Reduction of air- and liquid water-filled soil pore space with freezing explains high temperature sensitivity of soil respiration below 0 °C

At temperatures just below 0 °C, the temperature sensitivity of heterotrophic soil respiration (RH) is orders of magnitude higher than above 0 °C. Two primary mechanisms have been proposed for this high sub-zero temperature sensitivity: changes in soil microbial community composition and physiology, or the physical effects of the transition of water between liquid and ice phases. In this study, the effect of soil freezing on RH was modeled using a simple modification of the Dual Arrhenius Michaelis–Menten model, to account for both the reduced liquid water content of the soil pore space and the reduced air-filled pore space as water expands during freezing. Using parameters derived from previous studies, RH was modeled at a range of sites throughout Southeast Wyoming, ranging from prairie to high elevation forest. Across the study region, RH at sub-zero temperatures was low in the prairie (0.002 mg C m−2 h−1 at −1 °C and optimal water content) and sagebrush (5·10−7 to 0.012 mg C m−2 h−1 at −1 °C and optimal water content) sites, with lower organic matter and higher sand content, and much higher in the high sub-alpine forest (0.71 mg C m−2 h−1 at −1 °C and optimal water content) and meadow (3.5 mg C m−2 h−1 at −1 °C and optimal water content) sites with high soil organic matter content. The modeled Q10 (the multiplicative response of RH to a 10 °C increase in temperature) above freezing was ∼3.2, while below freezing the median value ranged from 15 to 255, and the maximum was 1.6·1024. These values capture the range of Q10's described in the literature, suggesting that the model based on changing liquid water contented presented here can explain much of these observed apparent temperature responses. Hence, this model may prove valuable for predicting soil C fluxes in environments that undergo seasonal freezing.