Priming Effects Induced By Organic Matter Input In Qinghai-Tibet Plateau Lakes And Their Microbial Mechanisms

Saline lakes are a crucial component of the world’s inland waters and play a significant role in the storage and mineralization of organic matter.Saline lakes are important sources of greenhouse gas emissions and are a key component of the global carbon cycle.Saline lakes are widely distributed.The Tibetan Plateau possesses a great number of saline lakes.The salinity of these lakes will change with water level changes,impacting the carbon cycling and OM mineralization processes.The salinity of saline lakes can be affected by climate change and understanding the effects of salinity change on OM mineralization is crucial for evaluating the carbon feedbacks to climate change.The positive priming effect（PE）is a key mechanism in regulating global organic carbon storage and stability,but it remains unclear which kind of OM（allochthonous or autochthonous）can induce stronger PE intensities and which PEs are more sensitive to salinity change.Lakes play a significant role in the global carbon budget,emitting around0.6 Pg carbon to the atmosphere each year.The intensity of PE is considered to be a crucial parameter in predicting the carbon budget in lakes,but there is a lack of data on PE intensity in lakes,hindering its use in prediction.Positive PE can be caused by stoichiometric decomposition and nutrient mining,and studies have suggested that the early stage of PE generation is usually governed by stoichiometric decomposition while the late stage is controlled by nutrient mining.However,little is known about the difference between the regulatory factors of early and late PE intensities in lakes,or about the dominant factors regulating lake PE intensity under different temperatures.Further research is needed to address these knowledge gaps and provide a better understanding of lake PE intensity and its regulating factors.This will help in predicting the carbon dynamics of lakes under global change and in understanding the temperature sensitivity of PE intensity and its feedback to climate warming.The present study chose the saline lakes of Qinghai-Tibet Plateau as a study object,with the use of an integrated biogeochemical technology and microcosm experimental simulation.The purposes of this study are to verify that salinity and grass OM input influenced the composition of key taxa and to reveal the response of the salt lake microbial community to the increase of allochthonous organic matter input and its function.Carbon isotope techniques and microbial techniques were applied to corroborate that microorganisms are key factors driving the occurrence of the positive excitation effect,which directly affects the stability of autochthonous organic carbon storage and is important for understanding lake organic carbon mineralization and storage.The main research findings are listed as follows:1)Water surface expansion induced by climate changes has caused increased inputs of terrestrial organic matter into saline lakes.However,the ecological consequences of terrestrial OM input increase remain poorly understood.Here,microbial mineralization of terrestrial grass（Achnatherum splendens）OM at different quantity levels in lake sediments with different salinity was investigated by performing microcosm experiments.The incremental supply of grass OM in the studied sediments increased CO₂ production rates,average network connectivity between fungal taxa and other microorganisms,and relative abundances of Bacteroidota,Firmicutes,Spirochaetota and Ascomycota;whereas it decreased prokaryotic richness and community assembly stochasticity,average path length and modularity of networks,numbers of network nodes,edges and keystone taxa.The decrease in salinity will intensify the positive correlation between microbial mineralization of organic carbon and the terrestrial grass OM input.Among the measured variables,five significant predictors（i.e.,grass powder quantity,salinity,and relative abundances of Firmicutes,Desulfobacterota and Proteobacteria）gave a strong prediction for CO₂ production rates in the studied samples.Together,desalinization of saline lakes will enhance microbial mineralization of terrestrial grass OM,and its intensity increases with the increase of grass OM input.2)Ecological consequences of the increased input of allochthonous and autochthonous OM remains unknown in saline lakes.Here,we reported microbial mineralization of algal and grass OM（represented by autochthonous and allochthonous OM,respectively）in lake sediments with different salinity(1,40 and 120 g L^-1).The addition of algal and grass OM significantly increased the CO₂ production rates in the studied sediments.Algal and grass OM input can induce a positive priming effect.PE intensity induced by grass OM input was significantly higher than that by algal OM input.PE intensity induced by algal and grass OM input decreased with increasing salinity.Bacterial taxa affiliated with Actinomycetia,Alphaproteobacteria,Bacilli,Bacteroidia,Clostridia and Gammaproteobacteria played important roles in driving PE generation in the studied sediments.Our finding suggested that the priming effects induced by allochthonous and autochthonous OM should be considered in saline lakes that are intensively influenced by climate change.3)Priming effect is a key biogeochemical process regulating organic carbon mineralization.However,the impacts of climate change-related factors on PE intensity remain largely unclear in lakes with a large salinity range.The patterns of PE intensities induced by algal(¹³C-labeled Chlorella vulgaris)and terrestrial grass(¹³C-labeled Festuca ovina)organic matter at different temperatures were investigated in lake sediments with a salinity range from 0.7 to 376.3 g L^-1.In the studied lake sediments,positive PE predominated over negative PE.Climate-related factors（e.g.,salinity,temperature,OM types）exhibited significant influences on the intensity and/or direction of PE.Positive PE was more likely to occur in the sediments of hypersaline lakes than of freshwater/saline lakes.PE induced by algal OM was more sensitive to temperature change than that induced by grass OM,suggesting PE induced by autochthonous OM is more responsive to climate warming than that by allochthonous OM in affecting organic carbon mineralization.The OM types（algal OM vs.grass OM）significantly affected the intensity of early PE but did not affect that of late PE,indicating that early PE intensity was more susceptible to different OM types than late PE intensity.Together,our study provides empirical evidence that positive PE can accelerate organic carbon loss in lake sediments with a large range of salinity,and sheds light on the regulating mechanisms of lake PE intensity.The presented results are of great significance for understanding the carbon cycle in lakes under global climate change.In summary,the results of this thesis verified that salinity and OM input from grasses influenced the composition of key taxa and revealed the response of microbial communities in salt lake sediments to increased exogenous organic matter input and their functions.It further confirms that microorganisms are the key factors driving the occurrence of the positive priming effect,which directly affects the stability of local organic carbon storage.The positive priming effect can accelerate the loss of organic carbon in the sediments of a large range of salinity lakes and enriches the understanding of the mechanisms regulating the intensity of the priming effect in lakes.It has important implications for the deeper understanding of the lake carbon cycle under global climate change.