Natural Abundance of Nitrogen-15 in a Forest Soil
Because of measurement difficulties, only a few studies on natural 15N abundance (δ15N) of inorganic N in forest soil have been pursued despite its importance for interpretations of plant δ15N signatures. To investigate stable N isotope ratios in inorganic N, the δ15N 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 δ15N values of total N, NH+4-N, and NO−3-N increased with increasing soil depth. The values of δ15N 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 δ15N 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 δ15N values were valid. Moreover, on upper slope sites where soil inorganic N was predominantly NH+4-N, the order of δ15N was generally total N > NH+4-N > NO−3-N for each depth, whereas the order of δ15N 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 δ15N in forest-soil N.
Earth Systems Research Center
Soil Science Society of America Journal
Soil Science Society of America
Digital Object Identifier (DOI)
Koba, K., N. Tokuchi, T. Yoshioka, E.A. Hobbie, and G. Iwatsubo. 1998. Natural abundance of 15N in a forest soil. Soil Science Society of America Journal 62:778-781.