Manipulating cover crop growth by adjusting sowing time and cereal inter-row spacing to enhance residual nitrogen effects

Legume-based cover crop (CC) mixtures can increase nitrogen (N) availability in arable systems, reducing the need for external N inputs, as they retain soil N and fix atmospheric N-2. However, they need sufficient biomass accumulation to influence soil N availability. Early establishment through undersowing can improve CC growth and plant N accumulation, but competition with the main crop should be minimized. This study aimed to investigate the effect of main crop inter-row spacing (12, 18, 24 cm), manure application (without, with) and CC undersowing (early, late, no cover crop) on N accumulation in a legume-based CC mixture and the residual N effect on the following crop. We conducted a field experiment in Denmark with spring wheat as main crop and spring oat as following crop during two pairs of consecutive seasons (2015-2016, 2016-2017). Spring wheat yield was unaffected by inter-row spacing, but it was increased by manure application. In manure treatments, larger inter-row spacing and early CC undersowing increased CC N accumulation by up to 260% (30 kg N ha(-1)), due to increased light availability. Treatments without manure and with early undersowing gave the highest CC N accumulation, resulting in N fertilizer replacement values of 13-50 kg N ha(-1). Thus, increasing main crop inter-row spacing and early undersowing can improve CC growth and N accumulation, and well established legume-based CC could help to stabilize crop yield over time, as suggested by the similar cumulative grain yields (spring wheat + oat) without and with manure.