| Constructed wetlands(CWs)have been used for the treatment of various wastewaters in the past several decades.Among the multitudinous and complicated mechanisms of pollutants removal in CWs,the microbial pathway has been universally realized as a unique and critical one.Moreover,macrophytes not only can absorb pollutants from wastewater directly,but also play an active part in microbial abundance and composition in CWs via providing a substratum for microorganism growth,serving carbon compounds for heterotrophic bacteria,and delivering oxygen for aerobic microorganisms.However,some studies revealed that the wetland macrophytes had little statistical influence on the microbial abundance and community composition in the substrate,and limited or even negligible effect on the removal of contaminants from wastewater in CWs under certain conditions.In fact,the interaction between the macrophytes and microorganisms depends on various factors such as macrophyte species and growth status,as well as season.Therefore,the influence and detailed mechanisms of different macrophytes on wastewater purification in different type's of CWs deserve deeper exploration.In addition,it is well acknowledged that there is a great dependence on temperatures of the microbial activity and macrophytes growth status.Hence,the removal of the excessive nitrogen from polluted water remains a challenge in high-latitude areas or during winter in low-to middle-latitude areas with average temperatures of lower than 10?.Thus,in this study,the two questions were posed:Can macrophytes be applied to enhance the nitrogen removal rate in CWs during the low-temperature season(water temperature<10?)?Can macrophytes regulate the microbial community by providing organic carbon and oxygen and then facilitate the nitrogen removal in CWs during the low-temperature season(water temperature<10 ?)?In order to answer the two questions,the current study includes:1)Effects and mechanisms of cold-season macrophytes on purification capacity of subsurface flow constructed wetlands(SSF-CWs)in low-temperature season;2)Purifying effect of subsurface constructed wetlands(SSF-CWs)with Iris pseudacorus on saline wastewater during low-temperature season;3)Influences and related mechanisms of Oenanthe javanica on nitrogen removal in free-water surface constructed wetlands(FWS-CWs)under low-temperature season;4)Study on the Effect of Oenanthe javanica-planted constructed floating wetlands(CFWs)for nitrogen removal from secondary wastewater in low-temperature season.Results showed that:(1)In compared to the unplanted SSF-CWs(CWc),which had removal rates of 72.3%and 39.3%for NH4+-N and TN,respectively,the SSF-CWs planted with Iris pseudacorus(CW1)showed a prominent advantage(p<0.05)on average removal rates of NH4+-N(80.5%)and TN(47.5%),but no distinct advantage was found in the SSF-CWs planted with Typha orientalis Presl.(CWT)(p>0.05).Meanwhile,the ROL rate(5.95 ?molO2·g-1 Root·h-1)and root vitality(347.48 ?gTTC·g-1Root·h-1)of I.pseudacorus also showed striking difference compared to those of T.orientalis,which were recorded as 1.73 ?molO2·g-1 Root·h-1 and 56.44?gTTC·g-1 Root·h-1,respectively(p<0.05).Furthermore,the 16S rRNA gene Illumina Miseq sequencing analysis indicated the positive effects of I,pseudacorus on the bacterial community richness and diversity in the substrate.Moreover,the community compositions of the bacteria involved with denitrification presented a significant difference in the three systems.Additionally,higher relative abundances of nitrifying bacteria(0.4140%,0.2402%,and 0.4318%for Nitrosomonas,Nitrosospira,and Nitrospira,respectively)were recorded in CWi compared with CWT(0.2074%,0.0648%,and 0.0181%,respectively)and CWC(0.3013%,0.1107%,and 0.1185%,respectively).(2)The mean removal rates of ammonia nitrogen(NH4+-N)and total nitrogen(TN)reached 80%and 52%,respectively,in Iris pseudacorus-planted SSF-CWs fed with normal(salinity,1.3‰-1.5‰)wastewater,but both of them were reduced by 27%in Iris pseudacorus-planted SSF-CWs fed with saline(salinity,6.3‰-6.5‰)wastewater.Higher salinity not only reduced the plant biomass and nitrogen uptake by 32.1%and 50.1%,respectively,but also decreased the growth rate,activity,and oxygen release rate of plants root by 37.8%,68.0%,and 62.9%,which might change the bacterial community composition,especially the bacteria involved in nitrification-denitrification.(3)The presence of macrophytes observably improved the removal rates of ammonia nitrogen(65%-71%)and total nitrogen(41%-48%)(p<0.05),with a sharp increase in COD concentration(about 3-4 times)in FWS-CWs.Compared to the unplanted systems,the FWS-CWs planted with Oenanthe javanica not only presented higher richness and diversity of microorganism,but also significantly higher abundances of bacteria,ammonia monooxygenase gene(amoA),nitrous oxide reductase gene(nosZ),dissimilatory cdl-containing nitrite reductase gene(nirS)and dissimilatory copper-containing nitrite reductase gene(nirK)in the substrate(p<0.05).Meanwhile,the analysis on microbial community composition further revealed the differences.(4)The average removal rates of ammonium-,nitrate-and total nitrogen were 78.3%,44.4%,and 49.7%in CFWs planted with O.javanica(Tc),72.0%,40.0%,and 39.5%in controls without O.javanica(Ts),and 73.1%,33.7%,and 44.0%in controls without substrate(Tp),respectively.Besides the gradual increase of the chemical oxygen demand concentration,Tc showed higher microbial richness and diversity,and higher abundance of bacteria,archaea,anammox bacteria,and key genes(amoA,nosZ,nirS and nirK)involved in nitrogen metabolism in the substrate compared to Ts.Further analysis of the microbial community composition revealed a difference at multiple taxonomic levels among different systems.It suggests that:(1)the hardy plant I.pseudacorus,which still has active root oxygen release in low temperatures,positively affects the abundance of nitrifying bacteria in the substrate,and accordingly is supposed to contribute to a comparatively high nitrogen removal efficiency of the system during the low-temperature season;(2)high salinity inhibits nitrogen removal in SSF-CWs by suppressing wetland macrophyte growth and activity,and changing the microbial community composition;and thus,the application of SSF-CWs on saline wastewater(salinity,6.3‰-6.5‰)still needs to further optimize the process conditions,such as,extending the hydraulic retention time,for achieving perfect purifying treatment effect;(3)the enhanced abundances of microorganisms and the key functional genes involved with nitrogen metabolism in O.javanica-planted FWS-CWs,due to the abundant carbon release from the wetland macrophyte,promote the nitrogen removal in FWS-CWs;(4)the presence of O.javanica and vesuvianite in CFWs play positive roles on nitrogen removal from secondary wastewater during the low-temperature season(average water temperature<10?).In summary,the macrophytes in SSF-CWs,FWS-CWs,and CFWs can regulate the microbial community compositions and abundances of the key genes related to nitrogen metabolism in the substrate via supplies of organic carbon and oxygen,in turn promote the nitrogen removal from secondary wastewater,during the low-temperature season with an average water temperature less than 10?. |
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