Response Of Soil Organic Carbon Mineralization To Changes In Water Conditions In Coastal Wetlands Of The Yellow River Delta

Coastal wetlands are one of the world’s significant blue carbon sinks and play an important role in the global carbon cycle.As hydrological conditions control the structure and function of wetlands,making changes in moisture conditions can not only alter the accumulation of organic carbon in wetland soils,but also affect the carbon emission processes and thus the carbon balance of coastal wetlands.Coastal wetlands are highly susceptible to changes in moisture conditions in the context of sea level rise.Therefore,it is important to study the mechanism of the change of moisture conditions in coastal wetlands on the soil carbon mineralization process in wetlands to understand the soil carbon cycling process in coastal wetlands of the Yellow River Delta in the context of global climate change.In this study,the response characteristics of organic carbon mineralization in FPW（Freshwater restored Phragmites australis wetlands）and NPW（Non-flooded Phragmites australis Wetlands）,TPW（Tidal Phragmites australis Wetlands）and TSW（Tidal Suaeda salsa wetlands）in the Yellow River Delta were analyzed under the water level decline scenario.Through indoor simulation experiments,the influence pattern of soil organic carbon mineralization in three types of salt marsh wetlands under freshwater input was revealed,and the response characteristics of organic carbon mineralization to salinity stress in freshwater restored reed wetlands under brackish and semi-saline water intrusion.The results of the study showed that:（1）The soil organic carbon,dissolved organic carbon,readily oxidizable organic carbon,microbial biomass carbon,all decreased substantially within the 0～25 cm depth soil layer after the water table drop.The water table drop affected the profile distribution of dissolved organic carbon,but had little effect on the profile distribution of the remaining three carbon components.The decrease in water level caused different salt ions in the soil to move in different directions,with an overall increase in K⁺,Mg²⁺,Ca²⁺,SO₄^2-in 0～25 cm depth soil,a significant increase in the surface layer（0～10 cm）and little change in the deeper layer（10～25 cm）.Na⁺and Cl^-,by contrast,decreased as a whole,with the water table dropping changing their profile distribution characteristics,converging with the above four salt ions.The higher C/N and higher EC of the soil following the fall in the water table will be detrimental to the mineralisation and decomposition of microorganisms,and the higher percentage of clay particles will be more beneficial to the conservation of soil organic carbon.The effect of lower water levels led to a significant decrease in soil microbial biomass（P<0.05）,with significant decreases in bacteria,fungi,gram-negative bacteria,gram-positive bacteria and actinomycetes（P<0.05）.（2）The average rate and cumulative carbon mineralisation in the tidal reed wetlands following freshwater input was significantly higher and longer lasting than in the tidal saline alkali pond wetlands and non-flooded reed wetlands.The latter two types of wetlands showed similar rates of carbon mineralisation in response to freshwater inputs,and similar values of cumulative carbon mineralisation.The differences in carbon mineralisation rates and cumulative carbon mineralisation between the three wetlands did not show a correlation with salinity,but were more likely to be influenced by changes in water table status and the level of dissolved organic carbon in the soil.Freshwater input reduced the relative abundance of formerly dominant species in higher salinity tidal reed wetlands and tidal saline alkali pong wetlands,increasing species diversity,while expanding the relative abundance of formerly dominant species in lower salinity non-flooded reed wetlands,decreasing species diversity.Changes in functional gene abundance suggest that freshwater input reduced carbon fixation,starch hydrolysis and vanilloid catabolism and enhanced CH₄ oxidation in the three wetlands.Soil enzyme activities were not significantly different before and after freshwater input（P<0.05）.（3）Under both 35 ppt and 13 ppt Na Cl solution incubations,the carbon mineralisation rate of FPW soils showed a sharp increase followed by a decrease and then a plateau.The accumulation of soil salinity in the 35 ppt incubation group was substantially higher than that in the 13 ppt Na Cl solution group,and the cumulative mineralisation and CH₄ emissions were only slightly higher than those in the latter group,suggesting that 35 ppt Na Cl solution group may have a better inhibitory effect on carbon mineralisation in FPW,but the difference was small.The effects of 35 ppt and 13ppt water invasion on soil microbial communities were similar,with both changing the structure of the original soil microbial community to a greater extent,as evidenced by the relative abundance of the original soil dominant species,increasing the species richness of the microbial community,and promoting the soil nitrification process.The13 ppt Na Cl solution group was stronger than the 35 ppt Na Cl solution group in all of these aspects.In addition,the invasion of Na Cl solution weakened the autotrophic carbon fixation process and reduced the soil metabolic pathway,with a greater decrease in the 13 ppt Na Cl solution group,indicating that the salinity had a stronger inhibitory effect on the microbial metabolic pathway and carbon fixation process.β-glucosidase and phenol oxidase activities increased in both salinities.