Distribution And Dynamics Of Important Metals In Soil-water Paradigm: Roles Of Humic Substances And Their Transformation Mechanisms

Humic substances are heterogeneous mixture of organic compounds of natural organic matter,play significant role in key biogeochemical processes of nature,control the distribution and dynamics of metals in soil-water paradigm.However,the biogeochemical transformation mechanism of humic substances（HS）and metals as well as the role of climatic factors that influence the transportation of HS along with metals from terrestrial sources to the ambient water system still not clear.Therefore,this research work was conceptualized to ascertain three key fundamental research questions in soil-water paradigm.To investigate these key fundamental questions,this research work was designed and planned to set up the following aims and objectives:（1）To investigate the distribution of important metals and humic substances in soil-water paradigm and then to examine the complexation properties of metals and humic substances collected from different climatic regions of China.（2）To evaluate the optimum solubility of soil humic substances and characterize them authentically in contrasting soil environment.（3）To develop a new extraction technique to extract the humic substances from soil that correspondingly elucidate the existing biogeochemical phenomena of the respective soil.（4）To elucidate the biogeochemical transformation mechanisms of HS and metals in diverse soils.（5）To evaluate photoinduced and microbial degradation on soil HS under both natural sunlight and dark incubated（microbial）conditions and then to elucidate their alterations on the complexation properties of metal and humic substances（M-FDOM）.To achieve these aims and objectives,142 water samples were collected to assess distribution of metals（M）and their complexation with fluorescent dissolved organic matter（FDOM）,M-FDOM,from 15 provinces,which covered all five climatic zones across China.For developing a standard extraction technique,it was chosen three different soils,i.e.a forest,a maize and a paddy soil,to assess the solubility of soil HS based on their fluorescence（excitation-emission matrix,EEM）features and changes in nutrient(NO₃^?-N,PO₄^3?-P and dissolved Si)contents using multiple extraction approaches（time-dependent,various extractants,solid to liquid ratio,and sequential extraction）.To execute new extraction technique and for examining the characteristics of humic substances and their biogeochemical consequences on metals in diverse soils,it was collected 21 diverse soil samples,which included six forest sties,five river sides,six different types of cropland（maize,soyabean and paddy）,two artificial park areas（golf field）and two samples from degraded soils that were not cultivated,but exposed to sunlight as barren land from Tianjin.For the photoinduced and microbial degradation（dark incubated under same conditions）on soil humic substances and their complexation properties with metals,photo-irradiation and dark incubation experiments over 30-,75-,150-days under natural ambient conditions were conducted on forest soil.EEM-PARAFAC modelling was applied to characterize and quantify the FDOM components.The results from this whole research work produced four key studies.The first study describe the occurrences,transportation and characteristics of FDOM and metals under different climatic zones in inland waters.Interactions between metals and FDOM among samples of five climatic zones were analyzed by multivariate statistical analysis.For better understanding of M-FDOM interaction with physiochemical properties,this study also measured p H,electric conductivity（EC）,dissolved organic carbon（DOC）,and nutrients,including nitrate（NO₃^-）,total nitrogen（TN）,phosphate(PO₄^3-),total phosphorus（TP）and silicate(Si O₃^2-).The results of metals concentrations demonstrated that they are greatly varied in inland surface waters among 15 provinces across China,ranging from 3.1 to 287.7μg L^-1 for Al,from 0.12to10.80μg L^-1 for As,from 0.11 to 13.04μg L^-1 for Cr,3.56-561.10μg L^-1 for Fe,0.07-5.33μg L^-1 for Pb,0.31-14.99 mg L^-1 for Si,15.9-704.5μg L^-1 for Sr,0.01-15.29μg L^-1 for U and 1.56-116.38μg L^-1 for Zn.Such large differences of metals were also substantially differed among provinces under five climatic zones,for example,the average concentrations of Fe,Pb,As and Al were detected highest in temperate monsoon climate.Similarly,the average concentrations of Cr,Zn and U were observed highest in plateau and mountain climate whilst Sr and Si was detected highest in temperate continental climate.These results imply that a large difference of metals concentrations could originate from diverse sources.Conversely,PARAFAC modelling were followed two to four component model on the EEM spectra of individual