In arid and semi-arid ecosystems, dominant tree species create dramatic mosaics of plant islands of fertility and relatively barren plant interspaces that exert immense pressure on ecosystem processes and offers an ideal opportunity to explore the impact of bacterial communities. We evaluated potential links between soil respiration and N mineralization, and community co-occurrence networks and predicted gene function across three tree island microsites (i.e., beneath tree canopies, at the canopy edge, and in interspaces) in a replicated field experiment in thirty-eight woodlands sites in the Great Basin Desert in UT, USA. Additionally, we potentially intensified the effects of tree islands by creating a treatment where whole trees were shredded and the resulting fine woody debris (FWD) was deposited onto the soil surface and measured a suite of characteristics relating to the metabolic functional state of communities (i.e., microbial efficiency as the microbial quotient, C substrate quality, biomass, and dissolved organic C) to improve our interpretation of potential links between function and structure. We found that tree islands were the predominant driver, creating highly complex and connected assemblies of bacterial populations and easily discernable differences in abundance and composition of predicted functional genes. Specifically, communities directly beneath Juniperus and Pinus canopies were comprised of at least 5.2-times more connections between bacterial taxa than present in networks from interspace and edge. Using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) to predict the gene expression, differences in the functional potential mirrored shifts in network complexity. Tree island communities expressed 236 genes with many related to the degradation of polyaromatic or polycyclic compounds, while interspace communities expressed only 66 genes associated with the decomposition of more labile C substrates. We observed a robust tree island microsite effect on all ecosystem processes, with soil respiration rates increasing 12% and N mineralization decreasing 29% in canopy than interspace soils demonstrating that a more recalcitrant substrate from a sole C source provided high amounts of low quality of DOC and lead to a decrease in metabolic efficiency, but ultimately selected for a specific community assembly. Alternatively, communities at the edge of canopies, experiencing both tree island and interspace soil conditions generated similar levels of soil respiration as canopy soils regardless of not selecting for a highly connected community and/or specific genes suggesting that a diverse composition of labile and recalcitrant C substrates from multiple sources (e.g., trees, perennial grasses, annual grasses, and forbs) potentially elevates function by promoting the activity of a wide range of taxa. Our results identify that tree islands exert enough pressure to create distinct interactions between bacteria and alter gene expression resulting in changes in ecosystem function, but the link between structure and function is mediated through the diversity and quality of C substrates.



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Life Sciences; Plant and Wildlife Sciences



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