Mechanisms of phosphorus acquisition for ponderosa pine seedlings under high CO₂ and temperature

To test the hypothesis that elevated atmospheric CO₂ and elevated temperature, simulating current and predicted future growing season conditions, act antagonistically on phosphorus acquisition of ponderosa pine, seedlings were grown in controlled-environment chambers in a two temperature (25/10°C and 30/15 °C) x two CO₂ (350 and 700 μl^-1) experimental design Mycorrhizal seedlings were watered daily with a nutrient solution with P added in organic form as inositol hexaphosphate (64 ppm P). Thus seedlings were challenged to use active forms of P acquisition. Elevated CO₂ increased the relative growth rate by approx. 5% which resulted in an approx. 33% increase in biomass after 4 months. There was no main effect of temperature on growth. Increased growth under elevated CO₂ and temperature was supported by increases in specific absorption rate and the specific utilization rate of P. The contribution of mycorrhizae to P uptake may have been greater under simulated future conditions, as elevated CO₂ increased the number of mycorrhizal roots. There was no main effect of temperature on root phosphatase activity, but elevated CO₂ caused a decrease in activity. The inverse pattern of root phosphatase activity and mycorrhizal infection across treatments suggests a physiological coordination between these avenues of P acquisition. The concentration of oxalate in the soil increased under elevated CO₂ and decreased under elevated temperature. This small molecular weight acid solubilizes inorganic P making it available for uptake. Increased mycorrhizal infection and exudation of oxalate increased P uptake in ponderosa pine seedlings under elevated CO₂, and there was no net negative effect of increased temperature. The increased carbon status of pine under elevated CO₂ may facilitate uptake of limiting P in native ecosystems.