Budget and Control of Nitrogen in the Changjiang River Catchment and Its Mouth

Abstract

The various sources of total nitrogen (TN) and dissolved inorganic nitrogen (DIN) in the Changjiang catchment and N transport in the Changjiang mouth were estimated. The export fluxes of various form of N were mainly controlled by the river runoff, and the export fluxes of NO3–N, DIN, and TN in 1998 (an especially heavy flood year) were 1438 × 103 t a−1 or 795.1 kg km−2 a−1, 1746 × 103 t a−1 or 965.4 kg km−2 a−1, and 2849 × 103 t a−1 or 1575.3 kg km−2 a−1, respectively. The TN and DIN in the Changjiang River came mainly from precipitation, agricultural nonpoint sources, N lost from fertilizer and soil, and point sources of industrial waste and residential sewage discharge, which were about 56.2 and 62.3%, 15.4 and 18.5%, 17.1 and 14.4%, respectively, of the N outflow at the Changjiang mouth, maximum transport being in the middle reaches. The inorganic N in precipitation in the Changjiang catchment mainly comes from gaseous loss of fertilizer N, N resulting from the increases of population and livestock and from high-temperature combustions of fossil fuels. N from precipitation is the first N source in the Changjiang water and the only direct cause of high content of inorganic N in the Changjiang River and its mouth. The lost N in gaseous form and from agriculture nonpoint sources fertilizer comprised about 60% of annual consumption of fertilizer N in the Changjiang catchment and were key factors controlling the high content of inorganic N in the Changjiang mouth. The fate of the N in precipitation and other N sources in the Changjiang River catchment are also discussed in this section.