On March 31, 2009, a draft discussion of the American Clean Energy and Security Act of 2009 was released to the public. The bill contains four sections addressing: 1) clean energy; 2) energy efficiency; 3) reducing global warming pollution; and 4) transitioning to a clean energy economy. Section 3 contains a description of the proposed cap-and-trade program designed to reduce industrial emissions of greenhouse gases (GHG) including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydro-fluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). Agricultural sources, although largely diffuse and not regulated directly by the bill, account for 8 percent of U.S. annual GHG emissions (U.S. EPA, 2008). The agricultural community can address climate change by reducing its emissions and adopting management practices that enhance storage of carbon in soil. The objective of this article is to describe the effects of grazing land management on soil carbon storage.
The amount of carbon stored in soil is a net result of additions minus losses. The amount of soil carbon in grazing lands depends on properties such as climate, soil type, grazing land productivity and plant species composition. Atmospheric CO2 is captured through photosynthesis, where it becomes the carbon that is in plants. Carbon enters the soil through production, growth and decay of plant roots and litter. Carbon in plant material consumed by grazing animals also enters the soil from animal excretions. Because living plants, animals and soil organisms respire, much of the carbon that initially was captured through photosynthesis and entered into the soil returns to the atmosphere as CO2. If the net amount of carbon entering the soil exceeds that lost back to the atmosphere, soil carbon storage increases.
Long-term experiments have revealed that cattle can affect the amount of soil carbon stored in grazing lands. The consumption and trampling of plant material, and cycling of carbon and nutrients by cattle affect amounts of litter, roots and other plant material available to enter the soil carbon cycle. Overstocking reduces potential productivity of grazing lands and the quantity of carbon that enters the soil from roots and litter. A significant loss of soil carbon also can occur through erosion if pastures are overgrazed. The exposure of soil aggregates during erosion enables microbial decomposition and losses of carbon to the atmosphere.
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