Abstract
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.
Highlights
Vegetation is known to affect the hydrology and slope stability (Osman & Barakbah, 2011; Smethurst et al, 2015)
CONCLUDING REMARKS This study has used the instantaneous profile method to quantify the effects of plant roots on unsaturated hydraulic properties of vegetated silty sand, under single- and mixedspecies planting conditions
There was no discernible difference in terms of the rate of water desorption, the air-entry value of the silty sand increased substantially due to the presence of roots
Summary
Vegetation is known to affect the hydrology and slope stability (Osman & Barakbah, 2011; Smethurst et al, 2015). Some studies (Table 1) have shown an increase in water retention capability when plant roots are present in the soil (Scanlan & Hinz, 2010; Rahardjo et al, 2014; Leung et al, 2015; Ng et al, 2016a, 2016b; Jotisankasa & Sirirattanachat, 2017), probably because of the blockage of soil pore space by roots (Buczko et al, 2007). Jotisankasa & Sirirattanachat (2017) show that root effects on hydraulic conductivity were prominent only when matric suction of the soil was less than 10 kPa, whereas the hydraulic conductivity measured by Song et al (2017) found that roots affect Some studies have reported an opposite result (Ng et al, 2016a; Jotisankasa & Sirirattanachat, 2017), arguably because of the formation of soil cracks due to, for instance, repeated soil shrinkage/swelling and root decay/growth (Vergani & Graf, 2015; Ng et al, 2016a; Ni et al, 2017; Leung et al, 2018).
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