MICROBIAL COMMUNITY CHANGES IN THE RHIZOSPHERES OF WHITE CLOVER AND PERENNIAL RYEGRASS EXPOSED TO FREE-AIR CARBON-DIOXIDE ENRICHMENT (FACE)

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 invest igated the response of heterotrophic bacteria, NH4+-oxidising bacteria , and Rhizobium leguminosarum by. trifolii populations to elevated atm ospheric 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 1 8 m dia, to a final CO2 concentration of 600 mu mol mol(-1), while thr ee control areas of the same size receive ambient CO2 concentrations ( similar to 350 mu mol mol(-1)). For this study, white clover (Trifoliu m 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-e nrichment), and rhizosphere soil was obtained from clover and ryegrass roots for enumeration of bacteria. While the total numbers of cultura ble heterotrophic bacteria (determined by plate counts) in the rhizosp heres of both plant species were little affected by CO2-enrichment, th e populations of R. leguminosarum by. trifolii (enumerated by MPN) wer e increased two-fold in the rhizospheres of white clover exposed to el evated atmospheric CO2. There was no effect of the CO2 concentration o n the populations of R. leguminosarum by. trifolii in the rhizospheres of perennial ryegrass, indicating that the increase of Rhizobium numb ers is a host-related response to elevated atmospheric CO2. The number s of autotrophic NH4+-oxidizing bacteria in the rhizospheres (enumerat ed by MPN) were unaffected by the atmospheric CO2 concentration. There was also no effect of the CO2 concentration on the amount of microbia l biomass C in the bulk, non-rhizosphere soils in white clover or pere nnial 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 C O2 concentrations. (C) 1997 Elsevier Science Ltd.