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

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Abstract
The isotopic composition of N in soil organic matter from subalpine forest areas, upland and paddy fields in Japan was investigated. In the forest areas, 15N abundance in soil organic N increased with depth irrespective of soil types such as alpine brown forest soil and podzolic soil. Biological N fixation apparently as the main source of soil N. 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 N. In paddies, .delta.15N value in rice plant was variable depending on the kinds of applied fertilizers. Significant difference of .delta.15N was observed between rice plants and fertilizers, especially in a chemical fertilizer plot and in a green manure plot. Nitrification and denitrification seemed to be responsible for the difference. However, these seem to have little effect on the variation of soil .delta.15N, as rice plants completely assimilate available N 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 phenomena such as the growth of rice plants and hydrophytes. A long-term addition of a fertilizer increased the amount of soil organic N in the paddy fields enriched in organic matter. Based on isotope mass balance, N 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 .delta.15N decreased 15N content of soil nitrogen in a paddy field depleted in soil organic N. The organic C/clay ratio is an important factor that governs the .delta.15N value in paddy soils. The relation between .delta.15N value in soil organic N and organic and C/clay ratio could be presented by a hyperbola for paddy soils. A simple model is presented to elucidate major factors that govern 15N content of soil organic materials. The .delta.15N of fertilizer and plant N and 2 isotopic fractionations associated with epidiagenesis of soil organic matter and with adsorption of ammonia by clay minerals, constitute 3 major factors that determine the .delta.15N of soil organic nitrogen.