The nitrogen cycling within river basin has recently attracted attention, which is particularly stimulated through estuarine hypoxia problems caused by discharge of nitrogen from land. Proportional relationships have been found between N load and N discharge, but the latter is generally far less than the former, indicating possible sinks of nitrogen in the catchment. Although possible processes responsible for such sink are considered as denitrification, retention by plants or soil, concrete evidence has not been given yet. Here, existing researches on denitrification within river basin are reviewed and three relevant works are presented as follows. 1) The N budget study conducted for agricultural watersheds in hilly region without flat lands showed a 1 : 1 relationship between N load and N discharge, implying the absence of N sink in these watersheds. 2) Chemical analysis of ground water along an upland-lowland transect in an agricultural area demonstrated the decrease in NO₃^- and dissolved oxygen concentration and the increase in relative N₂/Ar ratio. 3) Denitrification is also appreciable through a number of evidence in the bottom area of hilly forests. Based on the above facts, the importance of denitrification in the underground in lowland for the N dynamics in river basin is emphasized.

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To identify the sources of nitrogen contamination of urban shallow groundwater in Kathmandu Valley in Nepal, groundwater, river and sewage water samples were collected and analyzed for oxygen and hydrogen isotopes in water, nitrogen and oxygen isotopes ratio in nitrate and major dissolved ions. Groundwater samples were collected from 15 dug wells and 19 driven wells; river water samples from 8 locations in the major rivers; and sewage water samples from two locations in the urban area. Results showed that nitrogen contamination in the shallow groundwater is composed mainly of nitrate and ammonium ions. The nitrate-nitrogen and ammonia-nitrogen are as high as 63.9 mg/L and 36.7 mg/L, respectively. The results of nitrate nitrogen isotope ratio suggest sewage as the source of nitrate contamination. The increase in hydrogen isotope ratio of shallow groundwater with chloride concentration indicates the occurrence of sewage water contamination to shallow groundwater. In addition to nitrate, nitrogen contamination is also revealed from the increasing trend of dissolved inorganic nitrogen (DIN) with hydrogen isotope ratio in the shallow groundwater. The level of concentration, however, is found as lower than expected concentration of DIN on the sewage-contaminated groundwater. A possibility of reducing nitrogen by denitrification under anoxic conditions in the shallow groundwater was also analyzed based on hydrogen isotope ratio and chloride concentration. The results showed that most of the groundwater has no rainwater dilutions, and therefore, denitrification may be occurring in the shallow groundwater of the Kathmandu Valley.

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Hydrological models have been applied to estimate water budget and nitrogen flux in watersheds. In order to discuss problems in the application of the models for the estimation, descriptions for nitrogen processes in the models were reviewed and summarize an aspect of improvement. As the results, there are possibilities of improvement of the processes in rainfall-runoff, groundwater, impoundments and denitrificaion. Lacking of concept of hydrological processes in rice paddy field and septic tank was found because the majority of models have been developed in Europe and the United States. Due to this problem, improvement of these processes was necessary if the models apply to agricultural or suburban watersheds in Japan. Consequently, it is necessary to pay attention for application of the models for estimation nitrogen flux, especially in the case of estimation of the contribution to river discharge due to the problems. For solution of the problems, confirmation of the model applicability, improvement of processes of nitrogen dynamics in the models and coupling with external specific models is suggested.

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