Effects Of Increased Flooding And Salinity On Soil Phosphorus Speciation And Microbial Phosphorus Limitation In The Tidal Freshwater Wetland Soils Of The Min River Estuary,Southeast China

In order to further understand the migration and transformation mechanism of soil phosphorus,extracellular enzyme activity and changes of microbial community structure in estuarine tidal freshwater swamp wetland under the background of sea-level rise in the future,here we established mesocosms loaded with soil from a tidal freshwater wetland of the Min River Estuary,southeast China,and subjected them to three salinity treatments（fresh control,oligohaline,mesohaline）and three flooding treatments（control,control+15 cm,control+30 cm）treatments.The basic characteristics of soil and plants were monitored,soil phosphorus speciation,soil physicochemical properties,microbial characteristics,and soil extracellular enzyme activities（EEAs）were analyzed,and microbial utilization efficiency（CUE）and microbial nutrient limitation were calculated according to the stoichiometric ratio of soil,microorganisms,and ecological enzymes.The results show that:（i）Inorganic phosphorus is the main form of soil total phosphorus.Moreover,Fe/Al-P was the main component of inorganic P,followed by Ca-P and Oc-P.Oligohaline treatment increased the contents of Fe/Al-P and Oc-P,whereas mesohaline treatment decreased these contents.As salinity increased,the TP contents decreased;however,the Ca-P contents increased.When the salinity changed from freshwater to oligohaline,a few parts of OP transformed into IP.Furthermore,when the salinity changed from oligohaline to mesohaline,the importance of IP decreased,whereas that of OP increased.（ii）Oligohaline treatment decreased the activity of NAG and AKP,whereas mesohaline treatment increased these contents.As salinity increased,the activity of BG and NAG decreased.Salinity and flooding treatments had a remarkable impact on the soil of EEA_C:N and EEA_N:P.soil EEA_C:Nand EEA_N:Pwere significantly affected by Salinity and flooding.Proteobacteria,Actinobacteria,and Gemmatimonadetes were the main dominant phyla in the microbial community.（iii）The values of Soil C:P and C:N were decreased by low salinity treatment compared with Oligohaline treatment,while soil C:P and C:N values were increased by mesohaline treatment.The value of MBC:MBN and MBC:MBP decreased with the increase of salinity.CUE has the same increasing trend as the flooding height,but it decreases first and then increases with the increase of salinity.The maximum value of PUE appeared in the Oligohaline treatment.As salinity increased,the activity of BG and NAG decreased;Microbial P limitation decreased firstly and then increased.The results showed different salinity and flooding changes could affect soil phosphorus content,extracellular enzyme activity,microbial community structure,and microbial carbon and phosphorus limits due to the induced sea-level rise.With the increase of salt,the release rate of IP is higher than the decomposition rate of OP,and the content of TP decreases.Soil extracellular enzyme activity and soil nutrient content decreased with the increase of salinity and flooding.As a result,the microbial phosphorus limit was increased,the microbial carbon limit was alleviated,and the microbial CUE increased.