Fresh grain legume systems with catch crops: promising soil C inputs and enhanced N fertility

Grain legumes (GLs) are essential for sustainable agriculture, offering nutritional, agronomic, and ecological benefits. GLs are predominantly grown to full maturity for animal feed in Denmark and across Europe. However, the EAT–Lancet Commission on healthy diets has underscored the necessity of a global dietary shift towards greater direct human consumption of plant protein to improve human health and promote planetary performance. The cultivation of GLs remains limited in Europe, challenged by facts like climatic constraints and nitrogen (N) leaching—driven by mineralization of their N-rich residues. Integrating early-harvested fresh GLs, grown for human consumption, with catch crops presents a promising strategy to overcome these challenges. Compared to late-harvest mature grain legumes for animal feed, such systems potentially reduce disease incidence, improve catch crop establishment and thus reduce N leaching, enhance N fertility for subsequent crops, and increase ecosystem services, such as carbon (C) inputs for soil C sequestration from both main crops and catch crops. However, empirical studies synthesizing these benefits of such systems in comparison to conventional cereals are missing at the crop rotation level. Based on a two-year crop rotation of fresh GLs (faba bean, pea, and the mixture of pea and barley), catch crops and the subsequent cereal crop (barley) in 2022-2023 in Denmark, we evaluated the productivity, N dynamics, and C inputs of GLs systems relative to a cereal crop. The results showed that GLs produced lower aboveground dry matter (DM) compared to the cereal (7-8 vs. 10 Mg DM ha^-1), but higher N yield (175-198 vs. 80 kg N ha^-1). Among GLs, faba bean fixed the most atmospheric N (166 kg N ha^-1) and left the highest residual soil N, which was effectively reduced by catch crops. Subsequent barley yields were, on average, higher following faba bean (4.7-5.3 Mg DM ha^-1) compared to the cereal reference (4.0-4.7 Mg DM ha^-1). Total C inputs to 1-m soil depth (main crops, from root biomass and plant deposition; catch crops, from shoot, root biomass and plant deposition) was 4.1-4.4 Mg C ha^-1 among GLs, which was comparable to the cereal reference (5.2 Mg C ha^-1). However, GLs based systems received no N fertilizer as opposed to the cereal reference, which was fertilized with 100 kg N ha^-1 (in slurry). This reduces the reliance on external inputs, and might minimize negative environmental impacts including greenhouse gases emissions, which needs future studies. Overall, our findings highlight the ecological and agronomic potential of fresh GLs systems with catch crops for sustainable agricultural production.