| Climate change-associated sea level rise and the impact of large wading projects have further intensified saltwater intrusion in the Yangtze estuary.Increased saltwater intrusion will change a series of biogeochemical processes such as soil microbial respiration and carbon degradation,affecting the carbon cycle of the community.The effect of saltwater intrusion on CH4 biochemical reactions and related microorganisms is a hot scientific issue in related fields such as environmental science and microbial ecology.Based on the National Natural Science Foundation of China?NO.41473094?"Effects of Estuary Wetland Vegetation on the Production and Emission of CH4 and N2O and Its Control Mechanism",the Yangtze estuary was selected as a typical research area.Combined research was conducted on 1)CH4 production and emission characteristics of the salinity gradient sediments in the Yangtze estuary;2)the effect of saltwater intrusion on organic carbon content and decomposition in estuarine wetlands;3)correlation between CH4-related microorganism and net CH4 production rate in response to saltwater intrusion.The main conclusions are as follows:?1?Throughout the study period,the pore water CH4 concentration in all sediments varied between 1.4×10-3 and 515.54 mg·L-1.Under the dual influence of intensified salinity intrusion and lower temperature in winter and spring in the Yangtze estuary,the temporal variation characteristics of pore water CH4 in sediments were high in summer and low in winter.In terms of spatial variation,the pore water CH4 in Wusongkou was the highest,while it in Qidong was lowest.Pore water CH4 concentration gradually increases to a stable level with the depth,and there was a jump increase at the corresponding depth.It was shown that the jump depth in winter was deeper than that in the other three seasons.Vegetation cover increased the pore water CH4 concentration in sediments.The CH4 diffusion flux at the sediment-atmosphere interface varied from0.03 to 4235.65 mg·m-2·day-1.The methanogens rate of the methane generation layer in the tidal flat sediment ranged from 0.79 to 1.67×104 mg·m-2·day-1.The temporal and spatial characteristics of the two were similar to the temporal and spatial characteristics of CH4 concentration in sediment pore water.Pearson correlation analysis showed that there was a significant negative correlation between CH4concentration in pore water and water content,NO3--N content and SO42-content?P<0.05?.Pearson correlation analysis showed a significant positive correlation between sediment-atmosphere interfacial CH4 diffusion flux and surface pore water CH4concentration?P=0.000?.Pearson correlation analysis results showed that there was a significant negative correlation between CH4 production rate and salinity in the production layer?P<0.05?,indicating that increased salinity could significantly inhibit CH4 production in sediments.There was a significant negative correlation between the CH4 diffusion flux at the sediment-atmosphere interface and salinity?P<0.05?,indicating that increased salinity could significantly inhibit CH4 emissions in sediments.There was no significant correlation between oxidation rate and salinity in CH4concentration mutation layer?P>0.05?,indicating that salinity was not the main factor influencing the CH4 oxidation of surface sediments in this study.?2?The organic carbon content and carbon storage level of the sediments in different salinity areas of the Yangtze estuary were higher in the sediments covered by vegetation and lower in the bare mudflat.In five salinity treatments experiment of salt water intrusion,the organic carbon content and carbon storage level of the sediment showed a change characteristic of 0‰>5‰>10‰>20‰>30‰?P<0.05?.The surface organic carbon content and carbon storage level were higher than those in the bottom layer.In the field implantation experiment,different transplant environments had a significant effect on the organic carbon and carbon storage of sediments?P<0.05?,and the organic carbon and carbon storage of sediments in the experimental group?Nanhui-Wusongkou?were significantly higher than those in the control group?Nanhui-Nanhui?.The sum production of CO2 and CH4 was used to characterize the decomposition loss of organic carbon in sediments.Seasonal differences of sum production of CO2 and CH4 in different sediments areas of the Yangtze estuary was significant?P<0.05?,and the highest sum production of CO2 and CH4 was in summer,reaching 5.87±0.72 mg,while lowest in winter.The spatial difference was significant?P<0.05?,the highest in Xitan and the lowest in Nanhui bare mudflat.In the natural environment,CO2 was the main organic carbon decomposition product,and the proportion reached more than 99%.In five salinity treatments,the sum production of CO2 and CH4 at each salinity level showed a change gradient of 0‰<5‰<10‰<20‰<30‰?P<0.05?.CO2 was the main organic carbon decomposition product,reaching more than 99%,and as the salinity increases,the proportion of CO2 also increases.In the field implantation experiment,the sum production of CO2 and CH4 in the surface and bottom sediments of the experimental group was higher than that of the control group.Pearson correlation analysis results showed that in the natural environment,there was a weak negative correlation between sediment organic carbon content and salinity,but the correlation was not significant?P>0.05?,while in the indoor experiment,there was a significant negative correlation between sediment organic carbon content and salinity?P<0.05?.The relationship between carbon storage and salinity was similar to that of organic carbon.There was a slight positive correlation between sum production of CO2 and CH4 and salinity in sediments under natural environment,but the correlation was not significant?P>0.05?.However,in five salinity treatments,there was an extremely significant positive correlation between the sum production of CO2 and CH4 and salinity?P<0.05?,indicating that with the further intensification of saltwater intrusion and the further intensification of organic carbon decomposition,the carbon storage of tidal shoal would significantly decrease.?3?There was a consistent response in both the number of methanogens and methanooxidizing bacteria and the net rate of CH4 production in the sediments.Seasonal differences in the net CH4 production rate in different salinity sediments of the Yangtze estuary were significant?P<0.05?,highest in summer and lowest in winter.In terms of spatial differences,it was highest at Wusongkou and lowest at Nanhui bare mudflat.Vegetation can significantly increase the net CH4 production rate in sediments?P<0.05?.In five salinity experiments,the net production rate of CH4 showed a change law of 0‰>5‰>10‰>20‰>30‰?P<0.05?.In field experiments,the net CH4rate production in the surface and bottom sediments of the experimental group was higher than that of the control group In five salinity experiments,the number of methanogenic bacteria was 1.09×1061.75×106 copies·g-1,and the number of methanogenic bacteria was 1.00×1051.65×106copies·g-1.As the salinity gradient increases,the absolute numbers of methanogens and methanooxidizing bacteria continued to decrease.In the field implantation experiment,the number of methanogens and methanooxidizing bacteria in the experimental group buried in Wujingkou increased compared with the control group buried in Nanhui.Pearson correlation analysis results showed that there was an extremely significant negative correlation between the net CH4 production rate in sediments and salinity under both natural environment and five salinity experiments?P=0.000?,indicating that increased salinity could significantly inhibit the net CH4 production rate.Besides,pearson correlation analysis showed a significant positive correlation the net CH4 production rate and the organic carbon content?P=0.000?,indicating that salinity can influence the substrate and energy required for CH4 production through the content of organic carbon in sediments,and ultimately affect the net CH4 production rate in sediments.Pearson correlation analysis showed a significant positive correlation between the number of methanogens and the organic carbon content?P=0.002?.It was shown that the decrease of organic carbon content caused by intensified saltwater intrusion will affect the number of CH4-related microorganisms,especially methanogens,thereby inhibiting the net production of CH4. |
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