Modeling nitrate leaching during the winter halfyear from sandy arable soils under intensive cultivation

Three years (1989-91) of post harvest and winter nitrogen dynamics (August to March) were simulated in 20 arable sandy soils to quantify nitrate leaching during winter time. Easily accessible soil, weather and management data were used for a simple but deterministic model. The calculated mineral N (N-min) content and distribution in the soil (0-90 cm) were compared to more than 100 measurements from September to March each season. An overall agreement of approximately 50% between measured and simulated N-min values was obtained. The simulation over- or underestimated the measure N-min depending on the rainfall and temperature distribution which varied from year to year. Practically, the effect of fertilizer application was largely (60%) responsible for deviations greater than +/-20 kgN ha(-1) from the 1:1-line. Ignoring these instances, 80% of the simulated N min contents were within these ''confidence limits'' of +/-20 kgN ha(-1). Considering the nitrogen distribution in the profile, the N-min content is underestimated in the top soil, but overestimated in the subsoil. Based on the 95% confidence intervals (measured versus simulated) the estimate was better for the lower (30-90 cm) than for the upper part of the profile (0-30 cm). It is concluded that winter leaching can be reduced from 130 kgN ha(-1) (corn, winter grain) to about 10 kgN ha(-1) growing winter hard forage crops. Two major processes were identified as reasons for the disagreement and are proposed for further model improvement: (1) The simulation underestimates the short term transport velocity on the basis of field capacity derived from survey data, (2) Nitrogen is mineralized quickly in sandy soils, especially after catch crops, and sometimes due to freeze-thaw effects. Furthermore, as ammonium remains in the surface, nitrification needs to be explicitly simulated.