Sources And Influencing Factors Of Chromophoric Dissolved Organic Matter(CDOM) In The Main Stream Of Lijiang River In Guilin

Coupled carbonate weathering(CCW)is a model of the carbon cycle in carbonate(rock)areas based on interactions among water,rock(soil),gas,and biota.The key mechanism of this model lies in the transformation of dissolved inorganic carbon(DIC)into organic carbon(OC)by aquatic phototrophs,which produces autochthonous OC.It is very important for further understanding the stability of karst carbon sink.However,the identification of OC source,especially the further refinement of autochthonous organic carbon(AOC)source and formation process,is of great scientific significance to understand the source and quantitative calculation of karst carbon sink.According to the calculation,chromophoric dissolved organic matter(CDOM)accounts for about 70%of OC in freshwater water accounted.The UV-Vis absorption spectra and three-dimensional excitation-emission matrices(EEMs)of CDOM can reveal the source and composition of OC,and it can be used to analyze the characteristics of OC qualitatively and semi-quantitatively.In this paper,we sampled the water in the main stream of Lijiang River in Guilin,a surface river in a typical karst area.CDOM spectral features were used to identify the source,composition and temporal and spatial variation of OC.The influential factors and formation process of OC input from different sources in water were analyzed based on the spatial and temporal variation characteristics of hydrological environment in Lijiang River.Carbon sink flux produced by CCW(CSF)in Lijiang river was calculated.This paper provides a new method for evaluating the carbon sink generated by rock weathering.The main conclusions are as follows:Using EEM-PARAFAC model to identify the fluorescent component of CDOM,the compositions of CDOM were aquatic plant input tryptophan fluorescent OM,allochthonous humic fluorescent OM and microbial input tryptophan fluorescent OM.Microbial input accounted for the largest proportion(mean 37.39%).The absorption and fluorescence spectra parameters of CDOM were similar to the spectral parameters of the CDOM that were autochthonously originated.The input amounts of OC from aquatic plants and microorganisms in Lijiang River were the highest in spring and autumn,and the lowest in summer.Seasonal variation trend of allochthonous OC input was opposite.The seasonal variation of OC from aquatic plants was related to the seasonal variation of chlorophyll a(Chl a)concentration and hydrological environment.The seasonal variation of OC from allochthonous source was related to the seasonal variation of precipitation,flow and soil erosion.The seasonal variation of OC from microbial source was related to the seasonal variation of hydrologic environment and the input amount of OC from aquatic plants.The OC input from the three sources increased along the river,and the content in karst area was higher than that in non-karst area.The input of the tributary of the covered karst area(Gantang river)resulted in the maximum increment of the OC input from three sources.The OC input from aquatic plants was influenced by the amount of aquatic plants and the effect of DIC fertilization,which made more contribution in the downstream karst area.The microbial OC was affected by the DIC fertilization effect and the content of OC from aquatic plants source.The amount of microbial OC input in the karst water with strong carbonic anhydrase(CA)activity was significantly higher than that in the upstream non-karst water.Moreover,there were big increases between different sampling sites in the downstream.However,the increment of the allochthonous OC input at other sampling points was small.Carbon sink flux produced by CCW(CSF)of the Lijiang River was 4.78×10~5 t/a.If the microbial source and the aquatic plants source OC were taken as the autochthonous input,the percentage of autochthonous input to total OC was calculated to be an average of 67.27%.By calculating the OC fluxes at the outlet section(Yangshuo section)of main stream of Lijiang River,it was found that the annual autochthonous and allochthonous source fluxes of the Yangshuo section are 0.22×10~5 t/a and 0.09×10~5 t/a respectively,so the OC output was calculated to be 0.31×10~5 t/a.