| Constructed wetland has been widely used in wastewater treatments with its characteristics of inexpensive construction,highly efficiency and simple maintenance,However,nitrogen removal rate is limited by low dissolved oxygen(DO)concentration in traditional subsurface constructed wetlands,and the biological process has become one of primary sources for nitrous oxide(N2O)which is a kind of significant greenhouse gas.Biochar has a large specific surface area and porous structure,and has a strong water-holding and adsorption properties.It is often used as a kind of soil conditioner,which plays an important role in the circulation of nitrogen in soil.Based on the characteristics of biochar and the effective use of wetland plants,this study aims to harvest the plants and to add them to the constructed wetland and study the effect of biochar on N2O emission reduction.The influence of biochar on N2O emission pathway was studied by 15N isotope tracer method.Using real time quantitative pcr(Qpcr)and other molecular biology techniques,explore the mechanism of the biochar on wetland N2O emissions from the perspective of Microbiology,to strengthen the N2O reduction in wetlands and provide a theoretical basis for it.A set of constructed wetlands were built under greenhouse,four different biochar addition were selected(40%,20%,10%,0%),with the influent COD:N=7:1.In this paper,the different biochar addition effects on The effects of biochar addition on N2O flux,N2O emission pathway,microbial community and gene expression levels related to N2O emissions in the constructed wetlands were studied.The main contents and results were as following:(1)The DO effluent of all wetlands was lower than 0.3 mg/L,and the addition of biochar significantly increased the effluent DO(p<0.05).The pH of effluent in wetlands was significantly lower than that in influent,and the addition of biochar could significantly increase the effluent pH in the wetland system(p<0.05).Under different treatments,the removal rate of COD reached more than 80%and the addition of biochar has a significant effect on COD removal in wetland systems.Biochar addition significantly increased the removal of NH4+-N,but had no significant effect on NO3-N removal.(2)The addition of biochar has a significant effect on the control of N2O emission from constructed wetlands.The average fluxes of N2O in all constructed wetlands were between 0.67±0.25-2.13±0.05 mg·(m2·h)-1,and the accumulation were 9.92±1.45mg ·m-2?59.81±6.45 mg·m-2.Compared with traditional constructed wetlands,CW40,CW20,and CW10 achieved 83.41%,65.91%,and 25.85%of N2O emission reductions,respectively.Cumulative N2O-N emission of total removal TN in CW40 accounted for 0.17%,which was significantly lower than that in traditional constructed wetlands(p<0.05).(3)The addition of biochar can significantly change the distribution of microbial communities in wetlands.The results of high-throughput sequencing showed that the nitrifying bacteria associated with N2O emissions in the wetlands were Nitrospira and Nitrosomonas,and the denitrifying bacteria were Methyloversatilis,Drechloromonas,Denitamisoma,and zooglea.The ratios of different biochar additions make a significant difference in their abundance.(4)Biochar can significantly increase the activity of ammonia nitrogen oxygenase(AMO)and nitrous oxide reductase(Nos)in constructed wetlands(p<0.05).The AMO activity of CW40-CW10 increased by 2.09,2.51,and 3.02 times compared with CK,respectively,and the amplitude increased gradually.The maximum Nos activity in CW40 was 18.09 U/L,which was 1.54 times higher than that of CK Higher biochar content has a more pronounced effect on Nos activity.The addition of biochar can also increase the activity of hydroxylamine oxidoreductase(HAO)and nitrite reductase(Nir)in the wetland system,but the addition of biochar has no significant effect on the two.The Qpcr amplification test of these four key enzyme functional genes showed that the abundance of amoA gene increased with the increase of the proportion of biochar,but the increase was not significant,and the nosZ gene copy number of CW40,CW20 and CW10 wetlands was higher than that of CK wetland.Increase 108.31%,58.08%and 35.57%(5)Study on the contribution of nitration and denitrification to N2O emissions by 15N isotope tracer shows that the addition of biochar affects the nitrification(NF),nitrification simultaneous denitrification(SND)and denitrification(DF)proportion of released 15N-N2O to total 15N-N20 emissions,respectively.When biochar was added,the nitrification reaction was the main source of N2O in the initial stage of the reaction(1h).The contribution of nitrification to N2O emissions in CW40 wetland(54.15%)was greater than that of CK system(51.19%).The N2O emission from all constructed wetlands in the later period of the reaction mainly comes from denitrification,and the contribution rate to 15N-N2O is 75.2%in CW40 and 93.65%in CK The highest N2O concentration(1168.30ppm)from denitrification in traditional constructed wetlands was significantly higher than that of CW40(225.93ppm). |
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