Effects of solar radiation and fine roots on suction of Amorpha fruticose-vegetated soil

The thickness of shallow landslides is generally less than 2 m, which is of the same order of magnitude as the growth range of vegetation roots. Vegetation roots can improve the stability of shallow soil through mechanical and hydraulic effects. Therefore, the landslide process is closely related to the plant roots growing on the slope surface. Plant roots play a dominant role in the regulation of soil suction through solar radiation induced transpiration. However, little is known about the correlation between cumulative solar radiation and soil suction. Moreover, the specific effects of fine roots on the suction distribution are not clear in most previous studies. In this study, a vegetated soil of a drought-tolerant and water-tolerant shrub, namely Amorpha fruticose, was adopted. The suction and volumetric water content of bare and vegetated soils were monitored under natural conditions for 4 months. The results demonstrate that there is a nearly linear relationship between cumulative solar radiation and suction ranging from zero to 100 kPa. Regarding the modeling of the soil-plant-atmosphere interactions, this relationship could serve a significant role in calculating the root water uptake under given solar radiation conditions. In addition, higher suctions were observed at the lower layer of the vegetated soil than those at the middle layer, which is different from the results of vegetated soil from previous investigations. This is due to the fact that the root area index (RAI) of fine roots at the lower layer is twice that of the middle layer. Importantly, the higher concentration of fine roots at the lower layer of vegetated soil sample resembles the root distribution of shrub near the soil-bedrock interface on shallow bedrock landslides. The fine roots would increase soil suction through transpiration, and hence reduce the permeability and increase shear strength of landslides. Eventually, these new findings serve as a preliminary step on the evaluation of the stability of this common type of landslides.