Dynamics Of Phosphorus Cycling In China And The Impacts On Resources And The Environment

Phosphorus(P)is an essential nutrient for living systems with no substitutes.P cycling relates closely to food security and life quality.With a large population and fast economic development,China demands more and more P.Meanwhile,since the global distribution of non-renewable phosphate rock is highly uneven,China plays an important role in international trade to redistribute global P resources.However,P cycling in China is unsustainable.China cosumes a tremendous amount of P rocks with low P use efficiencies,causing huge pressure on domestic P reserves.Excess P is discharged back into the environment,leading to eutrophication and damaging ecosystems in Chinese freshwaters.Therefore,it is necessary to find out the dynamics of P cycling in China and the impacts on resources and the environment,which would not only facilitate the understanding of the mechanism of P cycling,but also provide scientific evidence for developing national strategies to achieve P sustainability.First,from the perspective of the life cycle of P utilization,this study regards P as both a resource and a pollutant,develops a comprehensive hierarchical P cycling model at the national level and for the first time to our knowledge,analyzes the dynamic changes of Chinese P cycling during 1600-2012 using time series of activity data and parameters.In addition,the analysis of P cycling is further extended from mass accounting to environmental impact assessment by a quantitative relationship built between P-related activities,P emissions,P fate and eutrophication risks.This study characterizes the geographical patterns of anthropogenic P losses to mainland freshwaters and evaluates freshwater eutrophication potential with a spatial resolution of 5 arc-minutes in 2012.Finally,a global P supply-chain trade model is developed not only to analyze various forms of P exchanged among countries via international trade during 1988-2012,but also to evaluate its impact on patterns of P accessibility and inequality on P accessibility over a 25-year period.China’s P accessibility in the global context is evaluated over time,space and across supply-chain stages.The main results and conclusion are summrized as follows:(1)This study develops a hierarchical P cycling model based on substance flow analysis,which includes 10 kinds of anthropogenic activities and 4 kinds of natural compartments.A total of 102 separate P flows and their mathematical methods are determined with the priority of independent calculation to reduce interactions with other flows.This model advances beyond previous national-level models by basing on the mass balance principle,including more P-related activities with more detailed flow patterns over century time scales.(2)This study analyzes the dynamics of P cycling in China over a four-century period(1600-2012)and finds out that P cycling in China’s mainland has intensified from rather simple,nature-dominated stable situations in the earlier three centuries to complex,human-dominated scenarios in the past six decades.China has shifted from a net P importer to a net P exporter.P mass balance in China’s arable land has gone through depletion(P input lags behind P output)to accumulation(P input exceeds P output).Agricultural activities have become the major contributors to P losses to Chinese freshwaters.(3)To ensure the robustness of model results,this study conducts interactive cross-checks in P flow quantification,assesses data quality of input variables and compares P flow results with previous estimates from China.Despite some inconsistencies,the overall conclusions of this study and previous ones are basically the same.In addition,Monte Carlo simulation shows that all the key P flows have less uncertainties in the last century than before the 1910s.Uncertainties of most P flows are primarily associated with activity data rather than parameters.(4)Anthropogenic P losses to mainland freshwaters in 2012 mainly cluster in the eastern and central part of China,especially in the middle-lower reaches of the Yangtze River basin.The highest per grid cell P emission with 286 tons is found in cosmopolitan Shanghai.Compared to physical weights of environmental P flows,even more marked spatial disparities of eutrophication potential are observed,especially highlighting hotspots in the same area-These regions,with their large anthropogenic P emissions and dense water networks connected with sensitive freshwater ecosystems,are at high risk of ecosystem damage.(5)Global total P accessibility has increased from 25.8 Tg-P/yr in 1988 to 30.8 Tg-P/yr in 2012.China’s P accessibility has kept decreasing because of the net P export,but its share of the world’s P accessibility has increased dramatically.At the end of P supply chain,four populous countries(China,India,the United States and Brazil)account for 62%of the global total of P accessibility in 2012.Inequality analysis further indicates that the world has become more equitable in P distribution,not only over time but also through supply-chain stages.