Species complementarity sustains productivity and multifunctionality in tropical cover crop mixtures

Cover crop (CC) mixtures can enhance multifunctionality in tropical agroecosystems, but the mechanisms underlying their performance remain poorly understood. We evaluated whether functional complementarity among Urochloa ruziziensis or Megathyrsus maximus cv. Mombaça combined with Raphanus sativus and Crotalaria spectabilis enhances biomass production, carbon (C) and nutrient accumulation relative to monocultures under tropical conditions. A mesocosm experiment was conducted in southeastern Brazil using an additive design with monocultures and mixtures at two relative sowing rates. Shoot and root biomass, C and nutrient accumulation, residue quality, plant height, and root biometry were evaluated. Biodiversity effects were assessed by partitioning the net biodiversity effect (NE) into complementarity (CE) and selection (SE) components and multifunctionality was inferred from the consistent co-occurrence of positive NE values across variables. Functionally diverse CC mixtures achieved total biomass, C inputs, and nutrient accumulation comparable to the most productive grass monocultures. Although grasses exhibited reduced individual biomass in mixtures, brassica and legume biomass remained stable. Mixtures showed consistently positive NE values across variables, driven primarily by positive CE and slightly negative SE. Belowground traits showed strong complementarity and mixture residues balanced contrasting C:N ratios and lignin contents, reflecting differences in residue quality. These results are consistent with complementarity among species leading to positive NE and CE driven by synergistic interactions and reduced competition. Therefore, strategically combining different functional CC groups provides an effective pathway to support sustainable agriculture by optimizing C sequestration, nutrient cycling, and multiple ecosystem services in tropical agroecosystems. • Functionally diverse CC mixtures sustained productivity through species complementarity • Mixtures matched grass monocultures in biomass, C and nutrient accumulation • Lower grass biomass in mixtures was offset by stable brassica and legume growth • Root traits showed the strongest complementarity effects across all mixtures • Mixtures balanced residue quality through integrating contrasting C:N ratio and lignin content