Phosphorus status determines biomass response to elevated CO₂ in a legume: C₄ grass community

Thirty-six mesocosms, each containing a two-species community of Trifolium repens (C₃ legume) and Stenotaphrum secundatum (C₄ grass), were grown in sand with three nutrient regimes, zero N low P, zero N high P and supplied N high P, under ambient (aCO₂) and twice ambient CO₂ (eCO₂) for 15 months in two greenhouses. Aboveground annual production in the P limited mesocosms did not respond to eCO₂ and was reduced by 50% relative to mesocosms with an adequate P supply, where dry-matter production was increased by 12-24% under eCO₂. The stimulation of production by eCO₂ occurred throughout the year despite a clear seasonality in growth. There was no effect of eCO₂ on leaf area index (LAI), which was larger under high P than low P. Live root mass at the end of the experiment was higher under eCO₂ in all nutrient treatments, but the response of total belowground C (root + soil) to eCO₂ depended on P treatment. Under limiting P, belowground C was not significantly changed by eCO₂ (2-2.3 t belowground C ha^-1). Under high P supply, both root and soil C pools increased under eCO₂. Under aCO₂, low P supply increased belowground C by 0.7-1 t C ha_-1 above that added by the high P treatment. P is commonly limiting in Australian ecosystems and the majority of ecosystem N input is provided by biological N fixation. Consequently, the response of legumes to eCO₂ is of particular importance. These results demonstrate that at low P availability, there is likely to be only a limited response of biomass production by T. repens to eCO₂, which in turn may constrain any ecosystem response.