Abstract: [Objective]Chemical properties of soils at different altitudes and layers after long-term reservoir water level changes were analyzed to understand the degree of variation of soil environment after years of drastic changes in dry-wet alternation in the fluctuation zone of the Three Gorges Reservoir.[Method] Sample plots were set up in a secondary shrub forest within the typical hydro-fluctuation belt in the Three Gorges Reservoir. According to the water level operation plan of the reservoir area, the pH-value, organic matter, and contents of available N, available P and available K in the soil at different altitudes (145-155, 155-165, 165-175, and 175-185 m) in different soil layers (0-10 and 10-20 cm) were measured in 2008, 2009, 2012, 2014, and 2015 (that is, through 0, 1, 4, 6, and 7 fluctuations of water level, respectively).[Results]Results showed that soil pH-values at all altitudes increased obviously, the pH value at 145-155 m changed from acid (pH6.1) to alkaline (pH8.2), and the pH-values at other altitudes gradually tended to be neutral after 7 fluctuations of water level. The extent of pH variation at different altitudes was:145-155 m > 155-165 m > 165-175 m > 175-185 m. The content of soil organic matter in the flooded area was higher than that of the non-flooded area at 175-185 m. With the increase of flood duration, the content of organic matter in the 0-10 cm soil layer decreased year by year compared with that in 2008, but in the 10-20 cm soil layer it increased first and followed by a decrease. With the increase of the number cycles of water level fluctuation, the contents of soil alkali hydrolysable nitrogen and available potassium showed a downward trend of fluctuation, and the contents in the 0-10 cm soil layer were more sensitive to flooding compared with those in 10-20 cm layer. The alkali hydrolyzed nitrogen in 0-10 cm soil layer decreased most significantly in 2009 and 2015, and compared with 2008, the decrease at all altitudes was by 37.2%-54.7% and 50.6%-65.7%, respectively. Moreover, the decrease of available potassium in 0-10 cm soil layer was the largest in 2009, up to 17.1%-50.8%. The content of available P decreased year by year, and the loss of available P was the biggest among all the available nutrients. After 1, 4, 6 and 7 fluctuations of water level, the content of available phosphorus in 0-10 cm layer significantly decreased by 21.8%, 45.4%, 82.2%, and 90.2%, and the content of available phosphorus in 10-20 cm layer significantly (P<0.01) decreased by 44.3%, 61.5%, 83.5%, and 92%, respectively.[Conclusion]With continual extension of the operation period of the Three Gorges reservoir, the soil properties of the fluctuating zone change gradually, reflected by the increased soil pH-values, the decreased content of organic matter, and the depleted available nutrients. To a certain extent, this provides a new scientific basis for predicting succession process of ecological environment quality and the changes of soil properties in the reservoir area under long-term flooding conditions.

Key words: water-level-fluctuation zone, secondary shrub, annual fluctuation of water level, soil pH, soil organic matter, soil available nutrients, inter-annual variability