The Temporal And Spatial Dynamics Of Greenhouse Gases Emissions And Controlling Factors From Coastal Saline Wetlands In Jiangsu Province,Southeast China

Coastal wetlands account for a small proportion of the land surface,but contain much of the carbon in the soil,sediments and detritus ui'these waterlogged areas.At the same time,coastal wetlands are recognized as major contributors to atmospheric greenhouse gas?GHG:CO₂,CH₄,and N₂O?emissions due to the strictly anaerobic environment.Although coastal wetlands play an important role in governing the atmospheric concentration of CO₂,CH₄,and N₂O,thus controlling the global warming,however,research of the greenhouse gas emissions conducted in the coastal wetlands were not well-documented because of the difficulty in fieldwork in these harsh environment,and complicated controlling factors affecting the greenhouse gas emissions.The core region of Yancheng wetlands National Nature Reserve,rare bird?YNNR?,was selected as the research area after the careful field investigation on the Jiangsu coastal wetlands.In this dissertation,the CO₂,CH₄,and N₂O fluxes are investigated simultaneously in the intertidal zone within the core region of YNNR from August 2012 to April 2014,using the closed static dark chamber method.This study aims to?1?quantify and compare the temporal and spatial variations of CO₂,CH₄,and N₂O emissions under different hydrological conditions and vegetation covers?BF:Bare mud Flat;SAF,Spartina alterniflora Flat;SGF,Suaeda glauca Flat;PAF,Phragmites australis Flat;STF,Scirpus triqueter Flat?;?2?assess the effect of tidal,invasive species,and LUCC on the changes of GHG emissions specially.Based on these,we also analyze the global warming potential?GWP?of coastal wetlands and whether the wetlands presented to be the source or the sink.The main conclusions were summarized as follows:1.Significant spatial-temporal heterogeneity of GHG emissions were observed.The diurnal variations of CO₂ fluxes presented to be bimodal distribution in different growth-stage,and the peak was found in the moment of air temperature rising?6:00am or 9:00am?and falling?18:00?sharply.At noon,transitorily lower CO₂ fluxes occurred,and the lowest CO₂ values were observed early in the morning.Compared to the CO₂ fluxes,the peak of diurnal N₂O fluxes were also appeared to be the moment of dramatic changes,the difference was that the diurnal variations of N₂O fluxes not only presented to be bimodal distribution in October,December,and April,but also unimodal distribution in June.Significant differences?P>0.05?of mean N₂O fluxes were not existed in the different growth-stage.No obvious regularity was found in the diurnal variations of CH₄ fluxes,except for the June,the mean night-time CH₄ fluxes were higher than those from the day-time CH₄ fluxes in the other growth stages of S.alterniflora.In the metaphase and vigorous stage of S.alterniflora,the SAF presented to be the source of CH₄,however,in the preliminary and non-growing stage,the SAF was the sink of CH₄.The seasonal variations of CO₂ fluxes were highest in summer?588.811 mg m-2 h-1?,and lowest in winter?136.541 mg m-2 h-1?,in the spring and autumn seasons,the CO₂ fluxes were centered?305.846 and 370.193 mg m-2 h-1,respectively?.The Yancheng coastal wetlands presented to the weak sink of CH₄ in spring?-0.02 mg m-2 h-1?,and then shifted to the source in summer?0.250 mg m-2 h-1?,the peak occurred in autumn?0.364 mg m-2 h-1?,subsequently,the CH₄ fluxes began to decrease in winter?0.082 mg m-2 h-1?.The mean seasonal N₂O fluxes were 10.100,10.592,8.063 and 13.413 ?g m-2 h-1 in spring,summer,autumn,and winter seasons,respectively.The higher annual cumulative precipitation and lower annual mean temperature in 2012?2013 caused the lower CO₂ fluxes and higher CH₄ fluxes than those in 2013?2014.Nevertheless,with respect to the different vegetation-dominated flats,the inter-annual CH₄ fluxes were determined by hydrological conditions caused by the precipitation and soil moisture,in SAF and SGF,the higher precipitation cause lower CH₄ emission,but for PAF and STF,the higher precipitation resulting to the higher CH₄ fluxes.The inter-annual N₂O fluxes were higher in 2013?2014 than those in 2012?2013 in SAF and PAF,but for SGF and STF were observed the opposite tendency.The spatial variations of CO₂ and N₂O fluxes were present to the 'M' from