Effects of plant species and traits on metal treatment and phytoextraction in stormwater bioretention

To study effects of plant species selection on total and dissolved metal treatment performance of bioretention systems (BRS), 12 sets of columns were prepared, each planted with one of 12 species that are either widely used in BRS or have potentially important traits for metal removal (ability to hyperaccumulate metals, C4 photosynthesis, or ability to form mycorrhiza). Artificial stormwater was applied to half of the columns during all of a 31-week test period, while treatment of the others included a 5-week long dry period to test interactive effects of drying and plant traits on BRS metal treatment in more realistic alternating wet and dry conditions. Concentrations of metals (dissolved and total) in the effluent significantly differed between most columns with different plants, and the differences in concentrations of dissolved metals after the dry period were particularly important. Mean dissolved Cd concentrations exceeded Swedish reference values in effluents from BRS with two of the plant species, while mean dissolved Zn concentrations exceeded them in effluents from BRS with three of the species (and non-vegetated controls). Dissolved Cu leaching was observed in effluents from BRS with five of the plant species after the dry period, and mean concentrations exceeded Swedish reference values in effluents from all the BRS (including the constantly watered systems). Some support in terms of metal concentrations in shoots and shoot/soil ratios was obtained for using hyperaccumulators in BRS to remove metals from filter material. For example, Armeria maritima (a hyperaccumulator with the lowest shoot biomass) and Miscanthus sinsenis (a C4 plant with the highest biomass production) took up similar amounts of metals despite large differences in biomass. However, no significant correlations between effluent metal concentrations and plants’ metal uptake were found, possibly because of the short duration of the experiment. The results indicate that root biomass affected effluent metal concentrations more strongly. Root biomass was often positively correlated with total and (particularly) dissolved effluent metal concentrations. Further experiments with different soil metal concentrations, organic matter analyses and stronger focus on root characteristics are recommended, including additional tests of effects of hyperaccumulators and mycorrhiza on metal treatment and phytoextraction. •Plant traits (hyperaccumulators, C4 plants, mycorrhiza) were tested in bior