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Cropland Management

Growing crops generates greenhouse gas (GHG) emissions through the decomposition of organic matter in the soil and the application of fertilizer.

Each year, organic matter is added to the soil as crops grow, and it later decays, releasing CO2 and methane. Tilling (turning over and breaking up the soil) is often performed before planting, to loosen compacted soil and mix nutrients. However, this process exposes organic matter to air and accelerates the release of GHGs into the atmosphere. Tilling the soil less frequently or refraining from tilling altogether can allow more carbon to build up in the soil until the soil becomes saturated with carbon in about 20-25 years. However, reduced or eliminated tilling can adversely affect the productivity of some types of crops.

After the harvest each year, planting grass or a legume as a cover crop for the winter can help to reduce the release of organic matter from soils. Also, a winter cover crop may allow less fertilizer to be used in the next season.

Reduced tillage and the use of winter cover crops must be continued for as long as the carbon storage is to be maintained. Should the cropland be returned to typical farming practices, the stored carbon will be released to the atmosphere.

Application of fertilizer results in emissions of the greenhouse gas nitrous oxide (N2O), as only some of the nitrogen in the fertilizer is successfully taken up by the plants. There are techniques to increase the fraction of applied nitrogen that is used by plants, including improved fertilizer application methods and timing, reduction in the amount of fertilizer used, and the use of nitrification inhibitors. Nitrification inhibitors are chemicals that slow the conversion of ammonium (NH4+ in decomposed fertilizer) to nitrate (NO3-, a plant-available form of nitrogen), so that plants have time to capture more of the nitrate before it volatilizes or leaches away.

Farmers can be financially compensated for adopting reduced-tilling or no-tilling practices; the required level of compensation varies by crop, as some crop types are more affected by reduced tilling (such as those planted early in the season, when the soil is colder and harder than later in the season). A tax on nitrogen fertilizers (per lb of nitrogen) can encourage improved fertilization practices, so that less fertilizer may be used.

For a more detailed discussion, see the applicable chapter of Designing Climate Solutions, our book on smart energy and climate policy design.