What was done
In a multi-year experiment conducted out-of-doors in sunlit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system, the authors grew originally-one-year-old half-sib ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws.) seedlings from April 1998 to April 2001 at mean atmospheric CO2 concentrations of 420 ppm (ambient) and 690 ppm (elevated). The soil-litter system for this experiment was collected, by horizon, from a ponderosa pine stand near LaPine, Oregon, USA; and throughout the study the seedlings, in the words of the eight researchers who conducted the work, "relied on the native soil nutrients, mineralization of litter and soil organic matter and mineral weathering to provide the necessary nutrients."

What was learned
Tingey et al. report that "a mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting," and that when this was done, the analysis "showed that elevated CO2 significantly reduced [our italics] the soil respiration." More specifically, over the course of the study they found that the 64% increase in the air's CO2 content employed in their experiment reduced soil respiration by an average of 6% (P = 0.003).

What it means
The researchers note that their finding "contrasts sharply [our italics] from most other CO2 studies (e.g., Vose et al., 1997; Ball et al., 2000; Norby et al., 2002; Zak et al., 2000; King et al., 2004) in which elevated CO2 increased soil respiration." However, as they continue, "these studies did not use covariates to remove the potential influences of soil temperature or moisture before testing for a CO2 effect." And when Tingey et al. did so, overtly accounting for the effects of these concomitant complexities, the results turned out to be radically different from those of most earlier studies, suggesting yet another means whereby the ongoing rise in the air's CO2 concentration may be helping earth's terrestrial biosphere extract more carbon from the air and store it in the planet's soils.

References
Ball, A.S., Milne, E. and Drake, B.G. 2000. Elevated atmospheric carbon dioxide concentration increases soil respiration in a mid-successional lowland forest. Soil Biology & Biochemistry 32: 721-723.

King, J.S., Hanson, P.J., Bernhardt, E., Deangeli, P., Norby, R.J. and Pregitzer, K.S. 2004. A multiyear synthesis of soil respiration responses to elevated atmospheric CO2 from four forest FACE experiments. Global Change Biology 10: 1027-1042.

Norby, R.J., Hanson, P.J., O'Neill, E.G., Tschaplinski, T.J., Weltzin, J.F., Hansen, R.A., Cheng, W., Wullschleger, S.D., Gunderson, C.A., Edwards, N.T. and Johnson, D.W. 2002. Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage. Ecological Applications 12: 1261-1266.

Vose, J.M., Elliott, K.J., Johnson, D.W., Tingey, D.T. and Johnson, M.G. 1997. Soil respiration response to three years of elevated CO2 and N fertilization in ponderosa pine (Pinus ponderosa Doug. ex Laws.). Plant and Soil 190: 19-28.

Zak, D.R., Pregitzer, K.S., King, J.S. and Holmes, W.E. 2000. Elevated atmospheric CO2, fine roots and the response of soil microorganisms: a review and hypothesis. New Phytologist 147: 201-222.