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Atmospheric CO2 Enrichment:
Its Impact on the Abundance of Soil Organic Matter

"Soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial biosphere."  So wrote Cardon et al. (2001) in introducing their experimental study of the potential effects of the ongoing rise in the air's CO2 content on this vast store of material, which was once the "C" in the CO2 of much of earth's atmosphere, where it was freely wafted about prior to being assimilated by plants and sequestered in the soil.

Thus occupying such a pivotal position in the planetary carbon cycle, SOC is of great interest to scientists who worry about its stability.  Among other things, they want to know if allowing more CO2 to be emitted to the atmosphere would lead to even more carbon being sequestered in the soil (a logical hypothesis), or if would it somehow lead to a reduction in what is already there (a not-so-logical hypothesis but one that cannot be ignored, especially in view of what many people consider to be the incredibly high-stakes gamble we are taking with the world's climate as we continue to burn prodigious quantities of fossil fuels).

To explore this question, Cardon et al. studied soil carbon income and outgo in a number of small microcosms of two annual C3 grassland communities (sandstone and serpentine) of contrived high and low soil-nutrient availability that were maintained out-of-doors in open-top chambers at the Jasper Ridge Biological Preserve in Stanford, California from October 1994 through August 1996.  Key to their study was the utilization of isotopic tracer techniques to determine the sizes of the various SOC pools through time.  Helping them in this regard was the fact that they grew the C3 plants in a soil they obtained from a C4 grassland in Colorado, which ensured that the original organic carbon of their experimental soil would have a different isotopic signature from the organic carbon that would be injected into it by the C3 plants that grew upon it.  In addition, the carbon of the fossil fuel-derived CO2 supplied to the CO2-enriched chambers had yet a third unique isotopic signature.

So what was learned?  First of all, the extra CO2 supplied to half of the mini-ecosystems increased the total root biomass in the serpentine grassland microcosms by a factor of three in both the high and low soil-nutrient availability treatments, while it increased total root biomass in the sandstone grassland microcosms by a factor of four in both the high and low soil-nutrient availability treatments.  Hence, there was a tremendous CO2-induced increase in the amount of organic material that would eventually become available for incorporation into the soils of both grassland microcosms.

Second, with so much new organic matter being added to the soils of the CO2-erniched microcosms, Cardon et al. felt that previously carbon-limited microbes in these soils would alter their survival strategy and turn from breaking down older more recalcitrant soil organic matter to attack the more abundant and labile rhizodeposits being laid down in the newly-carbon-rich soils of the CO2-enriched microcosms.  This rhizodeposition, as they defined it, consists of "all deposition of organic carbon from living root systems to soils, including compounds lost through root exudation, sloughing of dead cells during root growth, and fine root turnover," which, as noted above, was dramatically enhanced by atmospheric CO2 enrichment.

The upshot of this scenario - which seems thoroughly vindicated in light of the observations about to be described - is that the experimentally imposed increase in atmospheric CO2 concentration actually retarded the decomposition of the older SOC of the imported soil, which had been deposited within that soil over who knows how many prior decades or centuries.  Also, this phenomenon effectively increased the turnover time of the original SOC.  Put another way, it significantly increased its stability.

What do these findings imply about the future?  "If this reduction in breakdown of older SOC is sustained," say Cardon et al., "an increased retention of carbon in older SOC pools might be expected under elevated relative to ambient CO2."  Hence, not only does atmospheric CO2 enrichment lead to higher rates of carbon input to soils, it apparently leads to slower rates of carbon withdrawal from them as well.  That's the classical "double whammy" that allows ever more carbon to be socked away in earth's soil bank as the air's CO2 content continues to rise.  And that phenomenon keeps the air's CO2 rate-of-rise from accelerating, even in the face of yearly increases in anthropogenic CO2 emissions.

How sweet it is!

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Cardon, Z.G., Hungate, B.A., Cambardella, C.A., Chapin III, F.S., Field, C.B., Holland, E.A. and Mooney, H.A.  2001.  Contrasting effects of elevated CO2 on old and new soil carbon pools.  Soil Biology & Biochemistry 33: 365-373.