『Abstract
　The longitudinal variations in the nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) isotopic compositions of nitrate (NO3^-), the carbon isotopic composition (δ¹³C) of dissolved inorganic carbon (DIC) and the δ¹³C and δ¹⁵N of particulate organic matter were determined in two Southeast Asian rivers contrasting in the watershed geology and land use to understand internal nitrogen cycling processes. The δ¹⁵NNO3 became higher longitudinally in the freshwater reach of both rivers. The δ¹⁸ONO3 also increased longitudinally in the river with a relatively steeper longitudinal gradient and a less cultivated watershed, while the δ¹⁸ONO3 gradually decreased in the other river. A simple model for the δ¹⁵NNO3 and δ¹⁸ONO3 that accounts for simultaneous input and removal of NO3^- suggested that the dynamics of NO3^- in the former river were controlled by the internal production by nitrification and the removal by denitrification, whereas that in the latter river was significantly affected by the anthropogenic NO3^- loading in addition to the denitrification and/or assimilation. In the freshwater-brackish transition zone, heterotrophic activities in the river water were apparently elevated as indicated by minimal dissolved oxygen, minimal δ¹³CDIC and maximal pCO2. The δ¹⁵N of suspended particulate nitrogen (PN) varied in parallel to the δ¹⁵NNO3 there, suggesting that the biochemical recycling processes (remineralization of PN coupled to nitrification, and assimilation of NO3^--N back to PN) played dominant roles in the instream nitrogen transformation. In the brackish zone of both rivers, the δ¹⁵NNO3 displayed a declining trend while the δ¹⁸ONO3 increased sharply. The redox cycling of NO3^-/NO2^- and/or deposition of atmospheric nitrogen oxides may have been the major controlling factor for the estuarine δ¹⁵NNO3 and δ¹⁸ONO3, however, the exact mechanism behind the observed trends is currently unresolved.

Keywords: Anthropogenic nitrogen loading; Denitrification; \Estuary; Nitrate; Nitrification; Tropical river』

Introduction
Study sites
Experimental
Results
Discussion
　Instream metabolic activities inferred from the dynamics of DIC and O2
　Turnover of NO3^- in the freshwater reach
　A steady-state turnover model of river-water NO3^-
　N turnover in the freshwater/brackish transition zone
　DIN turnover in the saline-water section
Acknowledgments
References