Microbial community changes in the rhizospheres of white clover and perennial ryegrass exposed to Free Air Carbon dioxide Enrichment (FACE)

Abstract

Increases in the global atmospheric concentration of CO 2 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 CO 2 concentrations in a model field-scale grassland ecosystem. The Free Air CO 2 Enrichment (FACE) facility in Eschikon, Switzerland, releases CO 2-enriched air into three large circular areas, each of 18 m dia, to a final CO 2 concentration of 600 μmol mol −1, while three control areas of the same size receive ambient CO 2 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 CO 2-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 CO 2-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 CO 2. There was no effect of the CO 2 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 CO 2. The numbers of autotrophic NH 4 +-oxidizing bacteria in the rhizospheres (enumerated by MPN) were unaffected by the atmospheric CO 2 concentration. There was also no effect of the CO 2 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 CO 2 concentrations.