The fate of basal nitrogen enriched with 15N was investigated in the paddy field under the various cultural conditions. Rice plants were transplanted at three different times, early May, late May and mid June, and were grown under two doses of basal nitrogen, 70 kg/ha and 150 kg/ha with 28 hills/m2 of plant density. Furthermore, shading plots and high plant density plots, 56 hills/m2 were established in case of transplanting at late May. Soil ammonium nitrogen derived from fertilizer disappeared by the end of June, in the plots transplanted at the early and late May, except the shaded plots. It disappeared by the mid July in the plots transplanted at the mid June. The rate of decrease of soil ammonium nitrogen derived from basal fertilizer was accelerated by dense planting and was decreased by the shading treatment. In all the plots, rice plants absorbed the basal nitrogen only the period when the ammonium nitrogen derived from the basal fertilizer stayed in plowed layer as previously reported. The recovery of basal nitrogen by rice plant was nearly 30 percent in all the treatments, in spite of absorption process and absorbing period being very different in each experimental plot. It was assumed that the percentage recovery and duration of absorption by plant was controlled by the absorbing rate of rice plant and the rate of denitrification in plowed layer.
