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 management of N resources in agricultural systems. NCSWAP (Nitrogen, Carbon, Soil, Water, And Plant) is a comprehensive, deterministic computer 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 Pennsylvania that investigated seasonal N dynamics in corn (Zea mays L.) provided with N as liquid dairy manure or as NH4NO3. Seasonal soil NO3 concentration in the upper soil layer, seasonal aboveground N accumulation by corn, and water leached past 1.2 m during the second year of the study were used to calibrate input values controlling soil water flow and NO3 production from mineralization of soil organic N sources. The validation 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 about as accurate as calibration simulations, reflecting the ability of the model to simulate C and N dynamics without recalibration from year to year. Much of the simulation error was related to an overestimation of NO3 leaching caused by the inability of the model's microporous flow submodel to simulate the macropore-influenced water flow in the well-structured soil used in the validation.