MODELING NITROGEN TRANSPORT AND TRANSFORMATIONS IN SOILS

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Abstract
A N model previously described was evaluated, using experimental data from a greenhouse lysimeter study for 2 soils, Windsor sandy loam and Charlton silt loam. Secondary treated waste water was applied to each soil at the rate of 3.8 cm twice weekly for 25 wk. 15N-enriched NH4+-N was applied at the beginning of the experiment in 1 waste water application. A mixture of grasses was grown on each lysimeter and was harvested every 2-4 wk. Solution samples were collected and analyzed for N and the soil-water pressure head was monitored frequently at different soil depths. Model predictions agreed well with pressure head data with depth and time and gravimetrically determined soil-water content with depth for the 2 soils. Model predictions of NO3--N concentration vs. time were adequate except for intermediate soil depths (30 and 45 cm). NH4+-N predictions were consistently higher than measured data. Reasonable agreement was obtained between calculated and experimental N uptake results, but after 60 days the model overestimated the N uptake by as much as 15%. Results from the 15N data were utilized to provide estimates for the nitrification rate coefficients. The rate coefficients obtained were within 2 and 4 times the values used to obtain best model predictions for Charlton and Windsor soil, respectively.

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