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The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO₂ concentrations depends on soil nutrient availability^,. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO₂ (refs. ^{, , –}), but uncertainty about ecosystem P cycling and its CO₂ response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change. 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 …