Agricultural Greenhouse Gas Emissions 101

• Agriculture contributes approximately 10 percent of total U.S. greenhouse gas emissions (not including emissions from onsite fossil energy use).
• Agricultural emissions of greenhouse gases include carbon dioxide, nitrous oxide, and methane. To evaluate the total impacts, emissions of the latter two gases can be converted to "carbon dioxide equivalent” (CO2e) based on their relative impacts on climate change.
• Agricultural emissions of greenhouse gases result from complex natural processes that are difficult to measure – in contrast with emissions from burning fossil fuels.
  • Methane comes primarily from livestock digestion (known as enteric fermentation) and the way livestock manure is managed. It contributes the most to agricultural emissions of greenhouse gases.
  • The second largest contributor is nitrous oxide, which results mostly from agricultural fertilizer application to soils and from manure management.
  • Carbon dioxide emissions come from increased decomposition of plant matter in soils and from converting lands to agricultural uses. Those emissions are partially offset by the increased plant matter stored in cropland soils.
• Carbon dioxide emissions can be reduced by planting additional crops outside of the primary growing season (known as cover cropping). Using cultivation methods that cause less disturbance to soil also can reduce carbon dioxide emissions.
• Nitrous oxide formation can be limited by reducing the amount of fertilizer applied and avoiding applications when conditions are more favorable to nitrous oxide formation.
• Methane emissions from manure can be reduced through the adoption of manure management that allows capture and use of the emissions (an anerobic digester).

Reducing emissions from crop cultivation and animal husbandry is an important part of efforts to achieve economy-wide decarbonization. According to the US Environmental Protection Agency, annual greenhouse gas emissions from crop cultivation and animal husbandry in 2021 constituted a little over 10 percent of total US emissions (not counting emissions from onsite energy use).

“Conservation programs” in Title II of the Farm Bill provide cost-share payments and technical assistance for implementation of practices to protect agricultural soils, water and air quality, and ecosystems and species habitats, as well as to lower agricultural emissions of greenhouse gases. The 2022 Inflation Reduction Act (IRA) increased funding dedicated to greenhouse gas mitigation in those programs by $19.5 billion over five years. Decisions about provisions in the 2023 reauthorization of the Farm Bill to reduce agricultural greenhouse gas emissions, including those in Title II, will have significant implications for reducing greenhouse gas emissions in the sector.

This explainer, the first in a series on agricultural greenhouse gas emissions and the Farm Bill, provides an overview of the sources of greenhouse gas emissions in agriculture and the technological options for mitigating those emissions. Subsequent explainers in the series describe capacities and challenges in reliably estimating agricultural emissions, review Title II conservation programs, consider other policy measures for agricultural emissions mitigation, and discuss renewable energy provisions in Title IX of the Farm Bill.

Agricultural activities cause emissions of three greenhouse gases: carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). The gases have different impacts on climate change. To evaluate the total impacts, emissions of the latter two gases can be converted to "carbon dioxide equivalent” (CO2e) based on their relative impacts on climate change (Box 1).

The figure below uses data from the EPA Greenhouse Gas Inventory Data Explorer to summarize sources of agricultural greenhouse gas emissions over the past 30 years (in carbon dioxide equivalents).

Carbon dioxide

As indicated in the figure, carbon dioxide emissions accounted for about 7.2 percent of (non-energy-related) agricultural emissions of greenhouse gases in 2021. Most carbon dioxide emissions from agriculture result from disturbance of soil organic matter (plant residues in various states of decomposition), that serves as an emissions repository, or “sink.” Tilling the soil (turning it over and otherwise preparing it for cultivation) accelerates the decomposition of the organic matter by microbial activity, and carbon dioxide emissions increase from greater exhalation by the microbes.

Nitrous oxide

Nitrous oxide accounted for about 49 percent of (non-energy-related) agricultural emissions of greenhouse gases (in carbon dioxide equivalents) in 2021. Nitrous oxide emissions predominantly come from chemical reactions between the atmosphere and nitrogen put onto soils via fertilizers, with a much smaller quantity of emissions resulting from animal manure. These emissions may be released directly from fertilizer application to fields, or from water runoff from fields. The amount of nitrous oxide formed depends on several factors, including the moisture and temperature of the soil, the microbes in it, and the presence of plants capable of fixing nitrogen in soil through their roots. Nitrous oxide formation also depends strongly on the amount of nitrogen applied to soils, which has increased drastically over time.

Methane

Methane, which according to the table constituted about 43.8 percent of (non-energy-related) agricultural emissions of greenhouse gases (in carbon dioxide equivalents) in 2021, is formed from “enteric fermentation” in the digestive systems of certain types of livestock called ruminants (cattle, sheep, goats); from decomposition of animal manure; and in lesser quantities from rice cultivation. Enteric fermentation is the source of about 70 percent of methane emissions from agriculture (and about one-third of all agricultural emissions). Cattle contribute the most methane, and growth in US methane emissions has been linked to the country’s growing population of beef cattle. All livestock (not just ruminants) contribute to methane emissions from the decomposition of manure.

Carbon dioxide

Reducing emissions from soil management and increasing storage of carbon dioxide in agricultural soils require actions to reduce soil disturbances and build up soil organic matter. One widely used practice for increasing soil organic matter is growing a “cover crop” that protects the soil between plantings, and then plowing the plant matter into the soil. Reducing soil disturbance by modifying or eliminating traditional tilling also has been promoted. However, there is scientific debate over the effectiveness of this approach for avoiding carbon dioxide emissions from the soil.

Nitrous oxide

As nitrous oxide formation depends strongly on fertilizer application, avoiding the overuse of nitrogen fertilizer or mistiming its application are key. There are several strategies to accomplish this. One strategy is to opt for several smaller applications of fertilizer over the growing season versus one larger application at the start. Another strategy is to take advantage of innovations in drone-based remote sensing of nitrogen levels in soils to target fertilizer applications to where there is greater need. Nitrous oxide formation can be curbed by avoiding application to very wet soils or at times when it is too cold for plants to effectively take up nitrogen. In addition, using legumes as cover crops and plowing them under naturally increases soil nitrogen content because legumes store significant quantities of nitrogen in their roots. That reduces the need for additional nitrogen application.

Methane

To reduce methane emissions from the digestive systems of ruminants, experiments have been undertaken with different feed additives. However, there are risks that these additives can inhibit digestive function and pose other health threats to the animals. Increased adoption of them can be expected in the future if some additives prove to be safe for livestock.

Methane emissions from manure decomposition can be reduced through use of devices called anaerobic digesters. These actually facilitate the creation of methane from manure decomposition then capture the resulting biogas (a mixture of methane and other gases) for use onsite or to be sold offsite as a source of energy. Anaerobic digesters have high capital costs, and more affordable digesters are less efficient at conversion of manure to methane. Digesters also can reduce water pollution from manure-containing runoff when manure is not adequately managed. However, anaerobic digestion also yields ammonia, which can inhibit the production of biogas and create a waste disposal issue by dissolving into the water emitted by the digester.

The economic feasibility of biogas production as an energy source remains unclear. It might seem like a free good for a farmer or rancher with a digester; and it is often successfully used as a heat source. However, biogas has a lower energy content per unit and more impurities than conventional (fossil) natural gas. This can limit the use of biogas and raise the cost of its use for a farmer or rancher. The same caveats apply to its sale to offsite energy users. At present, offsite demand is concentrated in California, where state-level regulation of greenhouse gases creates a large premium for biogas as a low-carbon fuel (for example, as “renewable natural gas”). Views are mixed on renewable natural gas.