Abiotic processes dominate CO2 fluxes in Antarctic soils

Abstract

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.