Nitrogen-removal ability and niche of Coix lacryma-jobi and Reineckia carnea in response to NO3−/NH4+ ratio

Wastewater comes from various sources with remarkably different nitrate (NO3−)/ammonium (NH4+) ratios. In order to choose coexisting plant species for treating wastewater with various compositions in constructed wetlands, it is important to know the performances of plant species under different NO3−/NH4+ ratios. In this study, we investigated the growth traits and nitrogen accumulation of Coix lacryma-jobi and Reineckia carnea under five NO3−/NH4+ ratios (100/0, 75/25, 50/50, 25/75 and 0/100) in a hydroponic system. Results showed that, the total biomass, aboveground biomass and aboveground nitrogen accumulation were the greatest under a NO3−/NH4+ ratio of 50/50 for C. lacryma-jobi, but those were the highest under a ratio of 75/25 for R. carnea. Under the NH4+-only (0/100) treatment, C. lacryma-jobi exhibited ammonium toxicity with leaf nitrogen concentration (43 mg g−1) exceeding the maximum limit for plants. In contrast, R. carnea showed less sensitivity to NO3−/NH4+ ratios as the coefficient of variance for biomass of R. carnea (17%) was less than half of that of C. lacryma-jobi (40%). The two plant species differed in nitrogen niche (niche overlap = 0.42).