Testing CERES model predictions of crop growth and N dynamics, in cropping systems with leguminous green manures in a Mediterranean climate

Computer models may be useful tools to design management practices for green manures, to maximize N uptake by the following crop while minimizing N losses to teaching. A previous paper tested CERES models predictions of soil processes in the absence of a crop, but did not consider effects of the crop, such as water and N uptake or shading of the soil surface. In this paper, we evaluated the ability of the CERES models to predict N dynamics during wheat and maize growth following legume cover crop (LCC) incorporation. Soil and crop data were collected in 2-year wheat and maize rotations from 1994 to 1998 at the Long Term Research on Agricultural Systems facility at the University of California, Davis. In unfertilized fallow/wheat and LCC/wheat rotations, soil inorganic N content and crop N uptake were mostly within 20% of predicted values, but the greater N uptake by wheat predicted after LCC incorporation (relative to no LCC) did not actually occur. Midwinter N uptake rate of unfertilized wheat was three times that predicted by the model, even though the soil inorganic N was only half that predicted. In maize, following earlier LCC incorporation, inorganic soil N was underestimated throughout growth in 1996 and 1997 (by 25-150 kg N ha^-1 in 1996 and 25-55 kg N ha^-1 in 1997), due to the underestimates of rapid N mineralization shortly before active maize N uptake. Maize N uptake was predicted within 20% in 1997, but was underestimated by 25-70 kg N ha^-1 in 1996. Overpredictions in soil temperature were sometimes of the order of 10°C, which could cause doubling of metabolic rates, e.g. soil N mineralization. This could explain slight but consistent overpredictions in inorganic soil N in the wheat system, but makes underprediction of soil N in the maize system even more surprising. The CERES models could give a rough estimate of N budget in a wheat rotation following a fallow or late-incorporated LCC. Further improvements of the CERES models should address predictions of winter-grown LCC decomposition after early incorporation (e.g. prior to maize), midwinter wheat N uptake, and soil temperature. (C) 2000 Elsevier Science B.V.