Effect And Mechanism Of Dissolved Organic Matter Derived From Livestock Manure Compost On Heavy Metal Immobilization In Black Soil

The black soil area in Northeast China is an important national food production base,and the quality of black soil directly affects food security.However,recent studies have found that it has been contaminated with heavy metals to varying degrees,especially in agricultural fields near roads,mines,industrial areas and suburban areas.Livestock manure compost is an efficient and sustainable organic fertilizer that can be used to improve soil quality and crop yield,and also has the effect of reducing the risk of heavy metal migration when applied in appropriate amounts.The bioavailability of heavy metals in non-acidic black soils is usually low and the potential for plant uptake through the root system is limited.However,the application of compost will introduce a large amount of dissolved organic matter(DOM),which is the most active component in compost.On the one hand,it can interact with the solid phase components of the soil to change the physicochemical properties and.On the other hand,it can interact with the solid phase components of the soil,changing the physicochemical properties and microbial community characteristics of the soil,influencing the morphology and environmental behavior of heavy metals.Due to the differences in composting materials and processes,the heterogeneity of DOM can be caused.Studying the migration and transformation process and mechanism of different properties of DOM on heavy metals in soil is of great significance for the safe application of livestock manure compost and the protection of black soil.Therefore,in this paper,DOM derived from composts with different manure sources and different maturity levels,two non-acidic black soils and three common heavy metals(Cu,Pb and Cd)in soils were studied by using microscopic chemical reactions,indoor simulated incubation and pot experiment.Combined with modern spectral detection and high-throughput sequencing techniques,such as fluorescence spectroscopy,Fourier infrared spectroscopy,X-ray diffraction.Supplemented by parallel factor analysis complexation model,two-dimensional correlation analysis,redundancy analysis,and structural equation modeling.The interaction between DOM derived from different composts and heavy metals,the key factors affecting soil solid mineral immobilization of heavy metals by DOM,the main regulatory mechanisms of DOM on the transformation and migration of heavy metals in the soil-plant system,as well as the response patterns of rhizosphere soil microbial community structure and diversity were systematically explored.The main research conclusions are as follows:(1)The sources of livestock manure and the type of heavy metals determine the binding capacity of DOM derived from composts(chicken manure MDOM,cow manure CDOM and pig manure PDOM)to heavy metals.Parallel factor analysis results showed that MDOM had high aromaticity,molecular weight,proportion of fluorescent substances and humic like components,showing stronger affinity with heavy metals and potential environmental migration risks of heavy metals.Cu and Pb can effectively bind different DOM fluorescence components,with stability constants(log k _M)ranging from 4.53 to 5.38 and 3.34 to 3.57,respectively.While Cd hardly produces fluorescence quenching effects on DOM.Two-dimensional synchronous and infrared correlation spectroscopy analysis showed that protein like components in the fluorescence structure of MDOM were preferentially involved in binding,while fulvic like components in CDOM and PDOM were preferentially bound.Amides and polysaccharides in MDOM had the fastest response to heavy metal disturbances,polysaccharides and phenols in CDOM have the fastest response,while phenols and amides have the fastest response in PDOM.Although the proportion of protein like components and non-fluorescent polysaccharides in DOM was relatively low,their binding effect with heavy metals cannot be ignored.(2)The effect of DOM on soil mineral immobilization of Pb increased with the increase of compost maturity.The adsorption capacity of Pb on montmorillonite decreased by 14.3%and 29.8%,respectively under the effect of DOM derived from chicken manure compost in the early stage(FDOM)and high temperature period(IDOM),while it increased by 44.4%under the MDOM.These differences were caused by the co-release or adsorption of DOM-Pb complexes,as well as differences in molecular weight,hydrophilicity,and self-structure of different DOM.Parallel factor analysis showed that Pb mainly bound to protein like components in FDOM and IDOM,and to fulvic like components in MDOM.In addition,X-ray diffraction showed that both DOM and Pb could enter the interlayer of montmorillonite,and the d-spacing increment of montmorillonite caused by the combined action of DOM-Pb complexes were the largest,from 1.33 nm to 1.42 nm-1.46 nm,of which the amplification effect of MDOM-Pb complex was the most obvious.At the same time,the adsorption between montmorillonite and FDOM-Pb and IDOM-Pb was mainly through Van der Waals force,while the adsorption with MDOM-Pb was more inclined to ligand exchange,especially in the presence of high concentrations of Pb.