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 (C-3 legume) and Stenotaphrum secundatum (C-4 grass), were grown in sand with three nutrient regimes, zero N low P, zero N high P and supplied N high P, under ambient (aCO(2)) and twice ambient CO2 (eCO(2)) for 15 months in two greenhouses. Aboveground annual production in the P limited mesocosms did not respond to eCO(2) and was reduced by 50% relative to mesocosms with an adequate P supply, where dry-matter production was increased by 12-24% under eCO(2). The stimulation of production by eCO(2) occurred throughout the year despite a clear seasonality in growth. There was no effect of eCO(2) 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 eCO(2) in all nutrient treatments, but the response of total belowground C (root+soil) to eCO(2) depended on P treatment. Under limiting P, belowground C was not significantly changed by eCO(2) (2-2.3 t belowground C ha(-1)). Under high P supply, both root and soil C pools increased under eCO(2). Under aCO(2), 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 eCO(2) 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 eCO(2), which in turn may constrain any ecosystem response.