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Effect of Elevated CO₂ and Drought on Soil Microbial Communities Associated with Andropogon gerardii

Kassem, Issmat I., Joshi, Puneet, Sigler, Von, Heckathorn, Scott, Wang, Qi
Journal of integrative plant biology 2008 v.50 no.11 pp. 1406-1415
Andropogon gerardii, C3 plants, C4 plants, bacterial communities, carbon, carbon dioxide, carbon dioxide enrichment, carbon nitrogen ratio, community structure, denaturing gradient gel electrophoresis, drought, ecosystems, exudation, grasses, microbial activity, microbial biomass, plate count, soil, soil bacteria, soil resources, soil-plant interactions, stomatal conductance, water content
Our understanding of the effects of elevated atmospheric CO₂, singly and in combination with other environmental changes, on plant-soil interactions is incomplete. Elevated CO₂ effects on C₄ plants, though smaller than on C₃ species, are mediated mostly via decreased stomatal conductance and thus water loss. Therefore, we characterized the interactive effect of elevated CO₂ and drought on soil microbial communities associated with a dominant C₄ prairie grass, Andropogon gerardii Vitman. Elevated CO₂ and drought both affected resources available to the soil microbial community. For example, elevated CO₂ increased the soil C:N ratio and water content during drought, whereas drought alone decreased both. Drought significantly decreased soil microbial biomass. In contrast, elevated CO₂ increased biomass while ameliorating biomass decreases that were induced under drought. Total and active direct bacterial counts and carbon substrate use (overall use and number of used sources) increased significantly under elevated CO₂. Denaturing gradient gel electrophoresis analysis revealed that drought and elevated CO₂, singly and combined, did not affect the soil bacteria community structure. We conclude that elevated CO₂ alone increased bacterial abundance and microbial activity and carbon use, probably in response to increased root exudation. Elevated CO₂ also limited drought-related impacts on microbial activity and biomass, which likely resulted from decreased plant water use under elevated CO₂. These are among the first results showing that elevated CO₂ and drought work in opposition to modulate plant-associated soil-bacteria responses, which should then influence soil resources and plant and ecosystem function.