[Dynamic response of riverine nitrate flux to net anthropogenic nitrogen inputs in a typical river in Zhejiang Province over the 1980-2010 period].

Based on long-term records of river water quality and discharge and nitrogen sources as well as the LOADEST model,annual riverine NO-3-N flux and net anthropogenic nitrogen input(NANI) were both estimated for a typical river catchment(2 474 km2) in Zhejiang Province over the 1980-2010 period. Historical trends in both riverine NO-3-N flux and NANI and their dynamic relationships were then fully addressed. Finally,the contributions of annual NANI,retained nitrogen pools,and natural background sources to riverine NO-3-N flux were indentified. Results indicated that both riverine NO-3-N flux and NANI showed parabolic changing trends with peak value of 5. 74 kg·(hm2·a)- 1for flux and 77. 5 kg·(hm2·a)- 1for NANI both occurring around 1998. In 1980-2010,net increase of riverine NO-3-N flux and NANI was ～ 42% and ～ 77%,respectively. Chemical nitrogen fertilizer application and atmospheric nitrogen deposition,which accounted for ～ 48% and ～ 40% of NANI,respectively,were the major sources of NANI. Although interannual change of riverine NO-3-N flux was significantly related to NANI(R2= 0. 27**) as well as the chemical nitrogen fertilizer application amount(R2= 0. 32**),it showed higher dependence on the river water discharge(R2= 0. 79**) or precipitation(R2= 0. 63**),implying that annual riverine NO-3-N was not only originated from current year's NANI,but also derived from retained N pools that were ultimately derived from NANI in previous years. A regression model developed by incorporating both NANI and water discharge could account for 94% of the variability of annual NO-3-N flux. This model predicted that NO-3-N flux could have been reduced by ～ 21% and～ 30% if the annual NANI and water discharge had been cut by 30%,respectively. Annual NANI,retained nitrogen pools,and natural background sources contributed to ～ 53%,～ 24%,and ～ 23% of the riverine NO-3-N flux,respectively,suggesting that ～ 77% of flux was derived from anthropogenic nitrogen sources. Although observed long-term interannual change of riverine NO-3-N flux was dependent on the combined influences of NANI and hydroclimate,a more immediate reduction of riverine NO-3-N flux may result from interception strategies than from cutting nitrogen source inputs due to the contribution of retained nitrogen pools.