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.
