Effects Of Saltwater Incursion On Rates And Pathways Of Organic Carbon Mineralization In Tidal Marsh Wetlands Of Min River Estuary, East China Sea

The saltwater intrusion caused by the rapid rise of global sea level changed the sediment geochemistry and hydrology of tidal marsh wetlands of coastal estuary,causing a series of complex changes in wetland ecosystem and had a profound influence on the global carbon cycle.This study took the tidal marsh wetlands of Min River Estuary（MRE）in southeastern coast of China as the studied object,sampling and analysis the sediment samples from the salinity gradient in MRE and a situ simulated saltwater intrusion and increasing Fe（Ⅲ）experiment.According the changes of geochemical characteristics in sediments and pore-water,and the rates and pathways（such as microbial iron reduction（FeR）,microbial sulfate reduction（SR）and methane production（MP））of organ carbon mineralization,it can be predicted how and what trend of tidal marsh wetland response to saltwater intrusion in the future.The main results and conclusions are as follows:（1）During the saltwater intrusion process,the soil density,porosity and water content of sediment in the marsh wetlands of MRE did not change significantly.The plant biomass in oligosaline wetland was about 2³ times that in freshwater wetlands.With the salinity increasing,the concentrations of pore-water SO₄^2–and NH₄⁺ increased,but the concentrations of dissolved CH₄,amorphous Fe（Ⅲ）,TOC,TN,and C:N ratio decreased with the quality of organic matter better.The iron reduction rate（FeRR）and sulfate reduction rate（SRR）were firstly controlled by water content,pH and belowground biomass,and then were the concentrations of pore-water SO₄^2– and Cl^–;the methane production rate（MPR）was mainly controlled by the C:N ratio.With the inflow of saltwater,the surface water of the tidal wetland was closed to neutral,and the plant biomass and pore-water SO₄^2–increased,which promoted the SRR（the contribution increased by 51%）;while neutral environment,strongly root activity,as well as the competition for organic substrates,and the MPR was also affected by salinity,resulted in a decrease in FeRR and MPR whose contribution decreased by 30%and 10%,respectively;however,the total of carbon mineralization rate（CMR）had not significant influence,indicating that the amount of carbon mineralization in tidal marsh wetlands of coastal estuary may not change significantly with moderate saltwater intrusion in the future.The main pathway of organic carbon mineralization in tidal marsh wetlands changed from FeR（52%）to SR（67%）.The increase of SR probably may have a positive effect for the accumulation of sulfide,but the Fe（Ⅲ）rich in marsh wetlands would relieve the toxic effect caused by sulfide to the ecosystem.（2）After the saltwater intrusion,there was no significant difference in the sediment organic matter substrate,but the concentrations of DOC decreased,and the content of non-sulfur Fe（Ⅱ）,crystalline Fe（Ⅲ）and FeS₂ in the sediment increased.From spring to summer,the pore-water SO₄^2–,non-sulfur Fe（Ⅱ）and FeS decreased by 57%,37%and41%,respectively,but amorphous Fe（Ⅲ）,crystalline Fe（Ⅲ）and FeS₂ were increased significantly.It figured that there would be have strong SR and sulfur oxidation in summer in tidal marsh wetlands of MRE.Crystalline Fe（Ⅲ）and pore-water SO₄^2–were the most important factors to FeRR and SRR,respectively.The SRR and FeRR added160%and 66%,respectively,as the concentrations of pore-water SO₄^2–and crystalline Fe（Ⅲ）increased significantly.However,when SO₄^2–and Fe（Ⅲ）simultaneously existed in a great amount in wetlands,the effect above would not be superimposed,but inhibit each other.MPR and CMR were mainly related to TOC and TN.There was no significant change in CMR,but MP was inhibited always with MPR reduced by 42⁵5%.From spring to summer,FeRR,SRR and CMR showed an upward trend.The main pathway of organic carbon mineralization in the tidal marsh wetlands of MRE changed from MP（52%）to MP（26³5%）and FeR（24⁴4%）,with SR（16²0%）also increased significantly.From spring to summer,the CMR in wetlands increased by 20%.The increased contribution of FeR was comparable with the decreased of MP,indicating that the input of river runoff Fe（Ⅲ）may offset the seasonal variation of MP after saltwater intrusion.