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 NO3(-)-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 NO3(-) -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 NO3(-)-N flux were indentified. Results indicated that both riverine NO3(-) -N flux and NANI showed parabolic changing trends with peak value of 5.74 kg x (hm2 x a) for flux and 77.5 kg x (hm2 x a)(-1) for NANI both occurring around 1998. In 1980-2010, net increase of riverine NO3(-) -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 NO3(-) -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 NO3(-) -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 NO3(-) -N flux. This model predicted that NO3(-) -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 NO3(-) -N flux, respectively, suggesting that -77% of flux was derived from anthropogenic nitrogen sources. Although observed long-term interannual change of riverine NO3(-) -N flux was dependent on the combined influences of NANI and hydroclimate, a more immediate reduction of riverine NO3(-) -N flux may result from interception strategies than from cutting nitrogen source inputs due to the contribution of retained nitrogen pools.