The progressive nitrogen (N) limitation hypothesis suggests that the uptake of N due to rapid tree growth under elevated CO₂ depletes pools of available N resulting in only short-term increases in productivity.  To date however, a down-regulation of forest productivity under elevated CO₂ has not been observed among the four forest FACE experiments suggesting that our understanding of the mechanisms by which trees influence soil N cycling needs further refinement.  We sought to test the hypothesis that trees exposed to elevated CO₂ increase soil N availability by via the release of exudates, and that the magnitude of such rhizosphere effects are greatest in soils with low N availability.  At the Duke Forest FACTS-1 site, NC, we collected exudates bi-monthly from intact fine roots of loblolly pine (Pinus taeda L.) trees exposed to elevated CO₂ and N fertilization.  In addition, we collected rhizosphere and bulk soil from the same plots to develop a time-integrated estimate of the plant-microbial response to the CO₂ and N treatments. 

Results/Conclusions

In general, soil N availability mediated root and microbial responses to CO₂ enrichment.  In non-fertilized plots, mass-specific exudation rates were 15% greater with CO₂ resulting in an increased flux of 4 g C m^-2 yr^-1.  In fertilized plots, elevated CO₂ had no effects on mass specific exudation rates relative to ambient rates, and total C fluxes were decreased by 5 g C m^-2 yr^-1 relative to non-fertilized plots.  In non-fertilized soils, rhizosphere effects on microbial activity, enzyme activity and net N mineralization were increased by CO₂ by 67%, 71% and 72%, respectively.  In fertilized plots, however, rhizosphere effects were far more variable and largely unresponsive to CO₂ and N treatments.  Collectively, these results suggest that increased inputs of labile C to the rhizosphere in forests exposed to elevated CO₂ may provide a mechanism for trees to accelerate soil N cycling and thus delay the onset of progressive N limitation.