Agricultural Soils and Climate Change Mitigation

Agricultural Soils and Climate Change Mitigation

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December 3, 2020
Agricultural Soils and Climate Change Mitigation
Policymakers, scientists, farmers, and other stakeholders
(derived from nonliving material, such as minerals). Soil
have debated the potential of agricultural soils to sequester
organic carbon (SOC) measures the carbon in soil organic
(store) carbon and help mitigate future climate change. This
matter (SOM), which consists largely of soil microbes (i.e.,
discussion includes various approaches to agriculture—
bacteria and fungi), and decaying and decayed plant and
referred to as carbon farming, regenerative agriculture,
animal material. In addition to its role sequestering carbon,
farming for soil health, and farming for soil carbon
SOM is important to soil health and agricultural
sequestration—and their potential to increase agriculture’s
productivity. Photosynthesis, decomposition, and
role as a greenhouse gas (GHG) sink and reduce its role as a
respiration are the major factors in determining SOC levels
GHG source. GHG sinks remove and store GHGs from the
(Figure 1). Photosynthesis fixes atmospheric CO2 into plant
atmosphere, and GHG sources emit (release) them.
material, which can lead to increased SOC. Decomposition
Figure 1. Carbon Cycling in Agricultural Soils
of SOM releases CO2 into the atmosphere and leaves a
small amount of carbon in the soil. Respiration of plants
and microbes releases CO2 into the atmosphere as a by-
product of using organic materials for energy and growth;
this process returns to the atmosphere some of the carbon
fixed through photosynthesis.
Agricultural practices have generally increased net GHG
emissions, but certain practices can reduce GHG emissions
in the atmosphere and increase net carbon storage in soils.
Such practices generally reduce soil exposure to air and
increase plant root growth. These practices include no-till
or reduced-till land management and use of cover crops,
compost, and manure. The combination of multiple

