Dynamic Analysis Of Nutrient Flow And Legacy Nutrient Pools In The Yongan River Watershed

Nitrogen(N)and phosphorus(P)are both key nutrient elements for sustaining ecosystem and human.To support the threefold growth in global population,anthropogenic N and P input on earth have been dramaticly enhanced over the past century.However,anthropogenic nutrient inputs have greatly exceeded nutrient uptake by crop and animal products,resulting in surplus nutrients in various ecosystems.As nutrients surplus in watersheds,there is considerable potential for nutrient losses to waters(rivers and groundwater)and the atmosphere that may result in a wide range of negative environmental and ecologicalimpacts(e.g.,eutrophication,drinking water risk,biodiversity loss,climate change,tropospheric ozone production,etc.).Therefore,it is imperative to improve utilization efficiency of N and P while to minimize their losses to the environments.Based on extensive data collection for the Yongan watershed in eastern China,this study provides a long-term(1980-2010)analysis of N as well as P inputs,flows,losses and accumulations dynamics by adopting a refined substance flow analysis approach.The primary objectives of this study were to:(1)explore the long-term change of N and P cycling pattern;(2)identify individual contributions of various human activities to rising N as well as P loss to waters;(3)address the quantities,locations and sources of legacy P pool;and(4)provide relevant management suggestions and measures for improving N and P utilization efficiencies.The main studied results acquired are as follows:(1)In 1980-2010,N and P flows have been largely strengthened as indicated that anthropogenic net N and P inputs increased by 32%and 34%,respectively.The majority of anthropogenic N and P inputs occured in the farming subsystem.The utilization efficiencies of both N and P in agriculture(planting and animal husbandry)system showed downward trends.The N utilization efficiency decreased from 44%in 1980 to 29%in 2010,while P utilization efficiency decreased from 54%in 1980 to 40%in 2010.The recycling efficiency of urban and rural residential N wastes deacresed from 40%in 1980 to 26%in 2010,while from 59%to 45%for P.It implies that a large amount of residenal wastes that contained high N and P levels directly flows into natural waters and non-agricultural soils.(2)The logarithmic mean di-exponential decomposition approach estimated that,the rate of rural manure discharge and the number of rural population were main factors for driving rapid increase of P loss quantity into the natural water body,which accounted for 33%,21%and 19%of variations of P loss quantity over the past 31 years,respectively.For change of P loss quantity into the natural water body,the number of livestock and the runoff coefficient of the unit farmland were main factors for driving rapid increase of P loss quantity into the natural water body,which accounted for 41%and 26%of variations of P loss quantity over the past 31 years,respectively.(3)Over the past 31 years,most of anthropogenic P input was stored in watershed cultivated soil,the non cultivated soil and the natural water body(especially sediments),with total cumulative legacy P magnitudes of 27080 tons P,14127 tons P,and 10772 tons P,respectively.Although the excessive N would be partly equilibrium due to removals by denitrification and volatilization,there was still net N accumulation in cultivated soil-groundwater,non arable soil-groundwater and natural water body,with total cumulative legacy N magnitudes of 114516 tons N,261530 tons N,and 27437 tons N,respectively.For the entire Yongan River watershed,total cumulative legacy nutrient magnitudes of 51979 ton P and 403482 ton N.(4)To improve N and P utilization efficiencies and to minimize their losses to the environments,relevant agricultural techniques should be adopte to recovery legacy nutrient in cultivated soils for supporting crop productions.Livestock and poultry farms should be enhanced in nutrient cycling management by combining crop farming and animal husbandry systments for recycling animal waste nutrient resources.Domestic wastes should be well classified and resource treated for improving recycling of biomass wastes in crop planting.Wastwater treatment plants should be also established and operated in cities and towns for avoiding direct discharege of wastewater into near waters.