Background
Over the years, it has periodically been claimed (see, for example, Hungate et al., 2003 ) that when atmospheric CO2 enrichment enhances plant growth and results in greater soil carbon inputs, decomposing microorganisms in the soil not only require more nitrogen, they take it, both to sustain themselves and to expand their populations, which phenomena are further claimed to reduce nitrogen mineralization rates and thereby decrease the amount of nitrogen available to plants. The "biological pessimists" who make these claims further contend that in a high-CO2 world of the future, the availability of nitrogen, in forms usable by plants, will probably be too low to support large increases in soil carbon storage, which they additionally claim will ultimately lead to more rapid and extreme global warming.

What was done
In a study germane to these many claims, the authors describe how they "sampled soils in a chaparral ecosystem [at the Sky Oaks Biological Field Station near Warner Springs, California, USA] at 18 intervals over a 3-year period in replicated field chambers ranging from 250 to 750 ppm CO2 at 100-ppm increments," assessing various ways by which carbon enters the soil and is sequestered there.

What was learned
Allen et al. report that "total allocation of carbon to soil increased significantly through the study period with elevated CO2," as did "new carbon inputs into macroaggregates," which latter observation is extremely important, as these aggregates, in their words, "have increasing concentrations of glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi (Rillig et al., 1999 )," which substance acts to create and stabilize soil aggregates and protect the carbon they contain (see Glomalin in our Subject Index). In addition, they say that CO2 effects on soil bacteria "were not detectable." In fact, they acknowledge that microbial mass was actually "negatively affected by increasing CO2," noting that "under extended nitrogen limitation the plants ultimately garner the nitrogen," and that the plants "ultimately outcompete microbes for these scarce soil resources (Hu et al., 2001)."

What it means
In concluding their discussion of their findings, Allen et al. remark that "undisturbed arid shrublands may not fix comparatively large amounts of carbon, but they may sequester a large fraction of that carbon." Noting that "carbon allocated to arbuscular mycorrhizal fungi forms a large part of the macroaggregate structure in the form of glomalin (Rillig et al., 2002)," and that those aggregates "may be protected from decomposition," they conclude that the enhanced formation of such aggregates in CO2-enriched air forms "an important [carbon] sequestration pathway" in chaparral ecosystems. Hence, we have another example of "biological pessimism" being far removed from reality.

References
Hu, S., Chapin III, F.S., Firestone, M.K., Field, C.B. and Chiariello, N.R. 2001. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature 409: 188-191.

Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.

Rillig, M.C., Treseder, K.K. and Allen, M.F. 2002. Global change and mycorrhizal fungi. In: van der Heijden, M.G.A. and Sanders, I.R., Eds. Mycorrhizal Ecology. Springer-Verlag, New York, NY, USA, pp. 135-160.

Rillig, M.C., Wright, S.F., Allen, M.F. and Field, C.B. 1999. Rise in carbon dioxide changes soil structure. Nature 400: 628.