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Soil carbon dioxide (CO2) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, Soil CO2 fluxes (approximately -0.1-0.15 mu mol m(-2) s(-1)) are generally low, and negative fluxes (uptake Of CO2) are sometimes observed. A combination of biological respiration and physical mechanisms. driven by temperature and mediated by soil moisture and mineralogy, determine CO2 flux and, therefore, soil organic C balance. The physical factors important to CO2 flux are being altered with climate variability in many ecosystems including and forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control Soil CO2 fluxes and C balances. We measured soil CO2 flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources Of CO2 flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO2 flux. Variation within these diel CO2 cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO2 flux if Moisture was not also increased. We conclude that CO2 flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO2 fluxes in and soils have recently been identified as potential net C sinks. We demonstrate the potential for and polar soils to take UP CO2, driven largely by abiotic factors associated with climate change. The low levels Of CO2 absorption into soils we observed may not constitute a significant sink of atmospheric CO2, but will influence the interpretation Of CO2 flux for the dry valley soil C cycle and possibly other and environments where biotic controls over C cycling are secondary to physical drivers. (C) 2009 Elsevier Ltd. All rights reserved.