Effects of grass roots on soil-water retention curve and permeability function

Abstract

The effects of Vetiver grass (Chrysopogon zizanioides) roots on the soil-water retention curves (SWRCs), permeability (k) function, and saturated permeability, ksat, have been investigated on clayey sand (SC) and low-plasticity silt (ML). For ML soil, when the root biomass per soil volume was lower than 6.5 kg/m3, the saturated permeability increased, the air-entry suction decreased slightly, and the SWRC became steeper with increasing root contents, probably due to the formation of cracks caused by wetting and drying cycles during the plant growing period. Nevertheless, roots appeared to decrease the saturated permeability and increase the air-entry suction of ML soil, after reaching this threshold with a root content of about 6.5 kg/m3 as roots occupied the macropores and tended to suppress cracks and swelling. For SC soil for all root contents, only a slight variation of the saturated permeability with root content could be observed for the upper bounds on saturated permeability. However, the lower bounds on saturated permeability appeared to decrease as root content increased. It has also been shown that for a suction range beyond 30 kPa, the influence of roots on permeability appeared to be less significant.

Plant age effects on soil infiltration rate during early plant establishment

“…Studies that focus on relatively young plants found slowing of infiltration (Gish & Jury, 1983;Leung et al, 2015;Jotisankasa & Sirirattanachat, 2017) presumably due to root occupancy of soil pore space, which blocks water flow paths (Scholl et al, 2014;Ng et al, 2016). In contrast, increased infiltration rate is more often reported in mature plants (van Noordwijk et al, 1991;Mitchell et al, 1995;Ng et al, 2017), and is attributable to the formation of (a) root channel related macropores associated with root decay (Ghestem et al, 2011) or (b) desiccation cracks upon drying of medium-to high-plasticity clay (Zhan et al, 2007;Jotisankasa & Sirirattanachat, 2017;Song et al, 2017). The existing research often considered plant effects only at one particular plant age.…”

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

“…Jotisankasa and Sirirattanachat (2017) shows that root effects on hydraulic conductivity were prominent only when matric suction was less than 10 kPa, whereas the hydraulic conductivity measured by Song et al (2017) found that roots affect unsaturated hydraulic conductivity for the entire suction range considered (< 100 kPa). Thus, the presence of plant roots does not necessarily always reduce or increase unsaturated hydraulic conductivity, depending both on the plant and soil types.…”

“…Some studies (Table 1) showed 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, b;Jotisankasa & Siririrattanachat, 2017), probably because of the blockage of soil pore space by roots (Buczko et al, 2007). However, some studies reported an opposite result Jotisankasa and Sirirattanachat, 2017), arguably because of the formation of soil cracks due to, for instances, repeated soil shrinkage, swelling and root decay and growth (Vergani & Graf, 2015;Ng et al, 2016a;Ni et al, 2017;Leung et al, 2017).…”

“…Thus, the presence of plant roots does not necessarily always reduce or increase unsaturated hydraulic conductivity, depending both on the plant and soil types. Indeed, although Rahardjo et al (2014) and Jotisankasa & Sirirattanachat (2017) tested the same grass type, the soil hydraulic properties of the vegetated soils measured were different possibly because of the different soil types considered in these two studies. Moreover, there is also a lack/no study that investigates the effects of multiple plant functional groups (i.e., mixed planting of herbaceous and woody species that typically exists in the field) on both SWRC and SHCF (Tables 1 and 2).…”

Quantification of root reinforcement in bio-slope stabilization: laboratory and field studies

“…Plants can provide additional slope stabilization effect further through transpiration, whereby soil moisture is extracted from the ground, thus increasing the soil suction and shear strength [15][16]. The roots can also affect soil's hydraulic conductivities, soil-water retention, and infiltration capacity in several opposing ways depending on root contents and growing stage [15,17]. Active growing roots tend to decrease permeability while decaying roots can increase macrovoid content, soil permeability, and infiltration into the soil.…”