Plant-soil interactions can enhance earth barrier systems in urban spaces

Climate change is expected to introduce increasing threats to human health and the urban built environment, due to extreme events such as heavy precipitation. In the urban environment, impermeable hard-engineered surfaces may exacerbate climate change effects and increase the risk of floods. Adaptation solutions are essential to limit the climate change impacts on the urban environment. Research is needed to design new environmentally friendly multi-layer earthen barrier systems that can mimic the natural hydrological processes (e.g., plant-soil interaction) removed by urbanization.

In this study, potential barrier materials were selected from both natural soils and recycled waste materials (e.g., recycled concrete aggregates). Contrasting herbaceous species (legumes, grasses and forbs) were selected and grown for five months in compacted soil columns and saturated hydraulic conductivity (K_sat) was tested for each soil column. Following K_sat tests, all soil columns were saturated and left for evapo-transpiration. Plant water uptake, matric suction and soil strength (penetration resistance) were measured.

Among the materials tested in this study, recycled concrete aggregate (RCA) was the most suitable material for the barrier drainage layer, having a K_sat equal to natural gravel, but with 14% lower dry density (2.3 Mg/m³) and seven-fold greater water holding capacity (0.08 g/g). However, a portion of the water stored in the RCA was strongly bound to micropores and not available for plants. Plant growth in soil columns increased K_sat. On average K_sat of four-month old vegetated soil (3.2e^-5 ± 2.0e^-6 m/s) was four times larger than that of control fallow soil (6.9e^-6 ± 1.4e^-6 m/s). However, tested species differed in their effect on K_sat, ranging from 9.9e^-6 ± 1.3e^-6 m/s of Festuca ovina (Grass) to 4.1e^-5 ± 3.7e^-6 of Lotus pedunculatus (Legume). In the fallow soil, daily evaporation led to an average water loss of 0.49 ± 0.04 g per 100 g of soil, evapo-transpiration led to a daily water loss up to 2.58 ± 0.10 g per 100 g of soil in Lotus corniculatus columns. Thus, soil drying and induced matric suction strengthened the vegetated soil and further increased its ability to store water. For instance, soil vegetated with L. corniculatus had seven times faster water absorption and twenty-five times greater strength compared with control fallow soil. Plants affected the hydraulic conductivity and water relation of the barrier system. Root systems can increase soil hydraulic conductivity through root-induced channels. This may enable faster drainage during floods, but we found large differences between species. Transpiration restored the water holding capacity of barrier systems after heavy rain events and induced strengthening of soil.

We suggest that vegetation should not be simply selected for aesthetically “greening” the barrier system, but specifically selected for its role in improving soil engineering function. There is a substantial scope to choose species to manipulate hydrological properties of the barrier system and improve its performance during extreme climate events.