A Study On The Loss Of Nitrogen And Phosphorus And Its Influencing Factors In The Upper Yangtze River Basin

Nitrogen（N）and phosphorous（P）are essential nutrients for plant growth,but excess nutrients will affect the stability of the water environment.With the development of economy,the loss of nitrogen and phosphorus in the upper reaches of the Yangtze River is increasing.Therefore,understanding the pollution impact of these nutrients and influencing factors in the upper reaches of the Yangtze River will benefit water quality efforts in this region.This study investigates pollutions sources of N and P in the upper reaches of the Yangtze River basin using statistical and literature data obtained from cities in this region.The N and P losses in the 7 sub-basins of the upper reaches of the Yangtze River were calculated from hydrological and water quality data using the LOADEST model.The influencing factors of N and P pollution in the upper reaches of the Yangtze River were analyzed using PLS-SEM on relevant meteorological,topographic,and economic data.Both point and nonpoint sources contribute to N and P pollution in the upper aches of the Yangtze River,but the non-point sources account for more than 90%of the total.The non-point source total N input in the following order:rural life emission（2.21%）<livestock breeding（15.66%）<atmospheric deposition（25.63%）<agricultural fertilization（56.48%）,while urban life emission（92.97%）and industry emission（7.03%）account for the total point source inputs.The Non-point source total P input in the following order:rural life emission（0.87%）<atmospheric deposition（4.54%）<livestock breeding（12.55%）<agricultural fertilization（82.03%）,while urban life emission（87.99%）is larger than industry emission（12.01%）.The input intensity of total N and P were 4802.79 kg·km^-2·yr^-1and 813.36kg·km^-2·yr^-1,respectively in the upper reaches of the Yangtze River.In the sub-basin,the input intensity varied in the order of Yalong River<Jinsha River<Minjiang River<Wujiang River<Jialing River<Mainstream Area of the Upper Yangtze River<Tuojiang River.The total N load and P loads were 1401.70 kg·km^-2·yr^-1and 26.89 kg·km^-2·yr^-1,respectively with the maximum load occurring in July and August;while the maximum N load occurred in May and June.The pollution load in rivers occurs in the order of Yalong River<Jinsha River<Jialing River<Wujiang River<Minjiang River<Tuojiang River.The ratio of loss to discharge of total N in the upper reaches of the Yangtze River is 29.19%,and the ratio of loss to discharge of total P is 3.31%.In each sub-basin,N mobility is high in the environment with loss/discharge between 18.63%and 64.09%,while P utilization rate（or retention rate）in the environment is high with loss/discharge between 1.79%and 15.92%.The influencing factors of total N loss were meteorological conditions（including temperature and rainfall,with a total effect of 0.589）,terrain conditions（referring to topographic undulations with a total effect of-0.249）,economic conditions（including population density and gross domestic product,with a total effect of 0.894）and total N input（including both the point and nonpoint input sources,with a total effect of 0.898）with R²of0.517.While the influence factors of total P loss were meteorological conditions（including temperature and rainfall,with a total effect of 0.396）,terrain conditions（referring to topographic undulations,with a total effect of-0.145）,economic conditions（including population density and gross domestic product,with a total effect of 0.910）and total P input（including both the point and nonpoint input sources,with a total effect of 0.913）,with R²of0.354.These findings provide an essential theoretical basis for the control of N and P pollution in the upper reaches of the Yangtze River and offer valuable information for policy decisions related to water quality protection.