Abstract:A comparative study was made on the soil active organic matter in a broadleaved forest and two Chinese fir (Cunninghamia lanceolata) plantations in subtropical region of China, aimed to understand the effects of forest conversion and continuous plantation on soil organic C and nutrient status. After the conversion from broadleaved forest to Chinese fir plantation, the contents of soil total organic C, humus C, humic acid, and fulvic acid decreased by 278%〖KG-*2〗-〖KG-*7〗521%, 322%〖KG-*2〗-〖KG-*7〗528%, 364%〖KG-*2〗-〖KG-*7〗590%, and 297%〖KG-*2〗-〖KG-*7〗500%, respectively. Continuous plantation also resulted in the decrease of soil organic C and humus contents. The contents of soil total organic C, humus C, humic acid, and fulvic acid in second generation of Chinese fir plantation were 90%〖KG-*2〗-〖KG-*7〗250%, 250%〖KG-*2〗-〖KG-*7〗380%, 286%〖KG-*2〗-〖KG-*7〗392% and 231%〖KG-*2〗-〖KG-*7〗364% lower than those in the first generation of Chinese fir plantation, respectively. More obvious effects were observed on the soil active organic matter. After the conversion from broadleaved forest to Chinese fir plantation, the maximum decrement of soil microbial biomass C and N and dissolved organic C and N was 618%, 382%, 433%, and 690%; while comparing with the first generation of Chinese fir plantation, the second generation of Chinese fir plantation had the maximum decrement of soil microbial biomass C and N and dissolved organic C and N being 347%, 293%, 304%, and 184%, respectively. Soil nutrient contents also decreased due to forest conversion and continuous plantation. In comparing with broadleaved forests, Chinese fir plantations had a decrease of soil N, P, and K contents being 157%〖KG-*2〗-〖KG-*7〗312%, 115%〖KG-*2〗-〖KG-*7〗493%, and 151%〖KG-*2〗-〖KG-*7〗338%, respectively. There were close relationships between soil nutrients and soil active organic matter fractions except cold water extractable organic N.