Stable Isotopic Signatures Of Organic Carbon And Released Carbon Dioxide In Different Types Of Water Bodies In The Xilin River Basin

The interference of human activities has gradually become an important factor affecting the global carbon cycle.However,less is known about different degrees of human influence on the sources of organic carbon and emitted CO₂ in grassland inland rivers and the changes of carbon conversion process within rivers.In this study,we takes the Xilin River Basin of the Inner Mongolia Autonomous Region as the research object.River water bodies were divided into four categories:natural water bodies,artificial water bodies,sewage discharge area water bodies,and mainstream water bodies according to different surface cover types and human disturbance levels.The field and indoor experiments conducted in April,June,August,and October 2019 to analyze the organic carbon content,CO₂ partial pressure,and stable isotopes of different types of water bodies in the Xilin River.The correlation analysis was carried out between carbon content,carbon emission,stable carbon isotopes and environmental factors,to discussed the differences and influencing factors of organic carbon and emitted CO₂ sources under natural process and human influence.The main results are as follows:1.The DOC content of different types of water bodies in the Xilin River was between 0.24 and 42.63 mg·L^-1,with an average value of 10.23 mg·L^-1.The POC content of different types of water bodies in the Xilin River was between 0.23 and30.90 mg·L^-1,with an average of 3.88 mg·L^-1.The DOC and POC content of different types of water bodies had obvious seasonal differences,and the DOC and POC content of different types of water bodies had significant differences.Precipitation and runoff were the influencing factors for the seasonal variation of DOC and POC content,while human activities were the main influencing factors for the differences in DOC and POC content in different types of water bodies.2.The ?¹³C-DOC value of the Xilin River water bodies varied from-31.72‰to-24.05‰,with an average value of-27.71‰.The?¹³C-POC value varied from-31.82‰to-23.4‰,with an average value of-28.1‰.The source of DOC in natural water bodies was terrestrial C3 plant and the source of POC were C3 plant and soil organic carbon erosion.The source of DOC and POC in artificial water bodies was autochthonous input from aquatic plants,among which DOC was affected by photochemical degradation,which makes the?¹³C-DOC value higher.The source of DOC and POC in the sewage discharge area was the input of sewage organic matter,while nutrients affected the?¹³C-POC by controlling the relative intensity of aquatic photosynthesis and respiration.DOC and POC in the mainstream water were derived from C3 plants,and the?¹³C-DOC value was affected by the influence of grazing disturbance.3.The pCO₂ of the Xilin River Basin ranged from 69 to 10913 ppm,with an average value of 2201 ppm.The pCO₂ of different types of water bodies in the Xilin River had different seasonal difference,and different types of water bodies had significant differences in pCO₂.The seasonal differences of pCO₂ in water bodies were affected by the temperature,runoff,precipitation,and the input of soil respiration.The differences of pCO₂ in different types of water bodies were affected by the input of groundwater and human activities.4.The ?¹³C-CO₂ value of the Xilin River water bodies ranged from-28.93‰to-13.26‰,with an average value of-20.48‰.The source of CO₂ emitted from natural water bodies was groundwater input.The source of CO₂ emitted from artificial water bodies was decomposition of organic matter by microorganisms,among which emitted CO₂was affected by photosynthesis,which makes the?¹³C-CO₂ value higher and pCO₂lower.The CO₂ emitted from the sewage discharge area came from the decomposition of sewage organic matter by microorganisms,and the input of nutrients promoted aquatic photosynthesis,so that the?¹³C-CO₂ value was higher and CO₂ emission was lower.However,the continuous input of high-concentration nutrients promoted the growth and reproduction of algae and other plankton in the water body,which will cause eutrophication of the water body,significantly increase the amount of CO₂released from the water body and cause the?¹³C-CO₂ value lower.5.Research on the response of organic carbon content,pCO₂ and stable carbon isotopes to environmental factors had shown that water environmental factors controlled the organic carbon content,pCO₂ and stable carbon isotopes of water bodies by controlling the relative intensity of aquatic photosynthesis and respiration,and the interference of human activities affected the above processes by affecting the water environmental factors.