Comparing the effects of plant diversity on the nitrogen removal and stability in floating and sand-based constructed wetlands under ammonium/nitrate ratio disturbance

Maintaining efficient and stable nitrogen (N) removal in constructed wetlands (CWs) that experience disturbance from their influent pollutant variations is crucial. The ammonium/nitrate (NH4+/NO3−) ratio of influent in CWs often varies widely. The N removal and stability in floating CWs have been found to be enhanced by manipulating plant species diversity. However, whether the positive effects occur in sand-based CWs remains unknown. Here, we established sand-based and hydroponic microcosms to investigate the differences in the responses of N removal and stability to plant species diversity under the disturbance of increasing influent NH4+/NO3− ratio in late period of plant growth. Results indicated that, (1) increasing plant species richness enhanced N removal but did not affect N removal stability in sand-based CWs under disturbance; (2) sand-based CWs had 46% higher average N removal stability than floating CWs, but the stability in floating CWs reached that in sand-based CWs at higher species richness levels; (3) under disturbed conditions, floating CWs with Phragmites australis or Typha latifolia achieved N removal and stability equivalent to those in sand-based CWs. This study indicates that, when treating wastewater with a variable NH4+/NO3− ratio, floating CWs with high plant species richness and specific species can achieve a win-win situation for high and stable N removal and bioenergy production.