UC Davis

This study quantified the long-term response of riverine total nitrogen (TN) export to changes in net anthropogenic nitrogen inputs to forest/agricultural (NANIFA) and residential (NANIR) systems across three catchments affected by low (LD), medium (MD), and high (HD) human impacts in eastern China. Annual NANIFA increased by 63–87% in 1980–1999, followed by 0% (LD), −23% (MD) and −40% (HD) changes of NANIFA in 2000–2010, resulting in a net increase of 56–78% in NANIFA in 1980–2010. Annual NANIR increased by 101–152% in the three catchments in 1980–2010. Land-use showed a 58–65% increase in developed land area (D%) and a 96–108% increase in agricultural lands with improved drainage systems (AD%) over the study period. In response to changes in NANIFA, NANIR and land-use, riverine TN flux continuously increased 3.0- to 6.1-fold in the three catchments over the past 31 years. For each catchment, an empirical model incorporating annual NANIFA, NANIR, water discharge, D%, and AD% was developed (R2=0.93−0.97) for predicting and quantifying sources of annual riverine TN fluxes. The model estimated that NANIFA, NANIR and other N sources (e.g., natural background, legacy, and industrial N sources) contributed 27–90%, 0–45%, and 10–28% of riverine TN fluxes, respectively. Model results were consistent with spatio-temporal changes of riverine chloride, ammonium, nitrate, dissolve oxygen and pH, as well as changes in available N levels in agricultural soils. In terms of N source management, reduction of NANIFA in catchment LD and NANIR in catchment HD would have the greatest impact on reducing riverine TN fluxes. Furthermore, changes in land use and climate as well as legacy N should be considered in developing N pollution control strategies.