Increased C-gain by an endemic Australian pasture grass at elevated atmospheric CO2 concentration when supplied with non-labile inorganic phosphorus

Limited phosphorus (P) availability in Australia's highly weathered soils m ay constrain an increase in terrestrial net primary productivity (NPP) with the globally increasing atmospheric CO2 concentration. We examined whether an Australian temperate pasture grass (Danthonia richardsonii) grown in sa nd culture and supplied solely with virtually insoluble Al- and Fe-phosphat e was able to increase C-gain when exposed to elevated (731 mu mol mol(-1)) compared with ambient (379 mu mol mol(-1)) CO2 concentrations. When suppli ed with 8 mg kg(-1) insoluble P concentration, total citrate efflux by root systems (mu mol h(-1)), plant P uptake, shoot photosynthesis rates and pla nt mass were all significantly greater at elevated than at ambient CO2 afte r a growth period of between 55 and 63 days. In this treatment, although th e P concentration of the rooting medium limited growth at ambient CO2, elev ated CO2 increased P-uptake from the non-labile source, increased photosynt hesis rates per unit shoot soluble-P and increased plant mass. At P concent rations lower than 8 mg kg(-1), plant mass, specific citrate efflux and max imum leaf carboxylation rates were limited by the amount of P available in the rooting medium and no CO2 effect was observed. In all treatments, carbo n supply did not appear to limit citrate efflux. Where an increase in P upt ake at elevated CO2 was achieved, it was due to an increase in root mass (i ndicative of a potentially larger soil volume explored) rather than to incr eased specific rates of citrate efflux. Above 8 mg kg(-1), the supplied P c oncentration was sufficient that minimal rates of specific citrate efflux a lone solubilised enough P for growth and a strong CO2 effect on plant mass, photosynthesis and P uptake was observed.