OOS 42-2 - Integrating new paradigms in C and N cycling: Rhizo-accelerated mineralization and priming in an elevated CO2 forest

Thursday, August 6, 2009: 1:50 PM
Mesilla, Albuquerque Convention Center
Richard P. Phillips, Biology, Indiana University, Bloomington, IN, Adrien C. Finzi, Department of Biology, Boston University, Boston, MA and Emily S. Bernhardt, Department of Biology, Duke University, Durham, NC
Background/Question/Methods

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.

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