Soil enzyme response to permafrost collapse in the Northern Qinghai-Tibetan Plateau

Permafrost degradation can result in the formation of permafrost collapsed ground features, and thus greatly alter soil variables such as moisture, pH, soil carbon and nitrogen content, and the biogeochemical cycling of soil carbon. However, little is known about the biogeochemical processes in these features within mountainous permafrost areas. We examined activities of six different soil enzymes (invertase, amylase, catalase, polyphenol oxidase, urease, and alkaline phosphatase) in three micro-topographical settings (i.e., collapsing, collapsed, and an unaffected control site) of a typical thermokarst feature on the northern Qinghai-Tibetan Plateau. Our results show soil moisture is substantially lower within the permafrost thaw-induced collapsed feature. In addition, soil organic carbon, light fraction organic carbon and total nitrogen in the upper 10 cm soil depth were lower in soils where permafrost was in the process of collapsing. Accordingly, soil enzyme activities varied considerably among the three settings, indicating biogeochemical processes have been altered by permafrost collapse. The invertase activities in collapsing soils were significantly lower than those of the control and collapsed soils for the upper 0-20 cm layer. Coefficient of variation values for amylase and polyphenol oxidase were 44.1% and 6.7%, respectively. For 0-10 cm soil depth, the catalase in collapsing soils were highest while the urease activities were lowest among the three settings. Statistical analysis demonstrated that light fraction carbon content, C:N ratios, and moisture were the most important predictors for enzyme activities. These results suggest that soil enzyme activities are good indicators for the decomposition of organic matter in permafrost collapse-affected areas.