Effects Of Soil Nitrification/Denitrification On N₂O Emission And Its Microbiological Mechanism In Typical Poyang Lake Wetland Under Hydrological Alteration

Nitrous oxide（N₂O）is one of the most important greenhouse gases causing global warming.Wetland becomes an‘ecological hotspot’of nitrogen cycle due to its special water environment characteristics,which in turn leads to N₂O emission.At present,continuous global warming and changes in water environment aggravate the N₂O emission from wetland soils.However,the microbial driving mechanism of N₂O emission from wetland soils in the context of hydrological alteration remains unclear.Therefore,this study takes Poyang Lake wetland,an inland freshwater lake system with complex geomorphology and variable hydrological alteration,as the research object,and selects four subsystems of different ecological types（Estuary,Lake,Mudflat,Vegetation）to carry out the following three aspects:1)By acetylene inhibition method,the potential N₂O emission characteristics of wetland soil were determined,the relative contributions of nitrification and denitrification processes were distinguished,and the dominant process of N₂O generation was clarified;2)The abundance of nitrification（AOA-amo A and AOB-amo A）and denitrification（nar G,nir K,nir S and nos Z）functional genes and group of nitrifying/denitrifying microorganisms related to N₂O emission were analyzed by q PCR and 16S r RNA gene amplification sequencing,so as to explore the microbial mechanism of N₂O emission.3)The nitrogen metabolism network associated with soil N₂O emission and related gene diversity and species origin in wet-season wetland soils were analyzed by metagenomic sequencing.The main conclusions of this study were as follows:1.The soil of typical wetlands in Poyang Lake showed a certain N₂O emission potential,mainly denitrification,with a contribution rate of 52.7%to 89.3%.The contribution rate of nitrification process was 10.5%-47.3%.In contrast,lake and vegetation regions had stronger potential to generate N₂O during denitrification and nitrification processes in dry and wet seasons,respectively.In addition,affected by hydrological alteration,the soils of Lake,Mudflat and Vegetation subsystems had higher N₂O emission potential in wet season.2.Analysis based on the abundance of nitrification and denitrification functional genes,the abundance of nitrifying genes（AOA-amo A and AOB-amo A）in wetland soils ranged from 0.64 to 3.25×10⁷copies g^-1,much lower than that of denitrifying（nar G,nir K,nir S,nos Z）genes(4.55-12.68×10⁷copies g^-1),indicating that the denitrification process is more advantageous in Poyang Lake wetland.In the two hydrological seasons.nar G and AOA-amo A gene were the main biological factors of N₂O potential emission during denitrification and nitrification,and their abundance were the highest in Lake and Vegetation subsystem,respectively.In addition,Lake,Mudflat and Vegetation subsystems were significantly affected hydrological alteration,and the abundance of denitrifying genes were higher in wet season.SEM analysis showed that SWC could regulate potential N₂O emission by directly affecting nar G gene and indirectly affecting AOA-amo A gene abundance.In addition,the relative abundance of the three denitrifying functional bacteria（Sulfuricella,Ignavibacterium and Sulfuritalea）were significantly positively correlated with N₂O emission.3.Metagenomic sequencing studies found:There were abundant nitrogen cycling pathways in Poyang Lake wetland.The diversity of functional genes involved in denitrification was 1363,which was much higher than that of dissimilatory nitrite reduction to ammonium（355）,nitrification（243）,assimilatory nitrate reduction to ammonium（131）,nitrogen fixation（43）,and anammox（12）.The more N₂O was reduced to N₂by microorganisms in Estuary subsystem.The Lake subsystem contained more abundant nrf A and nir B,which would generate N₂O reduction substrate（NO）during the dissimilar reduction process,and may further enhance N₂O emission.In addition,Proteobacteria were found to be the main functional bacteria involved in nitrogen cycling in Poyang Lake wetland soil,contributing more than34%of the genes related to nitrogen cycling.In this study,through laboratory incubation experiments combined with molecular biology analysis,the dominant process of N₂O potential emission characteristics and the potential microbial driving mechanism of Poyang Lake typical wetland soil under the background of hydrological alteration were explained,and the genetic diversity and species origin of multiple nitrogen cycle pathways involved in N₂O emission were studied.Comprehensive consideration of the effect of different regions and hydrological seasons of Poyang Lake on N₂O emissions,which is of great significance for comprehensively understanding and revealing the N₂O emission mechanism of Poyang Lake wetland,and also provide theoretical reference for other lake wetlands facing the same hydrological alteration.