Study On The Generation,Release And Influencing Factors Of CH₄ And N₂O In Xincun Bay

Methane（CH₄）and nitrous oxide（N₂O）are the most important greenhouse gases next to carbon dioxide（CO₂）in the atmosphere,and their warming potential is25 times and 300 times that of CO₂respectively.A lot of studies have been carried out at home and abroad,and attempts have been made to draw global CH₄and N₂O source and sink lists.Coastal estuaries,lagoon affected by human activities is severe,is an important source of CH₄and N₂O in the atmosphere,the typical coastal,estuary,lagoon,etc to the biogeochemistry of CH₄and N₂O in water cycle research,can provide accurate estimate of different types of CH₄and N₂O emission flux and provide basic data on the list of global sources.In this paper,Xincun Bay Lagoon in Lingshui County,Hainan Province was selected as the research object,and investigated in July 2020（summer）,January 2021（winter）and July 2021（summer breeding area）.Surface and bottom water samples were collected to measure the concentration of dissolved CH₄and N₂O in water.To investigate the characteristics of seasonal change of CH₄and N₂O and influencing factors,estimation of CH₄and N₂O sea-gas exchange flux;The concentrations of CH₄and N₂O in sediment interstitial water of tropical seagrass beds and the greenhouse gas fluxes at sediment-atmosphere interface and their influencing factors were studied.The effects of different electron acceptors on the potential of CH₄production were investigated through indoor culture experiments.The main conclusions are as follows:The average concentration of dissolved CH₄in the surface of xincun Bay water body in summer,winter and summer aquaculture zones was 63.27±13.76,17.49±11.45 and 48.61±28.54 nmol/L,respectively,and the average concentration in the bottom was:There were 83.64±30.96,14.75±2.93 and 43.48±32.61 nmol/L,in summer aquaculture zones>in summer aquaculture zones>in winter,and there were obvious seasonal differences.Field generation and terrestrial transport were important sources of CH₄in water body.The average N₂O concentration in the surface layer was 5.52±0.50,7.11±0.84 and 7.49±0.42 nmol/L,and the average N₂O concentration in the bottom layer was 5.62±0.53,7.20±0.24 and 7.62±0.45 nmol/L,respectively.The low DIN level in the water in the bay leads to the overall low N₂O concentration,and the factors affecting N₂O in the water are complex.According to the diurnal variation of CH₄and N₂O concentrations,CH₄and N₂O concentrations are significantly negatively correlated with tide,and CH₄and N₂O concentrations in water are the result of comprehensive physical and chemical processes in water.The saturation ranges of CH₄（N₂O）in summer,winter and summer breeding areas are 3450±989%（100±9%）,683±466%（88±11%）and 2522±1491%（129±7%）,respectively.The water Of Xincun Bay is in supersaturated state,which is the source of atmospheric CH₄.In summer and winter,most regions of N₂O are unsaturated,and in summer,N₂O is low saturated,which may be the sink of atmospheric N₂O.CH₄is generally in a supersaturated state,which is the source of atmospheric CH₄.Most N₂O stations in summer and winter are in an unsaturated state,and the breeding areas in summer are in a low saturation state,which is not necessarily the source of atmospheric N₂O.In this paper,according to the LM86（W92,RC01）formula,the average release flux of CH₄in summer,winter and summer breeding areas was estimated as follows:12.01±7.59（71.32±78.32、212.97±284.91）、0.94±0.74（3.63±4.03、14.47±10.85）and 9.06±8.31（67.82±86.15、218.71±297.34）μmol/m²·d,The average release fluxes of N₂O are:0.01±0.09（0.10±0.53、97.96±162.35）、-0.07±0.12（-0.31±0.70、11.65±9.09）and 0.35±0.26（2.80±3.78、137.2±226.06）μmol/m²·d.It is estimated that the annual CH₄and N₂O transported to the atmosphere by the whole lagoon are 8.23×10³-1.52×10⁶and-2.72×10³-6.52×10⁵mol/a.At E2 and E3,the CH₄release flux at sediment-atmosphere interface is-0.058±0.095μg/（m²·h）)and 0.35±0.094μg/（m²·h）),and the N₂O release flux at sediment-atmosphere interface is 0.032±0.069μg/（m²·h）)and-0.0046±0.0034（μg/（m²·h））,respectively.The area with higher vegetation coverage in Xincun Bay has a absorption effect on CH₄,while the plain zone emits CH₄and N₂O is the opposite.However,in general,the release fluxes of CH₄and N₂O at the sediment-atmosphere interface are low.The concentration range（average）of CH₄in sediment interstitial water at stations E1,E2 and E3 is:0.59-1.07（0.82±0.16）、0.57-0.68（0.61±0.043）and 0.28-0.65（0.51±0.14）μmol/L,N₂O concentration range（average）are:0.09-0.1（0.095±0.0024）,0.09-0.1（0.093±0.0015）and 0.05-0.1（0.085±0.02）μmol/L,the concentrations of CH₄and N₂O were E1>E2>E3.The content of CH₄in sediments is mainly affected by OM and sulfate,while N₂O is mainly affected by OM and nutrients.However,due to the complex wetland environment,vegetation coverage,hydrostatic pressure and human activities will affect the distribution and release of CH₄and N₂O in sediments in different ways.SO₄^2-、NO₂^-、NO₃^-have certain inhibitory effect on CH₄.