Temporal And Spatial Variation Characteristics Of Nitrogen Deposition And Its Contribution To River In Small Watersheds Of Western Sichuan

At present,due to the environmental degradation and atmospheric deposition bring more and more serious ecological problems,atmospheric nitrogen deposition has gradually become a research hotspot.Therefore,this study selected different ecological regions（intensive agricultural areas,common agricultural areas,urban areas and forest areas）in the Xihe small watershed of the western Sichuan Plain as the research area,and continuously monitored the dry and wet deposition of atmospheric nitrogen and the nitrogen of river from May 2017 to April 2018.The spatial and temporal variability and influencing factors of atmospheric nitrogen deposition in the Xihe River Basin were studied.Based on the software of geographic information system and output coefficient model,the contribution of atmospheric nitrogen deposition to the nitrogen of Xihe River was further discussed.The main findings are as follows:（1）The annual dry fluxes of atmospheric NO₃^--N,NH₄⁺-N,DON and TN are 5.84,10.88,7.58,and24.30 kgN/（hm²·a）,respectively;the annual fluxes of wet deposition are 11.54.16.97,7.93 and 36.44kgN/（hm²·a）;the annual fluxes of total deposition were 17.38,27.85,15.51 and 60.74 kgN/（hm²·a）,respectively,in the study area.All the samples were mainly wet deposition,and the NH₄⁺-N flux accounted for the main part in the dry and wet and total deposition.（2）Atmospheric nitrogen deposition in the study area showed some differences in time.The dry deposition fluxes were the highest in autumn,followed by spring and winter,and the lowest in summer（P<0.05）.Among the wet deposition fluxes,NO₃^--N,DON and TN were the highest in summer,followed by spring and autumn,and the lowest in winter（P<0.05）,NH₄⁺-N showed the highest in autumn,followed by spring and summer,and lowest in winter（P<0.05）.Among the total deposition fluxes,NO₃^--N,NH₄⁺-N and TN is the same as wet deposition NH₄⁺-N,DON showed the highest in summer,followed by spring and autumn,and the lowest in winter（P<0.05）.（3）The atmospheric nitrogen deposition in the study area showed some difference in space.From the spatial variation of annual flux,the nitrogen deposition of various forms in dry deposition basically shows Farm point（intensive agricultural areas）,Mingzhu point（urban areas）>Wenjingjiang point（forest areas）>Huaiyuan point（forest areas）and Yuantong point（general agricultural area）（P<0.05）;the nitrogen deposition of various forms in wet deposition and total deposition basically show Huaiyuan,Wenjingjiang points（forest area）>Farm point（intensive agricultural area）,Mingzhu point（urban area）>Yuantong point（general agricultural area）（P<0.05）trend.（4）The annual average concentrations of nitrogen,NO₃^--N,NH₄⁺-N,DON and TN in Xihe of the study area were 1.24,0.12,0.15 and 1.51 mg/L,respectively.NO₃^--N is the main part of river nitrogen.The seasonal variation is basically as follows:autumn>winter>summer>spring（P<0.05）;the spatial variation of nitrogen concentration is basically represented by Xujiadu point（intensive agricultural area）,Mingzhu point（urban area）>Yuantong point（general Agricultural area）>Huaiyuan point（forest area）>Wenjingjiang point（forest area）（P<0.05）,that is,from the upstream to the middle to the downstream gradually rising trend.The annual loadings of NO₃^--N,NH₄⁺-N,DON and TN in the Xihe River were2175.2,193.4,349.2 and 2717.8 t,respectively.The contribution of nitrogen deposition to the nitrogen of the river were 4.56%,73.93%,25.32%and 12.17%,respectively,in the study area.Atmospheric NH₄⁺-N deposition is the main part of nitrogen entering the river,and the main source of NH₄⁺-N in the river is also the atmospheric deposition.