Long-term variations in the frequency and intensity of sheep (Ovis aries) grazing have led to the development of ubiquitous plant successional transitions in sub-montane regions of the UK. In this study, we measured a range of soil microbial properties across these successional transitions in three biogeographic regions of the UK, to establish how gradients of grazing-influence (in terms of the history and intensity of sheep grazing) alter the biomass, activity, and structure of soil microbial communities. We also measured soil physicochemical variables to relate changes in soil microbial community arrangement along these grazing-related successional transitions to key soil properties. Our results from three locations show that microbial communities of soils display some consistent and ‘broad-scale’ trends along successional transitions that are related to the history and intensity of grazing. We show that microbial biomass of soil is maximal at low-to-intermediate levels of grazing influence and that the phenotypic evenness (a component of diversity) of the microbial community declines as the intensity of grazing increases. We also provide evidence that soil microbial communities of heavily grazed sites are dominated by bacterial-based energy channels of decomposition, whereas in systems that are less intensively grazed, or completely unmanaged, fungi have a proportionally greater role. Further studies are needed to establish the significance of these changes in relation to soil-level ecosystem processes of decomposition and nutrient cycling. The data show that human disturbances can have profound effects on the biomass and structure of the soil communities that regulate soil processes in these ecosystems and that these effects are consistent across sites.
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