Abiotic processes dominate CO₂ fluxes in Antarctic soils

Ecosystems within the McMurdo Dry Valleys of Antarctica are highly sensitive to environmental change. Increases in soil temperature and/or moisture content may dramatically change rates of soil respiration and soil carbon (C) turnover. Present estimates of soil respiration rates and C turnover times are based on surface carbon dioxide (CO₂) fluxes and soil organic C content. However, the assumption that surface CO₂ fluxes are purely biological in origin has not been rigorously tested. We use concentration and, for the first time, the stable C isotopic composition of surface soil CO₂ fluxes and subsurface CO₂ profiles to: 1) examine mechanisms of soil CO₂ uptake and release, 2) identify the location of potential CO₂ sources and sinks within the soil profile, and 3) discriminate between biotic and abiotic contributions to CO₂ fluxes in soils of Taylor Valley. Surface CO₂ fluxes and subsurface CO₂ profiles confirm that these soils take up and release CO₂ on a daily basis (during the austral summer), associated with small changes in soil temperature. Shifts in the C isotopic composition of soil CO₂ are inconsistent with biological mechanisms of CO₂ production and consumption. Instead, the isotopic shifts can be accounted for by Henry's Law dissolution and exsolution of CO₂ into a solution of high pH, driven by changes in soil temperature. Our results constrain the biological component of soil CO₂ fluxes in Taylor Valley to less than 25% (and likely to be significantly less). This finding implies that previous measurements of surface soil CO₂ fluxes are overestimates of soil respiration, thus C turnover times calculated from them are underestimates. Discriminating between biotic and abiotic contributions to CO₂ fluxes in Antarctic dry valley soils is essential if the effects of climate change on these sensitive ecosystems are to be accurately identified.