EVALUATION OF CROPGRO FOR PREDICTION OF SOYBEAN NITROGEN BALANCE IN A MIDWESTERN ENVIRONMENT

There is increasing interest in using crop growth models to study soybean (Glycine max L. Merr.) seed yield and quality. Because the availability of nitrogen (N) during late season growth is a critical factor affecting both seed yield and quality of the soybean crop, accurate prediction of N balance is important for process oriented models. The objective of this study was to evaluate the accuracy of the N balance components of the CROPGRO soybean model for Iowa-grown soybean, and where appropriate to modify the model in order to improve its accuracy. Field samples of ‘Kenwood’ soybean, taken during reproductive growth in four consecutive seasons (1992-1995), were analyzed for leaf, stem, pod wall, and seed N concentration. Soil and weather data from these trials were used to evaluate the ability of CROPGRO to predict N on both land-area and concentration bases for leaf, stem, pod wall, and seed tissues. The original N2-fixation routine was too sensitive to cool soil temperature, which caused it to underpredict N accumulation in cool seasons. The original model also predicted remobilization of N from leaf tissue to occur earlier than was observed in the field. This reduced predicted photosynthesis which led to underprediction in the rate of N assimilation late in the season. Several simple modifications were introduced to enhance accuracy of the model: (1) decrease the lower limit of the range of optimum soil temperatures for nodule growth (from 28 to 22°C), and nitrogenase activity (from 23 to 20°C); (2) decrease the base temperature of pod set (from 14 to 10°C), and increase the base temperature for emergence and early vegetative development (from 7 to 9°C); (3) delay of N remobilization from leaves and stems to seeds; (4) increase the maximum N concentration of various tissues to reflect observed values for ‘Kenwood’ soybean, and set initiation of N remobilization to occur earlier in pods than in other tissues.