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 may constrain an increase in terrestrial net primary productivity (NPP) with the globally increasing atmospheric CO 2 concentration. We examined whether an Australian temperate pasture grass (Danthonia richardsonii) grown in sand culture and supplied solely with virtually insoluble Al- and Fe-phosphate was able to increase C-gain when exposed to elevated (731 µmol mol −1 ) compared with ambient (379 µmol mol −1 ) CO 2 concentrations. When supplied with 8 mg kg −1 insoluble P concentration, total citrate efflux by root systems (µmol h −1 ), plant P uptake, shoot photosynthesis rates and plant mass were all significantly greater at elevated than at ambient CO 2 after a growth period of between 55 and 63 days. In this treatment, although the P concentration of the rooting medium limited growth at ambient CO 2 , elevated CO 2 increased P-uptake from the non-labile source, increased photosynthesis rates per unit shoot soluble-P and increased plant mass. At P concentrations lower than 8 mg kg −1 , plant mass, specific citrate efflux and maximum leaf carboxylation rates were limited by the amount of P available in the rooting medium and no CO 2 effect was observed. In all treatments, carbon supply did not appear to limit citrate efflux. Where an increase in P uptake at elevated CO 2 was achieved, it was due to an increase in root mass (indicative of a potentially larger soil volume explored) rather than to increased specific rates of citrate efflux. Above 8 mg kg −1 , the supplied P concentration was sufficient that minimal rates of specific citrate efflux alone solubilised enough P for growth and a strong CO 2 effect on plant mass, photosynthesis and P uptake was observed.