Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Abstract

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 NO3 – pool at this site, we concluded that plant δ15N 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.