SIMULATION BY NCSWAP OF SEASONAL NITROGEN DYNAMICS IN CORN .1. SOIL NITRATE

Computer simulation models of crop-soil systems offer the potential to increase understanding of soil N cycle processes, thereby improving m anagement of N resources in agricultural systems. NCSWAP (Nitrogen, Ca rbon, Soil, Water, And Plant) is a comprehensive, deterministic comput er model of the plant-soil system that simulates seasonal soil C and N cycles under the control of temperature, moisture, microbial activity , and crop growth. The objective of this study was to validate NCSWAP using data collected during a 3-yr N-rate study in central Pennsylvani a that investigated seasonal N dynamics in corn (Zea mays L.) provided with N as liquid dairy manure or as NH4NO3. Seasonal soil NO3 concent ration in the upper soil layer, seasonal aboveground N accumulation by corn, and water leached past 1.2 m during the second year of the stud y were used to calibrate input values controlling soil water flow and NO3 production from mineralization of soil organic N sources. The vali dation of NCSWAP identified several limitations in the water flow and C and N cycling submodels as well as in the potential of the model to simulate seasonal N dynamics in corn. Validation simulations were abou t as accurate as calibration simulations, reflecting the ability of th e model to simulate C and N dynamics without recalibration from year t o year. Much of the simulation error was related to an overestimation of NO3 leaching caused by the inability of the model's microporous flo w submodel to simulate the macropore-influenced water flow in the well -structured soil used in the validation.