Abstract: Soil active organic matter is the main source of soil nutrients,and plays an important role in the formation and stabilization of soil aggregate.Chinese fir (Cunninghamia lanceolata) is the most important fast-growing timber tree species in southern China,but its continuous plantation has caused soil deterioration.The study on the active fractions of soil organic matter under the first and second generation plantations of Chinese fir and the native broad-leaved forest at Huitong Experimental Station of Forest Ecology,Chinese Academy of Sciences,in Hunan Province showed that their contents were higher under native broad-leaved forest than under Chinese fir plantations,and higher under the first than under the second generation plantation.The content of soil active organic carbon (SAOC),microbial biomass carbon (MBC),water-soluble organic carbon (WSOC) and water-soluble carbohydrate (WSC) was 18.79 g穔g^-1,421.7 mg穔g^-1,252.2 mg穔g^-1 and 136.3 mg穔g^-1 under the first generation plantation of Chinese fir,and 22.31 g穔g^-1,800.5 mg穔g^-1,361.1 mg穔g^-1 and 220.1 mg穔g^-1 under the native broad-leaved forest,respectively.The corresponding values under the second generation plantation of Chinese fir were 73.6%,87.9%,66.3% and 53.2% of those under the first generation plantation of Chinese fir.The results also indicated that there existed some extent correlations among the active fractions of soil organic matter,and higher correlations between soil microbial biomass carbon and other active fractions of soil organic matter.

Key words: Chinese fir plantation, Soil active organic matter, Microbial biomass C, Water-soluble organic C, Water-soluble carbohydrate