Influence of plants on anammox process in constructed Wetland: Irrelevance, inhibition or enhancement

Anaerobic ammonia oxidation (anammox) can become the dominant process of nitrogen removal in constructed wetland, yet whether plants that are signature composition of wetland play a significant role in this process is still unknown. Therefore, this study investigated the influence of plants (Phragmites) on the anammox-driven wetlands by comparing the differences between plant group and non-plant group. The results indicated that the proportion range of anammox process in nitrogen metabolism of both groups was estimated to be 45 %-63 % according to the detected abundance of functional genes, and there was a significant positive relationship between anammox proportion and total nitrogen removal efficiency (R = 0.81, P < 0.05), which confirmed anammox process dominating the nitrogen transformation in all systems. The average total nitrogen removal efficiencies in the systems with plants addition (84-85 %) were significantly superior than those in the corresponding control systems (74-82 %), respectively (P < 0.05). The presence of plants significantly contributed to the increase in the anammox proportion of nitrogen metabolism and the total absolute abundances of anammox bacteria (P < 0.05). Partial Least Squares Path Analysis Model (PLSPM) furtherly revealed that the estimated weight coefficients of plants to anammox process (R = 1.24) showed the larger level than denitrification process (R = 0.25). Overall, plants played an enhancing role in the anammox process of anammox-driven wetlands, rather than irrelevant or potential inhibiting role. The reason was mainly related to the plant role for improving biofilm formation by extracellular polymeric substances (EPS), and a significant positive relationship between EPS concentrations (7-224 mu g.g(-1)) and anammox bacteria abundances (1.63-7.66 x 10(8) cells.g(-1)) was found (R = 0.44, P < 0.05). Besides, the subsidiary reasons also lie in the minimal role of plant roots under higher nitrogen loading and the appropriate nitrogen component of wastewater.