Soil and Climate Change
Soil Carbon Sequestration
Soils are one of five principal global carbon pools, which also include the oceans, fossil fuel deposits, biotic (plant-based carbon), and the atmosphere. Soils are the foundation of life on land and represent one of the largest global carbon reservoirs. Because of the vast amount of carbon that they store and the continuous fluxes of carbon with the atmosphere, soil can either be part of the solution or problem with respect to climate change. One of the potential ways that is readily available to mitigate carbon additions to the atmosphere is carbon sequestration by soils. Carbon sequestration implies transferring atmospheric carbon dioxide into long-lived pools and storing it securely so it is not immediately reemitted. Thus, soil carbon sequestration means increasing soil organic carbon and soil inorganic carbon through judicious land use and recommended management practices.
Scientific Debate
Although some scientists are optimistic there are others who advise caution when considering agriculture’s potential to measurably mitigate global greenhouse gas emissions (GHG). The utility and effectiveness of mitigating GHG emissions via carbon sequestration by agriculture in part depend on the following conditions.
Carbon-Storage Potential of Agricultural Soils
Agricultural soils have the capacity to take up carbon from root exudates, litter, harvest residues, and animal manures used in agricultural production. Soil organic matter content is dependent on soil type, climate, current and past crop, and soil management. For example, soils in cool, wet climates such as the Midwest tend to have higher amounts of organic matter than soils in hot and wet climates like the Southeast or the hot and dry climates in the Southwest.
Carbon-Storage Potential of Agricultural Practices
Various sustainable agricultural practices are particularly favorable to carbon sequestration in soil. Some of the key components in a system-based approach include: (1) minimal soil disturbance or none, (2) adoption of complex rotations, (3) establishment of a cover crop during the off-season, (4) retention of crop residues on the soil surface as mulch, (5) use of integrated systems of soil fertility management, and (6) integration of crops with trees and livestock. No-till systems may result in lower carbon dioxide emissions from and greater carbon sequestration in the soil as compared to management systems based on intensive tillage, although some recent studies have indicated that no-till systems may simply result in higher carbon accumulations in the upper soil profile with no increase in carbon when the entire soil profile is considered.
Carbon Storage Over Time
Over the last two decades, soil scientists have addressed a number of challenges associated with soil organic matter in the context of climate change. In particular, some in the scientific community have argued that since soils often exhibit a sizeable carbon deficit relative to historical levels 50 or 60 years ago, there is a significant potential for them to re-store large amounts of carbon, and thereby, in principle, contribute to decelerating climate change or even halting it. This theme of the possible sequestration of carbon in soils, also referred to as the “recarbonization” of soils is controversial. For starters, the scientific community is uncertain about how long carbon can be stored in the soil, how much carbon can be sequestered by different practices, and how to effectively measure and track the carbon that is sequestered.
Carbon Sequestration Cost
Many farmers live season-to-season with very little margin of error, so they may not have the resources or be willing to take the risk of investing in soil carbon sequestration technologies, particularly those that reduce near-term yields. Despite these limitations and uncertainties, a compelling argument can be made for continuing to encourage soil carbon sequestration. Many of the same practices that are believed to store carbon have other beneficial environmental and economic effects. For example, improving overall soil health can increase agricultural yields while reducing the need for agricultural inputs, saving farmers money and reducing nitrous oxide (N2O) emissions—another potent GHG—from synthetic fertilizer application.
Soil Carbon Saturation
Agricultural carbon sequestration cannot be expected to offset anthropogenic GHG emissions indefinitely because of the phenomenon of soil carbon saturation. Implementation of improved soil health management practices on cropland soils typically leads to steady increases in total soil organic carbon over a period of 10 to 40 years, after which it reaches a new steady state or plateau. At saturation, a soil will cease to be a sink and can either become a carbon dioxide source or reach a steady state wherein it draws in as much carbon as it emits on an annual basis.
Carbon Farming
Carbon farming focuses on the “management of carbon pools, flows and greenhouse gas fluxes at farm level, with the purpose of mitigating climate change. This involves the management of both land and livestock, all pools of carbon in soils, materials and vegetation, plus fluxes of carbon dioxide and methane, as well as nitrous oxide. For the land managers, this definition means that carbon farming covers farming practices and land use changes that deliver one or more of the following outcomes: (1) carbon removal (sequestration) and subsequent storage in biomass above/below ground and in agricultural soils; (2) the avoidance of future carbon dioxide and other GHG emissions; and/or (3) the reduction of existing carbon dioxide and other GHG emissions.
Challenges for Carbon Sequestration
The potential for regenerative agriculture to contribute to climate change mitigation is not without its shortcomings. As discussed earlier, soil carbon sequestration is a complex, nonlinear process influenced by environmental conditions and current and historic management. The primary criticisms are centered around biophysical. First, detractors cite biophysical limits to carbon sequestration. While degraded lands may rapidly accrue carbon, the rate of sequestration decreases over time. Beyond the tapering of carbon sequestration rates, the impact of such a practice on climate change mitigation is only as large as the proportion of land use it occupies. Second, soil carbon sequestration requires that carbon added to the soil remains there long-term. Considerable uncertainty exists around soil sampling and soil carbon quantification methods (e.g., soil depth sampling, accounting for expansion and contraction of the soil profile, etc.).
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