Grassland responses to elevated CO₂ determined by plant–microbe competition for phosphorus

Rising atmospheric CO₂ has stimulated plant productivity, with terrestrial ecosystems currently absorbing nearly one-third of anthropogenic CO₂ emissions. Increases in photosynthesis can subsequently lead to increased carbon (C) storage in plants and soil. However, there is growing evidence that nitrogen (N) availability constrains elevated CO₂ (eCO₂) responses, yet we know much less about the role of phosphorus (P) limitation on productivity under eCO₂. This is important because P-limited ecosystems are globally widespread, and the biogeochemical cycles of N and P differ fundamentally. In the Peak District National Park of northern England, we conducted a free-air CO₂ enrichment (FACE) experiment for three years on two contrasting P-limited grasslands under long-term nutrient manipulation. Here we show that competition between plants and microbes for P can determine plant productivity responses to eCO₂. In a limestone grassland, aboveground productivity increased (16%) and microbial biomass P remained unchanged, whereas in an acidic grassland, aboveground productivity and P uptake declined (11% and 20%, respectively), but P immobilization into microbial biomass increased (36%). Our results demonstrate that strong competition with microbes can cause plant P uptake to decline under eCO₂, with implications for the future productivity of P-limited ecosystems in response to climate change.