the sea to the inland.The peak of CO₂ fluxes were observed in SAF,but for N₂O,the peak was in PAF.Significant differences of CH₄ fluxes in different vegetation covers were observed,although the CH₄ fluxes were almost maintain the zero fluxes in the whole tidal flats.The higher fluxes were observed in PAF,in STF,SAF,and SGF,were medium,and appeared to be the source of CH₄,however,BF was the sink of CH₄.2.The vegetation types and the development degree of aerenchyma have significant role in governing the GHG emissions.In the preliminary and vigorous stages of S.alterniflora,the CO₂ fluxes without aboveground vegetation were reduced by 41.2%and 33.3%,compared to those with aboveground vegetation.For N₂O fluxes,65.3%and 92.5%.In the non-growing seasons,cutting-out the aboveground vegetation increased the CO₂ fluxes,but reduced N₂O fluxes by 36.6%.Same to the S.alternilfora,in the preliminary and vigorous stages of Phragmites,cutting-out the aboveground vegetation reduced the CO₂ fluxes by 43.4%and 80.7%,and the N₂O fluxes by 67.4%and 40.2%.In the non-growing seasons of Phragmites,the CO₂ fluxes maintained the same,but the N₂O fluxes reduced by 34.9%.In addition,cutting-out the aboveground vegetation of S.glauca,the CO₂ fluxes reduced by 85.3%in the preliminary stage,and in the vigorous stages,kept the steady,for non-growing seasons,increased the CO₂ fluxes.The N₂O fluxes always higher than those with vegetation covers,no matter what growth stages of the S.glauca.The effect of cutting-out vegetation on CH₄ fluxes presented to be more complicated:in the preliminary stage,the aerenchyma has been cut off and the substrate for producing CH₄ was removed due to the vegetation cutting-out,thus the CH₄ fluxes shifted from the emission to the uptake.In the vigorous and non-growing stage,the soil was covered by the abundant litter,once the vegetation cut out and the litter removed,CH₄ produced in the surface soil was released to the atmosphere momently.3.The NPP of S.alterniflora was estimated to 31.32 t ha-1 a-1 via combining the ANPP?aboveground net primary production?,BNPP?Belowground net primary production?and annual SOC increment in the soil.The annual CO₂ and CH₄ fluxes was calculated to 66.438 ? 8.945 t ha-1 a-1 2.719 ± 7.140 kg ha-1 a-1 and NEE of CO₂ was estimated to-22.7 t ha-1a-1,indicating the S.Alterniflora-dominated flats could sequester 22.7 t CO₂ ha-1 a-1.The area of S.alterniflora in Jiangsu and China coastal wetlands were estimated to be 24 047.67 ha,and 53 000 ha,respectively.Consequently,the carbon storage in Jiangsu and China coastal wetlands were estimated to be 148 877 and 328 118 t.4.No obvious correlations existed between GHG emissions and tidal elevation.And CO₂ and N₂O fluxes were significantly reduced due to the water flooding.The effect of water flooding on the CH₄ was more complicated,it was determined by the types of vegetation,development degree of aerenchyma,the period of water flooding and temperature.5.LUCC also played an important role in controlling the GHG emissions.Take the rice paddy for example,the annual CO₂,CH₄,and N₂O fluxes were calculated to be 25.900 ? 4.420 t ha-1 a-1,79.486 ? 114.592 kg ha-1 a-1,1.872 ? 0.422 kg ha-1 a-1,indicating that when natural wetlands converted to constructed wetlands,the CH₄ and N₂O fluxes increased.6.The seasonal variations of GWP were higher in summer?2.975 kg ha-1 s-1;54.6%?,followed by autumn?1.175 kg ha-1 s-1;21.6%?,in winter and spring,the GWP was lowest?11.6%? 12.2%?.The spatial variations of GWP were presented to be'M' pattern,with the peak occurred in SAF and PAF.The Yancheng coastal wetland was the sink of GHG,and Phragmites could fix the highest CO₂,17.97 t CO₂ ha-1 a-1,the lowest was found to be STF.The higher CO₂ fixed and emitted together resulted to the lower carbon storage,compared to Phragmites.The results in this paper provide the norms,continuous,and systematic monitoring data for evaluating the GWP in Jiangsu coastal wetlands,and theoretical guidance for the establishment of fluxes station in Jiangsu.Besides,our results here also fill the blank research area of silt muddy tidal flat,and further provide the data support and case analysis to evaluate the source or sink in China coastal wetlands.In addition,we can also offer scientific basis and management measures for mitigating the greenhouse emissions from coastal wetlands.