Response of Eriophorum vaginatum to CO₂ enrichment at different soil temperatures: Effects on growth, root respiration and PO₄^3- uptake kinetics

In a phytotron experiment, we examined responses of a tussock sedge, Eriophorum vaginatum L., to changes in atmospheric CO₂ concentration and soil temperature. We were particularly interested in phosphorus (P) acquisition and below ground plant characteristics that regulated its uptake in response to CO₂ enrichment. Plants were grown at two CO₂ 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 CO₂ increased total plant biomass production, but decreased tissue P concentration. Although high CO₂ enhanced root carbohydrate concentration, it inhibited root respiration with no significant effect on root PO₄^3- absorption capacity or root: shoot ratio. Surprisingly, there were no significant interactions between CO₂ and soil temperature. The inability of Eriophorum to exhibit root-level compensatory adjustments, e.g. increased root:shoot ratio or PO₄^3- absorption capacity, was largely responsible for the observed decline in tissue P concentration under elevated CO₂ conditions. This could ultimately limit long-term growth responses of Eriophorum to CO₂ enrichment in the field where P availability is limiting. We found that uptake of PO₄^3- in response to elevated CO₂ was independent of changes in root respiration, but changes in root respiration could have important implications for ecosystem carbon budget under elevated CO₂ levels. Our data indicated that although root respiration on a per unit biomass basis declined in response to CO₂ enrichment, this effect was counterbalanced by increased root biomass, so that high CO₂ stimulated root respiration on a whole-plant basis by 30%. This might help to explain why long-term exposure to high CO₂ increases CO₂ efflux from Eriophorum-dominated ecosystems.