The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO₂ concentrations depends on soil nutrient availability^1,2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO₂ (refs. ^3–6), but uncertainty about ecosystem P cycling and its CO₂ response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change⁷. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO₂, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO₂ and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO₂ fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage.

Summary: Mesophyll conductance (g_m) is the diffusion of CO₂ from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C₃ species, g_m is influenced by diverse leaf structural and anatomical traits; however, little is known about traits affecting g_m in C₄ species. To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimate g_m and microscopy techniques to measure leaf structural and anatomical traits potentially related to g_m in 18 C₄ grasses. In this study, g_m scaled positively with photosynthesis and intrinsic water‐use efficiency (TE_i), but not with stomatal conductance. Also, g_m was not determined by a single trait but was positively correlated with adaxial stomatal densities (SD_ada), stomatal ratio (SR), mesophyll surface area exposed to IAS (S_mes) and leaf thickness. However, g_m was not related to abaxial stomatal densities (SD_aba) and mesophyll cell wall thickness (T_CW). Our study suggests that greater SD_ada and SR increased g_m by increasing S_mes and creating additional parallel pathways for CO₂ diffusion inside mesophyll cells. Thus, SD_ada, SR and S_mes are important determinants of C₄‐g_m and could be the target traits selected or modified for achieving greater g_m and TE_i in C₄ species.