Higher soil organic carbon in diverse plant communities linked to sustained biomass production during short-term drought

Background and Aims Soil organic carbon (SOC) plays a central role in global carbon cycling and is a key component of nature-based climate solutions. However, climate change and biodiversity loss are interrelated threats that may jointly undermine SOC. While plant diversity is known to enhance ecosystem functioning, its potential to buffer SOC under climate extremes such as drought, remains poorly understood. This study investigates whether plants growing in mixtures can mitigate SOC losses under drought by maintaining biomass production and moderating soil microclimate conditions. Methods This study was conducted within a large-scale factorial grassland biodiversity and climate variability experiment (UU-BioCliVE) at Utrecht University, manipulating planted species richness (1, 4, 8, 12 species) and precipitation to impose drought. We used structural equation modeling (SEM) to understand how plant diversity alters and/or maintains SOC via aboveground biomass, soil temperature, and soil moisture under drought conditions. Results SOC was higher in soils under plant mixtures than under monocultures across both control and drought treatments. Under drought, SEM showed a positive association between aboveground biomass and SOC in mixtures, while soil microclimate variables were not related to SOC. Under control conditions, mixtures increased biomass and reduced soil temperature, but neither factor explained SOC. Conclusion Our findings show that plant diversity supports SOC retention under short-term drought by sustaining biomass production. This underscores the importance of conserving plant diversity to sustain carbon storage under future drought stress.