Study On Nitrifying And Denitrifying Processes And Microbial Community Structure In Typical Soils

Mangrove ecosystems,which are sensitive and fragile,are hotspots for microbes that constitute a large portion of biogeochemical cycles.They are also the source,sink and converter of the greenhouse gases?GHGs?.One such greenhouse gas is nitrous oxide?N₂O?.N₂O,the third largest greenhouse gas following CO₂ and CH₄,has been neglected despite its significant contribution to global warming.The two major processes in N₂O production are nitrification and denitrification by microorganisms.Additionally,the most recent years have resulted in greater N20 emissions due to the increase of carbon and nitrogen load caused by anthropogenic pollution.However,the influence of major environmental factors on nitrogen cycling in mangroves remains unclear.On the other hand,urban soils as the hotspots of nitrogen cycling are also one of the potential N₂O sources.The urban greenspace is expanding due to urban development,and the frequent fertilizing and irrigating contribute to the increase of N₂O emission from urban lawns.Moreover,urbanization is speeding up globally,and the transition of populations causes the changes in land use.This transition of rural agricultural lands to urban living lands is threatening the biodiversity and ecosystem functioning.The influence of urbanization on microbial community diversity and composition yet remains under-characterized.The objective of this study is to estimate the impact of major environmental factors and urbanization on soils nitrifying and denitrifying microbial communities,as well as the effect of population transition and land-use change caused by urbanization on microbial community diversity and composition.The major contents and results are described as below:?1?The temporal and spatial dynamics of key GHG fluxes?CO₂,CH₄,and N₂O?at an unvegetated mudflat,cordgrass?Spartina alterniflora?,and mangrove?Kandelia obovata?sites along an estuary of the Jiulong River in Southeast China were investigated over a 2-year period.Results showed that the CO₂ and CH₄ fluxes demonstrated a seasonal and vegetation-dependent variation while N₂O fluxes showed no such dependent pattern.The plants significantly stimulated the fluxes of CO₂ and CH₄.However,no difference was observed between cordgrass and mangrove.Air temperature was the main factor influencing GHG fluxes.Moreover,CH₄ was the biggest contributor to the global warming potential while N₂O showed a minor contribution.?2?The impacts of vegetation?bulk,K.obovate,and S.alterniflora?and tidal process?falling tide and rising tide?on denitrification activities,abundances,and community compositions of denitrifiers in the sediments from different depths?0-5 and 5-10 cm?were investigated in a microcosm experiment.A significant enhancement of denitrification activities and gene abundances?nirS,nirK,and nosZ?in the vegetated sediments was observed.Activities and abundances were significantly higher in the 0-5 cm sediments when compared with the 5-10 cm counterparts.The effect of interaction between vegetation and tide or depth was also significant.The results also revealed that not only vegetation but also plant species had a significant impact on the community compositions of nirK denitrifiers,while the tidal process affected the community compositions of nirS and nosZ denitrifiers.However,depth only significantly shaped the nirS denitrifier communities.?3?Based on the above experiments,sediment samples associated with different vegetation types from the microcosm were selected to detect the potation nitrification rate?PNR?,the abundances of amoA gene and transcript,and the nitrifier community structures.PNR and abundances of both ammonia oxidizing archaea?AO A?and bacteria?AOB?were greatly enhanced in the vegetated sediments.S.alterniflora showed a greater promoting effect on nitrification activity.Abundance of AOB transcript was hundreds of times higher than that of AOA.Moreover,AOA and AOB communities were distinctly grouped responding to vegetation with two plant species.?4?By manipulating the salinity levels?0,10,20 and 30 ppt?in sediments from a mangrove,we investigated the differences in denitrifying activity,abundances of nitrifiers and denitrifiers,and their community structures.Increase of salinity inhibited both denitrifying activity and the abundances of nirK and nosZ denitrifying genes.AOB dominated the nitrification,and flourished at intermediate salinity levels?10 and 20 ppt?,while salinity showed smaller impact on AOA.The community structures of AOA and AOB,as well as nirK,nirS and nosZ denitrifiers were all significantly shaped by salinity.?5?The variations of soil nitrification and denitrification,and overall microbial community associated with different urbanization gradients?urban,suburban and rural?were investigated in Xiamen City.The parks were categorized into urban turf-grass and suburban turf-grass according to the human population density,and were compared with rural agricultural lands.The potential N₂O emissions,potential denitrification activity,and abundances of denitrifiers were higher in the rural farmland soils compared with the turf-grass soils.AOA were more abundant than AOB in turfgrass soils.Within turfgrass soils,the PNR and AOA abundances were higher in the urban than in the suburban soils.Microbial community composition was distinctly grouped along urbanization categories?urban,suburban,and rural?.?6?We characterized soil bacterial communities from turf-grasses associated with urban parks,streets and residential sites across the Chicago City,including a gradient of human population density.Bacterial diversity was significantly positively correlated with the population density;and species diversity was greater in park and street soils,compared to residential soils.Population density and greenspace type also led to significant differences in the microbial community composition that was also significantly correlated with soil pH,moisture and texture.Co-occurrence network analysis revealed that microbial guilds in urban soils were well connected.Abundant soil microbes in high density population areas had fewer interactions,and abundant bacteria in high moisture soils had greater connectivity,while microbes associated with larger sized particles had weaker connectivity.