Measurement of nitrogen isotopic composition (δ 15 N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ 15 N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ 15 N between soil N pools and plants, and by using an existing model. δ 15 N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO 3 – pool at two sites with different nitrification rates in a temperature forest in Japan. δ 15 N of plants was similar to that of soil NO 3 – in the high-nitrification site. Because of high foliar NRA and the large foliar NO 3 – pool at this site, we concluded that plant δ 15 N indicated a great reliance of plants on soil NO 3 – there. However, many δ 15 N of soil N overlapped each other at the other site, and δ 15 N could not provide definitive evidence of the N source. The existing model was verified by measured δ 15 N of soil inorganic N and it explained the variations of plant δ 15 N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ 15 N signatures from different ecosystems and to understand N dynamics.
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