The increasing demand of rice crops leads to elevated levels of nitrogen in water bodies in Asia. The concentrations of organic and inorganic nitrogen and the nitrogen isotope ratio in nitrate (NO 3 -) were analysed to identify the sources of nitrogen in the outflow waters from a paddy field in Japan in 2013. Nitrogen concentrations in the drainage and percolation waters were the highest during the paddling period, NO 3 was the dominant form of dissolved total nitrogen and the δ 15 N-NO 3

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... in both types of water were close to that of the chemical fertilizer. During the irrigation and non-irrigation periods, NO 3 and dissolved organic nitrogen (DON) were the dominant in percolation water, whereas DON was dominant in drainage water. The strong correlation between the δ 15 N-NO 3 values in the irrigation and drainage water suggested that the irrigation water is one of the N sources in the drainage water during the irrigation period. Nitrate in percolation water during irrigation and non-irrigation periods might have been affected by the nitrified ammonium in the chemical fertilizer, or mineralized nitrogen in the soil or organic fertilizer.