Effects And Microbial Molecular Mechanisms Of FeCl₃ And Phoslock^? Additions On P Transformation In Lake Sediments

The release of endogenous phosphorus（P）from sediments was the main cause of eutrophication in shallow lakes.The alternation of drying and wetting processes commonly took place in sediments due to the water fluctuation of shallow lakes.As a result,Fetransformation and different iron fractions in sediments were significantly affected by the change in redox potential（Eh）caused by water fluctuation.However,the microbial mechanism about how iron-oxidizing and iron-reducing bacteria influence iron transformation,and further impact P release from sediments has not been not well documented yet.To compare the effectiveness of FeCl₃and Phoslock^?additions in decreasing the concentration of P in shallow lakes,a microcosm experiment was carried out with five treatments:（ⅰ）CK,（ⅱ）FeCl₃（40%,64.9 mL/m²）,（ⅲ）Phoslock^?,（ⅳ）FeCl₃（40%,64.9 mL/m²）+KH₂PO₄（3.62 mg/L）,（ⅴ）Phoslock^?+KH₂PO₄（3.62 mg/L）.The pH,soluble reactive phosphorus（SRP）and organic phosphorus（OP）of overlying water were determined.Meanwhile,a 121-day drying-wetting microcosm experiment was conducted including three treatments(i.e.,CK,FeCl₃,Phoslock^?).The dynamics of pH and Eh in sediments were monitored.The changes of dissolved organic carbon（DOC）,Fe²⁺,Hedley sequential phosphorus fractionation,microbial biomass carbon and phosphorus（MBC and MBP）,alkaline and acid phosphatase activities（ALP and ACP）at different incubation time were determined.Responses of FeOB/FeRB（abundance,diversity and co-occurrence pattern）to water fluctuation and FeCl₃addition were explored by high-throughput sequencing technology.The goals of the present study were to untangle the molecular microbial mechanism about Fetransformation driven by FeOB/FeRB under drying-wetting processes and its effect on P release from sediments.Moreover,the effectiveness of different amendments on reducing P concentration in lakes was also compared.The outcomes of this study will provide a theoretical basis for regulating the release of endogenous phosphorus in lake sediments.The main results were followed below:1.FeCl₃addition significantly decreased the overlying water pH of the two types of sediments（P<0.05）,while Phoslock^?addition had no obvious effect on the overlying water pH of the two types of sediments.The order of different amendments addition in decreasing SRP in overlying water of aquacultural lake was:FeCl₃>Phoslock^?,while the opposite phenomenon was observed in overlying water of urban lake.The order of different amendments addition in decreasing OP of overlying water was:FeCl₃>Phoslock^?.2.Drying process gradually decreased pH in aquacultural sediment,while it increased in urban sediment.During re-wetting process,pH in two type of sediments approached to neutral（7.0）.In addition,FeCl₃addition significantly decreased the pH in both types of sediments（P<0.05）.The Eh in the two types of sediments increased during the drying process,while the opposite trend was true during re-wetting process.3.Drying process significantly reduced resin-P and NaHCO₃-Pi in two types of sediments,while NaOH-Pi was substantially increased（P<0.05）in CK treatment.Furthermore,FeCl₃addition significantly reduced the active phosphorus in two types of sediments,while NaOH-Pi was largely increased（P<0.05）.The active inorganic phosphorus in the two types of sediments was significantly increased,but NaOH-Pi was substantially decreased during the re-wetting process（P<0.05）.Compared with CK treatment,Phoslock^?addition significantly reduced the active inorganic phosphorus in both types of sediments,while Di-HCl-Pi and Con-HCl-Pi were increased（P<0.05）.Re-wetting process increased NaHCO₃-Pi,while NaHCO₃-Po and NaOH-Po in urban type of sediment in Phoslock^?treatment were significant decreased（P<0.05）.4.Dyuring drying process,the values of DOC,Fe²⁺,MBC,MBP,ACP and ALP in the treatment of CK and FeCl₃addition in two types of sediments were significantly decreased;while Phoslock^?addition significantly increased ALP activity（P<0.05）in urban sediment.Caomparatively,during wetting process,these values were highly increased（P<0.05）.5.In the treatment of CK and FeCl₃addition,both drying and re-wetting processes had no significant effect on the diversity（Chao1 index）of FeOB and FeRB communities.However,drying and re-wetting significantly changed the structures of FeRB community in two types of sediments（P<0.05）,while the FeOB community structure in the two types of sediments was substantially affected during the drying process（P<0.05）.The average degree and relative modularity indices of FeOB and FeRB co-occurrence networks in urban sediment were strongly increased in both drying and re-wetting processes.However,this phenomenon was not observed in aquacultural sediment.6.During the drying and re-wetting processes,the transformation of NaOH-Pi and active inorganic phosphorus were closely connected with the changes in co-occurrence networks of FeOB and FeRB communities driving by the fluctuation of Eh in urban sediment.In contrast,in the aquacultural sediment,the transformation of NaOH-Pi and active inorganic phosphorus were closely linked with the community structures of FeOB and FeRB rather than their co-occurrence networks driving by Eh.Taken together,the transformation of P in sediments caused by drying and re-wetting processes was closely associated with iron-bound P transformation driven by FeOB and FeRB communities（diversity/interaction）.Our findings deciphered the molecular microbial mechanism that drying-wetting processes affected iron-transformation functional communities（FeOB/FeRB）through changing Eh,and thus driving phosphorus dynamic transformation.The outcomes of this study will provide a theoretical basis for controlling the release of endogenous phosphorus in lake sediments.