Study On The Denitrification Potential In The Sediment Of Different Units Of Bypass Constructed Wetland

In this study, the sediments in different units of a multi-stage bypass constructed wetland near Nanfei River were investigated based on the physicochemical properties, denitrification rate and microbial abundance of denitrification. Moreover, the impacts of polyacrylamide (PAM) on the sediment physicochemical properties, nitrification and denitrification were explored, because PAM was used to remove suspended materials and to prevent blockage in constructed wetland. Results are shown as follows.Results of laboratory experiment showed that PAM residues in sediment had significantly positive correlation (P＜0.05) with total nitrogen (TN), ammonia nitrogen (NH4+-N), electrical conductivity (EC) and increased TN, NH4+-N and EC by 1.93%～22.98%,12%～35.29%,33.13%～224.29%, respectively. The increase of nitrate nitrogen (NO3--N), nitrite nitrogen (NC2--N) and organic matter (OM) by PAM had no significant difference, whereas pH showed a rising trend. Results of microbial nitrogen transformation showed that PAM promoted denitrification by 19.83%～ 143.92% compared to CK, and the denitrification rate was significantly positively correlated with PAM residues (P<0.05), while the promotion of nitrification was also related with PAM.Based on the sediment physicochemical properties in constructed wetland in four seasons, the variation trends among different units and different seasons were shown as follows:NH4+-N, NO3--N, TN, and OM varied within 26.28～1759.25 mg/kg, 0.32～102.69 mg/kg,0.20～2.43 g/kg, and 12.95～58.93 g/kg, respectively. The change trend of different units was grit chamber> surface flow wetland> oxidation pond, and seasonal variation trend showed the contents in summer and autumn were more than those in spring and winter. The carbon/nitrogen (C/N) ratio in different units showed a tendency as oxidation pond> surface flow wetland> grit chamber. The content of N showed a low annual trend (47.31～474.2 μg/kg) and a comparatively higher level in oxidation pond. In addition, the water content (30.96%-83.50%) was highest in summer in grit chamber, EC (78.50-425.33 μs/cm) was highest in grit chamber in winter and summer, and pH (6.28-8.46) in grit chamber and oxidation pond showed no significant difference while pH in surface flow wetland showed the lowest level.The denitrification rate in sediment varied within 22.86-140.44 mg/kg-h and was higher in summer and autumn but lowest in spring. Besides, the denitrification rate in grit chamber was significantly higher than that in oxidation pond and surface flow wetland. Based on the correlation analysis between denitrification rate and sediment physicochemical properties, denitrification rate might increase with abundant carbon and nitrogen and reached higher level when C/N ratio was about 20 in this study. In addition, denitrification rate had significantly positive correlation with EC and water content (.P<0.05). Gene abundances of denitrification functional microbial groups indicated that nirS and nirK respectively ranged within 9.14x10～1.93x109 cpies/g dw soil and 4.42x105～1.33x108 cpies/g dw soil. Higher denitrification rate in grit chamber might derived from rich nitrogen and carbon source and showed significant positive correlation with gene abundances (P＜0.05), which indicated that denitrification rate was influenced by sediment physicochemical properties and microbial abundance.