CARBON AND PHOSPHORUS LOSSES FROM DECOMPOSING CROP RESIDUES IN NO-TILL AND CONVENTIONAL TILL AGROECOSYSTEMS

An increased knowledge of crop residue decomposition characteristics i s a critical component for nutrient cycling studies in agroecosystems. Carbon and P losses from shoot residues of maize (Zea mays L.), wheat [Triticum estivum (L.), emend. Thell], soybean [Glycine max (L.) Merr .), and shoot and root residues of crimson clover (Trifolium incarnatu m L.) were compared in no-till and conventional till systems. Grain cr op residues were generally collected from senescent plants following h arvest and placed in fiberglass mesh litter bags. Soybean leaf residue s, however, were sampled following preharvest abscission, while crimso n clover residues were collected at spring anthesis and buried only in a conventional till system. Generally, the changes in C and P content of residues were best described by exponential and/or logarithmic fun ctions. Losses of C and P from crop residues were consistently greater , and more rapid when residues were buried vs. left on the soil surfac e. Crimson clover shoots lost C and P more rapidly than root residues. Generally, greater initial losses of P than of C occurred in most res idues. The lack of correlation between C and P losses is believed to b e due to an initial and probably variable inorganic P content that is readily leached prior to the decomposition and mineralization losses o f C. Differences in the rate and magnitude of C losses were related to seasonal effects, the initial N and P content, and/or the proportiona l amount of lignin in the plant residues. Tillage is clearly an import ant regulator or driving variable for element cycling in agroecosystem s.