments had been added to arid, semiarid, or Mediterranean rangelands. We found that organic amendments, on average, provide some environmental benefits (increased soil carbon, soil water holding capacity, aboveground net primary productivity, and plant tissue nitrogen; decreased runoff quantity), as well as some environmental harms (increased concentrations of soil lead, runoff nitrate, and runoff phosphorus; increased soil CO2 emissions). Published data were inadequate to fully assess impacts to native plant communities. In our models, adding higher amounts of amendment benefitted four outcomes and harmed two outcomes, whereas adding amendments with higher nitrogen concentrations benefitted two outcomes and harmed four outcomes. This suggests that trade‐offs among outcomes are inevitable; however, applying low‐N amendments was consistent with both maximizing benefits and minimizing harms. Short study time frames (median 1–2 years), limited geographic scope, and, for some outcomes, few published studies limit longer‐term inferences from these models. Nevertheless, they provide a starting point to develop site‐specific amendment application strategies aimed toward realizing the potential of this practice to contribute to climate change mitigation while minimizing negative impacts on other environmental goals. On rangelands, application of organic amendments such as compost and biosolids is increasingly proposed for climate change mitigation. Considering a range of ecosystem outcomes, this meta‐analysis found that organic amendments, on average, provide some environmental benefits (increased soil carbon, soil water holding capacity, aboveground net primary productivity, and plant tissue nitrogen; decreased runoff quantity), as well as some environmental harms (increased concentrations of soil lead, runoff nitrate, and runoff phosphorus; increased soil CO2 emissions). Our models can be used to design site‐specific amendment application strategies, helping to realize the potential benefits of this practice while minimizing environmental harms.
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Put More Carbon in Soils to Meet Paris Climate Pledges
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  • Dec 2018
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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Managing for soil carbon sequestration: Let’s get realistic: XXXX
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  • Nov 2018
Improved soil management is increasingly pursued to ensure food security for the world's rising global population, with the ancillary benefit of storing carbon in soils to lower the threat of climate change. While all increments to soil organic matter are laudable, we suggest caution in ascribing large, potential climate change mitigation to enhanced soil management. We find that the most promising techniques, including applications of biochar and enhanced silicate weathering, collectively are not likely to balance more than 5% of annual emissions of CO2 from fossil fuel combustion.
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A global agenda for collective action on soil carbon
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  • Jan 2019
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.
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Soil organic matter underlies crop nutritional quality and productivity in smallholder agriculture
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Global crop yield gains have not be associated with increases in the many macro- and micro-nutrients needed for a balanced human diet. There is thus growing interest in improving agricultural practices to increase nutrient availability to people. Because nutrients in crops come from soil, soil management—such as building soil organic matter—could be a tool in managing agriculture to produce more nutritious food. To understand the relationship between soil organic matter and nutritional quality, we measured soil organic matter fractions, crop yield, and wheat nutrient composition on smallholder farms along a land-use and land-cover gradient in Ethiopia. We found that wheat yields and protein content were related to organic matter nitrogen, and zinc content was related to organic matter carbon. Increasing organic matter carbon by 1% was associated with an increase in zinc equivalent to the needs of 0.2 additional people per hectare; increasing organic matter nitrogen by 1% was associated with an increase in protein equivalent to the daily needs of 0.1 additional people per hectare. Soil organic matter—and its associated fractions—was greatest in soils closest to a state forest and in home gardens (as opposed to in wheat fields). Wheat fields closer to the forest had elevated soil organic matter fractions relative to wheat soils closest to the market town. Our results indicate that realistic gains in soil organic matter could make human-health-relevant increases in wheat nutrient content. Soil organic matter management can therefore be an additional tool for feeding the world well.
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