Changes in natural15N abundance in paddy soils under different, long-term soil management regimes in the Tohoku region of Japan

Long-term temporal changes in natural 15N abundance (δ15N value) in paddy soils from long-term field experiments with livestock manure and rice straw composts, and in the composts used for the experiments, were investigated. These field experiments using livestock manure and rice straw composts had been conducted since 1973 and 1968, respectively. In both experiments, control plots to which no compost had been applied were also maintained. The δ15N values of livestock manure compost reflected the composting method. Composting period had no significant effect on the δ15N value of rice straw compost. The δ15N values increased in soils to which livestock manure compost was successively applied, and tended to decrease in soils without compost. In soils to which rice straw compost was successively applied, the δ15N values of the soils remained constant. Conversely, δ15N values in soils without rice straw compost decreased. The downward trend in δ15N values observed in soils to which compost and chemical N fertilizer were not applied could be attributed to the natural input of N, which had a lower δ15N value than the soils. Thus, the transition of the δ15N values in soils observed in long-term paddy field experiments indicated that the δ15N values of paddy soils could be affected by natural N input in addition to extraneous N that was applied in the form of chemical N fertilizers and organic materials.