When plants are grown on the soil amended with sewage sludge, they utilize two different N sources, soil N and sewage sludge N. The main objective of this investigation was to determine the source of N utilized by plants grown on the sludge amended soil in pot and field experiments. The values of natural ^<15>N abundance(δ^<15>N)of soils, plants, sludges and chemical fertilizers were compared to know the source of the plant N. 1)Pot experiment The Andosol with the activated sludges prepared artificially was used to grow Brassica rapa L. (komatsuna)in pots. (1)The δ^<15>N values of the activated sludges were far greater than those of the soil. (2)Addition of the sludges to the soil increased the δ^<15>N values of komatsuna plants to the same level as those of the sludge used but did not increase those of the soil. The effect of the zinc concentration in the sludges were insignificant. (3)Decomposition of the sludge in the soil for 100 d before seeding did not affect the availability of the sludge N to the plants. 2)Field experiment Since 1978, sewage sludge composts have been applied to the upland field of Andosol twice a year where barley(Hordeum vulgare L.)and corn(Zea mays L.)have grown every winter and summer. (1)The δ^<15>N values of the composts were far greater than those of the field. Their continuous application over 13 y has not changed the δ^<15>N values of the field. (2)The application of composts to the field increased the δ^<15>N values in the barley plants to the same level as those of the applied composts, suggesting that the plants utilized N derived from the sludges or the composts in preference to soil N. (3)However, in the late growing stage of barley plants, the δ^<15>N values of the plants were decreased compared with the early and middle stages, indicating that N derived from the composts became less available to the plants with the decomposition of the composts in the field for about 6 months.
