Abstract. Forest ecosystems exchange large amounts of carbon (C) with the atmosphere each year, an exchange that is strongly constrained by nutrient cycling. Forest productivity is anticipated to increase with the projected doubling in the concentration of atmospheric CO2 over the next century. A critical area for research and synthesis concerns the feedback effects of elevated CO2 on soil N cycling. Elevated CO2 significantly increased plant productivity, the uptake of N from soils and the flux of C to soil microbes but did not change litter chemistry, decomposition, the mass balance of N among ecosystem pools, gross and net N mineralization or plant N availability. Even though plant N availability did not increase under elevated CO2, there is no indication that N is progressively limiting the enhanced productivity of this ecosystem now in it sixth growing season under elevated CO2. This result is surprising given that (i) Net Primary Productivity is demonstrably N limited in this ecosystem, (ii) that there is intense competition for available N among plants, microbes and abiotic sinks in soils, and (iii) that the long-term productivity of a single prototype FACE plot at this research site was stimulated by elevated CO2 only after the addition of N fertilizer. These results suggest a fundamental gap in our understanding of the coupled nature of C and N cycles in forest ecosystems, and an opportunity to expand our knowledge of ecosystem biogeochemistry. This research will have direct benefits to society by addressing a major potential limitation to C sequestration in ecosystems that cover over 24 million hectares of forest land in the southeastern US; ecosystems that are likely to be managed for their C-sink potential under the Kyoto Protocol. Broader Impacts: Given the interdisciplinary and multi-institutional nature of the Duke Forest FACE experiment, this research will have impacts that extend well beyond this collaborative team. This grant will train future scientists and recruit and retain women graduate students, and will make an effort to broaden the participation of minorities by actively participating in the SEEDS program (a joint program of ESA and UNCF). This project will enhance infrastructure for research by contributing data sets and an enhanced model to current synthesis activities at NCEAS and an international network of 15 N tracer study sites. We expect the impact of this work to continue to be high, both in advances to fundamental ecosystem science, and in the demonstrable application of ecosystem science to issues of global change; issues that will increasingly impact society over the next century.
