N fate and transport under variable cropping history and fertilizer rate on loamy sand and clay loam soils: I. Calibration of the LEACHMN model

The need for efficient use of agricultural chemicals and their potential ad verse impact on critical water resources have increased the use of simulati on models of the soil and plant system. Nevertheless, there is currently li ttle or no agreement concerning model validity and applicability in varied soils and environments. The research version of LEACHMN (the N subroutine o f LEACHM) was calibrated using field data including soil physical, hydrauli c, and chemical properties, and maize (Zea mays L.) N uptake collected from a 3-yr nitrate leaching experiment. The field site consisted of plot-size lysimeters on clay loam and loamy sand soils with N fertilizer rates of 22, 100 and 134 kg N ha(-1). The calibration involved adjusting nitrification, denitrification, and volatilization rate constants to optimize the fit bet ween predicted and measured data. When calibrated for each treatment-year c ombination and soil type, the model simulations of soil profile NO3-N distr ibution were generally successful. The N transformation rate constants yiel ded by the calibration efforts were similar or close to those used in other model simulation studies. At both sites, the calibrated rate constants for the first year (following sod plowdown) were different from those for the subsequent two years. Denitrification rate constants were consistently high er for the clay site than for the sand site, while the nitrification rate c onstants were lower. N rate of application appeared not to affect the rate constants within each year-site combination, suggesting that cropping histo ry and soil type had the greatest effect on N transformation rates.