Strengthened resource limitation driven by accelerated microbial growth dampens response to elevated CO₂ in a mature forest

Anthropogenic elevated atmospheric carbon dioxide (eCO₂) increases carbon (C) availability to plants. Whether ecosystems store or release this additional C depends on the balance between net primary production (NPP) and soil respiration, where eCO₂-stimulated NPP can be constrained by the provisioning of nutrients by microbial decomposers. Ecosystems gradually become more phosphorus (P)-limited as weathering of primary minerals and occlusion of available phosphate increase with age, making plant-microbial competition for P a potential key driver of the fate of C in mature ecosystems under eCO₂. In a P-limited mature forest (EucFACE, Australia), we found that soil microbial growth was primarily limited by C, but that P was the secondary limiting resource. This suggests that increased plant-derived C inputs under eCO₂ could relieve microbial C limitation and intensify microbial demand for P. Surprisingly, a decade of eCO₂ instead exacerbated microbial C limitation. We posit that a fast growing copiotrophic decomposer community under eCO₂ required more C. This shift also resulted in exacerbated microbial P limitation under eCO₂. These together primed microbial use of fresh litter inputs, truncating the ecosystem P cycle, and explaining the observed faster turnover of organic matter and more intense competition for P between microbes and plants under eCO₂.