SIGNIFICANCE OF NITRATE LEACHING AND LONG-TERM N IMMOBILIZATION AFTERDEEPENING THE PLOW LAYERS FOR THE N REGIME OF ARABLE SOILS IN NW GERMANY

Field studies were conducted to assess the turnover and the leaching o f nitrogen in arable soils of Lower Saxony (NW Germany). The mean surp lus N (difference between N inputs by fertilization and N export by th e yield; 146 field plots) from 1985-1988 amounted to 38 kg ha(-1) yr(- 1) in fine textured (clay, loam, silt) and to 98 kg ha(-1) yr(-1) in c oarse (sandy) soils. Leaching of nitrate calculated by a simple functi onal model for simulation of the N regime over the winter period (i.e. mineralization and leaching) was 16 kg ha(-1) in the fine and 63 kg N ha(-1) in coarse soils (mean values of the winter periods 1985-1988 f rom 256 plots). Before the 1960s, the depth of the A(p) horizons rarel y exceeded 25 cm in arable soils of the former FRG. During the last th ree decades, ploughing depth has increased to at least 35 cm. The mass balance calculations for total N after ploughing to 35 cm in loess so ils of southern Lower Saxony (105 farm plots) yielded a mean increase in total N by about 900 kg ha(-1) in 20 years. With respect to soil or ganic matter equilibria, N accumulation will continue for at least ano ther 10 years on 67% of the examined farm plots. This study suggests t hat long term N immobilization is one of the most important sinks for nitrogen in arable soils of Germany. For simulation of the N dynamics over the growing season and for long time periods total nitrogen dynam ics need to be considered.