Study On Migration And Transformation Of Sediment Carbon As Well As The Driving Mechanism In Yulin River,A Tributary Of The Three Gorges Reservoir

The carbon emission of river system is an important part of the global carbon cycle,and its contribution to the global greenhouse effect can not be underestimated.Sediment is the main carbon pool of river system and an important place for the generation of carbon dioxide（CO₂）and methane（CH₄）.At present,most of the research on carbon emission from river system focuses on single-medium water body and water-air interface,ignoring the research on the transformation,upward migration and emission process of carbon in river sediment.The Yulin River,a tributary of the Three Gorges Reservoir（TGR）was used as the object of this study.We investigated the spatiotemporal distribution patterns of various species of carbon in sediment,analyzed the microbiological driving mechanism of CO₂ and CH₄ production and emission in sediment from the perspective of molecular biology.The main source of organic carbon in sediment was identified by means of stable isotope characterization,and the response behavior and mechanism of carbon in sediment under dry-wet alternating environment were discussed.The main research contents and conclusions of this study are as follows:（1）The spatial and temporal distribution characteristics of carbon in sediment were investigated during the stable impoundment period and the stable discharge period of the TGR,respectively.The main influencing factors of CO₂ and CH₄ production in sediment were analyzed by structural equation model.Combined with the fluxes of CO₂ and CH₄at sediment-water interface and water-air interface,the production and discharge processes of CO₂and CH₄ in sediment have been preliminarily discussed.The results showed that the organic carbon content of sediment was enriched in the upstream of the Ecological Regulation Dam of Yulin River,which promoted the CO₂ and CH₄ production in sediment and led to a greater diffusion fluxes of CO₂ and CH₄ at the sediment-water interface of the upstream than downstream.The structural equation model results showed that organic carbon was the main factor affecting the concentration of CO₂ and CH₄ in sediment of impoundment period,the negative effect of dissolved oxygen on CH₄concentrations in sediment was significantly enhanced in discharge period,which indicated that the methane oxidation process had an important impact on the migration and transformation of CH₄ in sediment.During the impoundment period,the CH₄ in sediment entered the atmosphere by diffusion and the CH₄ flux at the water-air interface of upstream was greater than that of downstream.During the discharge period,the CH₄in sediment was mainly released by bubbling.The water depth of downstream was shallow in discharge period,which led to more CH₄ bubbles that successfully escaped from the water,therefore the CH₄ flux at water-air interface of downstream was much higher than that of upstream.Meanwhile,the downstream where the water body was characterized by a greater decline of water level and a faster water flow during the discharge period,which was conducived to the input of terrestrial CO₂ into the water body,consequently resulting in a higher CO₂ flux at the water-air interface.（2）The high-throughput sequencing and quantitative PCR were used to analyze the microbial community structure,the abundance of methanogens and methanotrophs in sediment and water,as well as the carbon metabolism and methane metabolic pathways based on functional prediction.The microbiological driving mechanism of CO₂ and CH₄production and emission in sediment was analyzed from the perspective of microbial metabolism.It was found that the methanogenic microorganisms in the sediment were mainly hydrogenotrophic methanogens.With the increase of temperature in the discharge period,the abundance of functional enzymes involved in carbon metabolic pathway in sediment increased,which promoted the production of CO₂ in sediment.As a substrate for hydrogenotrophic methanogens,the CO₂ further promoted the methanogenesis process in sediment of discharge period,while the increase of the abundance of aerobic methanotrophs in sediment led to a lower concentration of CH₄ diffusing into the overlying water.The photosynthesis of Oxyphotobacteria in the upstream water during the discharge period reduced CO₂ in the water,thereby resulting in a lower CO₂ flux at the water-air interface of upstream.The abundance of Actinobacteria which came from the rhizosphere soil and plants in the downstream water was significantly higher than that of upstream,it was further verified that terrigenous input was the main reason for the higher CO₂ flux of the downstream.（3）Based on the stable isotope characterization methods,the traceability analysis of organic carbon in sediment was carried out.Theδ¹³C,δ¹⁵N and C/N value of sediment were between-26.72‰～-26.41‰,0.98‰～3.52‰,and 8.31～12.87,respectively.The qualitative analysis indicated that the organic carbon in sediment of Yulin River was predominantly derived from algae and terrigenous C3 plants.The calculation results of two end-members mixing model showed that the contribution of terrigenous C3 plants to organic carbon in sediment（>75%）was greater than that of endogenous algae（<25%）.（4）A laboratory simulation experiments was conducted to study the migration and transformation of carbon in sediment of riparian zone,the conversion process of“source-sink”and the driving mechanism of carbon in riparian zone were analyzed.During the drying period,plants grew and sequestrated carbon.After beening flooded,carbon in plants was released into sediment and water.This increased the abundance of methanogens and functional enzymes involved in carbon metabolic pathway in sediment and promoted the production of CO₂ and CH₄ in sediment,which had a positive driving effect on the carbon emission of riparian zone.Once the above-ground parts of plants were harvested before flooding,the riparian zone was transformed from a carbon“source”to a carbon“sink”.In the continuous dry-wet alternating environment,the plant regulation could reduce carbon emissions in riparian zone by 48%.In conclusion,the migration and transformation process of carbon in sediment of tributary of the TGR was closely related to the changes of hydrological environment and plant succession caused by river damming and TGR regulation.This study complements the understanding of the carbon cycle in river system under complex hydrological conditions.It is of great significance for a comprehensive understanding of the process and impact mechanisms underlying the effects of TGR regulation mode on the carbon emission of dammed tributaries.Additionally,it provides a scientific basis for the carbon emission accounting and carbon mitigation strategy of the riparian zone in the TGR.