Characteristics of the Relationship between Natural ¹⁵N Abundances in Organic Rice and Soil

Abstract

The characteristics of the relationship of natural ¹⁵N abundances (δ¹⁵N values) between rice and soil in organic farming and conventional farming were examined, and the possibility of discriminating organically grown rice from conventionally (non-organically) grown rice using this relationship was explored. Organically grown rice, conventionally grown rice and soil samples were collected from farmers’ fields in Daisen city and Yokote city, Akita prefecture, Japan as well as from the experimental fields of NARO Tohoku Agricultural Research Center located in Daisen from 2007 to 2009. Organic fertilizers and synthetic fertilizers available on the market in Akita prefecture were also collected. The δ¹⁵N values of those samples were measured. Regardless of the farming method, the δ¹⁵N values of rice reflected those of the soil. The δ¹⁵N values of the organic fertilizers available on the market were likely to be higher than those of the soil. Meanwhile the δ¹⁵N values of the synthetic fertilizers were negative and lower than those of the soil. The δ¹⁵N values of organically grown rice tended to be higher than the regression line obtained from the δ¹⁵N values of rice and soil without N source application. The δ¹⁵N values of conventionally grown rice tended to be lower than the regression line. These results indicated that the relationship of the δ¹⁵N values of rice and soil without an applied N source could aid in discriminating between organic rice and conventional rice.