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Soil Science Society of America Journal Abstract -

Natural Abundance of Nitrogen-15 in a Forest Soil


This article in SSSAJ

  1. Vol. 62 No. 3, p. 778-781
    Received: Oct 17, 1997

    * Corresponding author(s): kkoba@kais.kyoto-u.ac.jp
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  1. Keisuke Koba ,
  2. Naoko Tokuchi,
  3. Takahito Yoshioka,
  4. Erik Alan Hobbie and
  5. Goro Iwatsubo
  1. Lab. of Forest Ecology, Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto Univ., 606-01, Japan
    Inst. for Hydrospheric-Atmospheric Science, Nagoya Univ., Chikusa-ku, Nagoya 464-01, Japan
    U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333
    Faculty of Agriculture, Kinki Univ., Nara 643, Japan



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 NO3-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 NO3-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 NO3-N, respectively. Additionally, the δ15N values were different between NH+4-N and NO3-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 > NO3-N for each depth, whereas the order of δ15N was NH+4-N > total N > NO3-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.

This work was partly supported by a grant from the Ministry of Education, Science and Culture, Japan, relating to Japan Society for the Promotion of Science Fellowships for Japanese Junior Scientists (no. 6788).

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