Effects Of Organic Fertilizers From Different Sources On DOM Molecular Characteristics And Pb Speciation In Pb Contaminated Soil

Organic fertilizer plays an important role in agricultural production in China.Due to its abundant sources and low cost,it has long been used to enhance soil fertility,crop yield and quality.Organic fertilizers themselves contain a large amount of active dissolved organic matter（DOM）components,when entering into soil environment,which can not only change the physicochemical properties of the soil,but also directly chelate with heavy metals（HMs）,thus changing a series of environmental behaviors such as migration,biological effectiveness and toxicity of HMs in the soil.The composition of DOM derived from different sources’organic fertilizers,thus affecting the content and composition of soil DOM and the environmental behavior of DOM and HMs.Therefore,it is necessary to qualitatively and quantitatively understand the binding behaviors between HMs and DOM from different organic fertilizers,and reveal the effect on the molecular characteristics of soil DOM and the environmental behaviors of HMs,which can provide theoretical support for the assessment of the environmental effects of organic fertilizer remediation of HMs-contaminated soil.In this thesis,four different sources of commercial organic fertilizers in line with national standards（NY525-2012）,namely sheep manure（SM）,shrimp peptide（SP）,seaweed（SW）and biochar made from wheat straw（WSB）,were selected to study the molecular characteristics of DOM from different sources by extracting and fractionating organic fertilizer DOM,and the binding mechanism between organic fertilizer DOM and Pb by quenching and titration experiments.Meanwhile,the collected Pb contaminated soils at Changhua mine and commercial organic fertilizers,were incubated for three months to study the response of molecular characteristics of soil DOM to the decomposition of organic fertilizer and the effect of DOM on the Pb speciation in mine soils.The main findings were as follows:（1）Differences in DOM content and composition of four organic fertilizers from different sources.The DOC content order of four organic fertilizers from different sources was SM(2989.17 mg·kg^-1)>SW(1265.83 mg·kg^-1)>SP(1060.36 mg·kg^-1)>WSB(307.9mg·kg^-1).The DOC content firstly decreased and then increased with the decrease of particle size,and reached the highest value at particle size<1 k Da.The percentage of DOC content was also dominated by the smallest particle size（<1 k Da）,which accounted for 50%or more.The UV and fluorescence characteristic values of the four organic fertilizer DOM from different sources indicated that the hydrophobic components of the WSB-derived DOM accounted for a high proportion and the humification degree was relatively low,while the humification degree of the other three kinds of DOM was relatively high.The DOM sources of SP and WSB were endogenous and terrestrial inputs generated by microbial activities,respectively,while DOM of SW and SM had both endogenous sources from microbial activities and exogenous sources from terrestrial inputs.Protein-like components of SP-derived DOM contributed the most and their bioavailability was higher.DOM of the four organic fertilizers decreased with particle size,hydrophobic components and humification degree decreased,and bioavailability increased slightly.The proportion of fluorescence components and infrared spectra showed that DOM of four organic fertilizers from different sources was mainly humus,and contained a large number of amino acid N-H bond,hydroxyl O-H bond,C-O bond and other functional groups,and the regularity of small particle size（<1 k Da）was consistent with that of the whole amount.（2）Binding mechanism of the DOM released from different organic fertilizers and Pb.By studying the complexation process of Pb and DOM fluorescence substances,it was found that in<0.7μm（high molecular weight）fraction,the binding of Pb to DOM from organic fertilizer first occurred in the Protein-like substances and fulcrum acids,and in<1k Da fraction,the affinity of humus substances was stronger than that of fulcrum acids and proteins.Based on the UV differential logarithmic transformation spectrum and 2D-FTIR-COS analysis,it was found that the binding of organic fertilizer DOM and Pb mainly consisted of aromatic group,amide group,phenol group and carboxyl group functional groups.These observations are helpful to improve the understanding of the environmental behavior of organic fertilizers to improve heavy metals in soils.（3）Response of soil DOM molecular characteristics to organic fertilizer decomposition.Applying organic fertilizer can significantly increase soil DOC content.In the early stage of culture,the DOC content of Soil _SW,Soil _SM and Soil _SPincreased significantly and peaked on the 15th and 45th day.However,in the later period of culture,due to soil microbial degradation and soil mineral absorption,the DOC content gradually decreased.Soil _WSB and Soil _CKcontrast were similar,fluctuation dropped all the time,this concerned with the material property of organic fertilizer itself.Through Matlab parallel factor analysis,Soil DOM under the five fertilization treatments were all humus-like fluorescent components and C1 component（oxidation quinones of terrigenous material）was higher,compared with the control,the proportion of C2 component（aromatic carbon with high molecular weight）in the DOM of the other four fertilization treatments increased significantly with the culture period.The two-dimensional infrared spectrum analysis showed that the functional groups of soil DOM also changed successively with the cultivation period,and the types of functional groups of soil DOM were more abundant after applying organic fertilizer from different sources.In treatment Soil_ck without fertilization,the C-O functional group in carboxylic group is preferred to the olefin CH₂-stretching vibration than the polysaccharide C-O functional group,while the fertilization treatment resulted in the change of polysaccharide C-O and alkene CH₂-stretching vibration was earlier than that of carboxyl C-O functional group,and fertilization also increased the vibration of Al octahedron and Si-O-Al and other soil nano-mineral functional groups.（4）Effects of organic fertilizers from different sources on Pb forms in Pb contaminated soil.The order of Pb content was Soil _SW>Soil _SM>Soil _SP>Soil _WSB>Soil _CK,different fertilization treatments significantly increased the content of water-soluble Pb in Soil.Compared with the end of culture period,the soil water-soluble Pb content increased significantly by 1.75～21.50 times when organic fertilizer was applied at the early stage of culture.The changes of EDTA-Pb and different chemical forms of Pb under different fertilization treatments were not significant compared with the control.Correlation analysis showed that there was a very significant linear correlation between soil DOC and the contents of water-soluble Pb and EDTA-Pb in soil,indicating that these two forms of Pb were mainly controlled by DOM in soil solution,which brought environmental risks.Theμ-XRF fluorescence distribution and chemical form percentage content of soil Pb in soil under five different fertilization treatments showed that Fe-Mn-bound（T2）was the main form of Pb.The C NEXAFS spectral chart illustrated the application of organic fertilizers(except Soil _WSB Treatment)mainly increased aromatic carbon and phenolic carbon functional groups in the soil,while decreased carboxyl carbon and oxyalkyl carbon functional groups.