Nutrient removal capability and growth characteristics of Iris sibirica in subsurface vertical flow constructed wetlands in winter

Aquatic plants easily wither and go into dormancy in winter when temperature is extremely low. Little is known about nitrogen and phosphorus uptake by plants and its role in removing nutrient in constructed wetlands (CWs). Investigating the performance of CWs planted with overwintering plants and selecting out which plants will support an effective CW over the winter period are important. In this study, microcosmic subsurface vertical flow constructed wetlands (MVFCWs) were planted with Iris sibirica for treating simulated polluted river water and evaluated for nutrient removal, plant growth characteristics and plant nutrient uptake under different nutrient concentrations. Treatment performances indicated that the highest average total nitrogen (TN), ammonia nitrogen (NH4+-N), total phosphorus (TP), soluble reactive phosphorus (SRP), and chemical oxygen demand (COD) removal efficiencies were obtained in MVFCW units with medium nutrient (10-16 mg/L TN, 7-10 mg/L NH4+-N, 1.8-2.5 mg/L TP, and 80-120 mg/L COD); the removal efficiencies were 45.8 ± 15.4%, 62.1 ± 8.8%, 57.7 ± 8.3%, 59.1 ± 10.1%, and 39.3 ± 12.1%, respectively. MVFCW units with low nutrient (5-8 mg/L TN, 3.5-5 mg/L NH4+-N, 0.9-1.25 mg/L TP, and 40-60 mg/L COD) exhibited the worst treatment effects. Plant nutrient uptake in the different MVFCW units ranged from 19.86% to 50.19% of N removal and from 13.19% to 22.32% of P removal at the end of experiment. The N and P accumulation ability of the below-ground part of plants was better than that of the above-ground part. The I. sibirica plants demonstrated high nutrient uptake in winter, and MVFCWs facilitated a certain degree of nutrient removal. Thus, I. sibirica can be considered an effective overwintering plant selection in CWs for restoring polluted river water and potentially be suitable for generalizing elsewhere, especially over the winter period.