| Human activities and climate change have greatly changed the nitrogen cycle in rivers and lakes around the world,and changed the production and emission of the greenhouse gas nitrous oxide(N2O).Hydrodynamic conditions and environmental factors(temperature,p H,etc.)will affect the concentration of dissolved oxygen(DO)in rivers and lakes,and DO is an important factor affecting the production of N2O in various biochemical processes.At present,the research in this area is not in-depth.Therefore,The thesis changes the hydrodynamic and environmental conditions by constructing a sediment-overlying water-gas system,setting different agitating speeds and aeration amounts,and obtaining different DO concentrations in the system.Furthermore,the changes of ammonia nitrogen,nitrite,nitrate,organic carbon and other substances in the system under different DO concentrations and their effects on the production and diffusion flux of N2O were studied.Finally,combined with the distribution and composition of microorganisms in the sediment,the N2O production pathway and mechanism were explored.The main conclusions are as follows:(1)When the stirring speed increases from 0r/min to 60r/min,the DO concentration of the overlying water in the system increases from 0mg/L to 3.5±0.5mg/L,and when the aeration amount increased from 0m L/min to 800m L/min,the DO concentration of the overlying water in the system reached 7.625±0.375mg/L,indicating that increasing the stirring speed and aeration can significantly increase the DO concentration of the overlying water in the system,and the DO concentration increases more rapidly under aeration conditions.As the concentration of DO in the overlying water changes,the depth of DO penetrating the sediment varies from 0 to 3cm,and the penetration depth of DO under stirring condition is generally higher than that under aeration condition,indicating that the concentration of DO in the overlying water and different hydrodynamic conditions will affect the vertical distribution of DO in the sediment,and the stirring conditions are more conducive to the diffusion of DO into the sediment.(2)With the increase of DO concentration,the TOC concentration of sediments and overlying water in the system both showed a downward trend,and the TOC concentration of sediments decreased the most under static conditions,and the TOC concentration of the overlying water decreased the fastest;When the DO concentration is low,NH4+in the sediment and overlying water,and NO2-in the overlying water have accumulated,because under low DO environment,nitrification is inhibited;The concentration of NO2-and NO3-in the sediments showed a downward trend;DO has a significant effect on the concentration of NO3-in the overlying water.As the DO concentration increases,the accumulation of NO3-in the overlying water increases,and under low DO conditions,the NO3-concentration in the overlying water does not change significantly,which is closely related to nitrification.(3)The instantaneous concentration of N2O at SWI in each reactor ranged from0.2213±0.0137mg/L,and the cumulative value of N2O flux at the water-gas interface was positive,indicating that the sediment-overlying water system was the source of N2O.Although the increase in DO concentration will also accelerate the oxidation consumption of N2O in the overlying water,the increase in agitation speed and aeration will accelerate the release of N2O into the gas phase,indicating that the N2O consumption process and its related rate are as important as the production process.The output of N2O at SWI shows:stirring>standing>aeration.It shows that the aerobic environment will increase the production of N2O,but in the high-oxygen environment,the N2O produced is easily oxidized and consumed in the aerobic zone of the sediment.The result of N2O emission shows:stirring>aeration>stand still.It shows that the disturbance caused by stirring or aeration will benefit the emission of N2O,and the high oxygen environment formed in the system by aeration will accelerate the oxidation of N2O,resulting in a reduction of the amount of N2O discharged into the gas phase.In addition,the linear fitting result of DO and N2O flux is y=(0.79091±0.07828)+(-0.01979±0.018)x(R2=0.02912),and the linear fitting result of DO and the N2O concentration at SWI is y=(0.2211±0.00154)+(-5.61138×10-4±3.48024×10-4)x(R2=0.18602),indicating that the DO concentration has no obvious correlation with the N2O flux at the water-gas interface and the N2O concentration at the SWI.Although the DO concentration will affect the composition and distribution of the microbial community in the system,and thus the way to produce N2O,but with the increase of DO concentration,there is no obvious regularity of the N2O concentration at the SWI and the N2O flux at the water-gas interface,but at a certain DO concentration level,the N2O concentration and the N2O flux at the SWI reach the maximum,which provides evidence that the pathway for N2O production is driven by different types of microbial communities rather than a single key species.(4)The depth of DO penetration in sediments will also affect the vertical distribution and composition of microorganisms.The microbial species and abundance in the reactor sediments under static conditions are higher than those in other reactors,and as the DO concentration increases,the number of species in the sediment samples shows a downward trend.Under static conditions,the entire system is in an anaerobic environment.The penetration depth of DO in sediments is 0cm.Among them,Thiobacillus is the most detected sequence,followed by Bacillus.At an aeration rate of800 m L/min,the penetration depth of DO in the sediment was 1.375±0.875 cm.Among them,the highest number of detected sequences was Halomonas,followed by Thiobacillus.The highest number of sequences detected in the sediments of the remaining reactors were Thiobacillus,followed by Halomonas.Thiobacillus is a genus of bacteria that can perform denitrification or aerobic denitrification,indicating that these two processes occupy an important position in the production of N2O.In a high-oxygen environment,high denitrification and aerobic denitrification bacteria were detected,indicating that the presence of deep anaerobic and surface aerobic sediments may stimulate denitrification and aerobic denitrification.In addition,the genus of microorganisms related to N2O production detected in the sediment also includes Flavobacterium,Bacillus,Acinetobacter,Denitratisoma,Pseudomonas,Rhodobacter,Microbacterium,Eubacterium,Nitrosomonas,Nitrospira,etc.,which provides evidence for the existence of the main N2O production pathway.(5)By changing the hydrodynamic conditions and aeration to control the DO concentration in the overlying water and the penetration depth of DO in the sediment,the conditions and pathways related to N2O production in the system can be adjusted to reduce the production and release of N2O,which provides a scientific theoretical basis for regulating the contribution of rivers and lakes to global N2O release. |
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