Background/Question/Methods Plant-associated microbes can regulate ecosystem-level processes, such as decomposition. Understanding the ecological and evolutionary mechanisms by which microbiomes regulate ecosystem functions at local scales may facilitate predictions of the resistance and resilience of these functions to change. Our prior studies have shown that microbes can locally adjust to intraspecific variation among chemical defensive traits of riparian trees, causing accelerated microbial decomposition of leaf litter of local origin within riparian soils as well as in the adjacent riverine water column. In this striking pattern of a ‘home-field advantage,’ leaves from individual trees tend to decompose more rapidly immediately adjacent to their parent tree. We merged community ecology experiments with amplicon and shotgun-sequencing based surveys of microbial community composition and function to describe how bacterial communities adjust to within-species variation in leaves over spatial scales of less than a kilometer. We will describe how riverine bacterial community assembly and function corresponds with decomposition rates of leaf inputs from red alder trees into rivers of Washington State, USA. We will also provide preliminary results describing evolutionary divergence of bacterial decomposers across terrestrial and aquatic environments of riparian zones and discuss how this may elucidate how home-field advantage patterns arise. Results/Conclusions We found repeatable stages of bacterial succession on decomposing leaf litter in rivers, each defined by dominant taxa with predicted gene content associated with metabolic pathways relevant to the leaf characteristics and course of decomposition. Local leaves contained bacterial communities with distinct functional capacities to degrade aromatic compounds. Given known spatial variation of alder aromatics, this finding helps explain locally accelerated decomposition. We will also present preliminary results describing microbial life history using shotgun sequencing approaches to describe strain variation across aquatic and terrestrial environments of the Burkholderiales, which emerged as key players in leaf decomposition patterns. We will describe patterns of evolutionary divergence within these taxa across spatial scales relevant to the HFA patterns we have observed in this system. Overall, these results illustrate that bacterial decomposer communities adjust to intraspecific variation in leaves at spatial scales of less than a kilometer, providing a mechanism for rapid response to changes in resources such as range shifts among plant genotypes. We provide preliminary evidence into the ecological and evolutionary factors that may contribute to such rapid responses among microbes, that in turn maintain high rates of carbon and nutrient cycling through riparian ecosystems.
