Source Analysis Of Phosphorus And Transformation Of Phosphorus Fractions In The Water-sediment Phases In The Tuojiang River Basin

River is the main transport channel of phosphorus cycle.Phosphorus is the main limiting factor of water eutrophication.Excessive phosphorus will cause water eutrophication.It is very important to analyze the source of phosphorus,identify the distribution of phosphorus fractions and the migration and transformation process of phosphorus in different media for the prevention and control of phosphorus pollution in water environment.The Tuojiang River Basin is an important tributary in the upper reaches of the Yangtze River Basin.It is of great practical significance to carry out the research on the unclear source,distribution of phosphorus pollution,migration and transformation of phosphorus fractions and source-sink interaction of sediments in the Tuojiang River Basin.In view of the lack of systematic methods for tracing phosphorus source in river basins and accounting methods for the phosphorus emissions,as well as the relatively lagging research on the tracing technology of soluble reactive phosphorus（SRP）in China.This study aims to systematically clarify the source of phosphorus from both external and internal sources,and analyse the spatial-temporal distribution of phosphorus fractions in the water-sediment phases to reveal the migration and transformation of phosphorus,and explore the feasibility of multiple stable isotopes tracing the source of phosphorus.The results obtained could provide theoretical support and scientific basis for scientific prevention and control of phosphorus pollution in the Tuojiang River Basin.For exogenous phosphorus,a bottom-up total phosphorus（TP）accounting model with time scale characteristics was constructed,and the TP emission inventory of the Tuojiang River Basin was established;The spatial distribution characteristics of phosphorus fractions and their migration and transformation in water-sediment phases were analyzed in the wet and dry seasons.For the endogenous phosphorus pollution,the adsorption-release experiment of phosphorus in sediments and adsorption isotherm were simulated to analyse the adsorption-release characteristics of sediment phosphorus,the sediment source-sink interaction and influencial mechanism of sediment phosphorus adsorption-release were revealed.At the same time,nitrogen isotope(δ¹⁵N),hydrogen isotope（δD）,carbon isotope(δ¹³C)and phosphate oxygen isotopes(δ¹⁸O_p)and Iso Source and Bayesian（SIAR）isotope mixing model were used to identify the source of SRP in water and quantify the contribution of pollution sources.The main research results are as follows:（1）The classification system and accounting method of TP emission inventory composed of three-level pollution sources and four categories’sources with time scale characteristics were constructed.Four categories were industrial,agricultural,domestic and natural sources.Three-level pollution sources were the level-by-level subdivision of the four categories of sources.A bottom-up approach was adopted to develop the TP emission inventory using emission factors and loss coefficients for 2017 in the Tuojiang River Basin.In 2017,the TP emissions were 14254.70 t.The agricultural TP emissions were 5201.87 t,the TP emissions from soil erosion were 4947.00 t,the domestic TP emissions were 3621.19 t,and the industrial TP emissions were 484.64 t.Agricultural sources were the main sources of the TP emissions,followed by natural sources.Specificsally,among the secondary emission sources,soil erosion was the main source of TP emissions,followed by livestock breeding and rural sources,the TP emissions from crop farming were smallest.On the time scale,The TP emissions were higher in the wet season than those in the dry season.The spatial analysis showed that the largest TP emissions were distributed in Yibin,followed by Chengdu and Leshan,and the TP emissions were smallest in Meishan.The TP emission intensity was the highest in Zigong,followed by Deyang and Ziyang,and the TP emission intensity was the lowest in Chengdu.