The fertilizer nitrogen absorption and utilization

soil nitrogen residue and soil nitrogen pool deficit under different nitrogen levels (0

50

100

150

200

and 250 kg/hm2) were studied using 15N isotope tracer technique during nutrition transformation period of 5-year-old Fuji Apple. The results showed that with the increasing of nitrogen application

utilization ratio of fertilizer nitrogen decreased gradually

and the percentage of tree nitrogen absorbed from soil nitrogen gradually decreased

but from nitrogen fertilizer increased. After one month of nitrogen application

5.75%～12.99% of fertilizer nitrogen was absorbed by tree

and 29.62%～39.74% of it remained in 0—60 cm soil

and 47.27%～64.64% of it lost by other means. With the increasing of nitrogen application rate

the amount of fertilizer nitrogen absorbed by tree and soil residual nitrogen increased gradually

but the utilization rate of fertilizer nitrogen and soil residual rate decreased

and meantime

the loss amount and the loss rate of fertilizer nitrogen were both increasing gradually. Residual fertilizer nitrogen in the soil mainly distributed in the topsoil (0—20 cm)

and soil 15N abundance increased significantly with the increasing of nitrogen application. With the increasing of nitrogen application

soil total nitrogen balance changed from deficit to surplus

and this indicated that low nitrogen application rate could lead to the decrease of soil nitrogen fertility

and the excessive application of nitrogen could increase the accumulation of nitrogen. There was a significant positive correlation between soil total nitrogen balance and nitrogen application rate

and the regression equation was y=0.3147x–16.144（R2=0.990 2)

soil nitrogen pool reached balance when nitrogen application rate was 51.30 kg/hm2.