^<15>N tracer experiments in the experimental field of Chugoku National Agricultural Experiment Station were carried out. Mineralization of soil nitrogen and nitrogen absorption by rice plant derived from soil and from irrigation water were examined. Three types of plots were set up according to the application rate of rice straw compost as follows : C 0 (0 ton/10a), C 2 (2 tons/10a) and C 5 (5 tons/10a). The same applications has been continued for 24 years. The results obtained are summarized as follows : 1) The rate of nitrogen mineralization was in the following order : C 5 > C 2 > C 0. The amount of nitrogen mineralized for 10 weeks (Jun. 27th to Sep. 5th) in plots C 2 and C 5 were respectively 43 and 94% more than that of plot C 0. 2) The amount of nitrogen absorbed by the rice plant was in the following order : C 5 > C 2 > C 0. The amount of nitrogen absorbed gradually increased and reached the maximum level between July 18th to 25th, after which it gradually decreased. 3) The amount of nitrogen absorbed from soil for 9 weeks (Jun. 27th to Aug. 29th) in plots C 0, C 2 and C 5 were 3.49, 6.11 and 7.49 g/m^2, respectively. The nitrogen derived from irrigation water for the same period, which was calculated by deducting the nitrogen absorbed from soil from total nitrogen absorption, were 2.61, 1.40 and 1.68 g/m^2 for plots C 0, C 2 and C 5, respectively. These results suggest that it is important to take into consideration the level of nitrogen in irrigation water.