The objectives of this study were to investigate the effects of long term fertilization on soil microbial biomass carbon(SMBC)for the upper soil profile(0～40 cm)in the chestnut soil of northern China.The results showed that the average SMBC content was increased by different fertilizers compared to the unfertilized control(CK).Among these,treatment with a combination of organic and inorganic fertilizers(NPKM)yielded the greatest increase in SMBC content,followed by treatment with organic fertilizers only(M)and lastly by treatment with inorganic fertilizers only(NPK).The respective average SMBC contents produced by these three treatments were 57.1 mg·kg-1,51.0 mg·kg-1,and 46.4 mg·kg-1,which means 56.7%,39.9% and 27.3% respective percentages of increase.The SMBC content also varied depending on the season,as it was less during the potato seeding stage and after harvesting(40.6～51.4 mg·kg-1)and reached the maximum value(49.2～63.4 mg·kg-1)during the potato tuber formation and expansion period.The SMBC content as well as SMBC variations between different treatments was most significant in the 0～10 cm soil layer,and both the content amount and differences in treatment effects decreased in deeper soil layers.In conclusion,the application of organic manure,or,better,the combined application of organic manure plus chemical fertilizer,was an effective measure to improve SMBC in north area of Yinshan Mountain.