Abstract: This paper dealt with the changes of microbial biomass-N and its role in nitrogen nutrition through a 13-year field experment with different fertilizer treatmants on brown earth. Results showed that long term application of NPK fertilizers or manure could increase soil biomass-N, especially the effect of manure on biomass-N was obvious. The basic content of biomass-N (before fertilizer application) could reflect the capacity of offering N by soil. The seasonal fluctuation of biomass-N was mainly affected by applying fertilizer, soil temperature, soil moisture and other factors. The turnover period of biomass-N was between 0.25 and 0.52 year in different fertilizer plots, in which application of N fertilizer with low amount of manure the turnover period was 0.25 year and in which current plots without fertilization the turnover period was 0. 45 year in average. The amount of offering N by biomass-N was mostly from active organic N in soil. In Single N fertilizer plots the amount of offerng N by biomass was higher than that of N uptake by crop; in NP fertilizer plots the both was equal to same and in NPK fertilizer plots the amount of offering N by biomass was lower than that of N uptake by crop. There existed a close relationship between the offerng N by biomass and the uptake of N by crop and yield in various fertilizer plots.