Abstract
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 CO 2 (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.