Species identity but not richness affects effluent nitrogen, phosphorus, and potassium concentrations and the ratios in floating-constructed wetlands.

The nutrient ratio in wastewater discharge has a variety of ecological impacts on aquatic ecosystems. Plant species richness and identity (the presence of certain species in the community) affected the nitrogen (N), phosphorus (P), and potassium (K) removal efficiencies in constructed wetlands (CWs). However, the effects on the ratios of N/P/K are still unknown. This study conducted microcosms simulating floating CWs to explore the effects on these nutrient removal efficiencies and ratios. Results showed that (1) the presence of Canna indica decreased but the presence of Arundo donax increased effluent P and K concentrations, plant richness had no effect on effluent nutrients concentrations; (2) plant species richness only decreased the effluent P:K ratio but no effect on other effluent nutrient ratios; (3) the presence of C. indica increased but the presence of A. donax decreased effluent N:P and N:K ratios; (4) the presence of C. indica increased plant N, P, K pools through increasing plant biomass but the presence of A. donax decreased plant N, P, K pools through decreasing plant biomass. Overall, species identity surpassed species richness in affecting effluent nutrient concentrations and ratios. Assembling proper species composition could decrease effluent P and K concentrations and regulate effluent N/P/K ratios.