Modifying soil properties with herbaceous plants for natural flood risk-reduction

Background and aim: Nature-based solutions to engineering challenges are essential to limit climate change impacts on the urban environment. Quantitative understanding of multiple “engineering functions” provided by soil-plant interactions of different species is needed for species selection and re-establishing natural processes affected by urbanisation.

Methods: Contrasting herbaceous species (legumes, grasses, and forbs) were selected and grown as monoculture or species mix in soil columns for a five-month growing season. Saturated hydraulic conductivity was initially tested for each column, and then the columns were monitored for three-weeks of evapotranspiration. Water loss, matric suction, and penetrometer resistance were measured. Finally, soil was tested for aggregate stability and water retention.

Results: Saturated hydraulic conductivity of vegetated soil was generally larger than that of fallow soil (6.9e−6 ± 1.4e−6 m/s in fallow soil). Saturated hydraulic conductivity was significantly different between species (e.g., from 9.9e−6 ± 1.3e−6 m/s in Festuca ovina to 3.9e−5 ± 1.2e−6 m/s in Lotus corniculatus) and was negatively correlated with specific root length. The water stored in the soil was efficiently removed by plant transpiration (> 60% of evapotranspiration). Large changes in soil structure were observed in vegetated soil, with significant increases in soil strength, aggregate stability, and alteration of water retention properties.

Conclusions: Multiple soil-plant interactions influence species selection for optimising nature-based solutions (e.g., bioretention barriers). Substantial scope exists to choose species mixes to manipulate soil hydro-mechanical properties. Enhanced biodiversity did not compromise the engineering services of nature-based solutions (e.g., water removal), and may have multiple benefits.