Abstract:Two techniques were compared that uses the UV absorbance at 280nm of 0.5mol/L K_2SO_4 extracts of fumigated and unfumigated soils and fumigationwaterlogged incubation to estimate the concentrations of nitrogen in the SMB.Using 20 Ningxia and Shaanxi soils,with a wide range of organic carbon(5.16～24.6g/kg), total nitrogen(0.75～2.66 g/kg) and available nitrogen(25.7～119.0 mg/kg),it was demonstrated that the increase in UV absorbance at 280nm after soil fumigation was strongly correlated with the soil microbial biomass N(SMBN,r=0.79),as determined by fumigationwaterlogged incubation method.The 280nm UV absorbance technique has the advantages as followed,simple steps,less error,less time needed and better repeatability than fumigation/waterlogged incubation method.The SMBN determined by two methods all have remarkably positive correlations with total nitrogen,have positive correlation with organic carbon and no obviously relation with available nitrogen.In different soil types,it showed different correlation with total nitrogen,organic carbon and available nitrogen.In addition,the microbial biomass in fresh and dry soil were measured by 280nm UV absorbance technique,the results also showed that dry soil can be used to measure the(microbial) biomass after pre-cultivate,and the cultivating period is 10 days.