ABSTRACT

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.