Soil mineral N and N net mineralization during autumn and winter under an oilseed rape – winter wheat – winter barley rotation in different crop management systems

Abstract

Soil sampling in autumn gives important information on the soil N dynamic. In the growing seasons 1991/92 to 1995/96, the effects of different crop management systems on soil mineral N (NO₃-N plus NH₄-N:N_min) were investigated in a factorial field experiment at Hohenschulen Experimental Station near Kiel in NW Germany. The crop rotation was oilseed rape – winter wheat – winter barley, and soil tillage (conservation tillage without ploughing, conventional tillage), application of pig slurry (none, autumn, autumn+spring), mineral N fertilization (0, 120 and 240 kg N ha⁻¹) and fungicide application (none, three applications) were all varied. Each year, the treatments occurred in all three crops of the rotation and were located on the same plots. N_min was determined on four dates (‘After drilling’, ‘End of autumn growth’ before winter, ‘Beginning of spring growth’ before N fertilizer application, and ‘After harvest’) to 90 cm in 30 cm horizons.

Under all crops, N_min showed a large year to year variation. Highest values of 132 kg N ha⁻¹ were observed ‘After drilling’, which decreased until ‘End of growth’. The increase of autumn N_min (‘After drilling’, ‘End of autumn growth’) was mainly due to autumn slurry, whereas mineral N fertilizer mainly affected N_min ‘After harvest’. Soil tillage and fungicide application only slightly modified N_min at all dates.

The relationship between N leaching and N_min measured either ‘After drilling’ or at the ‘End of autumn growth’ in 1991/92–1994/95 remained too poor to be used to estimate N leaching. N net mineralization during autumn and winter varied with crops, as estimated by the N_min changes between ‘After drilling’ minus ‘Start of spring growth’ plus N uptake by the crop at ‘Start of spring growth’ plus N leaching during winter. On average over the years, 39 kg N ha⁻¹ were mineralized under oilseed rape and 42 kg N ha⁻¹ under wheat compared with 31 kg N ha⁻¹ under barley. However, a large year-to-year variation occurred. In addition, the ranking of the years differed with the crops. Slurry application led to different amounts of mineralized N. Under barley only 25 kg N ha⁻¹ were calculated for the autumn slurry, but 42 kg N ha⁻¹ for the autumn plus spring slurry treatment. In contrast, under oilseed rape the highest value of 41 kg N ha⁻¹ occurred in the autumn slurry plots. Under wheat, slurry application only slightly affected N mineralization. Increased mineral N fertilization decreased N release under oilseed rape, but significantly increased it under cereals. Application of fungicides did not affect N mineralization during winter.