| Thermokarst lakes and ponds are common landforms in permafrost regions.The permafrost degradation will accelerate the formation and expansion of thermokarst lakes and ponds,which can increase the release of carbon from permafrost regions.There is a close association between the sediment of thermokarst lakes and ponds and greenhouse gas release.Therefore,understanding of the physicochemical properties such as carbon and nitrogen content in the sediment is helpful to reveal the response of the carbon and nitrogen cycle in the Qinghai-Tibet Plateau.The physio-chemical variables of sediments play a critical role on methane production in the thermokarst lakes and ponds.Quantifying methane production from the sediment of thermokarst lakes and ponds will improve our understanding on the response of methane emissions to climate change over the Qinghai-Tibet Plateau.In this study,we selected 116thermokarst lakes and ponds located in the eastern part of the Qinghai-Tibet Plateau as our research subjects.These were further categorized into four different vegetation types:alpine wet meadow（60 lakes/ponds）,alpine meadow（39 lakes/ponds）,alpine grassland（14 lakes/ponds）,and alpine desert（3 lakes/ponds）.Subsequently,we collected water and sediment samples from these thermokarst lakes and ponds.We determined the physicochemical properties including total carbon（TC）,organic carbon（DOC/OC）,total nitrogen（TDN/TN）,and sediment particle size.The relationships among soil TC,OC,TN,sediment particle size and sediment physical and chemical factors were analyzed,based on sediment samples collected from eight thermokarst lakes or ponds in the central and eastern Qinghai-Tibet Plateau,we intended to investigate the relationship between the physio-chemical variables of sediment and methane production under laboratory incubation at 5℃,10℃,and 15℃,respectively.The results showed:（1）The concentrations of TC and TDN in water were highest in the alpine wet meadow,with values of 119.46 mg·L-1 and 2.05 mg·L-1,respectively,and lowest in the alpine steppe,with values of 52.89 mg·L-1 and 1.05 mg·L-1,respectively.The concentration of DOC in water was highest in the alpine wet meadow,with a value of25.44 mg·L-1,and lowest in the alpine desert,with a value of 7.56 mg·L-1.The concentrations of TC,OC,and TN in sediment were highest in the alpine wet meadow,with values of 35.92 g·kg-1,15.37 g·kg-1,and 1.48 g·kg-1,respectively,and lowest in the alpine desert,with values of 12.93 g·kg-1,1.01 g·kg-1,and 0.25 g·kg-1,respectively.The content of silt and clay in sediment followed the order:alpine wet meadow>alpine grassland>alpine meadow>alpine desert.（2）Correlation analysis of physicochemical factors showed that sediment sand was significantly negatively correlated with TC,OC,and TN（p<0.01）.Clay and silt particles exhibit a strong adsorption capacity for soil organic matter,providing protection against microbial decomposition.Consequently,sediments with higher clay and silt content tend to have higher organic carbon content,whereas sediments dominated by sand particles exhibit the opposite trend.The depth of thermokarst lakes and ponds was significantly positively correlated with TC,OC,and TN（p<0.01）,indicating that the deeper the lake or pond,the higher the C and N content.The deeper the thermokarst lakes and ponds,the higher the content of C and N.This observation may be attributed to the longer formation time of deeper lakes and ponds,leading to a greater accumulation of exogenous organic carbon input from the watershed.The DOC in water was significantly positively correlated with TC in sediment（p<0.01）.Water p H was significantly positively correlated with NO3--N in sediment（p<0.01）.Water PO43--P was significantly positively correlated with sediment p H（p<0.05）.There were no significant correlations between other physicochemical properties of water and sediment.（3）Anaerobic conditions and organic matter enrichment promote methane production,which mainly occurs in the anaerobic zone of sediment in thermokarst lakes and ponds.The main pathways of methane production are acetate fermentation and hydrogenotrophic methanogenesis.The maximum methane production occurred in sample MD-3 incubated at 10°C,with a production rate of 167.63μg·g-1 and a rate of methane production of 3.61μg·g-1·d-1.The minimum methane production occurred in sample AD-4 incubated at 10°C,with a production of 0.01μg·g-1 and a rate of methane production of 1.4×10-4μg·g-1·d-1.The methane production in sediment was significantly correlated with the depth of lakes and ponds,the NH4+-N content,and p H（positively correlated with the depth and NH4+-N content,and negatively correlated with p H）.（4）Temperature is an important factor affecting methane production in sediment in thermokarst lakes and ponds.In this study,the Q10 values indicate that the increasing temperature had promoting effects on 61.11%of methane production,an inhibiting effect on 18.06%of methane production,and no effects on 20.83%of methane production.Overall,warming had a more significant promoting effect on methane production in sediment in thermokarst lakes and ponds in the alpine grassland area.This study analyzed the effects of physicochemical properties and temperature on methane release from sediment in thermokarst lakes and ponds,providing insights into the response of thermokarst lakes and ponds to global warming. |
