Enzyme Kinetics Affecting Thermal Sensitivity of SOC Decomposition Differ in Hummocks and Hollows of a Permafrost Bog

Abstract

Northern peatlands are important pools of soil organic carbon (SOC) and show a fine-scale feature of hummocks and hollows. However, there remains uncertainty about the magnitude of SOC–climate feedbacks because of the knowledge gap about the fine-scale spatial pattern and temperature sensitivity (Q₁₀) mechanism of SOC decomposition. We collected peatland soils from the Greater Khingan Mountains in Northeastern China to investigate how soil enzyme kinetics control the hummock–hollow pattern of the Q₁₀ of SOC decomposition. Results revealed that soil enzyme kinetic parameters (maximum reaction velocity [V_max] and Michaelis constant [K_m]) were greater in hollows compared to hummocks. In the 0–15 cm depth, the catalytic efficiencies (K_cat) of β-1,4-glucosidase (BG) and acid phosphatase (AP) were greater in hollows than in hummocks, but the K_cat of 4-N-acetylglucosaminidase (NAG) was reversed. In the 15–30 cm depth, the K_cat of NAG and AP was greater in hollows than in hummocks, but the K_cat of BG was reversed. Except for the K_m of NAG, all enzyme kinetic parameters increased with rising temperature. The Q₁₀ values of V_max and K_m were greater in hummocks than in hollows, with Q₁₀ values of V_max ranging from 1.48 to 2.22 and from 1.12 to 2.12 for hummocks and hollows, respectively. Similarly, the Q₁₀ values of K_m ranged from 0.70 to 1.67 and from 0.55 to 1.50 for hummocks and hollows, respectively. A positive correlation was observed between V_max and K_m, indicating that increases in K_m may counterbalance increases in V_max as temperature rises. The Q₁₀ values for SOC decomposition were greater in hummocks than in hollows, with average Q₁₀ values of 3.5 and 1.5 when temperature increased from 5°C to 15°C, respectively. Soil TP and the Q₁₀–V_max of NAG and AP have emerged as the best predictors of Q₁₀ values for SOC decomposition, and they were positively correlated with the Q₁₀ values for SOC decomposition. The larger phosphorus content and the high temperature sensitivity of hydrolase activities on hummocks suggest they would have a high potential for carbon mineralisation in the background of climate warming. These findings suggest that soil enzyme kinetic parameters and their Q₁₀ provide valuable tools for predicting the response of microbially mediated SOC decomposition to climate warming scenarios.