Study On The Mechanism Of Radial Oxygen Loss And Root Exudates Affected Pollutant Removal In Constructed Wetlands

Constructed wetlands are widely used in sewage treatment because of its low construction and energy consumption, easy management. There are two questions limit the performance of constructed wetlands:lack of oxygen and carbon source. The plants are important component of constructed wetlands and play a significant role in pollutant removal processes amd relieving these two questions. Radial oxygen loss provides oxygen for the rizosphere area to satisfy the oxygen demand for aerobic microorganisms. Root exudates provide simple organic carbon as carbon source for microbial denitrification.In this work, harvesting of biomass aboveground was used to study the effect of radial oxygen loss on the removal of pollutant, and the mechanism was studied through migration of nitrogen, microbial abundance and species; the release rates of root exudates from three kinds of plants and the effect of root exudates on denitrification were studied, and the mechanism was studied through microbial abundance and contributions. The main research results are as follows:(1) Harvesting of biomass aboveground in summer enhanced the concentrations of dissolved oxygen (DO) and improve the efficiency of pollutant removal. The harvested group had the highest DO concentration,2.00±0.38 mg/L. The DO concentrations of harvested group and cleared group were 1.83±0.56 mg/L and 1.63 ±0.38 mg/L, respectively. The harvested group had the lowest concentrations of COD (18.35±5.23 mg/L) and NH4+-N (1.00±0.44 mg/L), which was followed by unharvested group and cleared group.(2) Summer harvesting improved nitrogen absorbance by plants and took up 40.63%. The other losses caused by microbial metabolism took up the highest amount in cleared group, which was 1.66 and 3.72 times higher than that in unharvested and harvested group, respectively.(3) The DO concentrations in constructed wetlands have an obvious effect on microbial abundance. The harvested group had the highest abundance of amoA gene, which was 1.95 times and 3.48 times higher than that in unharvested and harvested group, respectively. The harvested group had the highest microbial abundance, 8.26±0.11×1010 copies/g soil, which was followed by unharvested group (2.78±0.51×107 copies/g soil) and the cleared group (1.90±0.33×107 copies/g soil). The differences were significant (p<0.05).(4) The differences were significant in microbial species of different groups. Proteobacteria was the dominant phylum among all bacteria, with the highest proportion in cleared group (68.92%), followed by unharvested and harvested groups with 39.41% and 29.92%, respectively.(5) Different kinds of plants have different release rates of root exudates and the release rates are effected by the kinds of plants and light. The root exudates concentrations increased during the daytime and decreased in the night because of microbial degradation. The average release rates of root exudates of three kinds of plants were 5.40,11.04 and 10.09 ug g-1 h-1.(6) In simulation experiment, Phragmites australis had the highest concentration of NO3- (3.90±0.25 mg·L-1) and the lowest concentration of TOC (3.02±1.23 mg·L-1). Acorus calamus had the lowest concentration of NO3- (2.64±0.78 mg·L-1) and the highest concentration of TOC (5.27±1.24 mg·L-1). After nitrapyrin addition, the denitrification of all samples decreased due to the corresponding microbial inhibition, and concentration of NO3- and TOC increased. Three kinds of plants could provide microbial denitrification with 54.76%,29.19% and 53.19% of carbon source.(7) In the simulation experiment without nitrapyrin, the copy numbers of narG gene in control, Phragmites australis, Acorus calamus and Scirpus tabernaemontani soil samples were 2.72×109,1.62×109,2.18×109 and 2.15×109 copies/g soil, respectively. In the simulation experiment with nitrapyrin, the copy numbers of narG gene were all slightly decreased to 2.58×109,1.38×109,1.95×109 and 2.07×109 copies/g soil, respectively. The results indicate that the addition of nitrapyrin inhibits the expression of narG gene, and consequently the denitrification progress.