Natural abundance of 15N in soil organic matter with special reference to paddy soils in Japan

In order to explain the general principle which controls the 15N content of soil organic nitrogen, experimental rice fields which were fertilized for long term were investigated. The 15N abundance values of rice plants vary according to the kinds of fertilizer, e.g., chemical fertilizer and green manure. The significant difference of 15N abundance was recognized between rice plants and fertilizers. Nitrification and denitrification seemed to be responsible for the difference. But these have minor effect on the variation of 15N abundance of soil because rice plants assimilate all available nitrogen and are removed from paddy fields by harvest. Consequently, the effects of nitrification-denitrification and ammonia volatilization are observed only in biological process such as the growth of rice plants and hydrophytes. A long term addition of fertilizers clearly increased the amount of soil organic nitrogen in paddy fields. Based on isotope mass balance, nitrogen isotope fractionation factor of 0.9942 was estimated in the process of epidiagenesis which indicates the selective decomposition of heavy isotopic species. An addition of ammonium sulfate with low 15N abundance decreased the 15N content of soil nitrogen in the paddy fields with the depletion of soil organic nitrogen. OrgC/clay ratio was demonstrated as an important factor which controls the 15N abundance value in paddy soil. The relation between the 15N abundance value in soil organic nitrogen and OrgC/clay ratio was able to be presented by hyperbola for the paddy soild so far examined. The 15N abundance of source and plant nitrogen two isotopic fractionation associated with the epidiagenesis of soil organic matter and the adsorption of ammonia by clay minerals are the three major factors determining the 15N abundance of soil organic nitrogen. (Ishimitsu, A.)