The contribution of environmental selection on microbial community structure, function and biogeography

Agriculture, which accounts for approximately 75% of land use in the UK, relies on microbial communities for many key processes. Despite their importance to the sustainability of agriculture, our understanding of the long-term effects of agricultural practices on soil microbial communities is limited. Agricultural practices not only alter many edaphic variables such as nutrient content and pH, they also introduce chemical stressors such as pesticides to the soil environment. In the UK, approximately 2x105 kg of insecticides and molluscicides are applied each year, with only 0.1% reaching target organisms. Using a long-term (>20 years), fully-factorial grassland field experiment (Nash’s Field), along with a series of microcosm experiment, this thesis assessed the impact of anthropogenic activity, particularly liming and pesticide application, on soil microbial community structure, functioning, and biogeography. Microcosm experiments indicated a direct impact of pesticides on soil bacterial community structure and metabolic activity. Findings highlighted an asymmetric interaction between pH and pesticide, and indicated that shifts in bacterial community structure were pesticide specific. These results were mirrored in long-term exposure to pesticide, which also significantly altered both community structure and functioning. These commonly used agricultural practices significantly reduced the gradient of the distance-decay relationship, creating increasingly similar communities by way of environmental selection. Given the link between community structure and functioning, anthropogenic induced disturbances pose a serious threat to agricultural sustainability as well as potentially altering landscape heterogeneity.