This study was performed to clarify whether areal variation exists in the relationship between natural 15N abundances (δ15N values) of rice (Oryza sativa L.) and soil without an applied nitrogen (N) source, and to explore possible reasons for any areal variation. We investigated the relationships between δ15N values of rice and those of unamended soil with no applied N source in two locations; Daisen and Ogata, in Akita Prefecture, Japan. The δ15N values of rice in Daisen were higher than those in Ogata from 2007 to 2009, irrespective of the cropping year. Results demonstrated areal variation in the relationship between δ15N values of rice and those of unamended soil. The variation might be attributed to variation in the δ15N of natural N input and to ammonia nitrification and subsequent denitrification. When the relationship between δ15N values of rice and those of unamended soil is used to discriminate between organic and conventional rice, the areal variation of the relationship in the target area should be taken into account, from the point of the δ15N value of natural N input and N transformation in the soil.
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