(3)DOM derived from composting with different maturity levels could reduce the stability of Pb in different black soils by altering soil environmental factors.During the process of soil cultivation,DOM had a significant effect on the soil within 7-30 days,and the changes in soil properties tend to be more towards the soil background value after 30 days.DTPA-Pb in chernozem and dark brown soil increased by 0.37%-18.03%and 13.22%-22.17%,respectively after 60 days of cultivation,and the activation ability followed the order of FDOM>IDOM>MDOM.The input of MDOM in chernozem can reduce the proportion of easily mobile forms of Pb,while under other conditions,DOM promoted the transformation of Pb forms from organic bound and residual states to exchangeable and carbonate bound states.Correlation analysis and structural equation modeling results showed that p H and dissolved organic carbon(DOC)content in chernozem were the main factors driving DTPA-Pb changes,conductivity was an indirect driving factor,while DOC was the main driving factor in dark brown soil.In addition,DOM input could directly or indirectly drive the transformation of Pb forms by regulating soil p H,electrical conductivity,organic matter,DOC and enzyme activity(urease and phosphatase).(4)DOM derived from composting with different maturity levels altered the growth and ability to absorb and transport Pb in pakchoi by altering soil environmental factors.DOM treatments could be used to positively feedback the growth of chard by enhancing soil nutrients(organic matter and available phosphorus)and electrical conductivity.The application of low concentration MDOM in chernozem could reduce the Pb content in the edible parts of pakchoi by 39.60%.Other treatments enhanced Pb uptake in chard by 4.67%-105.70%and the enhancement increased with increasing DOM concentration,posing a potential environmental risk.FDOM was the most effective in promoting the uptake of transported Pb by chard,mainly due to its lower p H and small molecule ontogenetic structure.In addition,among the soil environmental factors determining the ability of pakchoi to uptake and transport Pb,DOC,the percentage of easily mobile forms of Pb and phosphatase activity were the main determining factors.While p H and cation exchange capacity play a special role in chernozem and dark brown soil,respectively.(5)DOM and heavy metals jointly affected changes in the structure and diversity of rhizosphere soil bacterial communities.High concentrations of DOM significantly reduced the Shannon diversity index of bacterial communities in both Pb-contaminated soils,as well as the Chao1 index of bacterial community abundance in chernozem(P<0.05),but had no significant effect on the abundance in dark brown soils.In addition,high DOM concentrations increased the abundance of dominant bacterial phyla(Proteobacteria and Bacteroidota)and dominant genera(Sphingomonas,Lysobacter and Arthrobacter)that are eutrophic and had strong resistance to heavy metals in soil.The Mantel Test showed a strong correlation between rhizosphere soil environmental factors and the level of bacterial genera,especially with the properties of soil DOM molecules(P<0.05).Redundancy analysis revealed that the association between easily mobile Pb forms,catalase activity,DOM molecular properties and microorganisms were key factors influencing changes in the microbial community.In summary,DOM derived from chicken manure compost exhibited stronger affinity with Cu and Pb.DOM derived from chicken manure composting with different maturity levels of increased Pb activity by altering the adsorption capacity of soil minerals to Pb,soil physicochemical properties,enzyme activity,as well as microbial community structure and diversity.The degree of activation decreased with the deepening of composting maturity.Furthermore,in relatively stable non-acidic black soils,except for the effect of low dose MDOM on stabilizing pakchoi absorption and transformation of Pb in chernozem,all other situations enhanced its absorption of Pb.Therefore,in practical applications,attention should be paid to the potential risk of heavy metals pollution caused by immature and excessive application of livestock manure composts.