Susan J Grayston, Amy M Treonis, Nick J Ostle and Brian G Ord

    In grassland ecosystems grazing by aboveground herbivores has been hypothesised to have a major influence on the size, activity and structure of the soil microbial community, through alterations in the quantity and quality of rhizosphere carbon flow as a result of shoot defoliation. However, few studies have linked these processes. Since the microbial biomass is critical in regulating soil ecosystem level processes, such as nutrient cycling, it is vital to understand the factors that influence its structure and activity.

    Application of 13C labelling of biomarkers has shown its potential to directly link microbial populations to specific biogeochemical processes. The aim of the research reported here was to study the impact of shoot defoliation (simulated by clipping) on rhizosphere carbon flow and its incorporation into soil microbial communities. This was achieved through application of 13C pulse-labelling to grass swards in the field, which were clipped immediately after labelling, or remained unclipped. This was followed by GC-IRMS of phospholipid fatty acids (PLFA) extracted from the soils, to assess which members of the microbial community were metabolically active, responding to the change in carbon flux.