Background
The authors note that for long-lived organisms such as trees, which are suddenly and continuously exposed to elevated atmospheric CO2 concentrations, sustained CO2-induced increases in vegetative productivity require a steady supply of plant available nitrogen, citing the work of Luo et al. (2004), who suggest that as time progresses, this important soil resource will gradually be reduced to where it is no longer sufficient to maintain the initial large increases in tree productivity, according to the dictates of the progressive nitrogen limitation (PNL) hypothesis, which posits that CO2-induced increases in carbon input to an ecosystem's soil will stimulate soil microbial activity and thereby immobilize a greater portion of the soil's nitrogen, rendering it less available to the ecosystem's plants and thus reducing their growth rates.

What was done
Working at the long-running (ten years) FACE experiment being conducted within an initially eight-year-old sweetgum (Liquidambar styraciflua L.) plantation at the Oak Ridge National Experimental Research Park in Roane County, Tennessee (USA), Austin et al. tested the PNL hypothesis by examining "bacterial community structure using culture-independent molecular techniques (16S rRNA gene cloning analysis), microbial community function using extracellular enzyme activity, and nitrogen cycling rates using laboratory incubations."

What was learned
The five researchers report that "elevated CO2 had no detectable effect on microbial community structure using 16S rRNA gene clone libraries, on microbial activity measured with extracellular enzyme activity, or on potential soil mineralization and nitrification rates," noting that "these results support findings at other forested Free Air CO2 Enrichment (FACE) sites."

What it means
Austin et al. conclude that "since no effects (adverse or otherwise) have been observed on bacterial communities and functional activity in this study," as well as in other forest FACE studies, "increased carbon inputs may continue to accumulate within the soil," noting further that "if excess carbon is sequestered in soil carbon pools, forests may act as a negative feedback to increased global carbon emissions," citing the work of Houghton et al. (1999) in this regard.

References
Houghton, R.A., Hackler, J.L. and Lawrence, K.T. 1999. The US carbon budget: contributions from land-use change. Science 285: 574-578.

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. Bioscience 54: 731-739.