N₂O And CH₄ Fluxes And Their Influencing Factors In Spartina Alterniflora Salt Marsh In Nanhui Shore Of Yangtze Estuary Under High Nitrogen Background

Rapid expansion of Spartina alterniflora along the southeast coast of China changed the local ecosystem of the coastal wetland.Under the background of global climate change,sea level rise,salt-fresh water mix and nitrogen surplus in Yangtze Estuary,analysing seasonal and diurnal patterns of nitrous oxide(N₂O)and methane(CH₄)fluxes and their"source-sink"dynamics,and their relationships with biological and non-biological factors can provide scientific reference for the protection,restoration and management of estuarine wetlands.Since March,2017 to December,2018,three plots dominated by S.alterniflora(P1,P2,P3),and one bare mudflat(P0)in a salt marsh of Nanhui shore in Yangtze Estuary have been established for monthly field observation.S.alterniflora colonised the sites P1 and P2 nine years,and P3 was only two years.Sediment salinity at P1 was higher than that at other plots.The polt with S.alterniflora colonised many years(P2)has been established for day-night observation and the experiment of exogenous nitrogen(Na NO₃)addition(N0:no addition;N1:1.28g N·m^-2·yr^-1;N2:2.56 g N·m^-2·yr^-1).The effects of S.alterniflora and exogenous nitrate addition on N₂O and CH₄ fluxes and microbial community structure in estuarine wetlands were analyzed by the static chamber method.The main results were as follows:1.The salt marshes dominated by S.alterniflora in the Yangtze Estuary shifted between N₂O sources(spring and summer)and sinks(autumn and winter),and was a relatively low N₂O source throughout the year.N₂O fluxes from three S.alterniflora vegetation plots ranged from-41.9 to 39.3?g·m^-2·h^-1,average flux(4.2?g·m^-2·h^-1)was higher than that measured in bare mudflat(1.3?g·m^-2·h^-1).In three S.alterniflora plots,N₂O was emitted in spring and summer and consumption in autumn and winter,showing obvious source-sink conversion.Conversely,N₂O fluxes in the bare mudflat(P0)consumed N₂O during all sampling periods except in summer.Diurnal N₂O fluxes showed obvious variation,and day-and nighttime N₂O fluxes were also different in different seasons.The daily maximum CH₄ flux was usually observed in the daytime.In summer,it appeared at night,which was also the peak of annual emissions.Diurnal variation of N₂O fluxes were affected by a combination of temperature,water table,sediment moisture,vegetation and other factors.The increase of temperature and NO₃^-concentrations promoted N₂O emission,while the increase of salinity inhibited N₂O emission.Vegetation promoted N₂O emission flux during the growing period,and the N₂O flux was higher in salt marsh where S.alterniflora was established for longer years.2.The S.alterniflora salt marsh was a CH₄ source and the mudflat was a CH₄sink.CH₄ fluxes from three S.alterniflora vegetation plots ranged from 0.2 to 72.2mg·m^-2·h^-1,with an average CH₄flux of 12.4 mg·m^-2·h^-1,indicating that S.alterniflora salt marsh was a CH₄ source,and showed obvious seasonal variation,with summer as the peak of emissions.Whereas the mudflat was a CH₄sink(-0.8 mg·m^-2·h^-1).Rising temperature promoted CH₄ emission.Sediment salinity affected spatial variability of CH₄ flux.The years of plant colonization and community types were the main factors affecting CH₄ flux.CH₄ fluxes were similar in day and night.Diurnal variation of CH₄fluxes were affected by a combination of temperature,water table,sediment moisture,vegetation and other factors.Comprehensive analysis of the warming potential of N₂O and CH₄ showed that bare flat was the sink of greenhouse gases(N₂O and CH₄)(-1.9 Mg CO₂·hm^-2·yr^-1),while S.alterniflora marshes were the source of greenhouse gases.In the early stage of colonization,it was 2.8 Mg CO₂·hm^-2·yr^-1,and after the long period of colonization,S.alterniflora marshes with high salinity and low salinity were 12.3 Mg CO₂·hm^-2·yr^-1and 76.6 Mg CO₂·hm^-2·yr^-1,respectively.Salinity inhibited the emission of N₂O and CH₄.3.The species diversity and richness of bacteria community and nir S-denitrifying bacteria community at S.alterniflora plots were higher than those at the bare flat.The composition of bacteria community and nir S-denitrifying bacteria community showed spatial and seasonal variation.The species diversity and richness of bacterial community and nir S-denitrifying bacteria community at the three S.alterniflora plots were higher than those at the bare flat.There was no significant correlation between the species diversity of bacterial community and nir S-denitrifying bacteria community with environmental factors.The composition of bacterial community and nir S-denitrifying bacteria community varied in different plots and seasons.The dominant phyla were Proteobacteria,Bacteroidetes and Chlorobacteria(relative abundance>10%),and the dominant families were JTB255?marine?benthic?group,Flavobacteriaceae and Anaerobicaceae(relative abundance>5%).nir S-denitrifying bacteria community was divided into four clusters(Cluster A-D).Cluster A has little monthly variation and was enriched in relatively high salinity environment;Cluster B was enriched in summer;Cluster C was enriched in low salinity environment.Cluster D showed an increasing trend in the year.Temperature,salinity,carbon and nitrogen content were important environmental factors affecting nir S-denitrifying bacterial community.4.Exogenous nitrate addition promoted N₂O emission and inhibited CH₄ emission.The annual N₂O fluxes of no-addition group(N0),low nitrate addition group(N1)and high nitrate addition group(N2)were 12.1?g·m^-2·h^-1,39.7?g·m^-2·h^-1 and 58.5?g·m^-2·h^-1,respectively.CH₄ fluxes were 41.3 mg·m^-2·h^-1,40.5 mg·m^-2·h^-1 and 36.9mg·m^-2·h^-1,respectively.Compared with N0 group,the annual N₂O emission fluxes in N1 group and N2 group increased by 228%and 383%,respectively,and the annual CH₄fluxes decreased by 1.9%and 10.7%,respectively.The response of CH₄ and N₂O fluxes to nitrate addition was more significant in summer.In terms of the overall warming potential of CH₄ and N₂O,nitrate input weakened the warming potential of gas emissions,and N1 and N2 groups decreased by 0.13%and 9.6%,respectively.5.Exogenous nitrate addition changed microbial species richness,and affected microbial community structure by increasing soil total nitrogen and decreasing soil C/N ratio.Exogenous nitrate addition decreased the species richness of bacterial communities,but increased the species richness of methanogens and nir S-denitrifying bacteria.However,it has minor effect on the species diversity of microbial community.The composition of microbial community showed seasonal differences,which were affected by temperature and water salinity.Exogenous nitrate addition affected the composition of microbial community by increasing soil total nitrogen and decreasing soil C/N ratio,but the difference was smaller than that caused by seasonal variation.Methanosarcinales were major orders in methanogens communitiy at P2,from about30%in May to more than 80%in September.The relative abundance of Cluster A in nirs-denitrifying bacteria communitiy did not show significant monthly variation,Cluster B decreased with month,while Cluster C and Cluster D increased with month.In conclusion,invasion of S.alterniflora increased CH₄ and N₂O fluxes,and CH₄and N₂O fluxes increased with the increase of its time of colonization,but the increase of salinity and nitrate nitrogen inhibited this effect.The colonization of S.alterniflora promoted the diversity of soil microbial community and changed the composition of microbial community.Therefore,the above factors should be considered in the comprehensive control of S.alterniflora.