EFFECT OF SLURRY APPLICATION AND MINERAL NITROGEN-FERTILIZATION ON N LEACHING IN DIFFERENT CROP COMBINATIONS

Nitrogen (N) fertilizer not used by the crop can increase the risk of nitrate leaching into the groundwater. In two growing seasons, 1990/91 and 1991/92, the relationships between N fertilization and yield, N u ptake by the grain and the N leaching in the subsequent percolation pe riod were investigated in a multifactorial field experiment at Hohensc hulen Experimental Station near Kiel in NW Germany. The crop rotation was oilseed rape - winter wheat - winter barley, and effects of soil t illage (minimum tillage without ploughing, conventional tillage), appl ication of pig slurry (none, application in autumn, application in aut umn and in spring), mineral N fertilization (none, 80 or 200 kg N ha(- 1) to oilseed rape and 120 or 240 kg N ha(-1) to cereals) and applicat ion of fungicides (none, intensive) were all tested. In each year, the rotation and the treatments were located on the same plots. Mineral N fertilization and fungicide application increased yield and N uptake by grain or seed in all crops. In contrast, the application of slurry, especially in autumn, had only small effects on yield and N uptake. N itrogen losses by leaching (measured using porous ceramic cups) were a ffected mainly by the year and the crop. In 1992/93, averaged over all factors, 80 kg N ha(-1) was leached compared with 28 kg N ha(-1) the previous year. Oilseed rape reduced N losses, whereas under winter whe at up to 160 kg N ha(-1) was leached. Due to a lower N-use efficiency, autumn applications of slurry increased N leaching, and mineral N fer tilization of the preceding crop also led to higher N losses. Since th e amount of leached N depends both on the nitrogen left by the precedi ng crop (unused fertilizer N as well as N in residues) and on N uptake by the subsequent crop, it is not possible to apportion the N losses to any particular crop in the rotation. The cropping sequence, togethe r with its previous and subsequent crops, must also be considered. To minimize leaching, N fertilization must meet the needs of the growing crop. In order to improve the efficiency further, investigations must be conducted in order to understand the dynamics of N in the plant-soi l system in conjunction with the weather and crop management practices .