MODELING NITRATE LEACHING DURING THE WINT ER HALFYEAR FROM SANDY ARABLE SOILS UNDER INTENSIVE CULTIVATION

Three years (1989-91) of post harvest and winter nitrogen dynamics (Au gust to March) were simulated in 20 arable sandy soils to quantify nit rate leaching during winter time. Easily accessible soil, weather and management data were used for a simple but deterministic model. The ca lculated mineral N (N-min) content and distribution in the soil (0-90 cm) were compared to more than 100 measurements from September to Marc h each season. An overall agreement of approximately 50% between measu red and simulated N-min values was obtained. The simulation over- or u nderestimated the measure N-min depending on the rainfall and temperat ure distribution which varied from year to year. Practically, the effe ct of fertilizer application was largely (60%) responsible for deviati ons greater than +/-20 kgN ha(-1) from the 1:1-line. Ignoring these in stances, 80% of the simulated N min contents were within these ''confi dence limits'' of +/-20 kgN ha(-1). Considering the nitrogen distribut ion in the profile, the N-min content is underestimated in the top soi l, but overestimated in the subsoil. Based on the 95% confidence inter vals (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 co ncluded 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 und erestimates the short term transport velocity on the basis of field ca pacity derived from survey data, (2) Nitrogen is mineralized quickly i n sandy soils, especially after catch crops, and sometimes due to free ze-thaw effects. Furthermore, as ammonium remains in the surface, nitr ification needs to be explicitly simulated.