MANAGING CROP RESIDUES FOR THE RETENTION OF CARBON

Soil organic matter, a major sink for carbon, is controlled by many fa ctors that have complex interactions. The management of crop residues is of primary importance. Reduced tillage and no-tillage practices res ult in a significant build-up of soil organic matter because they grea tly reduce the rates of decomposition of both the native soil organic matter and of the crop residues. The crop residues decompose slower be cause most remain on the soil surface where there is less biological a ctivity, and the native soil organic matter decomposes slower because there is less tillage for aerating the soil and for breaking the aggre gates that ''pose organic compounds to the soil microorganisms. Crop r esidues, however, are highly variable. Although most crop residues con tain about 40 percent carbon, the nitrogen contents range from very lo w to more than 3.5 percent. For carbon to be stabilized in the soil as organic matter, there must be adequate nitrogen available in the syst em and this factor is frequently overlooked. Climate is often the most critical factor determining the sustainability and enhancement of soi l organic matter. As temperatures increase, organic matter decompositi on, particularly in frequently tilled soils, is greatly accelerated. A s precipitation decreases, there is less biomass produced for replenis hing decomposed carbon. Consequently, soil organic matter maintenance becomes increasingly difficult in either hot or arid regions, and part icularly difficult in areas that are both hot and arid. Semiarid regio ns comprise almost 40% of the world's land area so management of crop residues in these fragile areas is important in relation to the global C picture.