【Objective】 Soils are an essential section of the terrestrial carbon cycle and act as either source or sink for carbon, depending on what fraction the carbon is in and its stability. However, few studies have been reported on relationship between soil carbon fractions and carbon fixing process (Calvin-Benson-Bassham cycle) in wetland soil systems. Ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) is a key CO₂ assimilation enzyme in the Calvin cycle, and its relationship with its large-subunit I gene and soil carbon fractions may provide some information about their relative importance to the genetic potential of CO₂ fixation. Meanwhile, RubisCO enzyme activity and soil physicochemical properties were determined, and statistical analyses were performed to identify key factors driving microbial CO₂ sequestration in wetland soils.【Method】In this study, soil samples were collected separately from wetlands under. Triarrhena lutarioriparia community (ND), Phragmites australis-Carex cinerascens community (HH), Carex cinerascens community (TC) and Zizania latifolia community (JB) and a bare shoal (LT) at Nanjishan of the Poyang Lake in China, for fractionation of soil carbon and analysis of soil organic carbon (SOC), soil microbial biomass C (MBC), and dissolved organic C (DOC), activity of RubisCO and its large-subunit I gene (cbbl ).【Result】Results show that content of total carbon, SOC, MBC and DOC in the soils varied in the range of 6 380~33 830, 1 209~2 259, 124.6~1 282 and 2.848~18.00 mg kg^-1, respectively, and the four patches of wetlands displayed an order of ND > HH > TC > JB in terms of variation of their means. RubisCO activity in the soils varied in the range of 38.08~125.1 nmol CO₂ kg^-1 min^-1 and was the highest in ND reaching 125.1 nmol CO₂ kg^-1 min^-1 and the lowest in TC falling down to 38.08 nmol CO₂ kg^-1 min^-1. Total number of copies of cbbl genes in the soils varied in the range of 36.07×10⁴~195.6×10⁴ copies g^-1, being the highest in ND (195.6×10⁴ copies g^-1) and the lowest in JB (36.07×10⁴ copies g^-1). RDA analysis indicates that the main factors affecting distribution of soil carbon fractions are Total N, pH, catalase activity and sucrase activity; and soil total N is the most important factor affecting soil carbon fractionation, followed by pH. Correlation analysis indicates that Total N and sucrase activity are positively related (p<0.01), while pH and catalase activity negatively related to carbon fractionation (p<0.01). The contact of the soil with the atmosphere might lead to a significant negative relationship between soil catalase and carbon. The correlation between MBC and carbon cbbl gene is the strongest, followed by DOC, which shows a significant positive correlation with cbbl gene (p<0.01), while total C and SOC are significantly and positively related to RubisCO activity (p<0.05). 【Conclusion】All the findings in this study suggest that soil carbon fixation enzyme and cbbl gene are positively correlated with soil carbon fractionation, which improves our knowledge of their roles in carbon sequestration and nutrient turnover. Obviously the study is of great significance to further researches on soil carbon cycle and its microbial mechanism in Poyang Lake wetlands.