Response ofEriophorum vaginatumto CO₂enrichment at different soil temperatures: effects on growth, root respiration and PO₄³⁻uptake kinetics

summary In a phytotron experiment, we examined responses of a tussock sedge, Eriophorum vaginatum L., to changes in atmospheric CO 2 , concentration and soil temperature. We were particularly interested in phosphorus (P) acquisition and below ground plant characteristics that regulated its uptake in response to CO 2 , enrichment. Plants were grown at two CO 2 , partial pressures, 35 and 70 Pa, three soil temperature regimes, 5, 15 and 25 °C and a constant ambient air temperature of 15 °C. Elevated CO 2 , increased total plant biomass production, but decreased tissue P concentration. Although high CO 2 , enhanced root carbohydrate concentration, it inhibited root respiration with no significant effect on root PO 4 3− absorption capacity or root:shoot ratio. Surprisingly, there were no significant interactions between CO 2 , and soil temperature. The inability of Eriophorum to exhibit root–level compensatory adjustments, e.g. increased root: shoot ratio or PO 4 3− absorption capacity, was largely responsible for the observed decline in tissue P concentration under elevated CO 2 , conditions. This could ultimately limit long–term growth responses of Eriophorum to CO 2 enrichment in the field where P availability is limiting. We found that uptake of PO 4 3− in response to elevated CO 2 was independent of changes in root respiration, but changes in root respiration could have important implications for ecosystem carbon budget under elevated CO 2 levels. Our data indicated that although root respiration on a per unit biomass basis declined in response to CO 2 enrichment, this effect was counterbalanced by increased root biomass, so that high CO 2 stimulated root respiration on a whole‐plant basis by 30%. This might help to explain why long‐term exposure to high CO 2 increases CO 2 efflux from Eriophorum‐dominated ecosystems.