Abstract: [Objective] In this study, the seasonal variation of carbon and nitrogen of microbial biomass in the Korean pine (Pinus koraiensis) mixed forests at different altitudes was investigated. This study would provide a scientific basis for comprehending the role of soil microbial communities in carbon and nitrogen cycles. [Method] In this study,we set 5 sites along the altitude gradient of Changbai Mountain, starting from the highest distribution altitude of Korean pine and down in one hundred meters altitude intervals. At the same altitude, 3 sample plots were set, and each of sample plot was 20 m×20 m in size. A total of 10 sampling points in a size of 15 cm × 15 cm were randomly set up in an S type at each plot. We collected the samples in the plots on May 21, July 19, August 23, September 20 of 2013, respectively. We analyzed the dynamic variation and different mechanism of the soil microbial biomass carbon (SMBC) and microbial biomass nitrogen (SMBN) in growing season. [Result] The SMBC, SMBN and the SMBC/SMBN had the same variation trend, that is, increased at first and then decreased with the increased altitude. The SMBC, SMBN and the SMBC/SMBN all reached to their maximum at 900 m (1287.18 mg kg^-1; 224.29 mg kg^-1; 9.29). There were significant differences in the SMBC and SMBN among the elevations (P <0.01). The microbial biomass carbon and nitrogen decreased with soil depth. The microbial biomass carbon in 0-5 cm layers of the soil was 1.15, 1.55, 1.29, 2.58, and 1.32 times higher than those in the 5-10 cm layers at the 5 elevations, respectively. The microbial biomass nitrogen was 1.50, 1.23, 1.45, 2.64, and 1.09 times higher than those in the 5-10 cm layers, respectively. In the growing season (from May to September), the SMBC and SMBN in the 0-5 cm soil layers decreased at first, then increased and decreased again. The shape of the curve is inverted "N". However, the SMBC and SMBN first increased and then decreased and presented a unimodal shape of curve in the 5-10 cm soil layer. The seasonal variation of microbial biomass carbon-nitrogen ratio among different altitudes and different soil layers were different. The microbial biomass carbon-nitrogen ratio was between 5 and 20 during the measuring periods except May. The results showed that fungi were more dominant than bacteria in microbial communities in the forests and the capability of soil humification was relative high. In August and September, the microbial biomass carbon-nitrogen ratios were highest, indicating that the capacity of carbon sequestration of soil was strongest. The content of soil available N, soil organic carbon, available P, available K and pH, and soil moisture was obviously different among the 5 elevations. Generally, all indexes increased gradually from 700 m to 900 m and reached to the maximum, and then decreased with the increased elevation. Soil microbial biomass carbon had extremely significant positive correlations with soil organic carbon, soil moisture, available N, available P and available K at different altitudes (P<0.01), and had significant positive correlation with pH at different altitudes (P<0.05). Soil microbial biomass nitrogen had extremely significant positive correlations with pH at different altitudes (P<0.01), and had significant positive correlations with soil organic carbon, soil moisture, soil available N, available P and available K at different altitudes (P<0.05). [Conclusion] The elevation gradient, soil depth and seasonal variation could impact on soil microbial biomass carbon and nitrogen content significantly. The physical and chemical properties of soil, and the difference of forest types were major factors which led to the changes of soil microbial biomass.

Key words: Korean pine, soil microbial biomass, seasonal dynamics, elevation gradient, active carbon