Unsaturated hydraulic properties of vegetated soil under single and mixed planting conditions

Effects of plant roots on changes of soil hydraulic properties, including soil water retention curves (SWRC) and soil hydraulic conductivity functions (SHCF), are not well understood, especially when soil is unsaturated and vegetated with multiple plant species. The aim of this note is to quantify the root effects on both SWRC and SHCF of silty sand using the instantaneous profile method. Four types of vegetated soil, namely bare, grass-only, tree-only and mixed tree-grass silty sand, were subjected to a controlled drying-wetting cycle in a plant room. Plant roots affect the air-entry value, saturated hydraulic conductivity and reduction rate of unsaturated hydraulic conductivity (with respect to suction) most significantly, but the roots do not affect the reduction rate of volumetric water content much. When planted with single species (grass or tree), the air-entry value of silty sand increased, while the saturated hydraulic conductivity and reduction rate of unsaturated hydraulic conductivity with suction decreased. However, under the mixed planting conditions, opposite results are found.