Linking root traits and soil moisture redistribution under Achnatherum splendens using electrical resistivity tomography and dye experiments

The rooting traits of deep-rooted alpine grasses strongly control the redistribution of soil moisture (i.e., the changes in the soil volumetric water content, ΔSVWC), which is the most important water resource supporting vegetation growth in cold semiarid regions. However, due to the difficulty of developing appropriate methods to characterize the hidden structures of root-soil systems, the linkages between root traits and ΔSVWC remain challenging to detect. This study aims to identify how the roots of Achnatherum splendens (A. splendens) affect soil water infiltration and spatiotemporal variations in the rhizosphere under different root densities and rainfall patterns using electrical resistivity tomography (ERT) and dye experiments, as well as to determine the relationship between root traits and soil water redistribution. Manipulative rainfall experiments 5, 10, 20 and 40 mm simulated rainfall were applied to sample plots with no plants (control), single and double A. splendens plantings (BAS, SAS, DAS, respectively). The results of ERT and the dyeing profiles showed that the roots of A. splendens greatly affected infiltration processes and the soil water distribution. The soil water was vertically and laterally funneled along the roots. In SAS, which had a lower root length density (RLD), the soil water accumulated mainly in the shallow layer (0–20 cm), while in DAS, which had a higher total root mass density (RMD), the soil water mainly infiltrated rapidly into the deep soil (>20 cm), especially during the extreme (40 mm) rainfall event. The temporal increases in soil moisture below 20 cm under DAS were higher than that those under BAS and SAS after rainfall for 2 and 24 h. The RMD, specific root length (SRL), specific root surface area (SSA) and RLD were all negatively and significantly correlated with the ΔSVWC at 0–20 cm. At 20–40 cm, the RMD and RLD were positively and significantly correlated with ΔSVWC, whereas the SRL and SSA showed the opposite trend. These findings provide direct evidence of the linkages between A. splendens root traits and ΔSVWC using ERT and the dye tracing method, and this evidence is important for the development of soil-plant-atmosphere continuum (SPAC) models.