Increases in the global atmospheric concentration of CO2 will not only directly affect the growth of plants, but might also alter the living conditions for soil biota. This could lead to shifts in the size and composition of the soil microbial communities. In this study we investigated the response of heterotrophic bacteria, NH 4/+ -oxidising bacteria, and Rhizobium leguminosarum bv. trifolii populations to elevated atmospheric CO2 concentrations in a model field-scale grassland ecosystem. The Free Air CO2 Enrichment (FACE) facility in Eschikon, Switzerland, releases CO2-enriched air into three large circular areas, each of 18 m dia, to a final CO2 concentration of 600 μmol mol-1, while three control areas of the same size receive ambient CO2 concentrations (~ 350 μmol mol-1). For this study, white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were grown as replicated monocultures within the FACE rings. Soil samples were taken from 0-10 cm depth in May and November 1994 (the second year of CO2-enrichment), and rhizosphere soil was obtained from clover and ryegrass roots for enumeration of bacteria. While the total numbers of culturable heterotrophic bacteria (determined by plate counts) in the rhizospheres of both plant species were little affected by CO2-enrichment, the populations of R. leguminosarum bv. trifolii (enumerated by MPN) were increased two-fold in the rhizospheres of white clover exposed to elevated atmospheric CO2. There was no effect of the CO2 concentration on the populations of R. leguminosarum bv. trifolii in the rhizospheres of perennial ryegrass, indicating that the increase of Rhizobium numbers is a host-related response to elevated atmospheric CO2. The numbers of autotrophic NH 4/+ -oxidizing bacteria in the rhizospheres (enumerated by MPN) were unaffected by the atmospheric CO2 concentration. There was also no effect of the CO2 concentration on the amount of microbial biomass C in the bulk, non-rhizosphere soils in white clover or perennial ryegrass plots. These data indicate that under a legume crop, at least in terms of inoculum quality in the rhizosphere soil, symbiotic nitrogen-fixing organisms might be favoured by elevated atmospheric CO2 concentrations.
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Acknowledgements-This study was supported by grants from the Swiss National Science Foundation (to MS.) and from the Minnesota Agricultural Experiment Station (to M.J.S.). We thank Michael Dolan for computing the root data and Robert Noyd and Barbara Fischer for technical assistance. We also thank Hana Santruckova for discussion of the manuscript. This is manuscript number 22,309 in the University of Minnesota Agricultural Experiment Station series.