Mechanism Of Nitrogen Forms Transformation And Its Effect On Nitrogen Transport Flux During Baoxiang River Runoff Process In Dianchi Lake Basin

Water security problem caused by water shortage and water pollution has been a key factor restricting the sustainable development of society,and has threatened regional public security and human survival.Due to the rapid urbanization process in Dianchi Lake Basin,the nitrogen entering the surface water increases sharply,causing water environment problems such as eutrophication and water quality deterioration,and making the public water resources security problem more serious.Therefore,it has become an urgent task to reduce non-point source nitrogen pollution,propose precise treatment measures for nitrogen pollution in water environment,reduce pollution burden of Dianchi Lake,restore and improve water environment of Dianchi river lake system,and alleviate regional water resources security problems.The Baoxiang River Basin of Dianchi Lake had been takes as the research area in this study,the nitrogen and phosphorus occurrence form concentrations,?¹⁵N and?¹⁸O of nitrate,water environment factor parameter values of contribution end-members and main stream runoff were be monitored systematically,the nitrification and denitrification process of nitrogen in water were investigated,and the contribution rates of different end-members were quantitatively calculated based on the SIAR model.Nitrogen pollution sources in different sections of the main stream were be traced based on the measurement of nitrate ¹⁵N and ¹⁸O isotopes.By analysis of nitrogen and phosphorus occurrence forms and pollution characteristics of different contributing end-members and main stream runoff in the basin,and combined with hydrological observation data of Baoxiang River,the transformation amount of nitrogen non-point source pollution entered into river channel,during the river transport process was calculated.And the nitrogen transformation process and its influencing factors were explored;Included the amount of nitrogen transformation in the process of river transport,the nitrogen transport flux of Baoxiang River was estimated.The results were as follows:?1?In the rainy season,nitrate?¹⁵N in the main stream of Baoxiang River shows a general trend of increasing first and then decreasing from the upstream to the downstream,with a range of 6.6‰to 9.7‰,while nitrate?¹⁸O shows a trend of decreasing first and then increasing,with a range of 3.4‰to 14.6‰.The main sources of nitrate in river source area were rainwater and soil runoff;In the upstream area,river runoff nitrate mainly come from the sources of villages,agriculture and rainwater,among which the agricultural sources were mainly consist of ammonium fertilizer and manure;in the middle reaches the nitrate mainly come from the sources of villages and agriculture;the nitrate in the downstream areas were mainly coming from agricultural sources,villages sources and urban sources.The trace results of nitrate ¹⁵N and ¹⁸O isotopes showed that there were no significant nitrification and denitrification occurred in the runoff of the Baoxiang River during the rainy season.The order of nitrate contribution rates in the Baoxiang river main stream were:agricultural source?31.2%?>village source?29.8%?>urban source?20.3%?>rain water?18.7%?.The nitrate contribution rates at different end-members show that from the upstream to the downstream of Baoxiang River,the mainly sources of nitrate were from the villages and towns that related to urbanization,which accounts for 40%-50.1%of the total contribution rate,followed by the agricultural source,which accounts for30.6%-31.2%,and the rain water,which accounts for 18.8%-29.4%.?2?In the mainly nitrogen contribution end-elements of the Baoxiang River Basin,the total nitrogen?TN?concentration of farmland ditch runoff,village sewage outlet and urban sewage outlet runoff were ranged from 0.21-33.55mg/L,0.69-54.72mg/L,1.52-30.61mg/L,respectively.Besides,the annual average concentrations of which were 8.97 mg/L,15.05mg/L,15.02mg/L,respectively.Dissolved inorganic nitrogen?DIN?was the main form of water nitrogen in all the three types of contribution end-elements,among the components of DIN in farmland ditches,rural sewage outlets and urban pollution outlets,the annual average values of ammonia nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and nitrite nitrogen?NO₂^--N?accounted for 12%,46%and 5%;74%,4%and 2%;85%,0.2%and 0.2%of the total nitrogen concentration,respectively.The rainy season?i.e.,May to October?was the peak period of nitrogen released and migrated in farmland,and the TN concentration of farmland ditch runoff in rainy season reaches 13.9 times of the dry season.The TN,total particulate