| Eutrophication in coastal waters and global climate change are two prominent environmental problems that human society is facing,and they have been widely concerned by the international community.The coastal wetlands are located at the intersection of land and sea,and massive nitrogen-rich water enter the wetlands periodically due to the influence of tides,surface runoff and other factors.In addition,due to the rich organic matter in the wetland sediment,the wetlands have a high CH4production and emissions.Therefore,to explore the nitrogen removal and CH4emission process,as well as the influencing factors,could help us better understand the significant of coastal wetlands for eutrophication of coastal waters and global climate change.In this paper,we took Fengxian wetland in Shanghai as the study object,studied the removal mode of DIN,the dominant factor of eutrophication and the emission flux of CH4 of the coastal wetland through the field monitoring and control experiments.Besides,we analyzed the effects of tide,salinity,plants,and explored the correlation between CH4 emission flux and denitrification.The main conclusions are as follows:(1)Through situ simulation of tidal process in Fengxian wetland,monitoring theconcentration change of overlying water and pore water,as well as in-situ measurement of denitrification rate of wetland sediment by calcium carbide acetylene inhibition method combined with Picarro portable greenhouse gas analyzer,we think the removal mode of DIN in tidal water by wetland is"diffusion+denitrification".During the experiment,the main component of DIN in sea water is NOx?(the concentration is about 1.2mg/L).In the process of rising tide,NOx?entering the wetland along with the tide is intercepted by the wetland and enters the pore water of sediment through diffusion,resulting in the concentration of NOx?rising to about 0.25mg/L in the pore water;after ebbing,NOx?in the interstitial water is mainly removed through denitrification.Before the next high tide,the concentration of pore water in the sediment decreased until the seawater brought new nitrogen input during the high tide.(2)According to the salinity characteristics of the coastal salt marsh wetland in Hangzhou Bay area,four salinity gradients of 0,2‰,5‰and 10‰were set up in this experiment,including small-scale experiment,greenhouse simulation experiment and field simulation experiment.The experimental results show that in the range of 0-10‰,the removal rate of DIN is obviously inhibited by salinity.When the salinity is10‰and the concentration of NOx?in the overlying water is 1.0 mg/L,the removal rates of NOx?in 12 hours are 31%,43%and 66%respectively.When the salinity is 0,the removal rates of NOx?are 44%,52%and 81%respectively.The removal rates of NOx?in the small-scale experiment,as well as greenhouse simulation experiment and field simulation experiment are 10‰group<5‰group<2‰group<0.In the range of 0-10‰,the denitrification rate of wetland decreased with the increase of salinity.The results show that the salinity has a significant inhibition on denitrification of wetland sediments.When the salinity is 10‰,5‰,2‰and 0,the denitrification rates are 61?mol/(m2.h),72?mol/(m2.h),79?mol/(m2.h)and98?mol/(m2.h),respectively.(3)There is obvious seasonal difference in methane emission flux of wetland,which is the highest in summer and the lowest in winter.The continuous monitoring of Fengxian wetland shows that the maximum CH4 emission flux appears in August in summer,which is 0.36?mol(m2.s),and the minimum CH4 emission flux appears in January in winter,which is 0.11?mol(m2.s).The monitoring results of 12 months show that CH4 emission flux appears in summer>autumn>spring>winter.There is obvious spatial difference in CH4 emission flux of wetland.Different flux levels are detected in the bare land and vegetation belt of Fengxian wetland,which is manifested in that the CH4 emission flux of bare land is slightly higher than that of vegetation belt under the condition of higher temperature,and the flux level of vegetation belt is significantly higher than that of bare land under the condition of lower temperature.(4)Through LGR portable greenhouse gas analyzer,CH4 emission flux of Fengxian wetland was monitored in situ,and the results showed that the flux was obviously"U"type.At the beginning of rising tide,CH4 emission flux decreased rapidly,and the concentration was lower than 0.1?mol/(m2.s).With the constant change of water level in wetland,CH4 emission flux fluctuated slightly,and recovered rapidly after ebbing,approaching the discharge level before rising tide.(5)The existence of the denitrifying methane anaerobic oxidation(DAMO)process was determined by lab control experiments,field data comparison of CH4 emission flux and denitrification rate,and macrogenomic sequencing.The results of this study provide a theoretical reference for a better understanding of the role of coastal salt marsh wetland in eutrophication and global temperature rise.Through comprehensive and continuous monitoring,research and the discovery of DAMO process in Fengxian wetland,it provides a data reference and early support for other research in this area. |
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