Decomposition of crop residues in the field: evaluation of a simulation model developed from microcosm studies

On 12 occasions during a 2-y field experiment, we determined the decomposition of barley straw, ryegrass foliage, white clover foliage, potato haulm and white cabbage leaves confined in buried mesh bags that were protected against leaching. After 1 y, 49, 18, 8, 25 and 5% of initial carbon in the five crop residues remained, respectively. The corresponding figures for nitrogen were 105, 32, 8, 36 and 11%. The data were used to evaluate a simulation model developed during previous studies of crop residue C and N turnover under controlled temperature and moisture conditions. Description of plant residue degradability and model parameter values were taken from these studies. Rate-modifying functions were then added to take account of effects of measured soil temperature and moisture. The model gave a good overall description of crop residue degradation but underestimated C release and, to a greater extent, N release during the first autumn and winter. The relatively rapid N loss during this period, suggested that low temperatures restricted microbial N immobilization more than it did gross decomposition. We hypothesized that this was caused by a reduced microbial growth yield efficiency (E) at low temperatures. When we reduced the value of E from 0.5 (default value) to 0.2 at 2 degrees C and below, model fit to C and N mineralization was improved substantially. Moreover, the model produced an excellent fit to remaining wheat straw C and a gave good description of N mineralization in an independent experiment, indicating that reducing the value of E was justified. The results suggested that crop residue degradability and the decomposer community were reasonably described in the model, that the rate-modifying functions of temperature, moisture and N availability were sound and that parameter values set under controlled conditions also represented field conditions, possibly with the exception of E. (C) 1999 Elsevier Science Ltd, All rights reserved.