stream,river and lake/reservoir samples,which identified total eight fluorescent components or FDOM in the surface waters.The average total fluorescence intensities for all fluorescent components were significantly differed among five climatic zones,which were fluctuated from 13 to 967 QSU（ave 378±401 QSU）in tropical monsoon climate（zone-1）,121 to 2490 QSU（ave 592±598 QSU）in sub-tropical monsoon climate（zone-2）,272 to 345 QSU（ave 290±28 QSU）in plateau and mountain climate（zone-3）,72 to1813 QSU（ave 503±391 QSU）in temperate monsoon climate（zone-4）,and 78 to 832QSU（ave 230±213 QSU）in temperate continental climate（zone-5）.These results suggest that the fluorescent components could markedly derived in the surface waters of sub-tropical monsoon climate and temperate monsoon climate zones.High DOC concentration was typically detected in south and eastern part of China（Guizhou,Guangdong,Yunnan,Shanghai）.Such high DOC could presumably originate from terrestrial soil along with occurrence of high vegetation in the respective regions,which could input into ambient waters through precipitation or surface runoff.Conversely,DOC concentration were found the lowest in northern parts of China（Qinghai,Xinjiang,Inner Mongolia）.Such low DOC could be attributed to low vegetation along with low precipitation in northern parts of China.Interestingly,highest level of PO₄^3-and TP did not exhibit any relationship with highest contents of NO₃^-and TN in the surface waters,strongly indicate that the biogeochemical uptake and mineralization processes of PO₄^3-and TP are entirely different from NO₃^-and TN.Surprisingly,highest level of Si O₃^2-was observed in southern parts of China which were partially similar to the highest level of NO₃^-or TN,implying that both nutrients(Si O₃^2-and NO₃^-)could contribute to the same biogeochemical processes and functions in the surface waters.The average concentrations of Fe,Sr,Si,Al,As,and Pb were detected the highest level in temperate monsoon climate and in temperate continental climate zones which were correspondingly followed the highest level of terrestrial humic substances（humic acids and fulvic acids）.Likewise,all metals concentrations were detected the lowest in tropical monsoon climate,which correspondingly followed the lowest level of terrestrial HS.It indicates that terrestrial HS are susceptible to transport of those metals from soil/land ecosystem into ambient inland waters.Conversely,the contents of U,Cr and Zn became the highest in plateau and mountain climate zone,which subsequently followed by second highest level of total average of HS,FA-like plus HA-like.These results imply that different metals could form complexes with various fluorescent components which could depend on the occurrence of FDOM functional groups.Among all metals,Fe and Al exhibited the maximum concentrations adsorbed by POM（～90%）whereas Al,As,Sr and Zn of freshwater of temperate continental climatic zone exhibited the maximum metal adsorption by POM.This could be attributed due to release of high POM from soil under drying climatic conditions as well as the lowest total intensity of FDOM content in that region.Climatic factors evidenced that temperate continental climate played a profound role on metal leaching from terrestrial to ambient aquatic system that subsequently influenced to enhance the metals concentrations in inland freshwaters.These results finally conclude that M-FDOM study could be useful indicator for better understanding of a holistic approach of terrestrial land-aquatic system,which is unquestionably vital for intelligent management of natural Surface-Earth system.Therefore,it is crucial to identify how does terrestrial soil humic substances and important metals linked with M-FDOM in surface water.To assess this question,it is vital to ascertain optimum solubility of soil HS in contrasting soil environments and their biogeochemical characteristics under different extraction methods along with discovering a proper extraction for HS,which are unknown in earlier studies.The next study belongs to solubility of soil HS under different extraction conditions and at the same time,to ascertain biogeochemical characteristics of soil HS along with nutrients changes in three characteristic soils.The second study focused on optimum solubility of soil HS using a standardised extraction method to extract HS with a minimum level of degradation effect.Three different soils,i.e.a forest,a maize and a paddy soil,were examined to assess the solubility of soil HS based on their fluorescence（excitation-emission matrix,EEM）features and changes in nutrient(NO₃^?-N,PO₄^3?