Natural ¹⁵N abundance of plants and soil N in a temperate coniferous forest

Measurement of nitrogen isotopic composition (δ¹⁵N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ¹⁵N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ¹⁵N between soil N pools and plants, and by using an existing model. δ¹⁵N 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. δ¹⁵N of plants was similar to that of soil NO₃^- 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 ¹⁵N indicated a great reliance of plants on soil NO₃ ^- there. However, many δ¹⁵N of soil N overlapped each other at the other site, and δ¹⁵N could not provide definitive evidence of the N source. The existing model was verified by measured δ¹⁵N of soil inorganic N and it explained the variations of plant δ¹⁵N 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 δ¹⁵N signatures from different ecosystems and to understand N dynamics.