Background
In a recent News & Views item published in Nature Geoscience, Agren (2010) says "it is often assumed that warming will stimulate carbon dioxide release from soils," but he notes that "soil-warming experiments suggest that warming-induced increases in soil carbon dioxide release are transitory, and that emissions return to pre-warming levels after a period of five to ten years," citing the work of Kirschbaum (2004) and Eliasson et al. (2005). And in much the same vein, Bradford et al. (2010) write that in actual field studies "elevated soil respiration rates under experimental warming are relatively short-lived," citing the works of Jarvis and Linder (2000), Oechel et al. (2000), Luo et al. (2001), Rustad et al. (2001) and Melillo et al. (2002).

What was done
Taking a laboratory microcosm approach, Bradford et al. (2010) tested the potential for the mass-specific respiration (Rmass) in six different soils to adapt to three experimentally-imposed thermal regimes (a constant 10, 20 or 30°C) by periodically assaying soil subsamples "using similar approaches to those used in plant, animal and microbial thermal adaptation studies."

What was learned
"As would be expected," the three researchers write, "after 77 days of incubation Rmass rates across the range of assay temperatures were greatest for the 10°C experimentally incubated soils and lowest for the 30°C soils, with the 20°C incubated soils intermediate."

What it means
Bradford et al. state that "well-established evolutionary trade-offs in controls on metabolic rates suggest that Rmass rates should be lower for organisms adapted to higher temperature regimes and vice-versa," citing Hochachka and Somero (2002); and they say their work "demonstrates that, for soils sampled from hardwood forests in the north- and southeastern United States, substrate Rmass rates of the microbial biomass when measured under conditions of excess glucose substrate availability, follow the expectations of established biochemical trade-offs." Thus, as ever more studies of the phenomenon are conducted, the old notion of soil respiration rising in response to global warming, and remaining high, is seen to be ever farther from the truth.

References
Agren, G.I. 2010. Microbial mitigation. Nature Geoscience 3: 303-304.

Eliasson, P.E., McMurtrie, R.E., Pepper, D.A., Stromgren, M., Linder, S. and Agren, G.I. 2005. The response of heterotrophic CO2 flux to soil warming. Global Change Biology 11: 167-181.

Hochachka, P.W. and Somero, G.N. 2002. Biochemical Adaptation: Mechanism and Process in Physiological Evolution. Oxford University Press, New York, New York, USA.

Kirschbaum, M.U.F. 2004. Soil respiration under prolonged soil warming: Are rate reductions caused by acclimation or substrate loss? Global Change Biology 10: 1870-1877.

Luo, Y.Q., Wan,S.Q., Hui, D.F. and Wallace, L.L. 2001. Acclimation of soil respiration to warming in a tall grass prairie. Nature 413: 622-625.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173-2176.

Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D. 2000. Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response to decadal climate warming. Nature 406: 978-981.

Rustad, L., Campbell, J.L., Marion, G.M., Norby, R.J., Mitchell, M.J., Hartley, A.E., Cornelissen, J.H.C. and Gurevitch, J. 2001. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and above-ground plant growth to experimental ecosystem warming. Oecologia 126: 543-562.