Abstract: 【Objective】 Labile organic carbon (Lab-C)and recalcitrant organic carbon(Rec-C)are two important components of soil organic carbon, and their ratio has an effect on organic turnover and sequestration properties. So, it has great significance for nutrient management and fertility improvement to study the characteristic of labile and recalcitrant organic carbon content under different long term fertilization systems.【Methods】 Using particle size density separation method, labile and recalcitrant carbon pools were studied under four treatments, namely no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer combined with straw (NPKS), and chemical fertilizer combined with manure (NPKM) from four long term experiment sites (three from 23 years old upland sites of black soil, fluvo aquic soil and red soil, one from 32 years old paddy soil) in eastern China.【Results】 The separation method was a simple and suitable method for carbon pool size analysis in which average soil mass recovery and average soil carbon recovery were both above 95%. The average labile organic carbon contents in upland soils was 1.91 g/kg, lower than those in paddy soil 2.42 g/kg. The average ratio of labile carbon to total carbon content (Lab-C/TOC) in upland soils was 15.4%, higher than in paddy soil of 9.9%. In upland soils (black soil, fluvo aquic soil and red soil), the increase of labile carbon content under NPKM treatment was significantly higher than NPKS, NPK and CK treatments, in which the corresponding increases were 98.4%, 43.7% and 71.2% compared to NPK treatments, respectively. The Lab-C/TOC was consequently increased greatly under NPKM treatment, although the increases differences among the treatments were not significant. The labile carbon contents in NPK and NPKS were no significant different with CK in black soil and fluvo aquic soil, but significant in red soil. The labile carbon content in NPK and NPKS were 33.1% and 29.6% significantly lower than that under CK (P0.05), respectively; There were no significant differences among four treatments of paddy soil in labile organic carbon content. Recalcitrant carbon content in four soils was consistent with their total organic carbon under four treatments as NPKMNPKSNPKCK. Compared with the NPK treatment, the recalcitrant carbon content in the four soils were greatly improved under NPKM treatment, their proportion were increased by 68.8%, 42.7%, 17.6% and 17.2%, respectively; In the meantime, the Rec-C/TOC was reduced. Under NPKS treatment, the recalcitrant carbon contents in black soil, fluvo aquic soil and paddy soil were increased by 10.9%, 15.1% and 18.0% compared to NPK treatment, respectively. There was a significant positive correlation between the total organic carbon content and labile/recalcitrant organic carbon content.【Conclusion】Comparing with paddy soil, the labile organic carbon content in upland soil are more susceptible to fertilization. Long term application of NPKS or NPKM will improve the quantity of soil labile organic carbon and recalcitrant organic carbon contents, meanwhile it could increase the Lab-C/TOC ratio. The improvement effect is in order of NPKMNPKSNPK