Dynamic Characteristics Of Net Anthropogenic Phosphorus Inputs And Riverine Phosphorus Export In The Yangtze River Basin From 1980 To 2015

Phosphorus is one of the most essential nutrients for all life on earth.Anthropogenic phosphorus(P)additions(e.g.,fertilizers and detergents)play an important role for meeting the demand of food and energy from growing population.However,excessive anthropogenic P inputs not only waste the limited P-rock resources,but also caused environmental issues such as eutrophication in different water bodies.Quantitative understanding of the response of riverine P export to anthropogenic P inputs is critical for guiding efficient P pollution measures.As the largest river in China,the Yangtze River has important strategic significance in strengthening its ecological environment protection.Recent studies have shown that P pollution has been the key water qaulity issue in the Yangtze River.However,there is still a lack of long-term analysis of the quantitative response of river phosphorus export to anthropogenic P input.This study estimated the spatial and temporal variations of the net anthropogenic phosphorus input(NAPI)and its components in the Yangtze River basin from 1980 to 2015.Temporal variability of riverine P export fluxes and forms in Datong station and its influencing factors was also addressed.Multiple regression models incorporated NAPI,dam storage capacity and water discharge were then developed for predicting annual riverine P exports.Using the developed models,this study further indentified the contributions of NAPI and legacy P to riverine P export and made scenario predictions of riverine P export in the next 35 years.The main findings of this study are listed as follows:(1)In 1980-2015,the NAPI in the Yangtze River basin continuously increased with a net increase of 1.4 times.In terms of the NAPI components,net food and feed input(28-35%)and fertilizer input(56-67%)are the most important sources.Across spatial distributions,NAPI and its components increased from western to eastern China,with the maximums observed in the downstream Shanghai and the middle reaches of Henan(>5000 kg P km-2 yr-41)and the minimums observed in the upper reaches of Qinghai and Tibet(<300 kg P km-2 yr-1).However,the NAPI growth rates in the upstream and midstream areas were significantly higher than that in downstream.Increasing population and livestock densities as well as land use change were the main drivers of NAPI spatiotemporal variations.(2)In 1980-2015,observed riverine export fluxes of total phosphorus(TP),particulate phosphorus(PP)and suspended sediment(SS)at the Datong hydrological station synchronously decreased by 52%,75%and 75%,respectively.The decreasing rates of riverine TP and PP exports were different during 1980-1985,1986-2002 and 2003-2015 time periods.However,observed riverine export fluxes of dissolved phosphorus(DP)showed a significant trend of increase with a net increase of 7 times from 1980 to 2015.The decrease of riverine SS,TP and PP exports might be mainlly due to increased riverine retention and reduced soil erosion by increasing damming volumes and extending vegetation coverage area.Increasing damming volumes and extending vegetation coverage area that is favor of the transformation of PP to DP combined with the increase of the P inputs from detergent and food and feed(point source pollution)resulted in a significant increase of riverine DP export flux.(3)The ratio of riverine PP to TP at the Datong station decreased continually from 97%in 1980 to 50%in 2015,while the proportion of DP to TP increased from 3%to 50%.Therefore,although both riverine TP and PP exports decreased,the increase of DP level might aggravate the risk of algal blooms in many impoundments along the Yangtze River system.(4)In 1980-2015,the cumulative riverine TP export in the Datong station was estimated as 3760 kg P km-2,accounting for 6.3%of the cumulative NAPI.Therefore,the remaining 93.7%(56018 kg P km-2)of NAPI accumulated in watershed soils,sediments and biomass,implying that legacy P sources may have a potential contribution to riverine P export.(5)Developed multiple regression models incoporated NAPI,dam storage capacity and water discharge could explain 79%and 86%inter-annual variability of riverine TP and DP fluxes,respectively.Contributions of NAPI to riverine TP and DP fluxes were estimated as 54%(3-87%)and 84%(78-89%),while contributions of legacy P sources to riverine TP and DP fluxes were 46%(13-97%)and 16%(12-22%),respectively.Model predictions indicated that riverine TP and DP fluxes at the Datong station would reduce by 0.7%and 1.3%by 2050 under the "climate change" scenario,respectively.Riverine TP and DP fluxes would increase by 60%and 62%under the "developing" scenario,respectively.Under the "tackling" scenario,riverine TP and DP exports would reduce by 13%and 39%,respectively.(6)To effectively control P pollution in the Yangtze River,more attention should be paid to reduce the fertilizers P inputs and increase P utilization efficiencies,especially in the middle and lower reach regions.Efforts should be also made to improve the ratio of recycled living and livestock organic wastes for replacing patial chemical P fertilizer inputs in crop productions.Attention should be paid to the re-utilization of legacy P remained in farmland soil,as well reduction of the risk of soil phosphorus losses.Finally,the collection and treatment of domestic and industrial wastewaters should be improved to effectively reduce the direct DP dischrages.