EVALUATION OF CROPGRO FOR PREDICTION OF SOYBEAN NITROGEN-BALANCE IN AMIDWESTERN ENVIRONMENT

There is increasing interest in using crop growth models to study soyb ean (Glycine max L. Merr:) seed yield and quality Because the availabi lity of nitrogen (N) during late season growth is a critical factor af fecting both seed yield and quality of the soybean crop, accurate pred iction of N balance is important for process oriented models. The obje ctive of this study was to evaluate the accuracy of the N balance comp onents of the CROPGRO soybean model for Iowa-grown soybean, and where appropriate to modify the model in order to improve its accuracy. Fiel d samples of 'Kenwood' soybean, taken during reproductive growth in fo ur consecutive seasons (1992-1995), were analyzed for leaf stem, pod w all, and seed N concentration. Soil and weather data from these trials were used to evaluate the ability of CROPGRO to predict N on both lan d-area and concentration bases for leaf stem, pod wall, and seed tissu es. The original Nz-fixation routine was too sensitive to cool soil te mperature, which caused it to underpredict N accumulation in cool seas ons. The original model also predicted remobilization of N from leaf t issue to occur earlier than was observed in the field. This reduced pr edicted photosynthesis which led to underprediction in the rate of N a ssimilation late in the season. Several simple modifications were intr oduced to enhance accuracy of the model: (1) decrease the lower limit of the range of optimum soil temperatures for nodule growth (from 28 t o 22 degrees C), and nitrogenase activity (from 23 to 20 degrees C); ( 2) decrease the base temperature of pod set (from 14 to 10 degrees C), and increase the base temperature for emergence and early vegetative development (from 7 to 9 degrees C); (3) delay of N remobilization fro m leaves and stems to seeds; (4) increase the maximum N concentration of various tissues to reflect observed values for 'Kenwood' soybean, a nd set initiation of N remobilization to occur earlier in pods than in other tissues.