Cover crop biomass production as a predictor of nitrogen fertilizer replacement value - legumes secure positive effects

Cover crops can contribute to climate change mitigation by promoting soil organic carbon sequestration, retaining nitrogen (N) in soil and lower greenhouse gas emissions from reduced fertilization of following crops. Their N fertilizer replacement value (NFRV) varies based on cover crop type and management. This study aimed at identifying factors affecting cover crop NFRV as a means to guide N fertilizers reduction. Ryegrass (Lolium perenne L.; RG), plantain (Plantago lanceolata L.; PL), red clover (Trifolium pratense L.; RC), and two mixtures (RG+PL and RG+PL+RC) were sown in May 2020 and 2021 in an organic field trial, and terminated by either rotovation+ploughing or ploughing. Spring barley (Hordeum vulgaris L.) was used as a test crop to quantify NFRV. Before sowing spring barley, cover crop plots were not fertilized and control plots received 0, 50 and 100 kg mineral N ha⁻¹. In both years, RG+PL+RC had the greatest aboveground biomass and biomass N yield in autumn (5.5 Mg ha⁻¹, 128 kg N ha⁻¹), while RG had the lowest (0.7 Mg ha⁻¹, 21 kg N ha⁻¹). Before termination in spring, differences among cover crop types were less pronounced with no effect of termination method on NFRV. However, the NFRV was significantly correlated to cover crop biomass (p < 0.001) in autumn (r = 0.83) and spring (r = 0.66). NFRV was as high as 100 kg N ha⁻¹ with RC and RG+PL+RC, while negative values were obtained with autumn biomass below 1 Mg ha⁻¹. We found a strong correlation between cover crop biomass and NFRV, which can guide N fertilization reduction if autumn cover crop biomass can be estimated. The threshold between positive and negative residual N effects at 1 Mg ha⁻¹ confirms the need for a minimum cover crop biomass for providing desired ecosystem services.