Transformations of fertilizer nitrogen in soil-plant system with special reference to comparison between submerged conditions and upland field conditions

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.