Terrestrial organic matter content and composition is crucial to quantify and characterize, as soils act as an essential carbon sink but, due to mineralization, can also act as a carbon source to ecosystems and the atmosphere. The McMurdo Dry Valleys (MDV) of Antarctica are a polar desert ecosystem, characterized by 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. Hyporheic microbial mats in the MDV fix carbon and nitrogen essential to downstream productivity, and hyporheic soils represent an area of relatively high soil biological activity due to enhanced access to water. Hyporheic and ephemerally saturated sediments could become increasingly significant as critical zones of productivity in the MDV, as enhanced melt leads to an expanded spatial distribution and longer duration of hyporheic saturation for microbial mat communities. Thus, it is crucial to characterize the composition and rate of turnover of organic matter in hyporheic and ephemerally hydrated soils in the MDV to understand the controls on and dynamics of biogeochemical cycling within this ecotone. Since this region of Antarctica is devoid of vascular plants, all primary productivity and decomposition pathways are within microbial mats. External influences on the soil ecosystem in the MDV are minimal and well constrained. Therefore, MDV are an ideal system to describe and assess the biotic influence on the processing and composition of organic matter within soils that are ephemerally wetted from snow melt or hyporheic processes. Using untargeted metabolomics on a Ultraperformance High-Pressure Liquid Chromatography Mass Spectrometry (UPLC-MS) and isotopic characterization (13C, 14C, and 15N) analyses, I aim to explore the influence of microbial mats on soil biogeochemical cycling. Through the identification of metabolites and biomarkers within the soils, I seek to elucidate and describe the chemical composition of the organic matter deposited into the soils, facilitating predictions on the fate and processing of organic matter in both soils and adjacent stream ecosystems.