Soil microbial biomass carbon(MBC)is an important index to reflect farmland soils quality.The research studied the impact of different fertilizing management and weed control on soil microbial biomass carbon of Tibetan plateau farmland ecosystem in Lhasa agroecological experimental station,chinese academy of sciences.Soil samples were taken by random mixing in long term fertilizing fields and abandoned farmland.Then indoor chloroform fumigation was taken.Based on the average of three growth stages,the soil microbial biomass carbon of the manure plot was 561% higher than that of blank plot,but the value of chemical fertilizer used only was 140% lower than the blank plot,which showed that manure had a significant effect on promoting to improve farmland soils quality in Tibet,chemical fertilizer would weaken the effect.The experiments in the abandoned farmland showed that the MBC in the natural abandoned farmland(without weeding)was about 852 mg/kg,while the soil microbial biomass carbon in the plot with frequent weeding decreased about 1/4.Then it further illustrated that input of organic matter played an important role in improving farmland soils quality in plateau.