Spatial-temporal Dynamics And Influencing Factors Of N₂O Fluxes In A Halophila Beccarii Seagrass Meadow

Since the industrial revolution and the development of urbanization,the burning of a large number of fossil fuels,the rapid growth of population and other problem have emitted a large amount of carbon dioxide（CO₂）,leading to the continuous intensification of atmospheric greenhouse effect.In recent years,seagrass meadow and other coastal wetlands have attracted global attention for their role in carbon sequestration and the importance of mitigating global warming.But when it comes to assessing the importance of these coastal wetlands in mitigating global warming,non-carbon dioxide greenhouse gas emissions from ecosystems are often overlooked,especially nitrous oxide（N₂O）and methane（CH₄）.Coastal wetland ecosystems have also been shown to be sources of N₂O emissions.Under the effect of exogenous nutrients input,the warming effect produced by N₂O emission from wetland cannot be ignored,and can even offset all the carbon sequestration benefits of ecosystem.Therefore,the consideration of N₂O flux,emission characteristics and influencing factors play a non-negligible role in the scientific assessment of the role of coastal wetland ecosystem on greenhouse effect.Although more attention has been paid to the carbon sink capacity of seagrass meadow ecosystem,few studies have been done on the temporal and spatial characteristics and influencing factors of greenhouse gases,especially N₂O flux,in seagrass meadow ecosystem,especially in intertidal seagrass meadow.In this study,the Halophila beccarii seagrass meadow in Maowei Sea,Guangxi was studied to investigate（1）the spatial and temporal distribution of greenhouse gases（CO₂,CH₄ and N₂O）in seagrass meadow,（2）the contribution of N₂O to the atmospheric warming effect of the greenhouse gas flux in seagrass meadow,and（3）the effects of seagrass vegetation,soil environment and microorganisms on the N₂O flux.In this study,seagrass meadow,bareflat,and seagrass meadow with different elevation were set up.Through seasonal surveys,greenhouse gas fluxes at the intertidal seagrass/bareflat-atmospheric interface during low tide were studied,as well as seagrass vegetation parameters,soil environmental parameters,and microbial functional gene abundance in soil.Seagrass meadow and bareflat were compared.Based on the warming potential of three greenhouse gases（expressed as CO₂ equivalent fluxes）,the contribution of N₂O to the atmospheric warming effect generated by greenhouse gas fluxes was analyzed.In addition,the relationship between N₂O flux and the spatio-temporal variation of the abundance of functional genes involved in nitrification and denitrification in seagrass plants and soil was also analyzed.Seagrass meadow(-0.02μmol m^-2 h^-1)and bareflat(-0.17μmol m^-2 h^-1)are both weak N₂O sinks,but the N₂O flux does not show significant temporal and spatial variation.The CO₂ equivalent flux of greenhouse gases at the wetland-atmosphere interface is mainly contributed by the CO₂ flux,which is jointly affected by seagrass and soil.The seasonal changes of vegetation parameters and soil parameters of seagrass also drive the changes of CO₂ equivalent flux of greenhouse gases in the ecosystem.The CO₂ flux in the plain showed more obvious seasonal variation only under soil regulation,which resulted in seasonal fluctuation of the contribution of N₂O to the warming potential of greenhouse gas flux in the plain.Seagrass parameters and soil parameters showed significant seasonal changes,and seasonal changes of seagrass and soil parameters also affected the abundance of nitrification and denitrification functional genes.For example,seagrass could promote the amplification of nar G,nir S and AOA-amo A genes.Increased moisture content will promote the release of N₂O in dark conditions.Although N₂O is mainly derived from nitrification and denitrification,seasonal changes of the abundance of functional genes in nitrification and denitrification do not affect the N₂O flux.The N₂O fluxes of seagrass meadow and bareflat under different elevation conditions are both low,showing as weak source or weak sink of N₂O.The interaction of elevation and light influence the N₂O flux of seagrass meadow,but there is no significant difference in the N₂O flux of ecosystem under different conditions.The CO₂ absorption capacity of seagrass stations near shore is strong,and the N₂O flux of seagrass stations near the sea contributes more to the CO₂ equivalent flux of greenhouse gases in the ecosystem.The presence of seagrass can reduce the seasonal variation of CO₂ flux in the ecosystem,which makes the contribution of N₂O flux to the greenhouse gas CO₂ equivalent flux in the ecosystem more stable than that of the bareflat.The biomass,density and soil parameters of seagrass plants varied with elevation,season or sampling site,and affected the abundance of microbial functional genes during nitrification and denitrification.The above results show that the N₂O flux of the seagrass meadow in the Maowei Sea is at a low level,and does not change significantly with the seasons,light and other conditions.Under the influence of light,season and other conditions,the CO₂ flux in the intertidal seagrass meadow will change significantly.Seagrass parameters,soil parameters,and the abundance of microbial functional genes during nitrification and denitrification had certain spatio-temporal changes,while seagrass and soil parameters mainly affected the N₂O flux.