Complementary cover crop mixtures as a management strategy to increase soil organic matter inputs in tropical agroecosystems

Cover crop diversification is increasingly used as a management strategy to enhance soil organic matter (SOM) inputs in tropical agroecosystems; however, the mechanisms linking species complementarity to dry mass (DM) production, carbon (C) inputs, and residue-mediated nutrient cycling remain poorly understood. We evaluated monocultures and functionally complementary mixtures of a tropical grass (Urochloa ruziziensis), a brassica (Raphanus sativus), and a legume (Crotalaria spectabilis) under crop rotation and maize intercropping systems in Brazil. In crop rotation, cover crop mixtures produced DM and C inputs comparable to highly productive U. ruziziensis monocultures (7.5 Mg DM ha⁻¹ and 3.15 Mg C ha⁻¹), reflecting positive complementarity driven by synergistic interactions and reduced interspecific competition. In maize intercropping, the same pattern was observed, although DM and C inputs were 45% lower due to maize shading. Within mixtures, U. ruziziensis accounted for approximately two-thirds of total DM and C, while brassica and legume species increased nitrogen, calcium, and sulfur accumulation and reduced residue C/N ratio and lignin concentration by an average of 27% across both experiments. Consequently, mixtures combined persistent, lignified grass residues with more labile inputs from legumes and brassicas. This functional diversity in residue quality promotes asynchronous decomposition and nutrient release, supporting sustained nutrient cycling while maintaining soil cover and physical protection. Overall, integrating high biomass production with diversified residue quality highlights complementary cover crop mixtures as an effective strategy to increase SOM inputs and support multifunctionality in tropical conservation agriculture systems.