Net Soil Exchange of CO2 and CO in a Mesocosm Experiment with Incipient Basaltic Tephra

Soils function as sources and sinks of trace gases due to various biotic and abiotic processes, influencing atmospheric composition. Understanding environmental drivers of terrestrial gas fluxes is necessary to predict potential feedbacks from soils to the Earth climate system. The Biosphere 2 - Landscape Evolution Observatory (LEO) is a large-scale simulated environment experiment investigating interactions of hydrology, biogeochemistry, and ecology in evolving soils. Utilizing LEO facilities as climate-controlled model systems can enhance understanding of gas exchange between soils and atmosphere. To that end, this research examined a miniature analog of LEO (miniLEO), a vegetation-free mesocosm uniformly filled with ground basaltic tephra at 10° slope. We explored net soil gas exchange of carbon dioxide (CO2) and carbon monoxide (CO) to identify and compare relationships of CO2 and CO soil fluxes to key environmental drivers such as soil temperature. MiniLEO was subjected to a rain schedule of two 3-hour rains, with a 2-hour separation, at 13 mm/hr every other day; growing lights mimicked daytime solar radiation. Net gas exchange was estimated using a chamber built of plexiglass and aluminum, divided into three sections: upslope, mid-slope, and downslope. Gas concentrations were measured within the headspace of the chamber at 1 Hz using a quantum cascade laser spectrometer and then used to calculate CO2 and CO surface fluxes. Our results conveyed that, in general, miniLEO soil acted as a net sink of both CO2 and CO. Net soil CO2 uptake could be due to biotic (e.g., microbes and nonvascular plants generating biomass, root and mycorrhizal activity) and abiotic (e.g., carbonate weathering) processes of CO2 utilization exceeding contribution of respiration and decomposition processes.Net soil CO uptake could be attributed to microbial consumption and CO oxidation, a well-known reaction in soils. Mechanistic understanding and quantification of potential sinks and sources of both CO2 and CO fluxes are important as global soil CO budgets remain highly uncertain, and soil CO2 uptake still sparks debate, especially when resulting from abiotic processes.