Autumnal N fertilization of late sown oilseed rape after minimum tillage

摘要

Oilseed rape (OSR) is considered as nitrogen (N) inefficient crop. Due to its low N harvest index and to favourable soil conditions, large N amounts remained in the soil, which, however, cannot completely be taken up by a subsequent wheat crop and increases the risk of N leaching into the groundwater during the following winter. In the long-term, N leaching correlates positively with the N balance. In order to reduce the environmental impact, in Germany therefore N balance must not exceed a threshold of 60 kg N ha-1 on a 3 year average as from 2009-2011. To reduce energy costs, farmers increasingly pass on ploughing during seedbed preparation, but they additionally apply about 30-50 kg N ha-1 in autumn, often directly upon the stubble of the preceding crop to ensure crop N supply before winter. However, OSR poorly utilizes autumn N. In order to test the effects of N applied in autumn on crop growth and N uptake before and after winter and on seed yield, a field trial was performed in 2002/03 till 2004/05 at the Hohenschulen Experimental Station located near Kiel in NW Germany. OSR was established at the end of August, later than recommended, following wheat and using minimum tillage without ploughing Spring N fertilization was also varied (80/80, 120/80, 80/120 kg N ha-1), but had no significant effects. Apart from an unfertilized control (N1), 40 kg N ha-1 as calcium ammonium nitrate were applied after the first stubble cultivation (N2), directly after sowing (N3) or at the 2-4 leaf stage (mid of September) (N4). On average of the three years, N applied to the crop (either after sowing or at the 2-4 leaf stage)significantly increased above-ground dry matter and N uptake at both sampling dates. Seed yield significantly differed by 0.22 t ha-1 between the Nl and N4 treatments. Assuming a seed N concentration of 3%, only 17 % of the applied N was removed by the seeds from the soil-plant system. We conclude that autumn N, if at all, should be applied to the OSR crop and not before sowing.