Characteristics And Impact Factors Of Carbon Release From Thaw Slump Landforms In Qilian Mountains

Carbon release from permafrost degradation is potentially the largest terrestrial feedback to climate change.Abrupt thawing of permafrost namely thermokarst is one of the most dramatic permafrost degradations,which may affect about half of soil organic carbon stored in permafrost regions.However,largely is unknown about the effects of thermokarst development on greenhouse gas fluxes and carbon stocks.Therefore,the overarching scientific question of this study is how the thaw slump development regulates environmental factors,organic matter components and microbial community characteristics to affect the distribution of soil carbon and nitrogen content and the process of ecosystem carbon release.We investigated seven thaw slumps developed the eboling areas in Qilian Mountains.Based on remote sensing data,ages of thaw slumps were determined and divided into three stages:the initial stage(development age<5 years),the middle stage(development age 5-8 years or 23-51 years)and the late stage(development age>51 years).We measured soil organic carbon(SOC),total nitrogen(TN)contents,as well as carbon dioxide(CO₂)and methane(CH₄)release rates,and analyzed the effects of thaw slump evolution on soil carbon storage and greenhouse gas emission fluxes.Through aerobic incubation experiment and measurement of soil carbon and nitrogen content and components,soil microbial community and enzyme activity to explore the effects of soil biological factors and abiotic factors on SOC decomposition from thaw slump.The main results are as follows:(1)The results showed that different stages of thaw slump development have different effects on soil hydrothermal,carbon and nitrogen loss.In the initial stage of thaw slump(<5 years),compared with the control area,the soil temperature(0-20cm)in the collapse area increased by 18%,and the soil water content(0-10cm)decreased by 3.8%.The soil temperature and water content decreased with the increase of the development age of thaw slump.In the middle stage(5-8 years),the soil temperature in the collapse area decreased by 12%compared with the control area,and the decrease was the largest.The soil water content in the middle stage(5-8 years)and the late stage(>51years);Compared with the control area,the soil water content in the collapse area decreased by 11%,and the decrease was the largest.It had no significant effect on topsoil carbon and nitrogen content at the initial and late stage of thaw slump(<5 years and>51 years).However,the loss of carbon and nitrogen in the topsoil and the subsoil are large at the middle stage(5-8 year and 23-51 year)of thaw slump,of which the maximum reduction of SOC and TN concentrations is 38%and 45%,respectively.In addition,we also found that the re-accumulation of carbon and nitrogen in the subsoil in the late stage(>51 years)of thaw slump.(2)In the initial stage of thaw slump(<5 years),the Reco increased by 8.5%.However,in the middle(5-8 years)and the late stages(23-51 years)of the thaw slump,Reco was significantly reduced,by 25%and 40%,respectively.Compared with the control zone(NEE=?3.79 mol m^-2 s^-1),the developmental age<5 years of thaw slump,the carbon absorption capacity increased and NEE was-6.01 mol m^-2 s^-1.However,with the increase of the development age of thaw slump,the ecosystem carbon absorption capacity weakened and even shifted from carbon sink to carbon source.For example,the NEE in the middle and late thaw slump were-1.60 mol m^-2s^-1 and+0.01 mol m^-2 s^-1,respectively.Compared with the control area,CH₄ increased significantly in the exposed area.The increase of thaw slump in the exposed area reached the highest at 5-8 years(6.74 mg CH₄ m^-2h^-1)and 23-51 years(2.37?mol m^-2s^-1),with an increase 2 to 10 times,respectively,but gradually decreased with the increase of the developing age.(3)Soil enzyme activity,microbial community and soil carbon composition changed greatly during the development and evolution of thaw slump.With the increase of soil collapse time,the activities of carbon and nitrogen related enzymes in the surface and subsurface soils increased first,reached the maximum in the soil one year after the soil collapse,and then gradually decreased.With the increase of the soil collapse time,the content of labile carbon(O-alkoxy-C)decreased gradually,and the content of recalcitrant carbon(alkyl-C)increased,while the content of aromatic carbon and carboxyl carbon in the collapse area was lower than that in the controlled area.The abundance of microorganisms in the surface and subsurface layer decreased with the increase of soil collapse time,and that in the surface layer was higher than the subsurface.(4)After 118 days of aerobic incubation experiment,it was found that the SOC in all regions produced a faster rate of CO₂ at the early stage of incubation,but decreased significantly after nine days.In the topsoil(0-15cm)and subsoil(15-30cm),the microbial decomposition potential of organic carbon was as follows:1 years of slump>control>10 years of slump>23 years of slump.The results of variation partitioning analysis showed that soil carbon quality(soil carbon and nitrogen content,components),soil water content and microbial community were the main factors affecting the carbon release,which explained 71%of the variation of the carbon release,and soil carbon quality was the main driving factor of the carbon release,explaining64%of the variation.Under the influence of climate warming and human activities,the thermokarst landforms on the Qinghai-Tibet Plateau are widely distributed and rapidly developing.By combining remote sensing,field monitoring and experimental analysis,this paper elucidates the biogeochemical process of carbon decomposition and release caused by thaw slump,and provides important data materials and theoretical basis for exploring the carbon-climate feedback effect of permafrost and predicting future climate change.