Soil Extracellular Enzyme Activities Along The Salinity Gradient Tidal Marshes Of The Min River Estuary,southeast China

To reveal the changes in the activities of soil extracellular enzymes in estuarine tidal marshes,we have investigated soil and plant properties,soil extracellular enzyme activity,soil carbon mineralization and microbial community structure along with the salinity gradient in the Cyperus malaccensis tidal marshes of the Min River estuary,southeast China.The results showed that:（1）The soil p H gradually increased along with the salinity gradient.The aboveground biomass at freshwater sites was 2～3 times than that at brackish sites,whereas the trend was opposite for the belowground biomass.The highest biomass of the aboveground and belowground both appeared in summer.Plant iron plaque increased with salinity,which indicates that salinity increases root respiration in marsh plants.The concentration of SO₄^2-,NH₄⁺,and NO₃^-in pore water were higher at offshore sites than inland sites.The concentrations of DOC decreased.The concentrations of DOC,NH₄⁺,and NO₃^-all showed peak values in spring.（2）The contents of TOC,TN,and TP in the soil all showed lower contents at the higher salinity sites than at freshwater sites.However,the contents of MBC,MBN,and MBP in soil increased significantly.All activities of six extracellular enzymes had peak values at the brackish water site.The activities ofβ-1,4-glucosidase（BG）,cellobiohydrolase（CBH）,phenol oxidase（PHO）,peroxidase（PEO）,β-1,4-N-acetylglucosamine（NAG）and acid phosphatase（AP）increased by 214%,255%,370%,170%,316%,and 308%,respectively along with the salinity gradient.The increase in activities of soil extracellular enzymes was not consistent across different seasons.the highest activities appeared in the spring and summer.（3）The cumulative mineralization curves of soil CO₂ and CH₄ both showed a rapid increase during the first（0～10 d）and then tended to be stable（10～25 d）in anaerobic culture experiment.which was in line with the first-order dynamic model.Salinity and season variations had significant effects on maximum production potential（M₀）of soil carbon mineralization.M₀ was positively correlated with the above-and belowground biomass and soil extracellular enzymes（BG,CBH,PHO,PEO,NAG,and AP）,while negatively correlated with TOC and MBC contents.With the increase of salinity in summer,the relative abundance of Proteobacteria in the dominant soil bacterial community increased by 19.48 percent,while Chloroflexi and Nitrospirae increased first and then decreased.The relative abundance of Euryarchaeota in the dominant archaea community gradually decreased with the increase of salinity,while Crenarchaeota and Thaumarchaeota showed an increasing trend.Combined,the results indicated that the increasing salinity leads to higher activities of extracellular enzymes and the promotion of soil carbon mineralization in tidal marshes of the Min River estuary,southeast China.According to the results,saltwater intrusion caused by future sea level rise will lead to a large loss of carbon pool in tidal marshes and wetlands of freshwater,which will further reduce the blue carbon pool of tidal marshes and enhance emissions of greenhouse gas.