Net Anthropogenic Phosphorus Inputs In Seven Major River Basins In China And Corresponding Phosphorus Fluxes Into The Sea

Phosphorus（P）is a necessary nutrient element for the growth and development of organisms.Excessive P inputs will cause eutrophication and other problems in water,and then threaten the vital movement of organisms.Yangtze River,Yellow River,Pearl River,Songhua River,Liaohe River,Haihe River and Huaihe River are China’s major outflow rivers,with a basin area of 451.19×10⁴ km²,accounting for 47%of the total land area of China.The total population of these basins is 11.03×10⁸,accounting for 79%of the total population of China and are located in the developed region in China.Human activities in the basin will cause a large amount of P to be discharged from the terrestrial environment into the river and carried by the river to the adjacent sea area,thus affecting the ecological environment quality in the coastal area.In this study,we calculated the net anthropogenic phosphorus inputs（Net Anthropogenic Phosphorus Inputs,NAPI）in China’s seven major outflow river basins,including P fertilizer input(P_fert),food and feed and non-food P input(P_im&nf)and TP flux in corresponding rivers.At the same time,a quantitive relationship was explored to relate TP flux to NAPI,river discharge and lake area proportion in the basin.This will provide more evidence for the management of river P pollution in China.The NAPI of the Pearl River basin,the Songhua River basin and the Liaohe River basin showed an overall increasing trend from 1999 to 2018.Among them,the NAPI of the Liaohe River basin increased greatly.The NAPI in the Yangtze River basin,the Yellow River basin,the Haihe river basin and the Huaihe River basin first increased and then decreased.The NAPI of the seven major river basins presented an obvious uneven distribution spatially in 1999-2018,and the NAPI value was higher in the river basins with high population density and fast socio-economic development.The highest NAPI was found in the Huaihe River basin,reaching 3988.28 kg P km^-2 yr^-1,which was caused by the intensive economic and agricultural production activities.The Songhua River basin has the lowest NAPI,only 327.17 kg P km^-2yr^-1.The different contributions of NAPI components were mainly attributed to the different land use types in each basin.The transformation of various nutrients could provide the basis for phosphorus control.The conversion rate of phosphorus in phosphorus fertilizer to the crops and phosphorus in crops to the livestock products were low,at 13%and 16%respectively.Therefore,we could increase fertilizer use efficiency,adjust the nutrients proportion in livestock feed and improve breeding methods to reduce phosphorus input.This study established an NAPI model（R²=0.86）based on total population,the amount of livestock and crop production,and simplified the calculation process of the model.This will be conducive to the extensive application of the model in future research.The TP fluxes of the Yangtze River,Yellow River,Pearl River,Songhua River and Haihe River showed an increasing trend on the whole,while the TP fluxes of the Liaohe River and Huaihe River showed less obvious increasing trend.From 1999 to 2018,the total average annual TP flux of the seven rivers was 98.31×10³ t P yr^-1,among which the TP flux of the Yangtze River was the highest(71.13×10³ t P yr^-1),accounting for 72.36%of the total TP flux of seven rivers.The lowest value of TP flux appeared in Haihe River,only 0.14×10³ t P yr^-1.TP flux had strong correlation with river discharge,the amount of livestock,population and phosphorus fertilizer.In this study,a quantitative relationship between TP flux and NAPI,river discharge,and lake area proportion in the basin was established（R²=0.82）.The model can well describe the influencing factors of TP flux in the river.The results of this study provide more basis and ideas for the effective management and control of P pollution in rivers in China.