-P and dissolved Si)contents using multiple extraction approaches（time-dependent,various extractants,solid to liquid ratio,and sequential extraction）.Three fluorescent components,i.e.HA-like,FA-like,and protein-like fluorophores（PLF）,were identified by parallel factor（PARAFAC）analysis of EEM spectra of the various soil extracts.Note that PLF were associated with humic-like substances（HLS）showing two peaks,i.e.peak M at Ex/Em=270-300/410–423 nm and peak A at Ex/Em=220–225/410–423 nm.Not only FA-like was absent in KCl extracts,but also peak C of HA-like shifted to a shorter wavelength region.This blue shift effect of HA-like could might be ascribed to the fluorescence properties of HA-like,were influenced by K⁺and/or Cl^-ions in solution depending on soil characteristics.The HA-like in alkaline extracts exhibited EEM images and fluorescence peaks,similar to those of HA-like in water extracts from the forest soil,but new EEM images of HA-like appeared in the extracts from cultivated maize and paddy soils,which appeared as two distinct peaks in the C-region（365,335/477,460 nm）for the paddy soil and as a longer emission range of peak A（460–477 and 442–468 nm,respectively）for maize and paddy soils.The solubility of HS,DOC and nutrients were shown to increase with extraction time,except for PLF.The FA-like fraction disappeared completely in KCl extracts of all three soils,suggesting the inefficiency of salt extraction.Conversely,HS and nutrients solubility substantially increased in alkaline extracts,and dissolved Si was significantly correlated with the fluorescent intensities of HA-like,FA-like and PLF.These results suggest that dissolved Si might be originated from insoluble HS complexes with amorphous silica（phytolith）and/or silicate minerals.This hypothesis was also supported by the fact that in tropical soils Si is mostly present either as phytolith or adsorbed to soil mineral particles（Fe/Al oxides/hydroxides）.The relative solubility of HS and nutrients was higher at lower solid to liquid ratio（1:250?1:100）,whereas their maximum yields was achieved at high solid to liquid ratio（1:10）for all three soils.Sequential extraction results showed that the first water extraction step contributed 42–55%of HS,which suggested that a single extraction was insufficient to recover HS.These results thus indicate that water and alkaline extraction could provide,respectively,the labile and insoluble complexed HS existing in soil.Water extraction appeared most efficient when short extraction time（ca.6-h）and higher solid to liquid ratio（1:10 to 1:20）are used at room temperature（25oC）to isolate the labile fractions of HS and study their biogeochemical phenomena and molecular compositions and structures in soils.Alkaline extraction using a solid to liquid ratio of 1:10 or 1:20 at room temperature（25°C）was the most efficient method to extract the insoluble HS complexes involved in biogeochemical processes and characterize them at the molecular-level.These results could be used to assess the typical soil characteristics,including the degradative nature of each individual HS,their biogeochemical consequences,including carbon and nutrient dynamics in soil environments.Based on the results obtained from this aforementioned study,it is important to know how does soil HS and metals exist in labile and complexed state in diverse soils by applying new developed extraction technique under both water and alkaline extractions in diverse soil environments.These issues could encounter to compare with biogeochemical occurrences and transportation of M-FDOM in surface waters.The third study includes occurrences of metals and soil HS along with their biogeochemical transformations in diverse soils.A novel extraction method was applied for the first time to extract labile and complexed state of DOM separately and then EEM-PARAFAC modeling applied to identify FDOM components authentically.DOM in this study is thus operationally separated as two fractions:labile portion of DOM that can soluble relatively low molecular weight humic substances and alkaline extracted complexed state of DOM that can dissolute relatively larger molecular weight humic substances.The DOC concentrations were ranged from 4.73 to 10.77 mg L^-1 in water and from 8.07 to 50.74 mg L^-1in alkaline extraction in 21 soils.Results of DOC concentrations from five different characteristics samples（forest,riverside,cropland,park and degraded）demonstrated that water to alkaline extraction ratio was predominantly occurred in degraded soils.The EEM-PARAFAC model on the soil samples identified maximum three fluorescent components,such as HA-like,FA-like and PLF in 21 soils.Interestingly,humic acid-like and protein-like fluorophore were absent in labile state both in degraded and Golf park soils.But in alkaline extraction,only HA-like were entirely degraded.In addition,peak C of FA-like in degraded and Golf park soils showed shorter wavelength region（310-315/425-426 nm）than forest（320/436 nm）,riverside（325/436 nm）and cropland soil（345/436 nm）during