Engineering transpiration-induced suction using mycorrhizal fungi with application to slope stability

The use of vegetation to mitigate rainfall induced landslide hazards is a form of a Nature Based Solution. Rainfall induced landslides occur when the pore-water pressure builds up in the soil in response to infiltrating rainwater. The vegetation can create a stabilising effect by the removal of soil water via transpiration. The removal of water keeps the soil in an unsaturated state and decreases pore-water pressure in the soil, thus reducing hydraulic conductivity. Lowering the hydraulic conductivity before a rainfall event will reduce the downward infiltration of water and, hence, hinder pore-water pressure build-up (drop in suction) that could result in a landslide.

To improve this stabilizing effect the transpiration during a drying period should be maximized. This study looks at implementing biological processes that occur in the root zone with the aim of engineering the transpiration process. A common biotic interaction that occurs in the soil is the symbiotic relationship between mycorrhizal fungi and plant roots. The fungus has been found to improve nutrient and water uptake in plants. This research looks at the effect of mycorrhizal fungi on transpiration induced soil suction.

A controlled laboratory experiment was carried out to determine if the application of arbuscular mycorrhizal (AM) fungi can increase transpiration of the herbaceous plants Medicago sativa and Lolium perenne. The plants were grown in mini-lysimeters with and without AM fungi and then exposed to drying conditions. The evapotranspiration was monitored via measurements of the change in mass of the mini-lysimeter and the soil water content was then back calculated from the final water content. Periodic soil volumetric water content measurements were also carried out using TDR-probes. The suction during drying was determined from the volumetric water content assuming that the water retention in the vegetated soil is the same as the bare soil, whose water retention properties were fully characterised in a separate experiment.

The inoculation with AM fungi in M. sativa plants increased the potential evapotranspiration during soil drying, this is likely due to the increase in the aboveground biomass. There was no significant difference between inoculated and non-inoculated L. perenne plants. The soil suction in the M. sativa plants increased by almost twice when inoculated with AM fungi in the same drying period. Inoculating with AM fungi can increase plant transpiration rates and generates a higher suction in the soil for M. sativa vegetation, although there was little difference with L. perenne plants. This suggests that the effect of AM fungi on plant water uptake can depend on root functional groups.