The content of soil microbial biomass nitrogen (BN) and its affecting factors were reviewed, the important roles of soil microbial biomass nitrogen in soil nitrogen cycling were described, and the relationships of soil microbial biomass nitrogen with soil mineralizable nitrogen, mineral nitrogen, organic nitrogen and fixed ammonium were emphatically discussed in this paper. It was indicated that soil microbial biomass nitrogen had a close relationship with the nitrogen-supplying capability of soil, and played an important role in controlling soil nitrogen supply, decreasing soil nitrogen loss,...