Purification effect of five kinds of hydrophytes on total nitrogen and nitrate in simulated vegetable field runoff water

In recent years, the planting area of vegetable greenhouse in Taihu Lake region had been increasing, and the high amount of fertilization had resulted in serious nitrate ( ${\mathrm{N}\mathrm{O}}_3^{-}$ -N) accumulation in vegetable soils. Nitrate was abundant in surface runoff at the beginning of rainfall during the opening of greenhouse in summer. In order to purify the surface runoff of vegetable field rich in ${\mathrm{N}\mathrm{O}}_3^{-}$ -N, a static simulated experiment with five common kinds of hydrophyte of Vallisneria natans, Elodea nuttallii, Ludwigia peploides, Myriophyllum verticillatum L. and Hydrocotyle chinensis in a tank containing simulated nitrate-rich runoff water was carried out. Results showed that at low ${\mathrm{N}\mathrm{O}}_3^{-}$ -N concentration with TN of 6.32-6.80 mg/L and ${\mathrm{N}\mathrm{O}}_3^{-}$ -N of 4.76-5.09 mg/L, all the five hydrophytes showed good removal effects in the early stage (the first week) with the removal rates of TN and ${\mathrm{NO}}_3^{-}$ -N over 80% and 90%, respectively. But in the late stage (the third and fourth week), the removal rate of TN and ${\mathrm{NO}}_3^{-}$ -N by Vallisneria natans, Elodea nuttallii and Hydrocotyle chinensis decreased obviously, while Ludwigia peploides and Myriophyllum verticillatum L. still maintained high ${\mathrm{NO}}_3^{-}$ -N removal rate (over 90%). At high ${\mathrm{NO}}_3^{-}$ -N concentration with TN of 15.62-18.00 mg/L and ${\mathrm{N}\mathrm{O}}_3^{-}$ -N of 13.19-13.96 mg/L, the growth of five hydrophytes all showed different degrees of stress. However, Ludwigia peploides and Myriophyllum verticillatum L. still maintained higher nitrogen removal capacity, which was mainly attributed to the large biomass and nitrogen uptake. There was no significant difference in nitrogen removal by denitrification between different treatments, therefore, Ludwigia peploides and Myriophyllum verticillatum L. were more suitable to purify the runoff water with high nitrate concentration.