USING NCSWAP TO SIMULATE SEASONAL NITROGEN DYNAMICS IN SOIL AND CORN

The objective of this study was to determine if a re-calibrated versio n of the computer model NCSWAP (version 36) could accurately predict c orn growth and soil N dynamics in conventionally tilled (CT) and no-ti ll (NT) corn supplied with legume green manure or ammonium nitrate as N sources. We also attempted to ascertain the reasons for limitations in the model's ability to simulate corn growth and soil N dynamics fou nd by our colleagues in a previous study and to propose potential impr ovements. The model was calibrated to accurately simulate total availa ble N (N in plant above-ground biomass plus soil nitrate in the 0 to 4 5 cm profile) for a control and a fertilizer CT treatment in the 1992 growing season. To do so, input values defining the quantities of acti ve soil organic N had to be reduced to 19% of the values proposed by t he model developers and a solute transport factor defining the mobile vs. immobile fractions of soil nitrate adjusted from 0.8 to 0.2. The d iscrepancies between the proposed values and the lower values employed in this study might be due to the uncertainties in quantitatively des cribing soil N mineralization processes and the way they are handled i n the model, as well as the lack of a component simulating macroporous -influenced water flow and solute transport in the model. With the cur rent version, until one knows how to predict what these values are, th e model needs to be re-calibrated for each experimental site and condi tion and thus is of limited value as a general model. With no further adjustment of input values, model validation success was mixed. The mo del accurately predicted total available N for treatments in the secon d year of the experiment that had the same N source and tillage as the treatments used for the calibration year but with the different weath er and growing conditions. However, total available N was underpredict ed where legume green manure was the N source and overpredicted with n o-till cultivation. The model was accurate in simulating seasonal corn growth for nearly all the treatments, judged by nonsignificant mean d ifference (MD) values and highly significant correlation coefficients (r). Prediction of seasonal soil nitrate concentration was less accura te compared to total available N and corn growth variables. Potential improvements in the model's simulation of a no-till system as well as for predicting corn harvest yield and seasonal soil nitrate concentrat ion where N deficiency occurs were discussed.