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 above- and 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.
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