Temperature, moisture, and oxygen interactions in a freezing-thawing incubation experiment

Abstract:

Soil organic carbon (SOC) responses to temperature change have been studied extensively; nevertheless, there are ongoing debates about how changes in temperature, especially in cold areas, concurrent with various biotic and abiotic environmental factors, govern the decomposition rate of SOC. In particular, there is uncertainty about the sensitivity of SOC decomposition near the freezing point of water. To improve our understanding on multiple controls on SOC decomposition, we conducted a full factorial incubation study using soil samples from a grassland in the Tibetan Plateau (TP), which holds approximately 2.3 % of the global C stocks. Here, we addressed the following questions: 1) How do temperature changes in the vicinity of the freezing point of water, together with different levels of soil moisture and O₂ content, affect the decomposition rate of SOC? 2) How different environmental conditions affect the temperature sensitivity of SOC decomposition rate at low temperatures? To address these question, we conducted a full factorial incubation experiment with two sets of freeze-thaw cycles by continuously changing temperature, from -5 to 10 ˚C. Additional treatments included 4 levels of soil moistures at 15, 30, 60 and 90 % of water-filled pore space, and two levels of O₂ concentration at 0 and 20 %. Our result shows that soil temperature has the strongest control on SOC decomposition rate at all soil moisture and O₂ levels. The heterotrophic respiration rate was higher in the dry-oxic and saturated-oxic treatments compared to the anoxic condition, by 49 and 66 %, respectively. This difference was less than 20 % when the soil was moderately moist (30 and 60 % WFPS). We also found that SOC decomposition rates accelerate faster when temperature increased from 0˚C compared to the case when temperature increased from -5 to 0˚C. Similarly, the rate of deceleration was lower in temperatures below 0˚C. The intrinsic temperature sensitivity of SOC decomposition rate was identical regardless of temperature, soil moisture or O₂ levels, except close to freezing point of water, where we observed high variability in temperature sensitivity. These results indicate that the rate of heterotrophic SOC decomposition in cold conditions such as those found in the TP, will respond strongly to the availability of O₂ as temperatures increase.