Abstract: A field experiment manipulating throughfall was constructed in a Pinus sylvetris var. mongolica plantation at PKU-SOGES. This study investigated the response of two indicators of belowground ecosystem processes, soil microbial biomass carbon and microbial quotient, to±30% throughfall amount. During the growing season from May to September, 2007, soil microbial biomass carbon (260.7 mg/kg) and microbial quotient (1.84 %) decreased with soil depth. Generally, the effects of ±30% throughfall amount on the means of soil microbial biomass carbon and soil microbial quotient in the growing season were not significant. Interestingly, the dynamics were much clear for the -30% throughfall treatment, with soil microbial biomass ranged from 243.1 mg/kg to 354.3 mg/kg and microbial quotient from 1.43% to 2.16%, and the highest value was in May and the lowest was in July. This indicates that the decrease of precipitation in a growing season may lead to a stronger fluctuation in soil microbial activity, thus alter the dynamics of belowground carbon cycle processes.

Key words: carbon cycles, ecosystem belowground process, throughfall, Saihanba, Pinus sylvestris var. mongolica plantation

摘要： 在北京大学地球环境与生态系统塞罕坝实验站樟子松（Pinus sylvestris var. mongolica）人工林内设置降水控制实验，研究地下生态系统过程的两个重要指标土壤微生物量碳和微生物商对穿透雨增加或减少30%的响应。在2007年5月到9月的生长季，土壤微生物量碳和微生物商平均值分别为260.7mg/kg和1.84%，二者随土壤深度增加呈下降趋势。总体上，穿透雨增加或减少30%对土壤微生物量碳和微生物商的生长季内平均值影响不显著，但穿透雨减少30%的土壤微生物量碳及微生物商的变幅较大，变化范围分别为243.1～354.3mg/kg和1.43%～2.16%，5月最高，7月最低，表明生长季内穿透雨减少将导致土壤微生物活动的较大波动，从而可能改变地下碳过程的季节变化规律。

关键词: 碳循环, 生态系统地下过程, 穿透雨, 塞罕坝, 樟子松人工林