Soil–Nitrogen Cycling in a Pine Forest Exposed to 5 Years of Elevated Carbon Dioxide

Empirical and modeling studies have shown that the magnitude and duration of the primary production response to elevated carbon dioxide (CO2) can be constrained by limiting supplies of soil nitrogen (N). We have studied the response of a southern US pine forest to elevated CO2 for 5 years (1997–2001). Net primary production has increased significantly under elevated CO2. We hypothesized that the increase in carbon (C) fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization. We tested this hypothesis by quantifying the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first 5 years of CO2 fumigation. We observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO2. Similarly, elevated CO2 had no statistically significant effect on the concentration or flux of organic N, including amino acids. Microbial biomass N was not significantly different between CO2 treatments. Thus, we reject our hypothesis that elevated CO2 increases the rate of N immobilization. The quantity and chemistry of the litter inputs to the forest floor and mineral soil horizons can explain the limited range of microbially mediated soil–N cycling responses observed in this ecosystem. Nevertheless a comparative analysis of ecosystem development at this site and other loblolly pine forests suggests that rapid stand development and C sequestration under elevated CO2 may be possible only in the early stages of stand development, prior to the onset of acute N limitation.