Does Phosphorus Stimulate the Effect of Elevated Carbon Dioxide On Growth and Symbiotic Nitrogen Fixation of Grain and Pasture Legumes?.

The effect of elevated [CO₂] (700 μmol/mol) and phosphorus (P) supply on the growth and symbiotic N₂ fixation of chickpea (Cicer arietinum L.), field pea (Pisum sativum L.) and barrel medic (Medicago truncatula Gaertn.) were investigated in the glasshouse. The effect of elevated [CO₂] on the growth and N₂ fixation at various growth stages of the chickpea and field pea plants (grown on a Vertosol) were also examined. Elevated [CO₂] generally increased the above-ground biomass of chickpea (by 18–64%), field pea (by 24–57%) and barrel medic (by 49–82%), but the effect was greater when P was non-limiting. Elevated [CO₂] only stimulated grain yield of chickpea (by 70%) and field pea (by 21%) if P supply was adequate. Elevated [CO₂] reduced the grain protein concentration of chickpea (by 17–18%) regardless of P input, but increased that of field pea (by 11%) when soil P was limiting but had no effect at adequate P. The percentage of shoot N derived from the atmosphere (%Ndfa) of the three legumes was unaffected by elevated [CO₂] regardless of soil P supply. Elevated [CO₂] increased the amount of N fixed by chickpea (by 20–86%), field pea (by 44–51%) and barrel medic (by 114–250%) under P fertilization, but had no significant effect when soil P was deficient. These results suggest that the predictions of future climates on the potential contribution of legumes to maintaining soil N fertility will depend on the particular response of a species to soil P status.