Soil moisture dynamic and desiccation effect of Robinia pseudoacacia plantation sequences in the loess hilly area

Soil moisture is the main factor limiting the revegetation and ecological restoration in the Loess Plateau region. Studying the change patterns of soil moisture across plantations is beneficial to the effective utilization of local soil moisture, meanwhile providing insights for scientific vegetation restoration. In this study, Robinia pseudoacacia (RP) plantations including 10, 15, 25, and 40 years (abbreviated as RP10, RP15, RP25, and RP40, respectively) were selected as the study objects, with the farmland (FL) and native grassland (NG) as controls. The variations of soil moisture and the intensity of soil desiccation at each site from 0–200 cm were analyzed in spring, summer and fall from 2015 to 2021. The results showed that the seasonal variation of precipitation had a strong influence on soil moisture, showing different degrees of lagging effect. The order of average soil moisture in seasons was fall > spring > summer and soil moisture in summer was significantly lower than that in fall and spring (p < 0.05). At depth, the vertical distribution characteristics of soil moisture were similar for RP plantations at the same season but varied within different seasons. The average soil moisture of RP plantations was significantly lower than that of FL and NG (p < 0.05) and showed a decreasing trend with the increase of stand age, which indicates that afforestation seriously increases soil moisture depletion. Furthermore, there were obvious soil drying layers in all RP plantations in different seasons, with the most severe intensity of soil drying in the 150–200 cm soil layer. This study highlights the long-term dynamics in soil moisture of RP plantations, providing insights for sustainable soil moisture use and rational land use management in the loess hilly areas of China.