Rhizo-Accelerated Mineralization and Priming in a Loblolly Pine Forest Exposed to Elevated Atmospheric CO2.

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 CO₂-induced increases in rhizosphere carbon fluxes (e.g. root exudates) accelerate soil N cycling by stimulating N-limited rhizosphere microbes to produce extra-cellular enzymes that release N from SOM. At the Duke Forest FACTS-1 site, NC, we collected rhizosphere and bulk soils from 25 year-old loblolly pine (Pinus taedaL.) trees exposed to elevated CO₂ for over ten years. We found stronger (two-fold) rhizosphere effects on microbial and enzyme activity in elevated CO₂ soils relative to ambient CO₂ soils, especially in plots with low N availability. In addition, we added model exudate cocktails to soil via artificial roots to examine the effects of substrate additions on soil N cycling. We found that ecologically-relevant substrate additions induced significant N transformations in soil. Collectively, our results suggest that roots may influence long-term soil quality and the sustained productivity of southern pine forests under elevated CO₂ through their effects on N availability.