Phosphorus supply enhances the response of legumes to elevated CO2 (FACE) in a phosphorus-deficient vertisol

Background & aims Understanding the mechanism of how phosphorus (P) regulates the response of legumes to elevated CO2 (eCO(2)) is important for developing P management strategies to cope with increasing atmospheric CO2 concentration. This study aimed to explore this mechanism by investigating interactive effects of CO2 and P supply on root morphology, nodulation and soil P fractions in the rhizosphere. Methods A column experiment was conducted under ambient (350 ppm) (aCO(2)) and eCO(2) (550 ppm) in a free air CO2 enrichment (FACE) system. Chickpea and field pea were grown in a P-deficient Vertisol with P addition of 0-16 mg Pkg(-1). Results Increasing P supply increased plant growth and total P uptake with the increase being greater under eCO(2) than under aCO(2). Elevated CO2 increased root biomass and length, on average, by 16 % and 14 %, respectively. Nodule biomass increased by 46 % in response to eCO(2) at 16 mg P kg(-1), but was not affected by eCO(2) at no P supply. Total P uptake was correlated with root length while N uptake correlated with nodule number and biomass regardless of CO2 level. Elevated CO2 increased the NaOH-extractable organic P by 92 % when 16 mg P kg(-1) was applied. Conclusion The increase in P and N uptake and nodule number under eCO(2) resulted from the increased biomass production, rather than from changes in specific root-absorbing capability or specific nodule function. Elevated CO2 appears to enhance P immobilization in the rhizosphere.