The Progressive Nitrogen Limitation Hypothesis
Austin, E.E., Castro, H.F., Sides, K.E., Schadt, C.W. and Classen, A.T. 2009. Assessment of 10 years of CO2 fumigation on soil microbial communities and function in a sweetgum plantation. Soil Biology & Biochemistry 41: 514-520.
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."
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."
Given such findings, 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.
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.