Phosphorus availability and elevated CO₂ affect biological nitrogen fixation and nutrient fluxes in a clover-dominated sward

• The response of biological nitrogen fixation (BNF) to elevated CO₂ was examined in white clover (Trifolium repens)-dominated swards under both high and low phosphorus availability. • Mixed swards of clover and buffalo grass (Stenotaphrum secundatum) were grown for 15 months in 0.2 m² sand-filled mesocosms under two CO₂ treatments (ambient and twice ambient) and three nutrient treatments [no N, and either low or high P (5 or 134 kg P ha^-1); the third nutrient treatment was supplied with high P and N (240 kg N ha^-1)]. • Under ambient CO ₂, high P increased BNF from 410 to 900 kg ha^-1. Elevated CO₂ further increased BNF to 1180 kg ha^-1 with high P, but there was no effect of CO₂ on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO₂ irrespective of supplied P. • The results suggest that where soil P availability is low, elevated CO₂ will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.