Effects of root exudates analogues of Robinia pseudoacacia L. on water retention capacity and capillary rise in loess: An experimental study

Root exudation is one of the key mechanisms by which plant roots modify the soil water retention capacity and capillary rise behavior. However, the specific effects of root exudates on the soil water retention capacity and capillary rise behavior, particularly for fine-grained soils, are not well understood. This hinders the mechanistic understanding and modelling of plant-mediated hydrological processes related to water contamination treatment, shallow groundwater recharge, and the stability analysis of slopes. In this study, we investigated the effects and underlying mechanisms of root exudates of Robinia pseudoacacia L. on the water retention capacity and capillary rise behavior of loess. The soil–water characteristic curve (SWCC), capillary rise rate and water absorption mass, electrical conductivity, zeta potential, contact angle, and water drop penetration time (WDPT) were determined through various tests on loess treated with root exudates at varying concentrations. The root exudates of Robinia pseudoacacia L. were found to increase the adsorbed water content, consequently leading to a decrease in the effective flow space, which contributes to enhancing the water retention capacity of loess. The AEV increased by 88.89 % to 872.22 % due to root exudates. In addition, the average contact angle increased from 18.06° and 114.54° as the root exudate concentration rose from 0.000 g/g to 0.050 g/g, which could be due to the presence of amino acids in root exudates and hyphae produced by microorganisms. The reduced hydraulic radius and the increased water repellency contribute to the decreased capillary rise rate and water absorption mass in loess with root exudates. Furthermore, the application of root exudates increased the soil electrical conductivity at equivalent vertical positions after the capillary rise test, which may be due to the synergistic effects of reduced water content, elevated ion concentration, and enhanced organic matter content or microbial activity.