CERES-N MODEL PREDICTIONS OF NITROGEN MINERALIZED FROM COVER CROP RESIDUES

Winter annual cover crops, widely used in no-till systems, can be an important source of N for the subsequent crop. Because many factors affect net N mineralization from cover crop residues, computer models can be powerful tools to predict it. The CERES models, which are some of the most widespread models for simulating the whole crop-soil system, have a common submodel (CERES-N) that describes N transformations. The objectives of this study were to determine decay rate constants under nonlimiting conditions for the carbohydrates and cellulose pools (CARE and CELL) of CERES-N for residues that decompose on the soil surface, and to test if two modifications to CERES-N could improve the simulation of N mineralization. The two modifications were to: (i) allow the user to vary the relative size of the residue pools (CARE, CELL, and lignin), and (ii) allow stems and leaves to decompose separately, having a common point of interaction through the inorganic N pool. Results of a 6-mo laboratory incubation experiment with four cover crop residues were used to adjust rate constants and test the effect of model modifications. The decay rates obtained were 0.14 and 0.0034 d(-1) for CARE and CELL, respectively. Allowing the user to vary the relative size of residue pools greatly improved the simulation of net N mineralized (root mean square error of the model decreased from 1.0 to 0.28 g m(-2)), whereas modeling the separate decomposition of leaves and stems only caused a slight improvement in the prediction of net N mineralized.