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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Natural abundance of 15N in soil organic matter with special reference to paddy soils in Japan: biogeochemical implications on the nitrogen cycle

Eitaro Wada, Reiko Imaizumi, Yasuo Takai
Geochemical Journal, Vol. 18, No. 3, P. 109-123, 1984


Isotopic composition of nitrogen in soil organic matters from subalpine forest areas, upland, and paddy fields in Japan was investigated. In the forest areas, 15N abundance in soil organic nitrogen increased with depth irrespective of soil types such as alpine brown forest soil and podzolic soil. Biological nitrogen fixation appear as the main source of soil nitrogen. Higher plants seem to promote the above vertical trend even in a cultivated field. 15N content of soil organic matters in Andosol soils was higher than that of Alluvial soils. Long-term fertilization experimental rice fields were investigated to elucidate the general principles that govern the variation of 15N content of soil organic nitrogen. In paddies, δ15N value in rice plant was variable depending upon the kinds of applied fertilizers. Significant difference of δ15N was observed between rice plants and fertilizers, especially in a chemical fertilizer plot and in a green manure plot. Nitrification and denirification seemed to be responsible for the difference. However, these seem to have a small effect on the variation of soil δ15N, as rice plants completely assimilate available nitrogen and later are harvested from the paddy fields. The effects of nitrification-denitrification, and of ammonia volatilization were, thus, observed only in biological processes related to relatively short-term phaenomena such as the growth of rice plants and hydrophytes. A long-term addition of a fertilizer clearly increased the amount of soil organic nitrogen in the paddy fields enriched in organic matter. Based on isotope mass balance, nitrogen isotope fractionation factor of 0.9942 was estimated for the process of epidiagenesis, indicating that the heavier isotopic species are preferably decomposed. An addition of ammonium sulfate with a low δ15N decreased 15N content of soil nitrogen in a paddy field depleted in soil organic nitrogen. Org.C/Clay ratio was demonstrated to be an important factor that governs the δ15N value in paddy soils. The relation between δ15N value in soil organic nitrogen and Org.C/Clay ratio could be presented by a hyperbola for paddy soils so far examined. A simple model is presented to elucidate major factors that govern 15N content of soil organic materials. It is emphasized that δ15N of source and plant nitrogen and two isotopic fractionations associated with epidiagenesis of soil organic matters and with adsorption of ammonia by clay minerals, constitute three major factors that determine the δ15N of soil organic nitrogen.

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