3.1 Profile changes in soil pH, soil moisture, and SIC
The profile changes of soil pH and soil moisture d during the growth of H. ammodendron plantation from MS to 46yrs are shown in Figs. 2a and 2b. The mean soil pH increased and then decreased along the depth of the profile as the age of the H. ammodendron plantation increased. In the 0 ~ 240 cm soil layer, the pH showed an overall increasing trend, and within the 240 ~ 300 cm soil layer, the pH showed a decreasing trend. The soil pH varied from 7.07 to 9.0 throughout the stand age stage, with an overall alkaline soil. Soil moisture, on the other hand, showed a trend of decreasing and then increasing. In the 0 ~ 200 cm soil layer, the soil moisture showed a decreasing trend with a varied range of 3.99 ~ 9.24 cm3 cm-3, and in the 200 ~ 300 cm soil layer, it showed an increasing trend with a varied range of 4.78 ~ 8.09 cm3 cm-3.
The profile variation of the SIC content is shown in Fig. 2c. With increasing soil depth, the SIC content showed an overall trend of first increasing and then decreasing. The SIC content first decreased in the surface layer from 0 to 40 cm and then increased again in the soil layer from 40 to 100 cm. Within the 100-260 cm soil layer, the SIC content first decreased and then increased, followed by a continuous decrease in the 260-300 cm soil layer. In the 0 ~ 300 cm soil profile, the SIC content varied from 0.751 to 4.413 g kg-1.