EFFECT OF ELEVATED CO2 ON RHIZOSPHERE CARBON FLOW AND SOIL MICROBIAL PROCESSES

Direct effects of increased above-ground CO2 concentration on soil mic robial processes are unlikely, due to the high pCO(2) of the soil atmo sphere in most terrestrial ecosystems. However, below-ground microbial processes are likely to be affected through altered plant inputs at e levated CO2. A major component of plant input is derived from litter f all and root turnover. Inputs also derive from rhizodeposition (loss o f C-compounds from active root systems) which may account for up to 40 % of photoassimilate. This input fuels the activity of complex microbi al communities around roots. These communities are centrally important not only to plant-microbe interactions and consequent effects on plan t growth, but also, through their high relative activity and abundance , to microbially mediated processes in soil generally. This review foc uses on approaches to measure C-flow from roofs, in particular, as aff ected by increased atmospheric CO2 concentration. The available eviden ce for impacts on microbial communities inhabiting this niche, which c onstitutes an interface for possible perturbations on terrestrial ecos ystems through the influence of environmental change, will also be dis cussed. While methodologies for measuring effects of increased CO2 con centration on plant growth, physiology and C-partitioning are abundant and widely reported, there is relatively little information on plant- mediated effects on soil microbial communities and processes. Importan tly, many studies have also neglected to recognize that any secondary effects on microbial communities may have profound effects on plant pa rameters measured in relation to environmental change. We critically r eview approaches which have been used to measure rhizodeposition under conditions of increased atmospheric CO2 concentration, and then consi der evidence for changes in microbial communities and processes, and t he methodologies which have been recently developed, and are appropria te to study such changes.