The complementarity hypothesis reversed: Root trait similarity in species mixtures promotes soil organic carbon in agroecosystems

Increasing species diversity in agroecosystems appears as a promising venue to restore or increase soil organic carbon (SOC). It has been hypothesized that this effect is largely driven by the greater variation of root systems in plant mixtures, which may promote complementarity. However, the magnitude of this synergistic effect and the root traits driving it are uncertain. The objective of this study is to determine which root trait composition optimizes plant mixture effects on SOC. To do so, we combined a global meta-analysis of 407 paired SOC content observations under mixed species vs. monocultures across grasslands and croplands, and root traits extracted from the GRooT database. The results show that high root mycorrhizal colonization and root tissue density for the species in the mixture have higher positive effects on SOC content. Our analysis also indicates that combining species with high similarity for these traits represents a preferable trait combination to increase SOC with plant mixtures, challenging the current paradigm around plant trait complementarity effects. We observed that the positive response of SOC content to species mixtures was tightly associated with increased root biomass and soil microbial biomass carbon, indicating an important contribution of belowground and microbial residuals to SOC. Additionally, SOC enhancements by plant species mixtures were more likely to be realized in regions with high precipitation, clay-rich soils, and when legumes are present. Our meta-analysis lays out a root-trait framework to enhance SOC with plant mixtures, which can serve as a guide for species and variety selection for field experiments and on-farm applications.