Increasing root trait complementarity in species mixtures may be detrimental for soil carbon storage

Designing plant mixtures with potential to increase soil organic carbon (SOC) appears to be a powerful nature-based tool to restore some of the carbon lost in agroecosystems. However, we are uncertain about the best way to design such benign plant mixtures. Trait-based approaches are increasingly used to explain the relationship between plant diversity and ecosystem functions, offering a conceptual opportunity to address this knowledge gap. In this study, we combine a global meta-analysis of 407 paired SOC content observations with a root traits database from GRooT, to explore the optimum way for the design of plant mixtures to increase SOC. We found that specific root traits at the community level were important predictors of the response of SOC to plant mixtures. Species mixtures could increase SOC content when the overall plant community had low variation in root mycorrhizal colonization and root tissue density. The positive response of SOC content to species mixtures was linked to increases in soil microbial biomass carbon and root biomass. Additionally, the SOC enhancements by plant mixtures were often found in regions with high precipitation and low sand content. Our meta-analysis presents a framework based on plant traits to enhance SOC sequestration using plant mixtures, which will enable farmers to optimize plant mixtures towards soil carbon sequestration.