Effects Of Exogenous Nitrogen Input On Flux Characteristics Of Volatile Haloalkanes In Soil And Plants Of Coastal Salt Marsh

Volatile haloalkanes（VHCs）are important trace greenhouse gases and organic pollutants commonly existing in the atmosphere.They can produce halogen free radicals through chemical reactions in the atmosphere,destroying the ozone layer and the greenhouse effect,and have great influence on global climate change.The coastal salt marsh is located in the transitional zone between land and sea.It is an important place for the production and mutual transformation of VHCs,and an effective blue carbon sink to mitigate global warming.In recent years,with the rapid development of urban industry,the massive burning of fossil fuels,and the overuse of nitrogen-containing fertilizers,a large amount of nitrogen has entered the coastal salt marsh wetland,posing a serious threat to the stability of the salt marsh ecosystem and its carbon sink function.The input of a large amount of exogenous nitrogen can cause changes in soil physical and chemical properties,microbial community structure and function,and enzyme activity,affect the growth of plants,change the structure and function of the salt marsh ecosystem,and then affect the emission flux of VHCs in the coastal salt marsh ecosystem.This study uses a combination of indoor simulation experiments and pot culture experiments to explore the emission flux characteristics and influencing mechanisms of CH₃Cl,CH₃Br,CH₃I,and CCl₄ in coastal salt marsh soil and Suaeda salsa under different concentrations of nitrogen application conditions.The main results are as follows:（1）In soil simulation experiments,under the four nitrogen treatment gradients of CK,1.5N0,2.0N0,and 3.0N0,the soil showed the net source of CH₃Cl and CCl₄,and both as a sink of CH₃Br and CH₃I.Different concentrations of exogenous nitrogen application have different effects on the emission flux of soil halomethane.Compared with CK,1.5N0inhibited the production of CH₃Cl and CCl₄ in soil,and promoted the absorption of CH₃Br in soil;2.0N0 and 3.0N0 promoted the emission of CH₃Cl and CCl₄ in soil,as well as the absorption of CH₃I in soil,and inhibited the absorption of CH₃Br in soil.（2）In the soil simulation experiment,the application of exogenous nitrogen can change the soil nutrient conditions,promote the accumulation of soil ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,soil total organic carbon content（SOC）.In the four different nitrogen treatment groups,the content of humus was the highest,the easily degradable small molecular aromatic protein was the second,and the soluble microbial by-products were the smallest.In addition,nitrogen addition promoted the transformation of SOC into dissolved organic matter（DOM）and accelerated the humification process of organic matter.The emission fluxes of CCl₄,CH₃Cl,and CH₃Br in the soil were more significantly affected by the content of NH₄⁺-N and SOC in the soil,while the CH₃I flux in the soil was more significantly affected by soil p H and NO₃^--N content.（3）In soil simulation experiments,the application of exogenous nitrogen increased the diversity,richness,and quantity of OUTs of soil microorganisms.The dominant microbial communities of soil microorganisms in different gradient nitrogen treatment groups were consistent at the level of phylum and genus in the early and late culture stages.The four dominant bacterial phyla were Proteobacteria,Bacteroidetes,Actinobacteria,and Gemmatimonadetes.The dominant genera were Marinobacter and Methylophaga.Overall,the communities with a higher abundance of microbial communities in different nitrogen treatment groups were mainly related to carbon and nitrogen cycle,nutrient decomposition and absorption,and degradation of animal and plant residues.This also promotes the emission of soil CH₃Cl and CCl₄ in the 2.0N0 and 3.0N0 treatment groups,as well as the absorption of CH₃I in soil.However,the promotion effect of the 1.5N0 treatment group was not significant,which may be affected by the non-biological production and degradation of halomethane in the soil.（4）In the plant culture experiment,Suaeda salsa was the source of halomethane under the three nitrogen treatment gradients of CK,1.5N0,and 3.0N0.The effect of different concentrations of nitrogen application on the flux of halomethane emissions from Suaeda salsa was not consistent.Compared with CK,1.5N0 promotes the emissions of CH₃Br and CH₃I,while 3.0N0 promotes the emissions of four halomethanes.Under different nitrogen treatment groups,the peak methane emission of Suaeda salsa in the whole growing season appeared in the early and late stages of seed setting,while the flux during the early and withered stages was relatively small.（5）The application of exogenous nitrogen promoted the growth of Suaeda salsa in the plant culture experiments.The fresh weight,dry weight,dry weight of different parts of roots,stems,and leaves,plant height,and root length of Suaeda salsa increased with the increase of nitrogen application concentration,which was most obvious in the late fruiting stage.There were positive correlations between the CH₃Cl,CH₃Br,CH₃I,and CCl₄emission fluxes of Suaeda salsa and growth indexes such as fresh weight,dry weight,plant height,and root length.Overall,the dry weight,plant height,and root length of Suaeda salsa were important factors affecting the halomethane emission flux.Among them,the biomass of Suaeda salsa leaves has a more significant impact on the halomethane emission flux than the total biomass.（6）In plant cultivation experiments,the application of nitrogen changed the dynamic cycle of carbon and nitrogen between Suaeda salsa and soil,promoting the accumulation of inorganic nitrogen,SOC content,and DOM components in the soil,and thus affecting the release of halomethanes in the potted system of Suaeda salsa.Among them,DOM fluorescence components in the root soil of Suaeda salsa potted in salt land in different nitrogen treatment groups were all composed of five fluorescence peaks,and the order of fluorescence peak intensity was the same,and the DOM fluorescence components of humus-like substances accounted for the highest proportion.The emission flux of halomethane from the salt marsh Suaeda salsa potted system is influenced by the interaction of multiple factors.In addition to the halomethane emissions from the salt marsh Suaeda salsa itself,the effects of soil NH₄⁺-N,NO₃^--N,and SOC on the CCl₄ and CH₃Br emissions from the salt marsh Suaeda salsa potted system were also significant.