Different utilisation of residual N from oil radish cover crop by maize or sugar beet, and subsequent winter wheat

Cover crops (CC) may take up substantial amounts of nitrogen and thereby reduce the soil nitrate pool in autumn and thus, the risk of nitrogen (N) leaching losses during winter. The dynamics of the remineralisation of this organic N into plant-available N pools affects the fertiliser N needs of subsequent crops and their economic optimal N rate (EONR). In a series of eight field trials (two years x four sites) across Germany, the effects of oil radish (OR) as CC on above-ground dry matter (DM) accumulation and N uptake of silage maize (SM) and sugar beets (SB) as first subsequent crops, and winter wheat (WW) as second subsequent crop were investigated in combination with four fertiliser N levels. Across all site-years, the N amount in pre-winter OR biomass clearly enhanced SM growth, while SB showed no or, in the unfertilized control, a negative response, indicating the occurrence of pre-emptive competition. The same pattern was observed for parameters derived from N response curves (DM yield and N uptake) without N fertilisation and at EONR. Net soil N mineralisation was stronger affected by soil type than by the first succeeding main crop with negative correlation of OR N uptake on sand and positive correlation on loess soils. In the second year, OR biomass N showed small effects on WW yield and N uptake without N fertilisation (positive after SM, negative after SB), but optimal N supply superimposed all OR effects. Residual N from OR did not significantly reduce EONR either in the first or the second succeeding main crop. For an economically beneficial effect in SM, OR N uptake must be at least 71 kg N ha⁻¹, while in SB typical N uptake from CC would not reach the break-even point.