Sustainability of Abandoned Slopes in the Hill and Gully Loess Plateau Region Considering Deep Soil Water

Soil desiccation of the deep soil layer is considered one of the main limiting factors to achieving sustainable development of ecosystems in the hill and gully Loess Plateau region. In this study, slope croplands were selected as the control, and deep soil water was studied on abandoned slopes, including natural abandoned slopes, Robinia pseudoacacia plantations, and Caragana korshinskii plantations. Then, we explored deep soil water characteristics of different vegetation types and slope aspects and the variation tendencies of deep soil water at different recovery stages. The results showed that there were no significant differences in deep soil water content between sunny and shady slopes, and thus, slope aspect was not the key impact factor affecting deep soil water. Deep soil water content on R. pseudoacacia plantations and C. korshinskii plantations was lower than that on natural abandoned slopes; there were no significant differences in soil water content between the natural abandoned slopes and slope croplands. Soil desiccation did not exist on natural abandoned slopes; thus, natural vegetation restoration is an appropriate way to achieve a sustainable ecosystem with respect to deep soil water. In contrast, soil desiccation intensified until it was difficult for vegetation to obtain available water in the deep soil layer on the plantations; soil desiccation began to appear at the 11–20-year stage, and it became increasingly severe until the deep soil water was close to the wilting coefficient at the ≥30-year stage on R. pseudoacacia plantations. Deep soil water was rapidly consumed, and soil desiccation began to appear at the 1–10-year stage and then was close to the wilting coefficient in the later stages on C. korshinskii plantations. According to the results, the plantations needed to be managed in a timely manner to prevent or reduce soil desiccation.