（2）The distribution characteristics of phosphorus fractions in water-sediment phase were analyzed in the Tuojiang River Basin.In water,the average concentrations of all phosphorus fractions were higher in the wet season than in the dry season,and SRP was the main phosphorus fractions.The spatial analysis showed that the concentrations of TP（TP-W）,total dissolved phosphorus（DTP）and dissolved organic phosphorus（DOP）were higher in the lower reaches than in the middle and upper reaches,and the PP concentrations were higher in the middle reaches than in the upper and lower reaches.In the dry season,TP-W,DTP,DOP and total particulate phosphorus（PP）concentrations were higher in the upper reaches than in the middle and lower reaches.In sediments,inorganic phosphorus（IP）and Fe-P were the main phosphorus fractions.The average concentrations of extractable organic phosphorus（Org-P）and phosphorus bound by Fe oxides（Fe-P）were higher in the wet season than in the dry season,and the average concentrations of TP（TP-S）,exchangeable phosphorus（Ex-P）,authigenic phosphorus（Ca-P）,detrital phosphorus（De-P）and refractory phosphorus（Res-P）were lower in the wet season than in the dry season.In the wet season,the concentrations of TP-S,Ex-P and Fe-P were higher in the lower reaches than in the middle and upper reaches,and the concentrations of Ca-P,Org-P,De-P and Res-P were higher in the upper reaches than in the middle and lower reaches.In the dry season,the concentrations of Ex-P and De-P were higher in the upper reaches than in the middle and lower reaches,and TP-S,Fe-P,Ca-P,Org-P and Res-P were higher in the lower reaches than in the middle and upper reaches.The phosphorus pollution of sediment was more serious in the dry season than in the wet season,and it was more serious in the lower reaches than in the middle and upper reaches.The concentrations of SRP and PP had a great influence on TP-W concentrations.There were migration and transformation between Fe-P and De-P and between Ex-P and Res-P,and between DTP and bioavailable phosphorus（BAP）and Fe-P.（3）The experiment for the adsorption and release of phosphorus in sediments of the Tuojiang River Basin was simulated.The modified Freundlich model was well fitted by the phosphorus adsorption process of sediments.The average values of phosphate adsorption-desorption equilibrium concentration（EPC₀）in the wet and dry seasons were 0.175 and 0.215 mg/L.The EPC₀ was higher in the dry season than in the wet season.The release process of sediments phosphorus involved fast and slow reaction stages,which was fitted with the quasi second-order kinetic model.The maximum phosphorus release amount of sediment was 0.0062 and 0.0320 mg/g in the wet and dry seasons,respectively.The phosphorus release amount of sediment was higher in the dry season.Fe-P was mainly involved in the adsorption and release of phosphorus in sediments.The phosphorus release amount of sediment increased with the increase of p H,temperature and disturbance intensity.Ferric chloride（Fe Cl₃）mainly converts unstable Ex-P,Fe-P and Org-P into more stable Res-P to inhibit the release of phosphorus from sediments,which could provide a scientific reference for in-situ phosphorus control technology of sedmient.The sediment mainly played the role of source,and the annual phosphorus release amount of sediment was 53.68 t.（4）Tacing the source of SRP in water using multiple stable isotopes was explored.The water of the Tuojiang River mainly came from atmospheric precipitation.The utilization of SRP by microorganism was low.Theδ¹⁸O_p values from various pollution sources were different.δ¹⁸O_p could be used to trace the sources of SRP in the Tuojiang River basin.δD,δ¹⁸O_H,δ¹⁵N andδ¹³C could assistδ¹⁸O_p to trace the possible main sources of SRP.The contribution rates of different SRP sources were quantified by Iso Source and SIAR isotope mixing model in the Tuojiang River Basin.Industrial and domestic sewage were the most significant source of SRP,followed by phosphate rock and phosphogypsum,and the contribution from agricultural sewage was the least.The contribution rates of SRP sources calculated by two mixed isotope models were basically the same.The range for the contribution rates of SRP sources calculated by the Iso Source model was large,which was mainly affected by the spatial difference ofδ¹⁸O_p between pollution sources and water bodies,and the uncertainty of SIAR model calculation results was relatively low.The research results could provide new research ideas and methods for watershed phosphorus source analysis and provide scientific basis for watershed phosphorus pollution control.