nitrogen?TPN?,dissolved organic nitrogen?DON?and DIN of Baoxiang River main stream were all characterized by increased from upstream to downstream,and the annual average concentration of which were 0.27-9.63mg/L,0.13-0.89mg/L,0.046-1.8mg/L and 0.089-6.94mg/L,respectively.Among the main forms of DIN in the main stream water body,the upstream was dominated by NO₃^--N,and the annual mean concentration of which was accounted for32%-52%of the TN.And the middle stream was dominated by NH₄⁺-N,and the annual mean concentration of which was accounted for 32%-38%of the TN.In the down stream NO₃^--N was the mainly form of DIN,and the annual mean concentration of which was accounted for 37%of the TN.NH₄⁺-N mainly occurs in rainy season?i.e.,May to October?,while NO₃^--N mainly occurs in dry season.Among the major phosphorus contributors on the drainage area,the average concentration of total phosphorus?TP?in runoff from farmland ditch,village sewage outlet and urban sewage outlet were 0.14,1.12 and 1.4mg/L,respectively.Total dissolved phosphorus?TDP?was the mainly form of each source.All of the average concentrations of TP,TDP and particulate phosphorus?PP?in the main stream runoff showed an increasing trend from upstream to downstream.The contributed of TDP along the river might be an important reason for TDP appearance as the mainly occurrence of water body from upstream Caohe region to downstream Baofeng wetland area of Baoxiang River.?3?The upper reaches and middle reaches of the Baoxiang River mainly show the absorption effect on TDN,and the absorption amount were 84.59 t/a and 102.44 t/a respectively,and the retention rate were 25%and 23%,While in the lower reaches it showed a release effect on TDN,and the release amount was 150.65 t/a.Peak periods of TDN absorption by upper and middle reaches and TDN release by lower reaches were mainly occurred in the rainy season?i.e.,May to October?.In the upper and lower reaches of Baoxiang River,the TPN mainly showed release effect,and the release amount were 3.09 t/a and 4.72 t/a,respectively.In the middle reaches,the TPN was mainly showed absorption effect,with the absorption of 29.59 t/a.The rainy season?i.e.,May to October?was the peak period for the retention and release of the TPN in the runoff of Baoxiang River.Among DIN components,NH₄⁺-N mainly showed absorption effect in upper,middle and lower reaches of Baoxiang River,which was more significant in rainy season.The annual retention rate and absorption amount of upper,middle and lower reaches were 31%,53%and 13%,and 50.15 t/a,156.38 t/a and 30.91 t/a,respectively.NO₃^--N was mainly manifested as absorption in the upper reaches,with the retention rate and absorption amount of 21%and 19.87 t/a,while in the middle reaches and the lower reaches NO₃^--N were mainly showed release effect,and its release amount were 23.28 t/a and 115.56 t/a.NO₂^--N was showed release effect in both upper,middle and lower reaches of Baoxiang River,and it's release amounts were 4.94 t/a,6.72 t/a and 7.69 t/a,respectively.The transformation of nitrogen occurrence forms in Baoxiang River was mainly dominated by the transformation of dissolved nitrogen,which was manifested as the absorption of NH₄⁺-N in the upstream and middle reaches in the rainy season,and the release of DON in the downstream in rainy season and the release of NO₃^--N in the end of the rainy season and the dry season.The transformation of nitrogen occurrence forms in Baoxiang River was mainly influenced by the hydrochemical conditions of water environment,followed by the action of microorganism and the temperature of water environment.?4?The total quantity of TN non-point source pollution flowed into river course from areas of all land use types on Baoxiang River watershed was 745.28 t/a.and nitrogen froms transformation were occurred during the river transport,after nitrogen non-point source entering the river channel.The nitrogen froms transformation lead to the NH₄⁺-N ratio reduced from 61%to 32%,and the NO₃^--N ratio increased from 14%to 33%,and the DON ratio increased from 11%to 21%.The transformation of nitrogen forms eventually make a reduction of the total TN non-point source quantity from the watershed surface by 58.16 t/a.Included the amount of nitrogen conversion during river transport into calculation,the final TN transport flux of Baoxiang River was 687.12 t/a.The research results could provide theoretical basis for the reduction of nitrogen non-point source pollution emissions in Dianchi Lake Basin,the formulated accurate treatment measures for nitrogen pollution in the water environment,and the improvement of regional water resources security issues such as water shortage and water use difficulty that caused by water quality-induced water shortage.