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

Abstract

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.