Effects of natural shrub of Caragana opulens Kom. on soil moisture in a semiarid area on the Loess Plateau

This study has chosen the shrubs of Caragana opulens Kom., a natural zonal vegetation in the semiarid area on northwest Loess Plateau, and compared its soil moisture from 0~9.9m to those of re-vegetated shrubs of C.korshinskii Kom., re-vegetated almond forest, natural grassland (protected) dominated by Stipa breviflora, intensively grazed grassland and continuous cropland. Soil moistures were measured at the depth over 3m with an interval of 0.2m beneath and with an interval of 0.3m. The purpose was to testify the hypothesis that the zonal natural shrub vegetation consumes less soil water than re-vegetated shrub and forest vegetations. The results show that the soil moisture of natural shrub of C.opulens Kom. was largely higher than those of re-vegetated shrub of C.Korshinskii Kom. and almond forest beneath 1m in soil profile, close to those of intensively grazed grassland and cropland,but slightly lower than cropland. The average soil moisture in 0~9.9m soil profiles of natural shrub of C.opulens, re-vegetated shrub of C.korshinskii, re-vegetated almond forest, natural grassland, intensively grazed grassland and cropland were 9.5%, 7.4%, 7.7%, 8.5%, 9.3% and 9.8%, respectively.and the average soil water moisture from 1~9.9m are 10.2%, 7.6%, 8.1%, 9.0%, 10.6% and 10.0% respectively. The soil moisture of natural shrub land of C.opulens was higher than the soil moisture of natural grassland between the soil layer of 0~4m, but beneath 4m, their soil moistures were essentially the same. All vegetation types have resulted in a continuous dry soil layer within which the soil moistures were hard or unavailable to vegetation. The thickness of hard or unavailable water moisture layers of natural shrub of C.opulens , re-vegetated shrub of C.korshinskii,re-vegetated almond forest, natural grassland, intensively grazed grassland and cropland were 2, 5.6, 4.2, 3.6, 3.3 and 1.0m, respectively. According to previous studies, the layer of unavailable soil moisture above 2m in soil profile is rechargeable by annual precipitation in the study area, but it hardly can be recharged by precipitation for unavailable soil water layers beneath 2m. From this initial study, we concluded that the natural shrubs of C.opulens consumes less soil water than re-vegetated shrubs of C.korshinskii, re-vegetated almond forest, underlining the importance of the role of zonal vegetation in the vegetation reconstruction in the study area.