Abstract: In order to clarify the effects of soil microbial biomass N on soil N availability of plants and N cycling, SMBN, SMBC, soil N, litterfall input, and other factors were determined in a secondary oak forest and pine plantation in Xiashu experimental sites in Jiangsu province from April 2006 to April 2007, the results are as follows: (1)There are obvious seasonal dynamics of SMBN both in secondary oak forest and pine plantation, and the variation is consistent, which shows the highest microbial biomass N in summer, lowest in winter, spring and autumn are middle; (2)In the condition of the undisturbed natural state(control treatment), SMBN in oak forest is significantly larger than in pine plantation (p<0.01), but the difference between the two forest types disappear after the litterfall is removed(litterfall excluding treatment); (3)The correlation analysis shows that soil microbial biomass N is significantly and positively correlated to SMBC both in secondary oak forest and pine plantation (p<0.05), but not to soil nitrogen, soil moisture or litterfall input. Our results show that the obviously seasonal changes of SMBN mainly controlled by the seasonal changes of soil microbial biomass, which may be influenced by the soil C, N availability and the competition between soil microbial biomass and plant growth with the deeper reasons, but the properties of litterfall had a significant impact on the SMBN.