Soil extracellular enzymes drive soil carbon accumulation under elevated CO₂

Human-driven increases in atmospheric CO₂ (eCO₂) are stimulating plant growth, thereby increasing the input of plant-derived carbon into soils. The fate of this additional carbon depends on the capacity of soil microbiomes to decompose and transform organic matter, a central process in regulating soil organic carbon (SOC) dynamics. However, how eCO₂ affects this microbial capacity remains poorly understood. Because soil extracellular enzymes catalyse the degradation of various SOC pools, their activities (extracellular enzyme activities, EEAs) could offer mechanistic insights into microbially mediated SOC dynamics. We synthesized 272 observations on SOC and EEAs from eCO₂ experiments across farmland, forest, grassland and shrubland, combining classical meta-analysis with random forest modelling. Our results showed that eCO₂ significantly increased SOC by 4.2%. Among all variables tested, increased cellulase activity, which targets the breakdown of labile carbon sources, emerged as the strongest predictor of SOC accumulation. Specifically, eCO₂ stimulated cellulase activity by 12.2% but had no effect on ligninase activity, which decomposes recalcitrant carbon. This enzymatic shift was likely driven by increased plant-derived labile carbon inputs under eCO₂ and was associated with changes in the soil microbiome, including a higher fungi-to-bacteria ratio. These results underscore the potential of EEA as a predictive indicator of SOC accumulation under eCO₂ and the importance of representing enzymatic processes in Earth system models. Read the free Plain Language Summary for this article on the Journal blog.