Abstract: In this paper, we present estimates of nitrogen (N) biogeochemical cycling in the Changjiang drainage basin, including N inputs, outputs and internal transfer for the period 1968 1997. The total N input, output and internal storage are approximately 10.0?10 9, 6.7?10 9, and 3.5?10 9 kg, respectively, in 1997, which is about fourfold, fivefold, and threefold of these in 1968, respectively. In terms of N inputs, N fixation was often a dominant input before 1977, providing about 1.0?10 9 kg year 1 ; while N fertilizer dominated N input after 1983, supplying an additional input of some 5.0?10 9 kg year 1 . In terms of N output, we estimate that the river dissolved inorganic nitrogen (DIN) concentration and flux have increased by about 10 fold from 1968 to 1997, reaching 1.60 mg L 1 and 1.40?10 9 kg, respectively, in 1997. Our study shows that about 25.0% of the total N input is transported through the river, while some 33% of the input is emitted by denitrification and volatilization, and the remaining 42% of the input is stored within the basin. The integrated N input, budget and storage have been linked to the increasing temporal trends of Changjiang River DIN. It is demonstrated that integrated N input, especially N fertilizer application, rather than human population density in the basin is the preferable predictor of the river DIN concentration and flux. Therefore, how N fertilizer is effectively applied is of crucial importance to the sustainable development of agriculture in the basin.

Key words: nitrogen, dissolved inorganic nitrogen, biogeochemical cycling, chemical fertilizer, Changjiang River, basin