Background
Burton et al. (2008) note that "increases in terrestrial ecosystem respiration as temperatures warm could create a positive feedback that causes atmospheric CO2 concentration, and subsequently global temperature, to increase more rapidly," as climate alarmists typically contend will occur, but that "if plant tissue respiration acclimates to temperature over time, this feedback loop will be weakened, reducing the potential temperature increase." With Burton et al. having demonstrated that plant tissue respiration does indeed so acclimate, Bradford et al. explore the same subject as it pertains to soil microbial respiration.

What was done
The authors acquired and analyzed pertinent data obtained as late as 15 years after the start of a soil-warming experiment established in 1991 on an even-aged mixed-deciduous tract of trees in the Harvard Forest (Massachusetts, USA), where heating cables were used to elevate soil temperatures to 5°C above ambient temperatures measured in non-heated control plots.

What was learned
Bradford et al. report that -- as has been found by many others in long-term field experiments -- "elevated respiration rates under soil warming return to pre-warming values within a few years," citing the corroborative findings of Jarvis and Linder (2000), Oechel et al. (2000), Luo et al. (2001), Rustad et al. (2001), Melillo et al. (2002) and Eliasson et al. (2005).

What it means
In the concluding sentence of the abstract of their paper, the nine researchers say their results imply that "stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted," while in the final sentence of the body of their paper they reiterate, in slightly different language, that "the changes in the biomass and physiology of soil microbial communities that we observed may decrease the expected strength of climate warming on soil respiration rates," which state of affairs, in the words of Burton et al., ends up "reducing the potential temperature increase."

References
Burton, A.J., Melillo, J.M. and Frey, S.D. 2008. Adjustment of forest ecosystem root respiration as temperature warms. Journal of Integrative Plant Biology 50: 1467-1483.

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.

Jarvis, P. and Linder, S. 2000. Constraints to growth of boreal forests. Nature 405: 904-905.

Luo, Y., Wan, S., Hui, D. and Wallace, L.L. 2001. Acclimatization 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. et al. 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. et al. 2001. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 126: 543-562.