Improving runoff quality in vertical greenery systems: Substrate type outweighed the effect of plant growth promoting microbes.

Due to limited urban green spaces and catchments, researchers are exploring the capacity of vertical greenery systems (VGSs) in stormwater management as complementary strategies. While the literature acknowledges the significant impacts of vegetated roof substrates on stormwater, comparing the stormwater management capacities of organic and non-organic substrates for VGSs remains largely unexplored. It is thus essential to gather empirical evidence to enhance the stormwater management capacity of VGSs. Here, we report on the impact of installation factors (substrate type and plant growth-promoting microbe (PGPM) inoculation) and environmental factors (simulated rainwater quantity and substrate moisture) of an innovative VGS on the concentrations and total loads of 15 elements (N, P, Al, V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Cd, and Pb) in the runoff. Results showed that substrate type was the most influential factor: concentrations and total loads were significantly higher from a reed-based substrate with high organic matter than from a sandy loam substrate. Substrate type also had profound interactive effects with other factors. For instance, PGPM inoculation significantly reduced the total loads of As, Cr, N, Ni, and Se, regardless of substrate type, and reduced the total loads of Cd, Co, Cu, Fe, Mn, and Pb in the reed-based substrate only. In addition, PGPM inoculation primarily reduced total loads, yet had little effect on concentrations. Substrate type also interacted with simulated rainwater quantity and substrate moisture: for example, in the reed-based substrate, a higher simulated rainwater quantity reduced concentrations but increased total loads, while concentrations and total loads remained constantly low from the sandy loam substrate under various simulated rainwater quantities. High antecedent substrate moisture increased both concentrations and total loads for most of the elements. We conclude that leaching from VGSs can be contained via substrate selection, maintenance of substrate moisture, and beneficial PGPM inoculation.