We investigated quantitatively the difference in N transformations in soil between submerged conditions and upland field conditions by using 15N-a mmonium. We also investigated the effect of the water conditions on the distribution of 15N fertilizer in rice plant and soil organic matter in greenhouse experiments. In the soil incubation experiment without plants, the amount of immobilized 15N in soil organic matter under upland field conditions increased rapidly during the initial period, but gradually decreased during the later period. The amount of immobilized 15N under submerged conditions continued to increase until the later period. In the pot experiment where plants were present, the amount of immobilized 15N in soil organic matter under submerged conditions was larger than that under upland field conditions, and the availability of the applied 15N to plants was larger under upland field conditions. Under upland field conditions, the 15N-ammonium immobilized by soil microorganisms was presumably transformed by the remineralization-nitrification sequence, and absorbed by plants. The submerged soil is likely to have a high capacity for accumulating the N fertilizer in soil organic matter. It was demonstrated that nitrification largely affects the distribution of N fertilizer in plant and soil organic matter.
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