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CO2 to the Rescue ... Again!
Atmospheric CO2 enrichment has long been known to help earth's plants withstand the debilitating effects of various environmental stresses, such as high temperature, excessive salinity levels and deleterious air pollution, as well as the negative consequences of certain resource limitations, such as less than optimal levels of light, water and nutrients (Idso and Idso, 1994). Now, in an important new study, Johnson et al. (2002) present evidence indicating that elevated levels of atmospheric CO2 do the same thing for soil microbes in the face of the enhanced receipt of solar ultraviolet-B radiation that would be expected to occur in response to a 15% depletion of the earth's stratospheric ozone layer. In addition, their study demonstrates that this phenomenon will likely have important consequences for soil carbon sequestration.

Johnson et al. conducted their landmark work on experimental plots of subarctic heath located close to the Abisko Scientific Research Station in Swedish Lapland (68.35N, 18.82E). The plots they studied were composed of open canopies of Betula pubescens ssp. czerepanovii and dense dwarf-shrub layers containing scattered herbs and grasses. For a period of five years, the scientists exposed the plots to factorial combinations of UV-B radiation - ambient and that expected to result from a 15% stratospheric ozone depletion - and atmospheric CO2 concentration - ambient (around 365 ppm) and enriched (around 600 ppm) - after which they determined the amounts of microbial carbon (Cmic) and nitrogen (Nmic) in the soils of the plots.

When the plots were exposed to the enhanced UV-B radiation level expected to result from a 15% depletion of the planet's stratospheric ozone layer, the researchers found that the amount of Cmic in the soil was reduced to only 37% of what it was at the ambient UV-B level when the air's CO2 content was maintained at the ambient concentration. When the UV-B increase was accompanied by the CO2 increase, however, not only was there not a decrease in Cmic, there was an actual increase of fully 37%.

The story with respect to Nmic was both similar and different at one and the same time. In this case, when the plots were exposed to the enhanced level of UV-B radiation, the amount of Nmic in the soil experienced a 69% increase when the air's CO2 content was maintained at the ambient concentration. When the UV-B increase was accompanied by the CO2 increase, however, Nmic rose even more, experiencing a whopping 138% increase.

These findings, in the words of Johnson et al., "may have far-reaching implications ... because the productivity of many semi-natural ecosystems is limited by N (Ellenberg, 1988)." Hence, the 138% increase in soil microbial N observed in this study to accompany a 15% reduction in stratospheric ozone and a concomitant 64% increase in atmospheric CO2 concentration (experienced in going from 365 ppm to 600 ppm) should do wonders in enhancing the input of plant litter to the soils of these ecosystems, which phenomenon represents the first half of the carbon sequestration process, i.e., the carbon input stage.

With respect to the second stage of keeping as much of that carbon as possible in the soil, Johnson et al. note that "the capacity for subarctic semi-natural heaths to act as major sinks for fossil fuel-derived carbon dioxide is [also] likely to be critically dependent on the supply of N." Indeed, in a previous essay in this series, wherein we discussed the findings of the literature review of Berg and Matzner (1997), we found that such is truly the case. With more nitrogen in the soil, the long-term storage of carbon is significantly enhanced, as more litter is chemically transformed into humic substances when nitrogen is more readily available; and these resulting more recalcitrant carbon compounds can be successfully stored in the soil for many millennia.

Clearly, earth's biosphere is effectively programmed to engage in a whole host of different phenomena that may act to slow - or actually stop - the ongoing rise of the air's CO2 content, especially if there is a chance it might otherwise attain a dangerously high level in terms of its potential to induce global warming, as we have indicated in earlier essays of this series. Furthermore, as was suggested in yet another related context well over a decade ago (Idso, 1990), lowly soil microbes may well play a major role in this biologically-mediated regulatory enterprise, as is so nicely demonstrated in the new and unique study of Johnson et al. in Swedish Lapland.

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

References
Berg, B. and Matzner, E. 1997. Effect of N deposition on decomposition of plant litter and soil organic matter in forest ecosystems. Environmental Reviews 5: 1-25.

Ellenberg, H. 1988. Vegetation Ecology of Central Europe. Cambridge University Press, Cambridge, UK.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B. 1990. A role for soil microbes in moderating the carbon dioxide greenhouse effect? Soil Science 149: 179-180.

Johnson, D., Campbell, C.D., Lee, J.A., Callaghan, T.V. and Gwynn-Jones, D. 2002. Arctic microorganisms respond more to elevated UV-B radiation than CO2. Nature 416: 82-83.