Background The hilly and gully region of the Loess Plateau, which is suffering from lacking in water resources, is the most serious soil erosion region in Loess Plateau. Generally, the soil moisture is an effective indicator to evaluate the influence of soil and water conservation measures. The objectives of this paper were to study the redistribution process of precipitation and the effect on the stability of soil reservoir under different modes of vegetation recovery.

Methods Based on paired watersheds, the effect of precipitation and slope aspect on soil moisture were quantitatively analyzed under different modes of vegetation restoration. Paired watersheds included two typical small watersheds in the hilly and gully regions of Loess Plateau:Qiaozi West Watershed (waste-grassland of natural restoration) and the Qiaozi East Watershed (black locust forestland of artificial restoration). Samples were collected at 9 soil layers in the upstream, midstream and downstream of two paired watersheds. The 9 soil depths were 0-2, 2-4, 5-15, 15-25, 25-35, 35-45, 45-65, 65-85, and 85-100 cm, respectively. This project was carried out about 4 times throughout the period from May to August. Characteristics of soil mass water content and isotope concentration of hydrogen and oxygen (δ_O¹⁸) within 0-100 cm soil layer were calculated.

Results The results showed that there were significant differences in the redistribution of soil water under different vegetation restoration modes (P < 0.05). Soil water content of watershed mainly comprising of black locust forestland (16.72%) is lower than that of the watershed mainly comprising of waste-grassland (20.42%). The variation coefficient of soil water content was relatively higher and the response of soil water content to precipitation was weaker in the former watershed. Critical depth for soil moisture was identified for the two watersheds. The water content was relatively stable (about 12.19%) under the soil depth about 10 cm in the black locust forestland. However, the water content increased with the soil depth when the soil depth blew 30 cm in the waste-grassland. The maximum evaporation depth of waste-grassland was 30 cm, which was shallower than that in the black locust forestland (about 55 cm). The soil mass water content in different slope aspects in Qiaozi West Watershed ranked in the follows:shady slope (19.60%), half negative slope (17.65%), half-sunny slope (14.24%), and the content were lower respectively about 6.68%, 4.98%, 1.03% in each slope aspect of Qiaozi East Watershed than the former watershed. Additionally, the concentration of ¹⁸O in different slope aspects ranked in the follows:negative slope(-5.57‰), half-sunny slope (-5.64‰) and shady slope(-6.08‰) in Qiaozi West Watershed, while the concentration were -5.74‰, -6.21‰, -6.03‰ in Qiaozi East Watershed, respectively.

Conclusions There were differences for soil moisture in the same slope aspect under different vegetation restoration modes in the hilly and gully region of the Loess Plateau. In natural restoration watershed, the influence of slope aspect on soil water content is greater than that of artificial restoration watershed. Compared with the artificial restoration mode, the natural restoration can greatly increase the capacity of soil water storage, which is beneficial to improve and sustain the watershed ecology.