Relationship between catchment characteristics and nitrogen forms in Cao-E River Basin, Eastern China.

The distribution of different nitrogen forms and their spatial and temporal variations in different pollution types of tributaries or reaches were investigated. Based on the catchments characteristics the tributaries or reaches can be classified into 4 types, including headwater in mountainous areas (type I), agricultural non-point source (NPS) pollution in rural areas (type II), municipal and industrial pollution in urban areas (type III), and combined pollution in main stream (type IV). Water samples were collected monthly from July 2003 to June 2006 in the Cao-E River Basin in Zhejiang, eastern China. The concentrations of NO3(-)-N, NH4(+)-N, and total nitrogen (TN) were measured. The mean concentrations of NO3(-)-N were decreased in the sequence type IV > type II > type I > type I, whereas, NH4(+)-N, total organic nitrogen (TON), and TN were in the sequence: type III > type IV > type II > type I. In headwater and rural reaches, CNO3(-)-N was much higher than CNH4(+)-N. In urban reaches, TON and NH4(+)-N were the main forms, accounting for 54.7% and 32.1% of TN, respectively. In the whole river system, CNH4(+)-N decreased with increasing distance from cities, and CNO3(-)-N increased with the increasing area of farmland in the catchments. With increased river flow, CNO3(-)-N increased and CNH4(+)-N decreased in all types of reaches, while the variations of CTON and CTN were different. For TN, the concentration may be decreased with the increase of river flow, but the export load always increased.