Permafrost carbon loss and chemical changes associated with thaw slumps in the northern Qinghai Tibetan Plateau

Abstract:

Permafrost thaw causes ground subsidence or thermokarst. Thermokarst terrain on hilly slopes can lead to the formation of thaw slumps, which dramatically alter soil properties and carbon emissions, but little is known regarding the effects of thaw slumps on the biogeochemical processes of soil carbon. In the present study, we measured the soil carbon contents and physiochemical properties in different thaw slump stages (no slump, slumping and slumped) in the upper reach of the Heihe River Basin in the northeastern Qinghai Tibetan Plateau (QTP). With these samples, the C mineralization rates were measured using laboratory incubations. Meanwhile, the chemistry changes of organic matter were examined using Fourier transform infrared (FTIR) spectroscopy analyzer before and after the incubation. The results showed that there was a significant decrease in soil carbon and nitrogen stocks in the slumping and slumped stages. The loss of organic carbon and total nitrogen was 29.6 ± 5.9% and 31.1 ± 8.8% in the upper 0-10 cm layer of the slumping soil compared to the no slump soil. The slumped soil had a significantly lower loss of carbon and nitrogen content than the slumping soil (t-test, p < 0.05). The incubation results implied that slumped soil has significantly higher cumulative CO₂ production than that of slumping soil (t-test, p < 0.05). In addition, the slumped soil had a higher intensity of hydrocarbons and lignin/phenol backbone composition than that of no slump and slumping soil for the 0-10 cm layer. This study demonstrates that abundant carbon and nitrogen loss occurs during the process of thaw slumps. Slumped soil can accumulate some organic matter, accompanied by substantial changes in its carbon chemical structure and characteristics. These results demonstrate that thaw slump plays an important role in the impact of permafrost thaw on chemical characteristics of organic matter and merits greater attention.