Soil microbes are the most important regulator and decomposer in the forest ecosystem. Soil microbial biomass is an important component of the carbon and nitrogen cycles in the terrestrial ecosystem. The Heilongjiang Liangshui Nature Reserve has full gradient interference, which includes virgin forests, secondary forests, and plantation forests. The reserve provides a good platform to explore soil microbial biomass and to determine the factors influencing soil microbial biomass in the Xiaoxing'an Mountains. We used the chloroform fumigation extraction method to measure and compare the microbial biomass carbon (MBC), the microbial biomass nitrogen (MBN), and the influencing factors (i.e., soil organic carbon, soil total nitrogen, soil water content and soil temperature) throughout the growing season. We also explored the effect of root removal on soil microbial biomass. The experiment included six typical forest types, namely, virgin mixed broadleaved-Korean pine (Pinus koraiensis) forest, selectively cut mixed broadleaved-Korean pine forest, secondary birch (Betula platyphylla) forest, artificial larch (Larix gmelinii) plantation, artificial Korean pine forest, and valley spruce-fir (Picea-Abies) forest. Each forest type consists of three 20 m × 30 m plots, and four 2 m × 2 m root trenching subplots and four 2 m × 2 m control subplots were randomly chosen in each plot. Soil samples from 0 cm to 10 cm soil layers were randomly selected monthly from the six forest types from June 2010 to October 2010. The samples in each plot were mixed as one sample to determine MBC and MBN. Results show that the seasonal changes in the soil microbial biomass of the six forest types were different during the study period. The maximum soil microbial biomass was mostly obtained in mid-August. The MBC and MBN generally showed similar seasonal dynamics in the control and root removal plots. The average values of MBC and MBN for the six forest types varied from 383.5 mg/kg to 1633.6 mg/kg and 47.6 mg/kg to 231.0 mg/kg, respectively, accounting for 1% to 2% and 2% to 4% of the soil organic carbon and nitrogen content. The average soil microbial biomass (MBC and MBN, respectively) of each forest type was recorded as follows: secondary birch forest (1318.8 and 215.5 mg/kg) > artificial Korean pine forest (1137.3 and 169.1 mg/kg) > selectively cut mixed broadleaved-Korean pine forest (980.3 and 153.4 mg/kg) > virgin mixed broadleaved-Korean pine forest (948.9 and 143.6 mg/kg) > artificial larch forest (927.2 and 131.0 mg/kg) > valley spruce-fir forest (606.2 and 95.0 mg/kg). Root removal significantly decreased soil microbial biomass (P < 0.01). The maximum MBC (44%) and MBN (34%) reductions were found in the valley spruce-fir forest and the artificial larch forest, respectively. The minimum MBC (12%) and MBN (11%) reductions were found in the selectively cut mixed broadleaved-Korean pine forest. MBC and MBN were significantly positively correlated with soil organic carbon, soil total nitrogen, and soil water content (P < 0.05), and MBN was significantly positively correlated with soil temperature (P < 0.01). We concluded that the combined effects of soil temperature, soil water content, and soil nutrients contributed to the differences in the soil microbial biomass found across the six forest types.