Decomposition of surface crop residues in long-term studies of dryland agroecosystems

Decomposition of surface crop residues is important for agricultural manage ment, especially under conservation tillage. The objective of this study wa s to test several models for describing crop residue decomposition under th ree eastern Colorado dryland agroecosystems at Sterling, Stratton, and Wals h with a yearly mean air temperature of 9.7, 10.4, and 12.0 degrees C, resp ectively. At each site, a soil toposequence common to its geographic region was chosen to include a summit, a sideslope, and a toeslope position, and several crop rotations were practiced under no-till conditions. Grab sample s were taken at planting and before harvesting for surface residue measurem ent since 1985, Simulation results showed that the Douglas-Rickman model de scribed surface crop residue decomposition better than the Gregory model du ring a 13-year period, based on a normalized objective function (NOF). Our fitted decomposition rate coefficients using the Douglas-Rickman model matc hed those originally published. The Douglas-Rickman model, which uses a fir st-order decay with respect to degree-days, was further evaluated against t wo other first-order decay models: one using a first-order decay equation w ith respect to decomposition-days and the other assuming a first order deca y with respect to time (d), Although the three approaches performed equally well in terms of NOF values (P = 0.354), fitted decomposition rate coeffic ients were significantly different (P < 0.012) among the three experiment s ites when models based on decomposition-days or on time (in days) were used . Therefore, the Douglas-Rickman model may be more applicable for describin g long-term crop residue decomposition because of its consistency in model parameters among experimental sites and simplicity in modeling approach.