To understand the spatial distributions of root systems and their potential controlling factors such as soil water content (SWC) and salinity in the arid regions of China, we conducted a field study in an Elacagnus angustifolia-Achnatherum splendens community developed on saline-alkali soils in Sand Lake, Ningxia Autonomous Region of the Northwest China. Specifically, eight locations from the base of E. angustifolia individuals to the adjacent open area were placed in a 50 m×50 m plot. The width of the gradient was set to be 20% of the crown diameter. We excavated five soil pits along the transect to measure root biomass, SWC and salinity at 5 vertical depths. We found the changes of the following variables as one moves away from the plant base: decreased mean root biomass density (RMD) in each soil layer, lowered SWC in each layer, and elevated variation among the layers and the electric conductivity (EC) increases in top layers. However, the EC in deep soil layers showed a concavity pattern with a minimal value at the edge of crowns. Interestingly, we also found that the soil layers with the highest RMD of the two dominant species tended to be vertically separated; and there existed a vertically-reversed change of SWC near the plant base. Our field data on the horizontal and vertical distributions of belowground biomass, SWC and EC supported us with a preliminary conclusion that plant absorption and evaporation are the two primary drivers affecting the distribution and movements of salts and water in the soil.