Research On Diffusive Transportation Of Greenhouse Gases And Their Carbon And Nitrogen Source From Agricultural Watershed In Subtropical China

This study was funded by normal project of National Natural Science Foundation of China（No.41475129,41775157）.We conducted a research on diffusive transportation of greenhouse gases and their carbon and nitrogen sources from a typical agricultural watershed in subtropical region,Hunan province,China.Spatio-temporal characteristics of diffusive transportation of methane（CH₄）and nitrous oxide（N₂O）from 4 reaches（S1,S2,S3 and S4）of Tuojia River systems was analyzed by double-layer diffusion model.Moreover,key production pathways of CH₄ and N₂O as well as their sources of carbon and nitrogen in the river waterbody were also discovered by using the stable isotopic method.Major conclusions of this study are as follows.（1）The nitrogen pollution in Tuojia River was obvious and has significant spatio-temporal variation.Water pollution was most seriousin urban and farmland areas.Tributaries had more pollution than main streams.Chemical properties of Tuojia River were significantly influenced by local anthoropogenic activities.Nitrogen loading from external input was the dominant nitrogen sources of Tuojia River waterbody.Sewage,agricultural non-point source pollution and animal waste may be the dominant impact factors of nitrogen pollution in this watershed.（2）Dissolved concentrations and diffusive transportation flux of CH₄ and N₂O of Tuojia River had significant spatio-temporal variation.The maximum and minimum values of methane flux were occured in spring and autumn,respectively.Moreover,the flux of methane in winter was higher than that of in summer.The mean diffusive flux of methane was 53.88μgC?m^-2?h^-1.CH₄ flux of different reaches presents a trend of S2>S3>S4>S1.While temporally and spatially,N₂O flux presents a trend of summer>spring>autumn>winter and S2>S4>S3>S1,respectively,with a mean value of 15.29μgN?m^-2?h^-1.The Tuojia River watershed was an apparent source of atmospheric CH₄ and N₂O.The flux of CH₄ was significantly and positively correlated with the concentration of NH₄⁺-N and DOC（P<0.01）,and the flux of N₂O was significantly and positively correlated with water temperature,EC,NH₄⁺-N and DOC（P<0.01）,respectively.However,the flux of N₂O was negatively correlated with pH（P<0.01）.（3）Contributions of the acetic acid fermentation for the production pathway of dissolved CH₄ in S1^S4 reaches were 87%,78%,76%and 81%,respectively,showing that CH₄ production was dominated by the acetic acid fermentation pathway.δ¹⁸O-NO₃^-values of nitrification in S1^S4 reaches were-7.01‰,-0.17‰,-0.28‰and-0.60‰,respectively.The concentration of nitrate in waterbody was positively correlated with itsδ¹⁵N-NO₃^-value.Furthermore,the ratio of theδ¹⁵N-NO₃^-toδ¹⁸O-NO₃^-in S1^S4 reaches was 0.66,-41.01,-30.23 and 9.39,respectively.Our results showed that nitrification dominanted the N₂O production in Tuojia River waterbody.（4）There are significant differences of NO₃^--N sources in different reaches of Tuojia River.The main nitrogen source of S1 was soil nitrogen,while the dominant sources of S2^S4 reaches were the combination of soil nitrogen,ammonium fertilizer and manure.The source of nitrogen in river sediments was related to the land use type of S1^S4 reaches.Thus,the dominant source of S1 near the source area was soil organic matter,while S2^S4 reaches were affected by agricultural non-point source pollution and sewage may be the main source of nitrogen.The source of carbon in sediments of Tuojia watershed may be C3 plant,C4 plant and terrestrial and aquatic mixed organic matter.