Nitrous Oxide Production and Emission on Multiple Interfaces and Related Influence Factors in Taihu Lake
Author:Liu De Hong
Supervisor:zhong wen hui
Nitrous Oxide(N2O)is one of the three important greenhouse gases in the atmosphere,contributing about 6%of the greenhouse gas’s radiation,and its greenhouse effect far exceeds that of carbon dioxide(CO2)and methane(CH4).Previous studies on N2O generation and emission mainly conducted in agro-ecosystems,and N2O emission from aquatic ecosystems has only received less attention.With the increase of water eutrophication,high-level of N2O concentration,saturation and emission flux has been detected,and some research findings were also obtained.However,the lake N2O generation and emission mechanism and the systematic research on multiple interfaces still remained unknown.In this study,the sediments,sedimentary columns,water bodies and sediments in different ecological types of Taihu Lake were selected,containing algae lake area(S1),open water area(S2),submerged plant area(S3)and floating leaf plant area(S4);the N2O emission flux at the water-air interface was measured;the nitrogen,organic carbon,denitrification rate and N2O emission rate of the sediment were analyzed;the aim ’was to investigate the N2O emission patterns and the influencing factors.The nitrification rates and denitrification rates on the sediment-water interface were determined by ATU and acetylene inhibition methods.The flux of N2O at the sediment water interface was also determined to clarify the N2O production and discharge characteristics and its dominant process on the sediment-water interface.The floating flux box was empolyed to determine the N2O emission flux on the water-air interface,and the relationship between water-soluble N2O contents and water environmental factors was also discussed to find out the main controlling factors affecting the N2O production and discharge in the lake.The gene abundance of sediment denitrifying microorganisms was determined by quantitative PCR(qPCR),and its relationship with sediment denitrification rate,N2O production rate and environmental factors was analyzed to reveal the effects of sediment denitrification.The environmental factors of gene abundance and the microbiological mechanisms of sediment N2O production.The Illumina Miseq high-throughput sequencing technology was used to determine the community structure and diversity of ammonia-oxidizing microorganisms,and the relationship between them and environmental factors was analyzed to explore the distribution characteristics and main controlling environmental factors of ammonia-oxidizing microorganisms in different ecological types of lakes in Taihu Lake.The main results are as follows:(1)The NH4+ content in sediment increased with the decrease of temperature in autumn and winter,and increased rapidly only in the algaetype zone(S1)in April,June and July.The NO3-content,with smaller fluctuation in the whole year,increased rapidly only in the June and July,while no significant difference was found in these sites.The NO2-content was lower in the spring and summer than that in the autumn and winter,but it remained at a lower level over a whole year.The organic carbon content is characterized by low winter,spring,summer and autumn,and has obvious seasonal variation.The denitrification rate of sediments is low at 8-12 months,with a mean of 0.29 nmol N g-1·h-1,which is rapid from January.Elevated,and in May,the denitrification rate of sediments in all regions reached the peak value,with a mean of 23.87 nmol N g-1·h-1;the sediment N2O was released only in 2-4 months,and February was the peak of the whole year.,the performance as the source,and the rest of the month as the sink.The denitrification rate was significantly positively correlated with sediment nitrogen and organic carbon from August to December.The denitrification rate increased rapidly from January to July and remained at a high level,but it was not correlated with sediment nitrogen and organic carbon.The denitrification rate is more characterized by seasonal changes.Overall,there is no significant correlation between denitrification rate and N2O production.We concluded that sediment denitrification may not be the main process for N2O formation in Taihu Lake sediments.(2)The nitrification and denitrification rate and the maximum N2O flux at the sediment-water interface occurred in the algal lake area in June 2015,respectively:17.80,235.51 and 31.49 μmol N m-2·h-1 In addition,the nitrification rate did not change much between months(0 to 1.18 μmol N m-2·h-1),and there was no seasonal variation.The nitrification rate was significantly correlated with the water nitrogen content The denitrification rate was lower from August to January(0.01 to 8.57 μmol N m-2·h-1,and increased rapidly from February to July(1.03 to 130.00 μmol N m-2·h-1).The annual variation of N2O exchange flux is small(-0.65 to 1.5 μmol N m-2·h-1).The water nitrogen level was significantly correlated with the nitrification and denitrification rates and N2O emission flux,but not with the sediment nitrogen level.Regression analysis found that the N2O flux was more closely related to the nitrification rate on the sediment-water interface.Therefore,we concluded that the nitrification and denitrification rates and N?0 emission flux on the sediment-water interface may be affected only by the nitrogen level of the water body.Both nitrification and denitrification processes have an important impact on the production of N20,but are mainly controlled by the nitrification process.The lower N20 flux at the water interface is mainly related to the lower rate of nitrification.(3)The dissolved N2O in water showed the trend of higher in winter and spring,lower in with the average of Summer and Autumn.The N20 saturation with low variation except for that in June in SI,with the average of 129%.The N2O flux ranged from-0.05 to 1.16μmol m-2·h-1,with low variation,except for a much higher rate(5.05 μmol m-2·h-1)in June in S1,with the average of 0.25 μmol m-2·h-1,and there was no significant differences among the other months and sampling sites.The nitrogen concentration was significant positively correlated with dissolved N2O content and N2O emission flux,however it had less effect on N2O emission.We inferred that the lower N2O emission flux and it’s no difference in any sampling sites were mainly due to the lower nitrogen concentration and the lower variation of nitrogen concentration.(4)The norB gene abundance was greater than nirS gene in sediments,the abundance of nirK gene and nosZ gene was similar,which was lower than that of norB and nirS genes.Overall,the abundance of denitrification genes was higher in winter and spring,and lower in the summer and autumn.Regression analysis indicated that TN and NO2-contents were the most important factors affecting the abundance of denitrification genes.Correlation analysis showed that the denitrification rate of sediment was positively correlated with the abundance of denitrification genes in summer and autumn,but there was no correlation in winter and spring,whcih suggested that no effect of abundance of denitrification genes on denitrification rate in Taihu lake was detected,and the denitrification rate may be affected by other factors.While the N2O production rate of sediment was significantly and negatively correlated with the denitrification rate,which suggested that denitrification can be a processes to consume N2O in sediment.There were no significant difference in the diversity index of ammonia-oxidation bacteria(AOB)in sediment among the sampling sits,and the Shannon diversity index of nitrite-oxidizing bacteria(NOB)in Summer was significantly higher than that in Winter.Regression analysis indicated that organic carbon contents of sediment was the most important factor affecting the community composition of nitrifiers.The organic carbon,PO43-and NO3-contents were the most important factors affecting the community composition of AOBs and Nitrosopira was the dominant nitrifier genus in sediment.RDA analysis showed that there were greater variations in nitrifying genus between S1 and the others sampling sites,and TN content was the most important factor affecting the spatial distribution of nitrifiers in Taihu lake.Moreover,biological communities did not affect the N2O production and emission from Taihu Lake.