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Grassland Conservation and Soil Carbon

Published: October 19, 2023 by Editorial Team

The role of anthropogenic effects on soil carbon dynamics as both a potential source of greenhouse gas emissions (GHG) and a sink for GHG reductions is a key player in climate change. Carbon is constantly moving between the atmosphere, living organisms, and the Earth in a process called the carbon cycle. Soil absorbs carbon from decomposing plant and animal tissues or the atmosphere, as plants fix CO₂ and release the carbon into the soil. Whether soil is a source or sink of carbon depends on land use, vegetation cover, and climate (1). In recent decades, there has been an increase in grassland to cropland conversion, a significant source of anthropogenic soil carbon emissions. Between 2008 and 2012, cropland expansion in the U.S. caused the release of about 38.8 million metric tonnes of carbon, with grassland conversion accounting for 90% of these emissions (2). Land use changes that alter grasslands to accommodate crops cause the soil structure to become disturbed, releasing much of the stored carbon and returning it to the atmosphere as carbon dioxide (CO2), the principle GHG causing climate change. While the destruction and degradation of grasslands and the release of their soil carbon can occur rapidly, complete recovery of these ecosystems occurs slowly, or not at all.

Historically, disturbing the soil for crop production was used to kill competing plants, soften the ground, and incorporate nutrients, creating a safe place for seeds. However, conventional tillage practices have substantial and long-lasting impacts on soils’ physical properties. When soil is tilled, the thermal and moisture conditions change, and aeration increases, promoting the decomposition and oxidation of soil organic matter and leading to the rapid depletion of soil carbon (3). The most significant soil carbon loss occurs within 20-50 years of conversion, with the cultivated field losing 20-67% of its original soil carbon stores (4). While adopting no-till practices following conventional farming can increase the depleted soil carbon, it can take a century or longer for the soil to sequester enough carbon to reach pre-agricultural carbon levels (4). Only an indefinite practice change to no-till will result in permanent reductions in carbon emissions, as even intermittent tillage can negate the carbon sequestered during periods of no-tillage (2). Soils within undisturbed grasslands, however, maintain their historic carbon stores while continuing to sequester carbon. In undisturbed grasslands, soil carbon sequestration occurs at a higher rate when compared to restored grasslands, potentially due to the long-term losses of plant biodiversity and a decrease in microbe abundance from cultivation (4, 5). Recent research indicates there is little expectation that disturbed grasslands will ever completely recover and resemble old-growth, intact grasslands (6). Recovering grasslands often lack the species richness and landscape heterogeneity to support birds, insects, and other animals seen in old-growth grasslands. Even decades, and sometimes centuries, after disturbances of belowground structures, restored grasslands lack the plant and soil microbe diversity, seed and bud banks, and soil carbon storage seen in undisturbed grasslands (6).

As soil carbon sequestration is largely time-dependent, soil carbon emissions due to land use changes may represent an irreparable climate impact for humanity (2). While the implementation of no-till practices and grassland restoration projects present meaningful climate opportunities, intact grasslands continue to store and sequester carbon at higher rates. Protecting grasslands from conversion allows for the most substantial climate benefits to be realized.