A. M. Treonis, S. J. Grayston, N. J. Ostle, B.G.Ord

Above-ground grazing of grasses is known to affect root exudation as well as the biomass, composition, and physiological profiles of the soil microbial community.  Generally, an increase in microbial biomass is seen due to an increase in rhizosphere carbon availability immediately following defoliation.  However, the mechanisms and details of this result are not consistent and need to be studied further.  The objective of this experiment was to determine how carbon flow from plants into microbial biomass (via root exudates) is altered when plants are defoliated.  We used a stable isotope (13C) to label plants and soils in the field and assessed label incorporation into PLFA biomarkers in order to follow the flow of photosynthetically fixed carbon from plant roots into rhizosphere microbial populations.

Grassland swards at the Sourhope NERC Soil Biodiversity Experimental site in the Borders of Scotland were pulse labelled in the field with 13CO2 for 2 5-h periods over 48h.  A defoliation treatment, simulated by clipping, was applied immediately after labelling was complete.  Soil samples were collected for analysis 24 h after clipping.  PLFAs were extracted from the soils, as well as from soils collected from labelled, unclipped control plots and unlabelled plots.  We determined the relative quantity and rate of transfer of rhizosphere carbon to soil fungi and different bacterial groups by comparing the amount of 13C label found specific PLFA components using GC-C-IRMS.  This technique allowed us to determine which groups of microorganisms responded to changes in root exudation caused by defoliation.  Additionally, soil microbial communities were assessed using analysis of the community-level physiological profiles (CLPPs, e.g, Biolog technique), determination of soil microbial biomass, and plate counts of specific rhizosphere microbial groups (fungi, bacteria).  The results of this analysis and their relevance to future studies of microbial community structure and function are discussed.