Nitrogen fertilization and nitrate leaching into groundwater on arable sandy soils

Nitrate leaching depending on N fertilization and different crop rotations was studied at two sites with sandy soils in N Germany between 1995 and 2000. The leaching of NO₃^- was calculated by using a numerical soil-water and N model and regularly measured N_min values as input data. Also the variability of N_min values on the sandy soils was determined along transects. They reveal the high variability of the N_min values and show that it is not possible to confirm a significant N_min difference between fertilizer treatments using the normal N_min-sampling intensity. Nitrate-leaching calculations of five leaching periods showed that even strongly reduced N-fertilizer applications did not result in a substantially lower NO₃^- leaching into the groundwater. Strong yield reductions of even more than 50%, however, were immediately measured. Mean NO₃^- concentrations in the groundwater recharge are >50 mg L^-1 and are mainly due to mineralization from soil organic matter. Obviously, the adjustment of the N cycle in the soil to a new equilibrium and a reduced NO₃^--leaching rate as a consequence of lower N inputs need a much longer time span. Catch crops are the most efficient way to reduce the NO₃^- concentrations in the groundwater recharge of sandy soils. Their success, however, strongly depends on the site-specific development possibilities of the catch crop. Even with all possible measures implemented, it will be almost impossible to reach NO₃^- concentrations <50 mg L^-1 in sandy soils. The only way to realize this goal on a regional scale could be by increasing areas with lower nitrate concentrations in the groundwater recharge like grassland and forests.