Organic Carbon Mineralization And Its Temperature Response Mechanism In Sediments Of Saline Lakes

Lakes are an important part of inland waters ecosystems and key nodes for the exchange of materials and energy in various ecosystems.Environmental conditions such as anoxic and less disturbance make the turnover rate of organic carbon in lake sediments slower,while the continuous supply of endogenous and terrigenous materials makes the organic carbon accumulation intensity of the lake sediments higher.The superposition effect of the above two characteristics makes lake ecosystems have a disproportional high organic carbon storage referring to their area.Saline lakes account for more than 50%of the total lake ecosystem area in the world,and are significantly different from freshwater lakes in terms of microbial community composition and biogeochemical cycle characteristics.Obviously,saline lake is an important part of lake ecosystem.However,studies on the stability of organic carbon in lake sediments,are still scarce,and the mechanism controlling the stability of organic carbon in saline lake sediments is unknown.In addition,as an important driver of the organic carbon cycle,the role of microbes in the turnover of organic carbon in lake sediments is not clear.Under climate change,global warming and the intensification of frequent extreme thermal events pose a huge threat to the stability of the lake ecosystem.Exploring the impact of temperature on the mineralization activity of organic carbon and the turnover patterns of different organic components and uncovering its underlying mechanism can help us gain insight into the stability of organic carbon pools in lake sediments under the effect of climate change.Therefore,saline lakes in the Qinghai-Tibet Plateau（QTP）,which have various environmental variables and are experiencing continuous warming,were selected as the research object in this study.The chemical composition and occurrence forms of organic carbon in the QTP lake sediments has been characterized by a variety of organic carbon fractionation methods.The temperature sensitivity of the mineralization activity of sediment organic carbon and the turnover pattern of representative organic carbon components at different temperatures were explored through the method of microcosm cultivation experiments.The underlying mechanism of the distinct responses of different organic carbon components in sediments to warming was analyzed from the perspective of lake physicochemical factors,organic carbon quality and microbial community characteristics.The following main results were obtained:1.Active organic carbon and mineral-associated organic carbon were the main contributors to the organic carbon pool of saline lake sediments.From the perspective of chemical composition,the proportion of active organic carbon with low chemical stability and high C/N ratio was slightly higher than that of recalcitrant organic carbon with extremely high chemical inertness in the total organic carbon pool of lake sediments on the Qinghai-Tibet Plateau.From the perspective of occurrence form,the vast majority of organic carbon in sediments existed in the form of mineral-associated organic carbon.Minerals were more likely to bind to active organic carbon in sediments.2.With the increase of ambient temperature,the organic carbon mineralization activity of saline lake sediments increased significantly.The temperature sensitivity index Q₁₀ of organic carbon mineralization in different lake sediments in the range of5-25℃is 1.65 on average The warming and expansion of lakes caused by climate change would greatly increase the organic carbon mineralization flux of lake sediment ecosystems on the Qinghai-Tibet Plateau,forming a more than 58%carbon flux increase by 2100.The sediment environment with high nutrient level,abundant labile organic carbon and weak mineral protection has higher organic carbon mineralization activity and temperature sensitivity.At the same time,the proportion of r-strategists in microbial community increased with the increase of temperature,which may increase the metabolic activity and the preference for small molecular substrate of microbial communities,and had a potential promoting effect on the temperature response pattern of sediment organic carbon mineralization process.3.Different components in the organic carbon pool of saline lake sediments had distinct response patterns to temperature.Although the high temperature promoted the decomposition and mineralization of all organic carbon components,organic carbon components with low molecular weight,low chemical stability,and microbial origin had higher temperature sensitivity in the decomposition and mineralization process,which were preferentially utilized by microorganisms.While organic carbon components with high molecular weight,high chemical inertness,and plant origin showed a growing contribution to sediment TOC pools with warming.4.The mineral protection mechanism could significantly reduce the temperature sensitivity of the overall organic carbon mineralization process and promote the accumulation of organic carbon with microbial or plant origin.The protective effect of minerals was more important for the stability of organic carbon with microbial origin than that with plant origin and high chemical inertness.The characteristics of organic carbon components in terms of different sources,molecular composition and mineral affinity jointly determined their contribution to the total organic carbon pool and the temperature response mode,which in turn shaped the stability of the total organic carbon pool of sediment.This study introduced the theory of organic carbon stability into saline lake sediment environment creatively,which quantified the sensitivity of organic carbon mineralization process to ambient temperature and its potential environmental effects in the lake sediment environment of the Qinghai-Tibet Plateau from a macroscopic perspective,and revealed the promoting effects of lake nutrient,labile organic carbon and microbial community evolution on mineralization activity and its temperature sensitivity.And this study revealed the difference in the turnover process of microbial and plant-derived organic carbon in saline lake sedimentary environment for the first time,and clarified the important contribution of plant organic carbon to the accumulation of organic carbon pools in saline lake sediments under the background of temperature rise,and the positive effect of mineral protection on the stability of organic carbon pools in saline lake sediments.The results of this study fully demonstrated that in saline lake sediment environment,the superposition of organic carbon molecular composition（source）,mineral protection,and microbial activity determined the response pattern of total organic carbon pools to warming,shaping the organic carbon pool composition over time.This study expands current knowledge of organic carbon stability mechanism in lakes and helps to better understand the evolution trend of organic carbon pools in lakes under the background of climate change.