Uncovering microbial metabolic trajectories during the snowmelt period in mountainous watersheds

Mountainous watersheds are the water towers for the world, the Upper Colorado River Basin for example, provides the majority contribution to the Colorado River, a source of water to one in 10 Americans across seven western states. Changing climate has altered snowpack dynamics, inducing changes in the hydrobiogeochemical cycles that regulate water and elemental flux from these watersheds. Plant-soil-microbial interactions are key, and microbial metabolism under the winter snowpack can regulate the mobilization, retention and loss of nitrogen and other elements. At the East River watershed we are exploring how montane meadow soil microbial metabolism responds to changing snowpack dynamics including interactions between carbon, nitrogen, phosphorus and sulfur cycles. A microbial biomass bloom and crash was observed during snowmelt and we used metagenome reconstruction, metatranscriptome assembly and mapping to identify the responding microbes and their expressed pathways. Analysis of metabolite dynamics via 1H-NMR and high mass-resolution metabolomics (FTICR-MS) demonstrated a sequential transition from metabolism of plant polymer decomposition products, to fermentation, followed by metabolism of C1 compounds including various methylamines. In parallel experiments using soils from conifer dominated hillslopes, continuous analysis of volatile compounds (VCs) via PTR-MS confirmed the key role for VCs in supporting the microbial biomass bloom during snowmelt and the potential intersection between methylamine metabolism and nitrification. These processes are highly sensitive to snowpack properties and snowmelt dynamics which vary spatially and temporally across the watershed. In order to extend these hillslope scale observations, we are currently exploring the watershed scale distribution of microbial functional traits and how they relate to watershed functional traits (geology, topography, hydrology, vegetation) that can be mapped using remote sensing.