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Microorganisms -- Summary
Plants grown in CO2-enriched atmospheres nearly always exhibit increased photosynthetic rates and biomass production.  Due to this productivity enhancement, more plant material is typically added to soils from root growth, turnover and exudation, as well as from leaves and stems following their abscission and falling to the ground during senescence.  Such additions of carbon onto and into soils often serve as the only carbon source for supporting the development and growth of microorganisms in terrestrial habitats.  Thus, it is important to understand how CO2-induced increases in plant growth affect microorganisms.

Several studies have shown that atmospheric CO2 enrichment does not significantly impact soil microorganisms.  Zak et al. (2000), for example, observed no significant differences in soil microbial biomass beneath aspen seedlings grown at 350 and 700 ppm CO2 after 2.5 years of differential treatment.  Likewise, in the cases of Griffiths et al. (1998) and Insam et al. (1999), neither research team reported any changes in microbial community structure beneath ryegrass and artificial tropical ecosystems, respectively, after subjecting them to atmospheric CO2 enrichment.

Other studies, however, have found that elevated CO2 can significantly impact soil microorganisms.  Van Ginkel and Gorissen (1998), for example, observed that three months of elevated CO2 exposure (700 ppm) increased soil microbial biomass beneath ryegrass plants by 42% relative to that produced under ambient CO2 conditions, as did Van Ginkel et al. (2000).  Likewise, soil microbial biomass was reported to increase by 15% beneath agricultural fields subjected to a two-year wheat-soybean crop rotation (Islam et al., 2000); and in a study by Marilley et al. (1999), atmospheric CO2 enrichment significantly increased bacterial numbers in the rhizospheres beneath ryegrass and white clover monocultures.  Similarly, Lussenhop et al. (1998) reported CO2-induced increases in the amounts of bacteria, protozoa, and microarthropods in soils that had supported regenerating poplar tree cuttings for five months.  In addition, Hungate et al. (2000) reported that twice-ambient CO2 concentrations significantly increased the biomass of active fungal organisms and flagellated protozoa beneath serpentine and sandstone grasslands after four years of treatment exposure.

In taking a closer look at the study of Marilley et al. (1999), it is evident that elevated CO2 caused shifts in soil microbial populations.  In soils beneath their leguminous white clover, for example, elevated CO2 favored shifts towards Rhizobium bacterial species, which likely increased nitrogen availability - via nitrogen fixation - to support enhanced plant growth.  However, in soils beneath non-leguminous ryegrass monocultures, which do not form symbiotic relationships with Rhizobium species, elevated CO2 favored shifts towards Pseudomonas species, which likely acquired nutrients to support enhanced plant growth through mechanisms other than nitrogen fixation.  Nonetheless, in both situations, the authors observed CO2-induced shifts in bacterial populations that would likely optimize nutrient acquisition for specific host plant species.

In a non-related study, Montealegre et al. (2000) reported that elevated CO2 acted as a selective agent among 120 different isolates of Rhizobium growing beneath white clover plants.  Specifically, when bacterial strains favored by ambient and elevated CO2 concentrations were mixed together and grown with white clover at an atmospheric CO2 concentration of 600 ppm, 17% more root nodules were formed by isolates previously determined to be favored by elevated CO2.

In the interesting study of Hu et al. (2001), fertile sandstone grasslands subjected to five years of twice-ambient CO2 concentrations exhibited increased soil microbial biomass while simultaneously enhancing plant nitrogen uptake.  The net effect of these phenomena reduced nitrogen availability for microbial use, which consequently decreased microbial respiration and, hence, microbial decomposition.  Consequently, these ecosystems displayed CO2-induced increases in net carbon accumulation.  Similarly, Williams et al. (2000) reported that microbial biomass carbon increased by 4% in a tallgrass prairie after five years exposure to twice-ambient CO2 concentrations, which contributed to a total soil carbon enhancement of 8%.

In summation, it is clear from the published literature that as the CO2 content of the air continues to rise, earth's vegetation will likely respond with increasing photosynthetic rates and biomass production.  As a consequence of these phenomena, more organic carbon will be returned to the soil where it will be utilized by microbial organisms to maintain or increase their population numbers, biomass and heterotrophic activities (Weihong et al., 2000; Arnone and Bohlen, 1998).  Moreover, it is conceivable that shifts in microbial community structure may occur that will favor the intricate relationships that currently exist between leguminous and non-leguminous plants and the specific microorganisms upon which they depend.

References
Arnone, J.A., III and Bohlen, P.J.  1998.  Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2Oecologia 116: 331-335.

Griffiths, B.S., Ritz, K., Ebblewhite, N., Paterson, E. and Killham, K.  1998.  Ryegrass rhizosphere microbial community structure under elevated carbon dioxide concentrations, with observations on wheat rhizosphere.  Soil Biology and Biochemistry 30: 315-321.

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 CO2Nature 409: 88-191.

Hungate, B.A., Jaeger III, C.H., Gamara, G., Chapin III, F.S. and Field, C.B.  2000.  Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2Oecologia 124: 589-598.

Insam, H., Baath, E., Berreck, M., Frostegard, A., Gerzabek, M.H., Kraft, A., Schinner, F., Schweiger, P. and Tschuggnall, G.  1999.  Responses of the soil microbiota to elevated CO2 in an artificial tropical ecosystem.  Journal of Microbiological Methods 36: 45-54.

Islam, K.R., Mulchi, C.L. and Ali, A.A.  2000.  Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil.  Global Change Biology 6: 255-265.

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.

Marilley, L., Hartwig, U.A. and Aragno, M.  1999.  Influence of an elevated atmospheric CO2 content on soil and rhizosphere bacterial communities beneath Lolium perenne and Trifolium repens under field conditions.  Microbial Ecology 38: 39-49.

Montealegre, C.M., Van Kessel, C., Blumenthal, J.M., Hur, H.G., Hartwig, U.A. and Sadowsky, M.J.  2000.  Elevated atmospheric CO2 alters microbial population structure in a pasture ecosystem.  Global Change Biology 6: 475-482.

Van Ginkel, J.H. and Gorissen, A.  1998.  In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide.  Soil Science Society of America Journal 62: 951-958.

Van Ginkel, J.H., Gorissen, A. and Polci, D.  2000.  Elevated atmospheric carbon dioxide concentration: effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition.  Soil Biology & Biochemistry 32: 449-456.

Weihong, L., Fusuo, Z. and Kezhi, B.  2000.  Responses of plant rhizosphere to atmospheric CO2 enrichment.  Chinese Science Bulletin 45: 97-101.

Williams, M.A., Rice, C.W. and Owensby, C.E.  2000.  Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years.  Plant and Soil 227: 127-137.

Zak, D.R., Pregitzer, K.S., Curtis, P.S. and Holmes, W.E.  2000.  Atmospheric CO2 and the composition and function of soil microbial communities.  Ecological Applications 10: 47-59.