Abstract: [Objective] DOM (dissolved organic matter) is an important labile carbon source in soil, and can be an important factor regulating CO₂ emission of forest soil. This study will improve understanding of the role of DOM on forest C cycle.[Method] We added DOM from leaf litter and dead roots of Cunninghamia lanceolata and Castanopsis carlesii to soil to examine the effects of carbon inputs on soil CO₂ efflux and microbial community composition by phospholipid fatty acid (PLFA) analysis through laboratory incubations for 36 hours. The treatments were as follows:soil with DOM from C. carlesii leaf litter, soil with DOM from C. lanceolata leaf litter, soil with DOM from C. carlesii dead root, soil with DOM from C. lanceolata dead root, and a control (soil with deionized water). Mineral soil (0-10 cm) was from an 11-year-old C. lanceolata plantation in Sanming of Fujian Province, China. Carbon mineralization was determined using CO₂ respiration method.[Result] The contents of dissolved organic carbon (DOC) from leaf litter were much higher than those from dead roots, and the humification index (HIX) values of the DOM were opposite. The maximum rates of C mineralization occurred in 2 hours following addition of DOM from dead roots of C. lanceolata and C. carlesii, and were 7.3 and 8.3 times higher than that of control respectively, then decreased to 78.9% and 66.3% of the maximum values by 24 hours. In contrast, the maximum rates of C mineralization were in 12 hours following addition of DOM from leaf litter of C. lanceolata and C. carlesii, and the magnitudes were 20.6 and 13.2 times that of control respectively, then decreased to 84.0% and 53.1% of the maximum by 24 hours. PLFA analysis showed that the contents of gram-positive bacteria, gram-negative bacteria, actinomycetes and fungi in soils added with DOM from C. carlesii leaf litter were 27%, 38%, 46% and 41% lower than those of soils added with DOM from C. lanceolata leaf litter, respectively (P<0.05). Compared to soils added with DOM from dead roots of C. lanceolata, the contents of gram-positive bacteria, gram-negative bacteria and fungi were 21%, 21% and 22% lower in soils added with DOM from dead roots of C. carlesii, respectively (P<0.05). After 36 h incubation, the ratios of gram-positive bacteria to gram-negative bacteria in soils added with DOM from C. carlesii leaf litter and the control were higher than those in untreated soil, while compared to untreated soil, the ratio of fungi to bacteria was lower following additions of DOM from leaf litter of C. carlesii.[Conclusion] There was significant difference in the microbial community composition following additions of DOM from various sources, and the maximum rates of C mineralization following addition of DOM depended on the quantity and quality of DOM.

Key words: Castanopsis carlesii, Cunninghamia lanceolata, CO₂ emission, dissolved organic matter, microbial community composition