Abstract: This paper studied the seasonal dynamics of soil microbial biomass C (Cmic) and N (Nmic), and of bacteria (cfub), fungi (cfuf), and actinomyces (cfua) in Larix gmelinii and Fraxinus mandshurica plantations in 2007〖KG-*2〗-〖KG-*7〗2008 under N fertilization. The two-year averaged soil Cmic and Nmic in L. gmelinii plantation were 138% and 183% lower than those in F. mandshurica plantation, respectively, but the soil Cmic and Nmic in the two plantations had similar seasonal patterns, being the lowest in May and the highest in September. The Cmic and Nmic, and the cfub, cfuf, and cfua were all greater in topsoil (0〖KG-*2〗-〖KG-*7〗10 cm) than in subsoil (10〖KG-*2〗-〖KG-*7〗20 cm), but the seasonal patterns of cfub, cfuf, and cfua were different from those of Cmic and Nmic. N fertilization decreased the Cmic and Nmic, and the
cfub, cfuf, and cfua significantly, with the decrements of Cmic and Nmic being 24% and 63% in L. gmelinii plantation, and 51% and 68% in F. mandshurica, respectively, which suggested that N fertilization limited soil microbial biomass and altered soil microbial communities in the two plantations.

Key words: Larix gmelinii, Fraxinus mandshurica, nitrogen fertilization, soil microbial biomass C and N, seasonal dynamics, southern China, seasonal drought, drought division, subzone assessment.