Measurement of nitrogen isotopic composition (delta(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 delta(15)N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of delta(15)N between soil N pools and plants, and by using an existing model. delta(15)N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO3- pool at two sites with different nitrification rates in a temperature forest in Japan. delta(15)N of plants was similar to that of soil NO3- in the high-nitrification site. Because of high foliar NRA and the large foliar NO3- pool at this site, we concluded that plant delta(15) N indicated a great reliance of plants on soil NO3- there. However, many delta(15)N of soil N overlapped each other at the other site, and delta(15)N could not provide definitive evidence of the N source. The existing model was verified by measured delta(15) N of soil inorganic N and it explained the variations of plant delta(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 delta(15) N signatures from different ecosystems and to understand N dynamics.