The experiment was conducted to study the decomposition characteristics of maize roots organic carbon(C）derived from N 0,120 and 240 kg/hm2 treatments(marked as R0,R120 and R240,respectively）in the 15 cm and 45 cm soil depths with various fertility,and to evaluate the effect of roots C decay on soil labile organic C,through indoor incubation and field decomposition conditions.The results showed that,cumulative CO2 release and potential C mineralization amount ranked in the order that R120＞R240＞R0 under indoor incubation condition.And in contrast to R0,the C mineralization rates of R120 and R240 increased by 21.1% and 12.7% in the topsoil(15 cm),and increased by 45.3% and 33.7% in the subsoil(45 cm),respectively.After 386 d field decomposition,the C residual ratios of R0,R120,and R240 were respectively 36.3%,25.2% and 28.7% in the topsoil,and were 38.4%,30.6% and 31.1% in the subsoil.It showed positive correlations between root C residual ratios and their C/N ratios,lignin contents,and lignin/N ratios,while performed a negative correlation between root C residual ratios and their total N contents.This result indicated that root decomposition ratio improved with its N content increasing.Soil microbial biomass C and soluble organic C contents improved significantly by 143%~297% and 19.9%~118.2%,respectively in the maize roots addition treatments.It was concluded that considering the influence of N fertilizer application on the nutrient compositions of crop roots,especially increasing root N content,the effects of longterm N addition on the nutrient accumulation of crop residues and their decay and cycling in farmland ecosystem should be paid more attention in soil C and N turnover evaluation.