Mechanisms of Phosphorus Acquisition for Ponderosa Pine Seedlings under High CO2and Temperature,

Abstract

To test the hypothesis that elevated atmospheric CO 2 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)×two CO 2 (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 CO 2 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 CO 2 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 CO 2 increased the number of mycorrhizal roots. There was no main effect of temperature on root phosphatase activity, but elevated CO 2 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 CO 2 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 CO 2 , and there was no net negative effect of increased temperature. The increased carbon status of pine under elevated CO 2 may facilitate uptake of limiting P in native ecosystems.