Significance of nitrate leaching and long term N immobilization after deepening the plough layers for the N regime of arable soils in N.W. Germany

Field studies were conducted to assess the turnover and the leaching of nitrogen in arable soils of Lower Saxony (NW Germany). The mean surplus N (difference between N inputs by fertilization and N export by the 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 coarse (sandy) soils. Leaching of nitrate calculated by a simple functional 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 from 256 plots).Before the 1960s, the depth of the A_p horizons rarely exceeded 25 cm in arable soils of the former FRG. During the last three decades, ploughing depth has increased to at least 35 cm. The mass balance calculations for total N after ploughing to 35 cm in loess soils 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 organic matter equilibria, N accumulation will continue for at least another 10 years on 67% of the examined farm plots. This study suggests that 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 dynamics need to be considered.