Chemodiversity Of Soil Dissolved Organic Matter(DOM) And The Effects Of Mineral Fractionation On DOM Properties

Soil dissolved organic matter(DOM)involves multiple biogeochemical processes,including adsorption/desorption,redox reaction and microbial metabolism.DOM consists of numerous small molecules and the distributions of molecular composition and properties(chemodiversity)and chemical functional groups are heterogeneity.Meanwhile,the chemodiveristy and characteristics of soil DOM can be affected by environmental variables,such as climate,soil properties and components,and microbial functions.Therefore,elucidating the effects of environmental varibales on the chemodiveristy and characteristics of soil DOM helps to predict carbon cycling and contaminants behavior in the environment.The purpose of this study mainly includes the two aspects:(1)we aim to quantitatively elucidate the effects of key environmental variables on the chemodiversity of soil DOM and the effects of DOM chemodiveristy on the Cu binding ability of DOM at the large-scale;(2)we attempt to reveal the effects of adsorptive fractionation by minerals on DOM chemodiversity and characteristics on the microscopic scale.The main findings of this study are as follows:Soil DOM samples were extracted from various soils collected from different regions of China,and we investigated the effects of climate and soil properties on the chemodiversity and characteristics of DOM,employing a combination of Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS),optical spectroscopy(UV-Vis and fluorescence spectroscopy),Cu titration and statistical analyses.On the one hand,our results indicated that,despite the heterogeneity of soil samples and complex influencing factors,aridity and clay can account for the majority of the variations of DOM chemical composition.Soil aridity significantly changes the DOM molecular composition from the microbial and plant origins by affecting the microbial functions,which is mediated by clay through selectively sorbing aromatic organic molecules and maintaining soil moisture content.Analysis of DOM molecular associations with environmental factors indicated that,in addition to the impact of both climatic factors and soil minerals,the intrinsic molecular properties(e.g.,O/C)of DOM also exerted significant roles in affecting the accumulation of aromatic molecules in soil solution.On the other hand,our results suggested that climate driven weathering of soil minerals may play a significant role in controlling Cu binding ability of soil DOM,since the weathering of soil minerals changes the contents of soil reactive minerals(e.g.,Fe and Al oxides),and reactive minerals can affect DOM molecular composition and properties through selective adsorption.Overall,the large-scale chemodiversity of DOM may be an ecosystem property that is more related to the environment factors than chemical persistence alone or biological sources,and the changes of DOM chemodiversity by environmental factors may further affect Cu binding ability of DOM.Above results can be used to develop large-scale models for predicting C cycling and heavy metal behavior in soils.Double spherical-aberration-corrected scanning transmission electron microscopy(Cs-STEM)and FT-ICR-MS were employed to elucidate the spatial distribution of DOM on allophane at sub-nanoscales and the mechanisms controlling the adsorptive fractionation of DOM molecules.Copper titration and WHAM VII calculation were used to elucidate the effects of DOM fractionation by allophane on Cu binding ability of DOM.Cs-STEM results revealed that within allophane aggregates,C mainly distributed on the edges of the pore spaces between allophane aggregates,while some DOM molecules localized at some small pore spaces,which may be ascribed to the physical isolution or the stacking of DOM onto the adsorbed molecules on the edges of allophane aggregates.Moreover,the Cs-STEM results suggested that DOM molecules were able to enter the interior spaces within tiny allophane nanoparticles/aggregates,implying a key process sequestering C in soil.FT-ICR-MS analysis indicated that molecules with high aromaticity,oxygen numbers and amounts of COO~-groups were preferentially adsorbed,which may be controlled by the surface ligand exchange reactions between allophane and DOM.Finally,the results of Cu titration and WHAM VII calculations indicated that the adsorptive fractionation by allophane decreased Cu binding ability of DOM through selectively adsorbing DOM moelcules with high amounts of carboxylic functional groups.Above results contribute to understanding the interactions between soil minerals and organic matter and their effets.The binding characteristics of five heavy metals(Cd,Cu,Ni,Pb,and Zn)to humic acid(HA)before and after adsorption by ferrihydrite were investigated by employing two-dimensional correlation spectroscopy(2D COS)integrated with synchronous fluorescence and Fourier transform infrared(FTIR)spectroscopies.2D COS analyses of the fluorescence results indicated that the susceptibility of the fluorescence of humic-like fraction to heavy metals significantly decreased after the adsorption of HA by ferrihydrite,which may be due to the fact that humic-like components were preferentially adsorbed by ferrihydrite.However,the fractionation processes did not alter the metal binding sequence and affinity to different HA components.2D COS analyses of the FTIR results suggested that fractionation processes decreased the susceptibility of COO~-groups to heavy metals,and changed the metal binding sequence to polysaccharides C-O and aryl groups,with the exception of Pb.Furthermore,model calculations showed that the binding ability of heavy metals to both humic-like and fulvic-like fractions decreased after the adsorption of HA by ferrihydrite.Overall,during the fractionation process,a portion of HA components with stronger metal binding ability were selectively adsorbed by ferrihydrite.Above results help to predict heavy metals behavior in the environment under the impact of mineral-DOM interactions.