Nitrogen and carbon isotopes in soil with special reference to the diagnosis of organic matter

Distributions of nitrogen and carbon isotopes in terrestrial ecosystems are described based on available data and our recent findings for soil organic matters. Major processes regulating N-isotope and C-isotope ratios in biogenic substances are discussed. The biological di-nitrogen fixation and the precipitation are major sources which lower the delta15N value for forested soil organic matters. Denitrification enhances delta15N value for soil in cultivated fields. An addition of chemical fertilizer lowers 15N content in soils. The permiation of soil water is an important factor controlling vertical profiles of delta15N in soil systems. Among soil organic matters, non-hydrolizable fraction seems to give unique low delta15N value, suggesting the utility of delta15N analysis in studying the nature of the fractions. delta13C of soil organic matter is significantly lower than that for marine sediments. delta13C for soil humus varies with respect to chemical forms as well as an age of soil organic matters. The variation is large in paddy fields. It is, thus, probable that delta13C is an useful parameter in studying the early epidiagenesis of soil organic matters. Based on the known delta15N-delta13C relationships, a two-source mixing model has been applied to assess sources of organic matters in coastal sediment. (author)