A field experiment was carried out in a six-year apple orchard to explore effects of 15N-urea absorption

utilization

loss and residue in the 0—60 cm soil layer of the orchard under different topdressing nitrogen applications (once

twice and other eight times) at fruit swollen stage using the 15N-labeled tracer method. Results showed that the highest Ndff value of different newborn organs (leaf

new shoots and fruits) were in the treatment received eight-time topdressing nitrogen application

and the lowest were found in the treatment received one-time topdressing nitrogen application at fruit expanding stage. At fruit maturity stage

the 15N absorption of the treatment received eight-time topdressing nitrogen application were 1.61 times and 2.10 times of the treatment received two-time and one-time nitrogen topdressing application respectively. The highest distribution ratio of 15N in vegetative and reproductive organs was in the treatment received eight-time topdressing nitrogen application

and the lowest was in the treatment received one-time topdressing nitrogen application at fruit maturity stage. With the passage of time

the amount of 15N residue in 0—60 cm soil layer of the treatment received eight-time topdressing nitrogen application was gradually higher than the treatments received two-time and one-time topdressing nitrogen application

and it was found mainly in the 0—40 cm soil layer. At fruit maturity stage

the plant 15N utilization rate of the treatment received eight-time topdressing nitrogen application was 17.65%

which was obviously higher than the treatment received two-time (10.99%) and one-time (8.37%) topdressing nitrogen application

and 15N loss rate the treatment received eight-time topdressing nitrogen application was 47.54%

which was obviously lower than the treatments received two-time (59.05%) and one-time (67.92%) topdressing nitrogen application. In short

all the findings indicated that eight-time topdressing nitrogen application at fruit swollen stage achieved the optimal effect

which could significantly reduce nitrogen loss rate

ensure steady and adequate supply of nitrogen and consequently improve nitrogen utilization rate

so as to exert its effect on the highest nitrogen demand of fruit trees.