Natural Abundance of Nitrogen‐15 in a Forest Soil

Abstract

Abstract Because of measurement difficulties, only a few studies on natural 15 N abundance (δ 15 N) of inorganic N in forest soil have been pursued despite its importance for interpretations of plant δ 15 N signatures. To investigate stable N isotope ratios in inorganic N, the δ 15 N values and concentrations of total N, NH + 4 ‐N, and NO − 3 ‐N of forest mineral soils in four profiles were measured along a slope (altitude of 765–870 m) in a coniferous (Japanese red cedar, Cryptomeria japonica D. Don) forest in Japan. Generally, the δ 15 N values of total N, NH + 4 ‐N, and NO − 3 ‐N increased with increasing soil depth. The values of δ 15 N ranged from 1.0 to 6.8‰, 2.5 to 15.6‰, and −14.8 to 5.6‰ for total N, NH + 4 ‐N, and NO − 3 ‐N, respectively. Additionally, the δ 15 N values were different between NH + 4 ‐N and NO − 3 ‐N for each soil depth. Thus, it was concluded that the assumptions about inorganic N used in interpretation of plant δ 15 N values were valid. Moreover, on upper slope sites where soil inorganic N was predominantly NH + 4 ‐N, the order of δ 15 N was generally total N > NH + 4 ‐N > NO − 3 ‐N for each depth, whereas the order of δ 15 N was NH + 4 ‐N > total N > NO − 3 ‐N on lower slope sites where NH + 4 ‐N was less dominant as soil inorganic N and relatively high net nitrification rates were measured. Our results suggested that nitrification plays an important role in regulating δ 15 N in forest‐soil N.