Roles of root porosity, radial oxygen loss, Fe plaque formation on nutrient removal and tolerance of wetland plants to domestic wastewater

Root properties including rates of radial oxygen loss (ROL), root porosity, and Fe plaque formation on the root surface, in six wetland plant species, namely Acorus calamus, Arundo donax var. versicolor, Cyperus flabelliformis, Canna indica, Iris tectorum, and Scirpus validus, and their relationships with nutrient removal and tolerance to domestic wastewater were investigated. Results showed that different species had different root porosities (18–36%) and Fe plaque formation (1.76–5.3 mg Fe g−1 root d.w.), which were significantly correlated with ROL (67–157 mmol O2 kg−1 root d.w. d−1) (p < 0.001). The ROL rates were also positively correlated with plant tolerance to domestic wastewater (p < 0.001) and removal of total nitrogen (35–76%), total phosphate (22–62%), and chemical oxygen demand (58–86%). Among the six species, C. flabelliformis and C. indica had significantly higher ROL, greater Fe plaque formation, higher tolerance to domestic wastewater, and better removal of TN, TP, and COD. These two species also had greater influences on changes in redox potential, pH, dissolved oxygen, and the quotients of [NH4+–N]:[NO3−–N] in rhizosphere soil solution, as well as greater N and P uptake in plant tissues, particularly belowground parts, than in the other species. For all plant species investigated, wastewater significantly decreased the rates of ROL and root porosity but induced greater Fe plaque formation. These results suggested that root properties are useful for selecting tolerant and efficient species in constructed wetland wastewater treatment facilities.