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Rising Atmospheric CO2 Concentrations Increase Soil Carbon Contents
Volume 9, Number 5: 1 February 2006

Many field-scale CO2-enrichment studies, in the words of Jastrow et al. (2005), "have failed to detect significant changes in soil C [carbon] against the relatively large, spatially heterogeneous pool of existing soil organic matter, leading to the general conclusion that the potential for increased soil C is limited (Hungate et al., 1997; Gill et al., 2002; Hagedorn et al., 2003; Lichter et al., 2005)."  An additional long-held opinion, as they relate it, is that "if CO2-stimulated increases in soil organic C do occur, they will be allocated to rapidly cycling, labile pools with little, if any, long-term stabilization (Hungate et al., 1997; Lichter et al., 2005)."  Now, however, after many long and arduous experiments have been conducted, and their data properly analyzed, the truth is being seen to be something quite different.

The long-awaited (for us!) confirmation of our more optimistic view of the subject, which is partially expressed in our previously-published reviews of two of the above-mentioned studies (Gill et al., 2002; Lichter et al., 2005) is presented by Jastrow et al. (2005), who describe and further analyze the pertinent findings of the first five years of the deciduous forest FACE study that is being conducted at Oak Ridge, Tennessee, USA (Norby et al., 2001), the entire eight years of the prairie grassland open-top chamber study that was conducted at Manhattan, Kansas, USA (Owensby et al., 1993), and 35 other studies of like nature.  So what did the seven researchers find?

Atmospheric CO2 enrichment to approximately 200 ppm above ambient, in the words of Jastrow et al., "increased C stocks in the forest soil at an average rate of 44 9 g C m-2 yr-1," while "in the prairie, the incremental increase in C stocks corresponded to an average accrual rate of 59 19 g C m-2 yr-1."  Why?  "Because," as they describe it, "both systems responded to CO2 enrichment with large increases in root production," and "even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity."  Likewise, their meta-analysis of the 35 independent experimental observations indicated that CO2 enrichment ranging from 200 to 350 ppm over periods ranging from two to nine years increased soil C over soil depths ranging from 5 to 20 cm by 5.6% (95% CI = 2.8-8.4%), "supporting the generality of the accrual measured in the forest and prairie experiments."

In commenting on their findings, the seven scientists say they "clearly demonstrate that mineral soil C, including microaggregate protected pools, can increase measurably in response to a step-function increase in atmospheric CO2 concentrations," and that "the C storage capacities of mineral soils - even those with large organic matter stocks - are not necessarily saturated at present and may be capable of serving as C sinks if inputs increase as a result of passive CO2 fertilization."  In addition, they say that "the meta-analysis, which included some multifactor studies and data collected over a wide range of climatic conditions, suggests that soil C accrual ... is likely to be a general response to CO2 enrichment."

This response, in Jastrow et al.'s words, "is not insignificant."  In fact, they note that "if mineral soil C in the surface 20 cm of the world's temperate forests, temperate grasslands, shrublands, and croplands (234 Pg C ... according to Jobbagy and Jackson, 2000) were to increase by 5.6% or at a rate of 19 g C m-2 yr-1, then 8-13 Pg of C might be accumulated within a 10-year period," which suggests that over a period of 180 years the amount of carbon found in the soils of these biomes could possibly be doubled (234 Pg C divided by 1.3 Pg C per year = 180 years).

Viewed in the light of these several observations, it can readily be appreciated that the soil-carbon-sequestering prowess of earth's vegetation can indeed act as a significant brake on the rate-of-rise of the air's CO2 content and thereby help to mute the magnitude of any CO2-induced impetus for global warming.

Sherwood, Keith and Craig Idso

References
Gill, R.A., Polley, H.W., Johnson, H.B., Anderson, L.J., Maherali, H. and Jackson, R.B.  2002.  Nonlinear grassland responses to past and future atmospheric CO2Nature 417: 279-282.

Hagedorn, F., Spinnler, D., Bundt, M. et al.  2003.  The input and fate of new C in two forest soils under elevated CO2Global Change Biology 9: 862-872.

Hungate, B.A., Holland E.A., Jackson, R.B. et al.  1997.  The fate of carbon in grasslands under carbon dioxide enrichment.  Nature 388: 576-579.

Jobbagy, E.G. and Jackson, R.B.  2000.  The vertical distribution of soil organic carbon and its relation to climate and vegetation.  Ecological Applications 10: 423-436.

Jastrow, J.D., Miller, R.M., Matamala, R., Norby, R.J., Boutton, T.W., Rice, C.W. and Owensby, C.E.  2005.  Elevated atmospheric carbon dioxide increases soil carbon.  Global Change Biology 11: 2057-2064.

Lichter, J., Barron, S.H., Bevacqua, C.E., Finzi, A.C., Irving, K.F., Stemmler, E.A. and Schlesinger, W.H.  2005.  Soil carbon sequestration and turnover in a pine forest after six years of atmospheric CO2 enrichment.  Ecology 86: 1835-1847.

Norby, R.J., Todd, D.E., Fults, J. et al.  2001.  Allometric determination of tree growth in a CO2-enriched sweetgum stand.  New Phytologist 150: 477-487.

Owensby, C.E., Coyne, P.I., Ham, J.M. et al.  1993.  Biomass production in a tallgrass prairie ecosystem exposed to ambient and elevated CO2Ecological Applications 3: 644-653.