Grain legumes combined with cover crops secure positive residual N effects in food pro-duction systems

Summary in the original language of the document

The global demand for high-quality plant-based food options is increasing, and fresh grain legumes are a great protein-rich alternative that offers environmental, climate, and health benefits. Local production of grain legumes can improve agricultural sustainability in Europe by diversifying crops and reducing the dependence on nitrogen (N) fertilizers due to their capacity to fix atmospheric N, which is particularly relevant for organic crop-ping systems. However, their cultivation remains low especially in Northern Europe. This study aimed to evaluate the impact of harvesting fresh versus mature grain legumes on subsequent crop productivity and soil mineral N in Denmark. The three-year field study was conducted according to organic standards and featured three grain legumes (pea, faba, soybean) harvested both fresh and mature, with and without an undersown ryegrass cover crop. The grain dry matter yields of 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. These yields were generally consistent with organic cultivation in the EU. The edamame soybean variety had a strong biomass production but was challenged in reaching maturity – especially in the first year where wildlife interference forced a late resowing. For both pea and faba bean, there was no impact of harvesting grain legumes fresh or mature on cover crop biomass and N yield, contrary to expectations of fresh harvest benefitting cover crop growth. Wider inter-row spacing allowed sufficient cover crop establishment despite mature legumes. Resowing of soybeans in the first year delayed cover crop growth, while in the second year, mature soybeans unexpectedly promoted higher biomass. The non-legume cover crops retained soil mineral N during winter and resulted in almost twice as high residual N for the following spring barley showing no signs of pre-emptive competition. The residual N effects of the three grain legumes were similar, reaching up to 60 kg N ha-1 when com-bined with cover crops. This will significantly influence the design of sustainable, low-input plant-based food production systems.