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 278%〖KG-*2〗-〖KG-*7〗521%, 322%〖KG-*2〗-〖KG-*7〗528%, 364%〖KG-*2〗-〖KG-*7〗590%, and 297%〖KG-*2〗-〖KG-*7〗500%, 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 90%〖KG-*2〗-〖KG-*7〗250%, 250%〖KG-*2〗-〖KG-*7〗380%, 286%〖KG-*2〗-〖KG-*7〗392% and 231%〖KG-*2〗-〖KG-*7〗364% 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 618%, 382%, 433%, and 690%; 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 347%, 293%, 304%, and 184%, 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 157%〖KG-*2〗-〖KG-*7〗312%, 115%〖KG-*2〗-〖KG-*7〗493%, and 151%〖KG-*2〗-〖KG-*7〗338%, respectively. There were close relationships between soil nutrients and soil active organic matter fractions except cold water extractable organic N.

Key words: Ecopath mass-balance model, stock enhancement, ecological carrying capacity, Chinese shrimp, Laizhou Bay of East China.