Soil Management for Pastures and Rangelands
Grazing and Soil Carbon Dynamics
Carbon sequestration refers to the process of transferring carbon dioxide from the atmosphere into the soil. Grasslands represent the most potential of any biological landscape on earth to sequester carbon and other nutrients. More than two-thirds of the carbon stored in grasslands is located below ground in soil organic matter pools. Grazing lands can be important sinks of atmospheric carbon dioxide and play a major role in the overall carbon cycle fluxes. Grasslands contain approximately 10 to 30 percent of the world’s soil carbon reserves. Unlike tropical forests, where the majority of the carbon is stored in the vegetation, as much as 90 percent of the carbon pools in grazing land ecosystems are located in the soil, hence it can be readily transferred into more permanent storage in the soil. Grazing exerts a major influence on grassland carbon cycling, affecting not only transfers among vegetation and soil compartments, but also ecosystem input and output flows. In the long run, these alterations may have important consequences for the capacity of managed grasslands to store carbon and contribute to the regulation of the global carbon cycle.
Soil Organic Carbon Levels
Soil organic carbon levels are dynamic. Grassland carbon stocks are primarily determined by temperature, rainfall, and soil texture. Temperature determines the rate of decomposition. Decomposition is slow in colder temperatures, and higher temperatures. In hot and wet areas such as the tropics, decomposition rates are so high that almost all organic carbon is decomposed, so soil organic carbon levels are low despite high plant productivity. Rainfall, along with other climatic factors such as evaporation, determines plant growth and therefore carbon inputs to soil. Rainfall also affects soil water content which determines the rate of decomposition by soil organisms. Decomposition is faster in moist soil than dry soil.
Managing Pastures to Increase Carbon Stocks
Pasture management can have a major role on ecosystem carbon balance. Management practices that increase productivity, such as fertilization, irrigation, use of productive perennial grass species, presence of legumes, and proper grazing management, are often associated with increased soil carbon stocks. Studies have shown that when low-fertility soils receive fertilizer or lime, forage productivity and soil carbon levels generally increase. As discussed, some actual, on-the-ground practices in pasture include limiting livestock access to areas where their hooves can expose soil. Seeding of grass or legume species into degraded grasslands can improve below ground production, and—in the case of legumes—improve soil fertility through nitrogen fixation.
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