Production,Dissolution And Emissions Of Nitrous Oxide From Reclaimed Shrimp Ponds In The Two Main Estuaries,Fujian

Aquaculture water which has massive amount of nutrient substances could release potentially N₂O,a serious greenhouse gas.The increasing of N₂O will bring about global warming which is a major global environmental issue at present.The characteristics of N₂O production,dissolution,emission and its influence mechanism in aquaculture ecosystem of coastal reclamation were studied in this paper in order to compile emission inventory of greenhouse gas for coastal wetland ecosystem,and control greenhouse gas emission theoretically.In recent years,the related research has caused the widespread concern of domestic and foreign scholars,but there is still lack of systematic,some key scientific problems have not been deeply revealed.Based on these above,this study selected the reclamation shrimp ponds of main estuary wetland?Min River and Jiulong River?of Fujian as the object,taking N₂O as the main line of research.This article plans to carry out researches from the aspects as follows:the emitting ways and fluxes of N₂O across the sediment-water interface and water-gas interface in shrimp ponds,characteristics of spatial-temporal change and influenced factors of dissolved N₂O concentration and saturation,the source and procedure of N₂O in the shrimp ponds,through in-situ observation,sample collection,laboratory analysis and simulation experiment.The main conclusions showed as follows:?1?There is no significant difference of N₂O float-tank fluxes in the shrimp ponds which are both N₂O emitting sources between the Min River estuary and the Jiulong River estuary.The N₂O fluxes across the water-air interface of the shrimp ponds in the Min River estuary were mainly affected by the air temperature and water temperature.It showed that the N₂O flux at middle aquaculture stage was significantly higher than the initial and final stage.Unlike the Min River estuary,the N₂O flux of Jiulong River estuary were mainly affected by the water salinity,NH₄+^-N and NO₃^--N concentrations,and the flux at the final stage of aquaculture was significantly higher than the initial and middle stage.In addition,the N₂O fluxes across the water-air interface of the two estuaries appeared to be a certain daily dynamic characteristic,with a certain hysteresis comparing with the temperature due to the influence of wind speed,DO,DOC,nitrogen,and Chl-a concentration.The N₂O fluxes at the water-air interface of shrimp ponds were all appeared to be N₂O emitting sources,and the N₂O diffusive fluxes at the water-air interface in the Min River estuary were extremely lower than in the Jiulong River's.The wind speed was the most impact factor of N₂O diffusive fluxes of the two estuaries.The N₂O fluxes of the three diffusive model?LM86,W92,and RC01?building for shrimp ponds of the two estuaries suggest that W92 model was near to float tank fluxes,the other two were above and below it respectively,especially the data of the Jiuong River estuary.There was a significant difference among each flux which appeared to be basically the same spatial-temporal changes.?2?The concentrations and saturations of N₂O in the shrimp ponds of the Min River estuary were extremely lower than those of the Jiulong River estuary,and both of which were all keeping a supersaturated state.The dissolved N₂O concentrations in the shrimp ponds was in the order of middle stage>final stage>initial stage in the Min River estuary,and final stage>middle stage>initial stage in the Jiulong River estuary.Though the dissolved N₂O of the shrimp ponds varied at every time during the day,there was no significant difference among change all the time.The dissolved N₂O concentration and saturation in the overlying water of the shrimp ponds of the two estuaries were various at different levels?surface,middle,and bottom?,but there was no significant difference in the vertical direction.The concentrations and saturation of N₂O in the pore water of the sediments were extremely significantly higher than that of the overlying water.The main factors influencing the temporal and spatial changes of dissolved N,O in the upper surface of the shrimp ponds in the Min River estuary included salinity,pH,Eh,concentrations of various forms of nitrogen,and Chl-a.While salinity,pH,Eh,the concentrations of C?N?P and Chl-a were making serious impact of the Jiulong River estuary.For the two estuaries,the main factors of influencing N₂O change in sediment pore water were DOC and various forms of nitrogen concentrations.?3?The measured fluxes across the sediment-water interface of the shrimp ponds in the Min River and Jiulong River estuary were 40 times higher than those of the diffusive fluxes,which means that the extra N₂O had being produced during the diffusion from the sediments to the overlying waters.Thus the lamina for N₂O generation had been becoming a hot spot.The measured N₂O fluxes across the sediment-water interface were only one half that of the floating tank fluxes across the water-gas interface.Therefore,part of the N₂O emitted from shrimp ponds was coming from overlying water.The contribution ratios of N₂O emitted from the sediments of shrimp ponds in the Min River estuary and the Jiulong River estuary were 64.96%and 66.96%,respectively.The N₂O emission of the pond water derived possibly from nitrification in the Min River estuary and denitrification in the Jiulong River estuary,respectively.The N₂O emitted from the sediment-water interface may both derive from denitrification in the estraries.?4?The N₂O in the sediments of the Min River estuary was generated from denitrification and abiotic effects,while nitrification and denitrification by nitrifying bacteria played a secondary role.Similar situation there was in the Jiulong River estuary.The discrepancy between them was as follows,for the latter N₂O production was mainly by denitrification and was strengthened by abiotic effects.pH,SOC and inorganic nitrogen content significantly affected the total N₂O production of sediments,which reached the maximum under high-temperature and low-salt conditions,and reached the minimum under low-temperature and high-salt conditions.In addition,nitrification and denitrification caused by denitrifying bacteria and nitrifying bacteria,abiotic effects were all significantly influenced by salinity,while there was no significant correlation between them with temperature.The interactions between salinity and temperature had significant influence on abiotic effects and denitrification caused by nitrifying bacteria,respectively.