SOIL NITROGEN CYCLING UNDER ELEVATED CO 2 : A SYNTHESIS OF FOREST FACE EXPERIMENTS

The extent to which greater net primary productivity (NPP) will be sustained as the atmospheric CO 2 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 CO 2 enrichment experiments (Duke Forest, Oak Ridge, Rhinelander). In these experiments, elevated atmospheric CO 2 has increased the input of above‐ and belowground litter production, which fuels heterotrophic metabolism in soil. Nonetheless, we found no effect of atmospheric CO 2 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 CO 2 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 CO 2 ‐enriched atmosphere.