Abstract: Taking the soils at different altitudes in the water-level-fluctuating zone (WLFZ) of Xiangxi River of the Three Gorges Reservoir area as the objects, this paper studied their physicochemical properties and microbial activities during discharging period (May-August), aimed to reveal the variation trends of the soil quality in the WLFZ of the Three Gorges Reservoir area. With the decrease of altitude and the increase of flooding strength, the soil water content and pH value in the WLFZ increased, soil microbial biomass and microbial quotient decreased, and soil metabolic quotient (qCO₂) increased significantly. The soil qCO₂ in the cyclic wetting and drying area at the altitude below 165 m was significantly higher than that the in non-inundated area at the altitude 175-185 m. At the lower altitudes of the WLFZ, soil was subjected to flooding stress, soil pH tended to become alkaline, and soil quality decreased, being not suitable for the growth of microorganisms. No significant differences were observed in the soil organic carbon (SOC), total N, and C/N ratio at different altitudes of the WLFZ, but the coefficient of variation (CV) of the three soil parameters was relatively smaller in cyclic wetting and drying area. It  was suggested that cyclic wetting and drying was the main factor affecting the distributions of soil C and N in the WLFZ. Correlation analysis showed that soil C/N ratio had a significant positive correlation with SOC, which implied that the changes of soil C/N ratio in the WLFZ of Xiangxi River were mainly determined by the SOC.

Key words: flue-cured tobacco, tobacco-planting soil, biochar, microorganism, soil respiration rate.