practices may further increase carbon storage in soils. The
Source: Figure created by CRS.
adoption of carbon-sequestering practices depends on
Agriculture: A GHG Source and Sink
factors that include requirements for equipment and labor
and vary widely in the United States (Table 1).
According to the Environmental Protection Agency (EPA),
the agriculture sector is a net emitter of GHGs ; agricultural
Table 1. Selected Carbon-Sequestering Management
practices, including crop and livestock operations, currently
Practices in Use in U.S. Croplands (2017)
emit more GHGs than they remove. The EPA’s annual
Inventory of U.S. Greenhouse Gas Emissions and Sinks
Acres
% of Total
reports estimates of anthropogenic GHG emissions and
Management Practice
(millions)
Cropland
sinks in the United States, using internationally
No-Til (includes Rotational Til )
105
27%
standardized sectors. On the source side, the agriculture
sector’s GHG emissions—primarily methane and nitrous
Reduced-Til
98
25%
oxide—include those from livestock and soil management.
Cover Crops
15
4%
The 2020 Inventory shows that in 2018, the agriculture
Source: USDA, 2017 Census of Agriculture (COA), 2019, Table 47.
sector contributed about 10% of total U.S. GHG emissions .
Note: Total U.S. cropland = 396 mil ion acres (COA, Table 1).
The Inventory reports estimates of net emissions (emissions
Scientific Debate
minus removals) from the Land Use, Land-Use Change,
The carbon sequestration potential of agricultural soils has
and Forestry sector (LULUCF)—primarily carbon dioxide
been an active research area for decades. Some scientists
(CO2) emissions and carbon storage. LULUCF includes net
are optimistic and others advise caution when considering
emissions for forestlands, agricultural croplands,
agriculture’s potential to measurably mitigate global GHG
grasslands, and other land types. Data from 2018 and prior
emissions.
years indicate that U.S. croplands are a net GHG source and
grasslands are a net sink. Land-use change, rather than land
The utility and effectiveness of mitigating GHG emissions
use, largely shapes these patterns—the conversion of other
via agriculture depends in part on the
land-use types to croplands (net emissions) and to
 carbon-storage potential of agricultural soils,
grasslands (net removals).
 carbon-storing potential of agricultural practices, and
Agricultural Practices That Store Carbon
 carbon storage over time.
Soils store carbon in two basic forms: organic (derived
Carbon-storage potential of agricultural soils. Recent
from living material, such as plant roots) and inorganic
estimates suggest that over the past 12,000 years, human
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Agricultural Soils and Climate Change Mitigation
land use has resulted in a cumulative global loss of about
Public sector. Some legislation introduced in the 116th
116 gigatons (GT) of SOC. Some researchers assert that
Congress would support farmers that implement carbon-
today’s soils have the technical potential to achieve the
sequestering practices. For example, the Growing Climate
amount of SOC that they held prior to these losses, and that
Solutions Act of 2020 (S. 3894/H.R. 7393) would create a
today’s soils have the attainable potential to store some
U.S. Department of Agriculture (USDA) program to certify
proportion of the lost amount. Scientists’ estimates of the
third parties as GHG technical assistance providers and
attainable potential vary considerably.
verifiers of carbon sequestration protocols. Such a program
Differences between the technical and attainable potentials
might facilitate farmer and forest owner participation in
derive from many factors, including socioeconomic and
carbon markets but would not create them.
policy constraints. As examples, farmers who rent rather
The Agriculture Resilience Act (H.R. 5861) would promote
than own their land may not have long-term economic
voluntary, incentive-based conservation measures. Among
incentives to implement soil management changes; farmers
proposed actions, the bill would amend the USDA
may not have the equipment needed to adopt new
Environmental Quality Incentives Program (EQIP, 16
management practices; or existing agricultural policies may
U.S.C. §3839aa et seq.) to add reducing GHGs and
incentivize management decisions that align with goals
sequestering carbon to existing program considerations.
other than carbon sequestration (e.g., maximizing
production or reducing labor and other inputs).
State-level initiatives also provide public sector
opportunities to encourage GHG mitigation through
Carbon-storing potential of agricultural practices. A
agricultural soils. California’s mandatory emissions trading
2019 report by the National Academies of Sciences,
system and the Regional Greenhouse Gas Initiative both
Engineering, and Medicine reviewed a variety of
allow agricultural offsets, though not for soil carbon.
technologies aimed at reducing and eliminating GHG
emissions and evaluated agricultural carbon sequestration
International. Most observers argue that addressing
as among the most cost-effective. It estimated that
climate change will involve some degree of internationally
agricultural practices could sequester up to 0.25 GT of CO
coordinated efforts. Specific to soil carbon, France
2
(0.07 GT of carbon) per year in the United States—
launched the 4 per 1000 Initiative in 2015, with the premise
equivalent to about 4% of total U.S. emissions from all
that increasing the carbon in global agricultural soils by
sectors in 2018—for a cost of less than $20 per ton of CO
four parts per thousand (~3.5 GT) per year would mitigate
2.
The report’s estimate for agricultural carbon sequestration
the annual increase of atmospheric CO2 due to human
assumes full adoption of soil conservation practices.
activity. The initiative invites its stakeholders (e.g., national
governments, private companies) to declare or implement
Carbon storage over time. Ongoing questions include how
practical actions related to soil carbon storage. Some
long sequestered carbon remains in the soil and how long
nonfederal U.S. entities (e.g., private companies,
management practices designed to store carbon continue to
foundations) are members, but the U.S. government is not.
sequester carbon. Research shows that some practices store
carbon only while they are in use. For example, carbon
Policy Challenges
accumulated through no-till management is released when
Many initiatives to increase soil carbon sequestration
the field is tilled again. Research suggests that no-till
through agriculture are predicated on accurately quantifying
management may increase net soil carbon sequestration for
SOC. Scientists recognize this as a technical challenge, as
an estimated 20 years before plateauing and declining to
such quantification needs to be extrapolated from remote
near-zero in later decades.
sensing data or discrete sampling over space and time.
Improving measurement accuracy may need additional
Selected Initiatives and Policy Proposals
research, innovation, investment, and technical assistance.
Existing and proposed approaches in the U.S. private and
If carbon-storing agricultural practices cost more than
public sectors, and internationally, may encourage climate
alternative practices (e.g., in terms of labor, equipment,
change mitigation in agriculture. Some cite climate change
productivity, or sale price), farmers are unlikely to adopt
mitigation as a goal, while others identify increased
them absent requirements or incentives. Various incentives,
economic opportunities for the agriculture sector. Selected
such as those provided through carbon markets, may
current examples are discussed below.
change the economic calculus.
Private sector. A number of private and nonprofit entities
Lack of awareness among agricultural producers of carbon-
are attempting to use markets to create business incentives
storing agricultural practices—what they are, what costs
to reduce net CO2 emissions in agriculture. For example,
and benefits they may provide, and how to implement
IndigoAg, a U.S.-based private company, launched its
them—may also impede adoption. USDA programs, such
Terraton Initiative in 2019. The initiative aims to remove 1
as the USDA Climate Hubs, cooperative extension, and
trillion tons of CO2 (~272 GT of carbon) from the
Natural Resource Conservation Service technical assistance
atmosphere by bringing 12 billion acres of global farmland
programs, may play a role in increasing awareness of these
under regenerative agriculture practices (e.g., no-till,
practices, as may other state and private efforts.
reduced synthetic fertilizers, and incorporating livestock
into croplands). The initiative includes a domestic carbon
Genevieve K. Croft, Analyst in Agricultural Policy
mark et focused solely on agriculture. Carbon markets
enable entities to buy or sell credits or offsets for GHG
IF11693
emissions reductions. Carbon markets may pay farmers for
the reduced emissions resulting from the use of specific
management practices or measures of soil carbon over time.
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Agricultural Soils and Climate Change Mitigation

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