Abstract: Based on pot experiment and isotope 15N tracer, the dynamics of soil microbial biomass nitrogenduring different corn growing stages in black soil was investigated and its biological availability was discussed The results indicated that the content of soil microbial biomass nitrogen of different treatments was different and its changes followed the similar trend during corn growing period Corn stalk and stubble rationing application of chemical fertilizer, in comparison with single application of chemical fertilizer, would result in the increase of the rate of fertilizer 15N immobilization and decrease of the rate of release of fertilizer 15N by microbes There existed significant correlations between soil microbial biomass nitrogen and soil total nitrogen, alkalihydrolysable N, corn uptake N( correlation coefficient were respectively 0.727**, 0.528* and 0.536*) Path analysis showed that the availability of soil microbial biomass nitrogen was similar to soil mineral nitrogen, above acidhydrolyzed organic nitrogen and acidinsoluble nitrogen The soil microbial biomass nitrogen was an available pool corn absorbing nitrogen from soil