De Deyn et al. established 32 microcosms of plant species mixtures characteristic of recently abandoned production grassland (early succession), grassland under restoration for twenty years (mid-succession), and species-rich natural grassland (the ultimate target state).  These microcosms were all inoculated with soil fauna from one of the three grassland successional stages.  The density and composition of the soil fauna added to the microcosms were the same as those of the three grassland successional stages and included microfauna (nematodes), mesofauna (microarthropods) and macrofauna (beetle larvae).  After four and six months of these treatments, the microcosm plants were clipped at 4 cm above the soil surface and the harvested dry weights of all individual plant species were determined, while after twelve months, the plants were clipped at the soil surface and their root dry weights determined.

What did the scientists discover?  As they describe it, "the soil fauna decreased the shoot biomass of the early succession plant species after 6 months, as well as plant species from the mid-succession stage, whereas the shoot biomass of the target plant species was increased."  Hence, as they note, "addition of the soil fauna also enhanced plant species diversity."  Results obtained at the end of the experiment further suggested that "the invertebrate root herbivores were selectively feeding on roots of dominant plants," which "provided an indirect advantage for the subdominant plant species, which were only marginally suppressed in the presence of soil fauna."  The researchers also report that the positive contributions of soil fauna and mycorrhizal fungi seem to be additive.

So what does all of this have to do with carbon dioxide?  It suggests that the ongoing rise in the air's CO2 content may also enhance ecosystem species richness, as a consequence of the tendency for atmospheric CO2 enrichment to increase both mycorrhizal fungi and soil fauna populations.

With respect to soil fauna, Rillig et al. (1999a) found that an approximate doubling of the air's CO2 content increased the numbers of microarthropods in sandstone and serpentine grasslands by 108% and 39%, respectively.  Likewise, in a study of well-fertilized poplar cuttings, Lussenhop et al. (1998) found that their approximately doubled-CO2 treatment supported twice as many microarthropods as their ambient-air treatment.  And in another microcosm study of terrestrial ecosystems, Jones et al. (1998) found that a 53% increase in atmospheric CO2 concentration led to a 52% increase in soil microarthropods.

With respect to mycorrhizal fungi, Rillig et al. (1998a) found that elevated CO2 increased percent root colonization by fungal hyphae in three grasses and two herbs that co-occur in Mediterranean annual grasslands; while Rillig et al. (1998b) determined that it increased percent root colonization by fungal arbuscles in the annual grass Bromus hordeaceus.  Lastly, Rillig et al. (1999b) found that elevated CO2 increased percent root colonization by arbuscules in serpentine and sandstone grasslands by three- and ten-fold, respectively.

In light of these several observations, there is a good likelihood that the ongoing rise in the air's CO2 content will hasten the conversion of abandoned agricultural fields and early and mid-succession grasslands into species-rich mature grasslands, while protecting the biodiversity of long-established natural grasslands.

References
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Crawley, M.J.  1997.  Plant Ecology.  Blackwell Science, Oxford, UK.

De Deyn, G.B., Raaljmakers, C.E., Zoomer, H.R., Berg, M.P., de Rulter, P.C., Verhoef, H.A., Bezemer, T.M. and van der Putten, W.H.  2003.  Soil invertebrate fauna enhances grassland succession and diversity.  Nature 422: 711-713.

Jones, T.H., Thompson, L.J., Lawton, J.H., Bezemer, T.M., Bardgett, R.D., Blackburn, T.M., Bruce, K.D., Cannon, P.F., Hall, G.S., Hartley, S.E., Howson, G., Jones, C.G., Kampichler, C., Kandeler, E. and Ritchie, D.A.  1998.  Impacts of rising atmospheric carbon dioxide on model terrestrial ecosystems.  Science 280: 441-443.

Lussenhop, J., Treonis, A., Curtis, P.S., Teeri, J.A. and Vogel, C.S.  1998.  Response of soil biota to elevated atmospheric CO2 in poplar model systems.  Oecologia 113: 247-251.

Olff, H. and Ritchie, M.E.  1998.  Effects of herbivores on grassland plant diversity.  Trends in Ecology and Evolution 13: 261-265.

Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B.  1998a.  Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients.  Oecologia 113: 252-259.

Rillig, M.C., Allen, M.F., Klironomos, J.N. and Field, C.B.  1998b.  Arbuscular mycorrhizal percent root infection and infection intensity of Bromus hordeaceus grown in elevated atmospheric CO2.  Mycologia 90: 199-205.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999a.  Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands.  Oecologia 119: 572-577.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999b.  Fungal root colonization responses in natural grasslands after long-term exposure to elevated atmospheric CO2.  Global Change Biology 5: 577-585.

van der Heijden, M.G.A., Klironomos, J.N., Ursic, M., Moutoglis, P., Streitwolf-Engel, R., Boller, T., Wiemken, A. and Sanders, I.R.  1998.  Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity.  Nature 396: 69-72.