Response Characteristics Of Sediment Phosphorus Conversion To Iron And Nitrogen Mediation Under Anoxic Conditions

Phosphorus is the key biogenic element of the lake.With the decrease of exogenous phosphorus input load,the contribution of endogenous phosphorus to the sediment is particularly prominent.The redox of iron is closely related to the phosphorus cycle.Fe（Ⅲ）in the hypoxic zone of sediments can be reduced to Fe（II）.Fe（II）is oxidized to Fe（Ⅲ）by NO₃^-imported from the lake under the action of nitrate-dependent Fe（II）oxidation bacteria（NDFOB）,which may cause the transformation of phosphorus Therefore,phosphorus conversion in sediments mediated by iron and nitrogen is of scientific significance for enriching the theory of lake endogenous control.In this study,Moshui Lake in Wuhan was selected as the research object.The sediment-overlying water system was used to study the transformation of iron,nitrogen and phosphorus in sediments with NO₃^-input,and the driving effect of iron and nitrogen on the change of iron-bound phosphorus was analyzed.The influence of organic electron donors on the transformation of iron and nitrogen-mediated phosphorus was discussed.Finally,the NDFOB pure culture system was used to study the effect of Fe（II）concentration on the relationship between iron,nitrogen and phosphorus.The main conclusions were as follows:（1）The input of NO₃^-increased the denitrification enzyme activity of the sediment and the NDFOB abundance,and the NO₃^--N concentration and soluble orthophosphate（SRP）in the overlying water and pore water decreased by 66.86～69.18%and21.1%～44.7%,The overlaying water TP of NO₃^-+Fe（Ⅱ）treatment group was significantly lower than that of control group and NO₃^-group（R²=0.440,P<0.01 and R²=0.440,P<0.01）.The input of NO₃^-changed the composition of microbial communities related to iron and nitrogen in the sediments,and the relative abundance of Firmicutes and Proteobacteria in the sediments increased at the phylum level.At the genus level,the abundance of iron and nitrogen-related Crenothrix,Sideroxydans,and Flavobacterium increased,while the input of NO₃^-+Fe（Ⅱ）increased the abundance of Actinobacteriota in the sediments.（2）Iron-bound phosphorus was the most dominant form of phosphorus in sediments（56-73%）,among which amorphous iron oxides(Fe_ox1)and crystalline iron oxides(Fe_ox2)played a key role in the fixation of phosphorus in sediments（53～59%）,resulting in the increase of P-Fe_ox2 and P-Fe_ox1content.The promotion effect was more obvious with the input of NO₃^-and Fe（Ⅱ）.P-Fe_ox2 content in the sediments after the input of NO₃^-and NO₃^-+Fe（Ⅱ）was 0.13 mg g^-1 higher than that in the control group,and P-Fe_ox1content was 0.02 mg g^-1and 0.08 mg g^-1 higher than that in the control group.（3）Acetate increased the abundance of NDFOB and denitrification enzyme activity in the sediments,and increased the NO₃^--N removal efficiency by 22.6%.There was a significant positive correlation between TP and Fe _T（P<0.01）.Fe_ox1and Fe_ox2were the iron forms with the strongest phosphorus adsorption capacity.Acetate promoted the reduction of Fe（Ⅲ）in the sediments.Due to the reduction of Fe_ox1 and Fe_ox2 in the sediments,a higher content of Fe（Ⅱ）was produced,accompanied with a drastic decrease of iron-bound phosphorus content.The P-Fe_ox1 content in the final sediments was 0.25 mg g^-1lower than that in the NO₃^-treated group.Acetate inhibited the formation of Fe_ox2 in the sediments,and the P-Fe_ox2 content was lower than that in the NO₃^-treated group Consequently,the TP concentrations in overlying water and pore water were significantly higher than those in NO₃^-treatment group（P<0.01）.（4）In the NDFOB pure culture system,with the increase of Fe（Ⅱ）concentration,the average oxidation rate of Fe（Ⅱ）increased,and the rate of NO₃^-reduction decreased.N₂ was the main gas product of the denitrification process,and its output decreased with the increase of Fe（Ⅱ）concentration.Fe_ox1 was the main form of iron oxide in the precipitation（80.63～86.42%）,in accordance with P-Fe_ox1 as the main form of phosphorus in the precipitation（81.96～90.39%）.With the increase of Fe（Ⅱ）concentration,the fixed TP in the precipitation increased gradually.FT-IR results showed that the iron oxides were Fe_ox1（ferririte）,Fe_ox2（hematite,goethite）,and magnetite.