Nitrogen sustainability and soil carbon sequestration in fresh grain legume-based rotations: The vital role of the cover crop mixture

Building soil carbon (C) while retaining nitrogen (N) is central to sustainable agriculture. Integrating fresh grain legumes (GLs) with cover crops could achieve both, yet documentation of combined C and N benefits is lacking at the rotation scale. In particular, no study has quantified total C inputs, including root fragments and rhizodeposited C, from both main GLs and cover crops within a single crop sequence. We conducted a two-year rotation in Denmark, comparing fresh GLs (faba bean, pea, pea-barley mixture) with a cereal reference (barley), each followed by two cover crop types (pure ryegrass or a mixture of ryegrass, plantain and chicory) and a subsequent cereal. We tracked N recycling (biological N₂ fixation, soil N availability, residual N fertility) and quantified root fragments and net rhizodeposited C from both main and cover crops (1-m), using a ¹ ³CO 2 -labelling approach. Among GLs, faba bean fixed the most atmospheric N₂ and left substantial residual N, that cover crops captured and translated into higher subsequent cereal yields. GLs supplied less belowground C inputs than the barley reference (1 vs. 2 Mg C ha −1 ), yet the cover crop mixture after faba bean remarkably added up to fivefold C (5 Mg ha −1 ) that of the preceding main crops. Total C inputs from faba bean-cover crop matched those of the barley-cover crop reference (4–5 vs. 5–6 Mg C ha −1 ). As the first empirical study, we demonstrated fresh GLs-cover crops, particularly faba bean-cover crop mixture, enhanced C inputs and sustained N recycling. • First rotation-scale study on C and N benefits of grain legumes with cover crops. • Faba bean-cover crops improved residual soil N availability to subsequent cereals. • The cover crop mixture after faba bean added 5-fold C inputs (5 Mg ha −1 ) of faba bean. • Total C inputs of grain legumes–cover crops equaled cereal–cover crops references. • The cover crop mixture sustained C inputs and N recycling in grain legume rotations.