The McMurdo Dry Valleys of Antarctica are characterized by desert soils underlain with permafrost, a glacier- and ice-dominated hydrosphere, and extreme cold and dry conditions for the majority of the year. Yet, each summer, melt connects and reactivates desiccated communities of heterotrophic and autotrophic organisms concentrated in microbial mats. Microbial mats are a key component of Dry Valley ecosystems, contributing disproportionally to autotrophic productivity and soil stabilization. Climate forecasts predict enhanced hydrological connectivity across Dry Valley landscapes, consequently expanding the spatial distribution of microbial mats. It is therefore crucial to characterize the impacts of microbial mats on soil biogeochemical cycling. We will investigate the relationship between microbial mats, meltwater, and soil organic matter (OM) by analyzing the δ15N and δ13C compositions of soil and mat samples collected from Taylor Valley, Antarctica. Both mat and soil OM will be characterized through δ15N and δ13C stable isotope analysis. By comparing resulting isotopic signatures, we will determine the source of OM, sinks, fractionation, and mixing among microbes and meltwater (both subsurface permafrost and surface snow). We hypothesize that microbial mats will have more depleted 13C signatures relative to the C that is abundant in legacy soils, reflecting increased photosynthate, and enrichment of 15N from N-fixation and depleted abiotic sources relative to legacy pools of N. In the soils directly beneath microbial mats, we expect δ13C and δ15N signatures similar to mats, albeit with lower OM concentrations. In soils with scant to no microbial mat cover, we hypothesize an enrichment in 13C signatures, representative of the legacy OM present in the Dry Valley soils. Collectively, this work will elucidate the influence of microbial mats on soil biogeochemical cycling within the Dry Valleys and potentially inform us how these dynamics could shift with a changing climate.