Grain legumes combined with cover crops secure positive residual N effects in organic food production systems

The rising global demand for high-quality, plant-based food highlights the potential of grain legumes as protein-rich alternatives that offer environmental, climate, and health benefits. Local cultivation of grain legumes can enhance agricultural sustainability in Europe by diversifying cropping systems and reducing reliance on synthetic nitrogen (N) fertilizers through biological N fixation. This is particularly beneficial in organic systems. However, grain legume cultivation remains limited in Northern Europe due to the short growing season, which can hinder mature harvests. Fresh harvesting offers a solution by extending the growing period for cover crops and improving N recycling. This three-year field study, conducted under organic standards in Denmark, assessed the impacts of fresh versus mature harvesting of grain legumes (pea, faba bean, soybean) on subsequent crop productivity and soil mineral N dynamics, with and without undersown ryegrass cover crops. Grain dry matter yields for mature faba bean and pea were 5.1 and 2.0 Mg ha⁻¹ in 2021, and 4.1 and 4.0 Mg ha⁻¹ in 2022, respectively. No significant differences were observed between fresh and mature harvests regarding cover crop biomass and N yield, attributed to wide inter-row spacing that promoted cover crop growth. Cover crops effectively retained soil mineral N during winter, resulting in nearly twice the residual N availability for spring barley compared to systems without cover crops, without indications of pre-emptive competition. Residual N contributions from the three grain legume species were comparable, reaching up to 60 kg N ha⁻¹ when combined with cover crops. These findings underscore the critical role of cover crops and grain legumes in designing sustainable, low-input, plant-based food production systems.