Temperature controls of microbial respiration in arctic tundra soils above and below freezing

Winter soil respiration can represent a significant fraction of the annual C cycle of arctic tundra, but the temperature response of microbial CO2 production in frozen soils is poorly understood. We used a short-term laboratory incubation to describe temperature effects on aerobic respiration in four organic tundra soils from Northern Alaska. Respiration conformed closely to simple first-order exponential equations with constant temperature dependence (r2=0.81–0.98). Temperature coefficients (Q10) increased abruptly with freezing, varying from 4.6 to 9.4 among the thawed soils (+0.5 to +14 °C), and from 63 to 237 among the frozen soils (−10 to −0.5 °C). The single abrupt increase in temperature dependence with freezing suggests a shift in the dominant process controlling respiration below 0 °C. The Q10s of frozen soils are too large to represent the direct kinetic effect of temperature, and more likely reflect extracellular barriers to diffusion and/or intracellular desiccation. In thawed soils, respiration Q10s decreased with increasing soil organic matter quality, as indexed by baseline respiration at 0 °C, consistent with the thermodynamic argument that reactions metabolizing structurally complex, aromatic molecules have higher activation energies and temperature dependence than reactions metabolizing structurally simpler molecules. In frozen soils, Q10s were unrelated to baseline respiration, suggesting that freezing uncouples the direct link between the C chemistry of microbial substrates and the temperature dependence of respiration. Our results underscore the potential for warming to stimulate microbial activity and the turnover of C and nutrients contained in tundra organic soils.