Soil Nitrogen Cycling under Elevated CO2: A Synthesis of Forest Face Experiments Author(s):

The extent to which greater net primary productivity (NPP) will be sustained as the atmospheric CO2 concentration increases will depend, in part, on the long-term supply of N for plant growth. Over a two-year period, we used common field and laboratory methods to quantify microbial N, gross N mineralization, microbial N immobilization, and specific microbial N immobilization in three free-air CO2 enrichment experiments (Duke Forest, Oak Ridge, Rhinelander). In these experiments, elevated atmospheric CO2 has increased the input of aboveand belowground litter production, which fuels heterotrophic metabolism in soil. Nonetheless, we found no effect of atmospheric CO2 concentration on any microbial N cycling pool or process, indicating that greater litter production had not initially altered the microbial supply of N for plant growth. Thus, we have no evidence that changes in plant litter production under elevated CO2 will initially slow soil N availability and produce a negative feedback on NPP. Understanding the time scale over which greater plant production modifies microbial N demand lies at the heart of our ability to predict long-term changes in soil N availability and hence whether greater NPP will be sustained in a CO2-enriched atmosphere.