Relationship was explored of N loss through ammonia volatilization,runoff and soil leaching (at the depth of 1 m in the soil profile) with nitrogen (urea) application rate in red soil planted with Smooth Crabgrass (Digitaria ischaemum).Results show that N lost through ammonia volatilization in Treatments N 90,N 160 and N 230 (kg hm^-2) reached 0.67,1.24 and5.16 kg hm^-2,respectively,accounting for 0.74%,0.77% and 2.24% of the total N applied,showing a rising exponential relationship between the total amount of N lost through ammonia volatilization (y) and the N application rate (x) (y=0.156 3e 0.014 6 x).The amount of N lost through runoff was N 1.05,0.88 and 1.01 kg hm^-2,accounting for 1.17%,0.55% and 0.44% of the total N applied,respectively,in the three N treatments.There was no significant relationship between the total amount of N lost through runoff and the N application rate during the growth stage of Smooth Crabgrass.In this experiment,the amount of N lost through leaching was 2.05,2.86 and 4.09 kg hm^-2,which accounted for 0.91%,1.02% and 1.24% of N applied,respectively,showing an approximate linear relationship between the total amount of N lost through soil leaching (y) and the amount of N applied (x) (y=0.012 2 x+1.0877).