Background
Most ecosystem models, according to the authors, predict that climate warming will stimulate microbial decomposition of soil carbon, thereby producing a positive feedback to rising global temperatures; and they say that in line with these predictions, field experiments have documented "an initial increase in the loss of CO2 from soils in response to warming." However, they also note that "the CO2 loss from soils tends to decline to control levels within a few years (Oechel et al., 2000; Luo et al., 2001; Melillo et al., 2002)." So what is the ultimate outcome?

What was done
Noting that the longer-term "attenuation response" could result from "changes in microbial physiological properties with increasing temperature" -- such as "a decline in the fraction of assimilated carbon that is allocated to growth" -- Allison et al. explored these mechanisms by means of "a microbial-enzyme model to simulate the responses of soil carbon to warming by 5°C."

What was learned
When all was said and done, the three researchers say they found that "declines in microbial biomass and degradative enzymes can explain the observed attenuation of soil-carbon emissions in response to warming." More specifically, they write that reduced carbon-use efficiency -- a decline in the fraction of assimilated carbon that is allocated to growth -- "limits the biomass of microbial decomposers and mitigates the loss of soil carbon." So, is it case closed? One would hope so; however ...

What it means
... Allison et al. go on to list a few confounding caveats. First, the model that they adapted, in their words, "couples carbon cycling to nitrogen, a linkage that may alter the magnitude and direction of carbon-climate feedbacks in global models." Second, "the temperature sensitivity of enzymatic degradation may increase as substrate quality declines." Third, "warming-induced changes in microbial community composition could also influence substrate quality through microbial turnover and soil organic carbon formation." And fourth, "community shifts could affect biomass and enzyme production directly."

Consequently, and in light of the long list of uncertainties that cloud the issue, we may not find a satisfactory answer to the core question until, as Allison et al. describe it, "a new generation of coupled models" comes along that can "account for these microbial properties" and thereby "improve estimates of soil carbon change and the magnitude of feedbacks in the carbon-climate system" ... which leaves us pretty much back where we were before Allison et al. ever began their modeling exercise ... except for the fact that we still have several empirical observations of accelerated soil respiration tending to decline back to pre-warming rates over a period of several years in the real world.

References
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.