Biologically driven fluxes of H2, COS, and CO2 across a temperate forest snowpack

Snowpack overlying temperate soils insulates soil microbial communities from wintertime subzero air temperatures that would otherwise halt most biogeochemical processes. Moreover, a porous snow matrix permits soil-atmosphere trace gas exchange to continue despite the snowpack cover. Consequently, below snow (subniveal) soil biogeochemical processes proceed throughout the winter season and continue to impact atmospheric trace gas composition. In this study, three climatically active atmospheric trace gases (H2, COS, CO2) that exhibit strong soil-atmosphere exchange are investigated to understand the following: 1) how snowpack properties affect the exchange of trace gases and 2) how different biogeochemical cycles behave throughout the low temperature subniveal winter. The selected trace gases represent largely decoupled and distinct biogeochemical cycles. Soil microorganisms act as a net sink for atmospheric hydrogen (H2) and carbonyl sulfide (COS) by oxidation (hydrogenase) and hydrolysis (carbonic anhydrase) reactions, respectively. In contrast, soil microbial respiration is a strong source of atmospheric carbon dioxide (CO2). We present atmospheric flux measurements of H2, COS, and CO2 from temperate deciduous Harvard Forest from the past two winter seasons, which differed greatly in terms of snowpack and air temperature. Additionally, we use a novel camera-based method to monitor snow depth, density, and fractional extent to understand how physical snowpack properties affect the exchange of these trace gases. The episodic nature of snowfall, snowmelt, and snowpack ventilation events are also considered. By comparative analysis of the H2, COS, and CO2 fluxes, we investigate differences in subniveal biogeochemical processes at different soil temperature and moisture levels throughout the winter season. Projections for global change anticipate changes in the temperate snowpack; therefore, understanding the processes by which the snowpack influences soil-atmosphere exchange of a variety of trace gases is crucial.