Environmental Gradients Drive Biogeographic Patterns In Soil Microbial Communities PDF Download

One of the central goals of the field of microbial biogeography is to better understand spatial patterns of microbial community diversity and how communities respond to gradients in environmental conditions, be they natural or anthropogenic in origin. The main aim of this thesis was to investigate how gradients in environmental conditions (i.e., across a mountain elevational gradient and across different land-use types) affect soil microbial community structure, diversity and functional traits, and to assess how these communities respond to differing environmental variables, using next-generation sequencing technologies. Elevation gradients are commonly used to explore impact climate impacts on biological communities since declines in temperature with increased elevation can generate substantial climate gradients over small spatial scales. However, inconsistent spatial patterns in soil bacterial community structure observed across elevation gradients imply that communities are affected by a variety of factors at different spatial scales. Here, I investigated the biogeography of soil bacteria across broad (i.e., a ~ 1500 m mountain elevation gradient) and fine sampling scales (i.e., both aspects of a mountain ridge) using 16S rRNA gene sequencing. Across equivalent distances, variation in bacterial community composition changed more with variation in site aspect than elevation. Bacterial community composition and richness were most strongly associated with soil pH, despite the large variability in multiple soil climate variables across the site. These findings highlight the need to incorporate knowledge of multiple factors, including site aspect and soil pH for the appropriate use of elevation gradients as a proxy to explore the impacts of climate change on microbial community composition. Similar to , inconsistent elevational patterns in soil fungal community diversity suggest that these communities are driven by a complex underlying mechanism. Thus, to enhance understanding of whether distinct biogeographic patterns can be distinguished between different microorganisms and how such gradients influence the potential interactions among individual taxa, I assessed variation in the co-occurrence of different fungal taxa at different elevations along the aforementioned mountain ridge, using fungal internal transcribed spacer (ITS1) DNA sequencing. Fungal community composition changed significantly along the gradient, and their co-occurrences were less frequent with increasing elevation. Such changes with elevation were associated with soil nutrient concentrations, likely driven by the relative ability of different taxa to compete for nutrients at different environmental concentrations. Evidence of nutrient-driven shifts in fungal community diversity and function in soil will enhance our understanding of underground nutrient cycling and the likely impacts of climate change and agricultural disturbance on soil microbial communities. To further explore gradients in the functional potential of soil bacterial communities along an elevation gradient, I devised a method to 'infer' metagenomics data from bacterial 16S rRNA gene sequences. I evaluated the applicability of my 'inferred metagenomics' approach, by comparing bacterial community composition derived from the original bacterial data to communities derived only from the 400 taxa for which genomic information is available. The results generated from these two datasets were highly similar, suggesting that the subset of 'inferred' community was largely reflective of that of the wider environmental community. Further analysis indicates that bacteria with larger genome size appear to prevail across the elevation gradient, suggesting that microorganisms might successfully cope with harsh or various environmental conditions by retaining a larger burden of potential genes and related functions. These findings highlight the potential for using inferred genomic information, based on bacterial 16S rRNA gene data, to generate a general functional trait-based picture of microbial biogeographical patterns. Apart from studies on elevational patterns of soil microbial communities, many other environmental gradients impact distributions of bacterial communities, including gradients of anthropogenic disturbance. Therefore, I studied how pastoral land management practices affect soil bacteria, both in agricultural soils and adjacent forest fragments along 21 transects bisecting pasture-forest boundaries. Decreased compositional dispersion of bacterial communities in the grazed pasture soils resulting in a net loss of diversity caused by community homogenisation after forest-to-pasture conversion. Additionally, a greater richness of pastureonly taxa for sites with a fence on the boundary between the two land uses revealed that boundary fences play an important role in protecting the integrity of soil bacterial communities in forests surrounded by agricultural land via restricting livestock invasion. The observed variation in bacterial community richness and composition was most related to changes in soil physicochemical variables commonly associated with agricultural fertilisation. Overall, my findings demonstrate clear, and potentially detrimental, effects of agricultural disturbance on bacterial communities in forest soils adjacent to pastoral land. This thesis reports the findings of a comprehensive evaluation of the impact of different environmental gradients on soil microbial community composition and functional potential, encompassing sample data collected across different spatial scales and land use types, as well as between different microbial phylogenetic groups. These results confirm that spatial patterns in both bacterial and fungal community structure are driven by various interacting environmental variables related with natural gradients or agricultural disturbances.