『Abstract
　The great spatial and temporal variability of nitrogen (N) processing introduces large uncertainties for quantifying N cycles in large scales, e.g. a watershed scale, and hence challenges the present techniques in measuring ecosystem N mass balance. The dual isotopes of nitrate (δ¹⁸O and δ¹⁵N) integrate signals for both nitrate sources and N processing, making them promising for studies on large scale N cycling. Here, the dual isotopes, as well as some ion tracers, from a subtropical river in south China were reported to identify the main nitrate sources and to assess the possible occurrence and degree of denitrification in the context of monsoon climate. Our results indicated that nitrification of reduced fertilizer N in soil zones was the main nitrate source, with sewage and manure as another important source in dry winter. Seasonal changes of denitrification was apparent by the 〜1:2 enrichment of δ¹⁸O and δ¹⁵N from April to August, and suggested to occur over the watershed rather than in the river. The lowest denitrification (10％) occurred in April, when the fertilizer application was strongest and the monsoon rainfall abruptly increased, causing enhancement of leaching. The highest denitrification (48％) took place in august due to the high soil temperature and moisture. In December, denitrification was significant (26％) perhaps due to the high enough temperature for microbial activities, whereas the low soil moisture appeared to limit the degree of denitrification. This study suggests that the seasonal variations in denitrification should be taken into account when estimating regional N mass balance.

Keywords: Nitrate; Dual isotope; Denitrification; River』

Introduction
Samples collection and analysis
　Background
　Samples and experiment
Results
Discussion
　Seasonal variations of nitrate sources and dynamics
　Denitrification
Conclusions
Acknowledgments
References