MANAGING CROP RESIDUES FOR THE RETENTION OF CARBON

Soil organic matter, a major sink for carbon, is controlled by many factors that have complex interactions. The management of crop residues is of primary importance. Reduced tillage and no-tillage practices result in a significant build-up of soil organic matter because they greatly reduce the rates of decomposition of both the native soil organic matter and of the crop residues. The crop residues decompose slower because most remain on the soil surface where there is less biological activity, and the native soil organic matter decomposes slower because there is less tillage for aerating the soil and for breaking the aggregates that ''pose organic compounds to the soil microorganisms. Crop residues, however, are highly variable. Although most crop residues contain about 40 percent carbon, the nitrogen contents range from very low 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 system and this factor is frequently overlooked. Climate is often the most critical factor determining the sustainability and enhancement of soil organic matter. As temperatures increase, organic matter decomposition, particularly in frequently tilled soils, is greatly accelerated. As precipitation decreases, there is less biomass produced for replenishing decomposed carbon. Consequently, soil organic matter maintenance becomes increasingly difficult in either hot or arid regions, and particularly difficult in areas that are both hot and arid. Semiarid regions 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.