Effects Of DOM And Microorganisms On Phosphorus Geochemical Characteristics In Surface Sediments From Caohai,Guizhou Province

Sediment-water interface（surface sediment）is very important for nutrient exchange between the sedimentary environment and overlying water and plays important roles of"source"and"sink"in the process of element cycling.Phosphorus（P）is a key element in eutrophication,and its speciation,spatial and temporal distribution,migration and transformation are the hot and priority fields in geochemical research.With the effective control of exogenous pollution in recent years,endogenous P in sediments may determine the eutrophication process.Previous studies mostly focused on the influence of physical and chemical factors on P speciation,migration,and transformation in sediments,but ignored the influence of biological factors such as microorganisms and dissolved organic matter（DOM）on the geochemical characteristics of P in sediments.Based on this,Guizhou Caohai,a typical karst wetland on the plateau,was taken as the study area.And the surface sediment samples of Caohai in the dry season,wet season,and anthropogenic disturbance area were collected.Serious methods,including the Psenner continuous extraction,Ivanoff organophosphorus（P_o）extraction,enzymatic hydrolysis,and liquid phase ³¹P-nuclear magnetic resonance methods were introduced to reveal the geochemical characteristics of P in sediments.The DOM spectral characteristics of sediments during dry and wet seasons were analyzed through UV-visible absorption spectrum,three-dimensional fluorescence spectrum,and parallel factor analysis.In addition,the effects of sediment DOM on P migration behavior（adsorption-release）were investigated by laboratory simulation experiments.High-throughput sequencing and metagenomics methods were used to analyze the microbial community and functional composition of sediments in the dry season,wet season.This study intended to reveal the contents,fraction,and distribution of P and P_o in the sediments of Caohai,and systematically evaluate the impact of DOM and microbial community on P migration and transformation in sediments.This study would be helpful to strengthen the understanding of the biogeochemical processes of P in sediments,and also provided a reference for ecological management and environmental management of plateau lakes.The main conclusions are as follows:（1）The total contents of P in Caohai sediments were 662.89-1188.57 mg kg^-1,and the higher contents were distributed from the west to the center of Caohai and from the south of Caohai.The general distribution of different P forms in sediments was Na OH-NRP>Res-P>HCl-P>BD-P>Na OH-SRP>NH₄Cl-P.Bioavailable P accounted for60.06-70.41%of total P.According to the P_o classification,the sediments in Caohai were mainly composed of Ful-P_o（intermediate-active organophosphorus）during the dry season,while Hum-P_o（inactive organophosphorus）was dominant in the wet season.The activity of organophosphorus in sediment decreased in the wet season compared with the dry season.The average proportion of enzyme hydrolysis P_o was 65.91%in the dry season and 53.62%in the wet season.The active monoester phosphorus was the main component of the enzyme hydrolysis P_o,accounting for 53.51%（dry season）and50.39%（wet season）,then diester P and phytate-like P were followed.The ³¹P-NMR showed that the P compound in sediments was Ortho-P>mono-P>DNA-P>Pyro-P.Mono-P was the main P_o form,with inositol hexaphosphate as the main,followed by adenosine-5’-monophosphate and glycerophosphate.Principal component analysis（PCA）and correlation analysis showed that anthropogenic pollutant input and local biological processes were both important sources of P_o in the sediments from Caohai.The P release risk index（ERI）indicated that the areas from the county seat to the West Sea Wharf were at a high risk of release,and tend to spread to the center of the lake.（2）The relative concentration,aromaticity,humification degree,and humification index（HIX）of DOM in Caohai sediments were higher in the dry season than that in the wet season.The relative molecular weight,FI,and BIX of DOM were higher in the wet season than that in the dry season.This indicated that the DOM of Caohai sediments is mainly endogenous,and the sediments in the wet season had stronger biological activity,higher authigenic contribution,and lower humification degree than those in the dry season.Components C1（indigenous proteins）and C2（terrestrial humus）were mainly in Caohai sediments,while the C3（biological humus）component disappeared in the wet season.Cooccurrence network analysis showed thatα₂₅₄,E₂₅₃/E₂₀₃,C1,α₃₅₅,S_R,E₃/E₄ affected various P forms（≥9）in sediments during the dry season.The results of redundancy analysis（RDA）showed that the fluorescence index（FI）got the best explanation for P speciation in the dry season.In the wet season,α₂₅₄,E₂₅₃/E₂₀₃,α₃₅₅,E₄/E_6,and HIX affected various P forms（≥8）in sediment.The BIX got the best explanation for P speciation in sediment during the wet season.In the simulation study,the three sediments showed similar P adsorption kinetics,but different adsorption isothermal characteristics and release characteristics.By analyzing the mechanism of the different adsorption-desorption behaviors,Scripus triqueter and Hydrilla verticillata affected the P migration behavior mainly through the fluorescence intensity and the content of DOM in sediments.（3）During the dry season,there were 5 domains,8 kingdoms,132 phyla,279classes,649 orders,1228 families,3145 genera,and 16,369 species in the sediments of Caohai.At the domain level,bacteria were dominant（94.09%on average）;at the phylum level,Proteobacteria were dominant（33.48%on average）;at the genus level,Caldilinea,Anaerolinea,and Streptomyces were dominant.The dominant domain,phylum,and genus in the sediment during the dry period were consistent with that during the wet period.Among the genes related to P cycling,genes related to the phosphoric acid transport system and polyphosphate synthesis were more abundant.There were 4 domains,7 kingdoms,87 phyla,152 classes,263 orders,453 families,1445 genera,and 4249 species of the microbial communities containing P genes in dry and wet seasons.The pho A,phn D and pho B genes,Shannon diversity index,and Aeromonas,Candidatus Accumulibacter,Pseudomonas,Anaerolinea,Longilinea had the great influence（the degree of exfoliation≥5）on P form in sediment during the dry season.pho A gene and Ilumatobacter,Ardenticatena,Anaerolinea,Levilinea,Longilinea,Thiobacillus extensively affected P speciation in sediment during the wet season.In the lakeside zone,the bacterial community mainly affected the sediment P form by iron oxidation/reduction and regulation of alkaline phosphatase activity,while the archaeal community affected the sediment Pspeciation by changing acid phosphatase activity and organic matter.Among them,Geobacter,Anaerolinea,Bryobacter,and Thiobacillus had a high degree of explanation for P speciation in sediments.（4）Comprehensive analysis of all indicators（water quality indicators,sediment physical and chemical indicators,DOM spectrum parameters,P genes,and microbial indicators）in the dry and wet periods,results showed that BIX highest interpreted the changes of P fraction in Caohai sediments,which accounted for 16.3%.DOM indicators（FI and E₃/E₄）also have high explanatory degrees（>10%）.pho B and pho R have high explanatory degrees among P cycling genes.In the microbial community,Candidatus Accumulibacter,Thiobacillus,Longilinea,and Levilinea also had a high interpretation rate of P form.In addition,the effect of p H on P form in sediments also showed a high explanatory rate（13.9%）.In summary,the dissolved organic matter,P genes,and microorganisms in Caohai,a shallow lake with rich vegetation,had a decisive influence on the biogeochemical characteristics of P in sediments.In addition,physical-chemical environmental factors such as the p H of sediments also have important effects on the biogeochemical characteristics of P in sediments.