water extraction.Fluorescence excitation-emission peaks（peak C and peak A）of HA-like and FA-like along with their intensities were largely varied during alkaline extraction among soils studied.These results indicate that degradation of humic substances in soils could occur simultaneously in their labile and complexed state.The fractionation study revealed that apart from Ca,Sr,Cd,U and Zn,all other metals were existed in a complexed state in soils.The highest solubility in alkaline solution was found for Si in cropland soils.Clay particles have highly significant relationship with all metals except Cd.Apart from As,Cd and Pb,all other metals have significant positive correlation with sulfur content of the soils.Most of the metals in soil samples have significant positive correlation among themselves except Cd.The results from Pearson correlation between metals and humic substances showed that most of the metals were strongly correlated with HA-like,FA-like and PLF during alkaline extraction.Moreover,some few metals（e.g.Si,As,Cu and Pb）were strongly correlated with HA-like and also PLF except Pb in labile state.These results strongly suggests that metals are coexisted with soil humic substances both in labile and complexed state in soils.Less correlation of labile state of humic substances with metals imply that the humic substances in labile phase can weakly coexisted with metals in the soils.Stable isotopic evidence and nitrogen content reveals that cropland,park and degraded soils pertaining to similar characteristics.Conversely,dissolved inorganic nitrogen（DIN）and dissolved inorganic phosphorus（DIP）contributed higher percentage of ratio to their corresponding soil total nitrogen（STN）pool and total dissolved phosphorus（TDP）in the degraded and crop soil samples.This indicates that more degradation of N-and P-containing humic substances that preferentially degraded to inorganic nutrients i.e.DIN and DIP.Factor analysis of soil parameters indicate that the cropland,slightly saline（degraded）and park soil are more degraded than forest and riverside soils.This is also evidenced by highest metals concentrations in forest and riverside soils that posits that non-degraded soils retain more metals than degraded soils.Riverside and park soils exhibited higher HA-like ratio in water to alkaline solution.Whereas FA-like ratio was higher in forest samples.A significant relationship was observed between metals and soil organic carbon.Finally,this study therefore concludes that the labile phase of humic substances during water extraction are responsible for weak complexation with metals,thereby facilitating the leaching of metals along with humic substances in soils.Finally,to exhibit the evidence how does metals and soil HS transform/alter between labile and complexed state in soil under experimental conditions,which are unknown to date.The fourth study includes simultaneous photo-microbial effects on labile and complex state of soil HS and metals along with their biogeochemical transformation under the conditions of natural sunlight and dark-induced experiments.Three sets of soil samples were examined to enumerate the photo and microbialtransformation of soil humic substances,metals,and nutrients under the conditions of sunlight and dark-induced incubation of forest soil as well as control at constant temperature（25℃）in thermostat at laboratory incubation conditions for 0（raw sample）,30,75 and 150-days.EEM-PARAFAC modeling was applied to identify the soil HS in photo-irradiated,dark-incubated and control samples in both water and alkaline solutions.Likewise,metals concentrations were measured to evaluate their solubility and biogeochemical changes under various experimental conditions studied.Similar novel extraction method was followed to extract soil humic substances or DOM from experimental samples.The results showed that photo-irradiated samples increased DOC concentrations approximately 3-fold in water extraction and<1%in alkaline solution than in raw/non-irradiated soil samples.Conversely,DOC contents were decreased in microbial and control samples than raw samples（decreased by 16-20%and 7-11%,respectively,in water and alkaline solution）over 150-days.These results suggest that photoirradiation plays important roles on top soils than microbial degradation.Silicate（Si O₃）were extracted～seven-fold higher in alkaline solution than water extraction but there was no significant changes observed by photo-microbial effects in complexed state.EEM-PARAFAC modeling on experimental soil samples demonstrated that humic substances（HA-like,FA-like and PLF）were increased by 2-fold,3-fold,3-fold,respectively under sunlight irradiation.But under dark and control condition PLF was decreased by 100%and 13%,respectively.HA-like increased by27%and decreased by 49%under dark and control experiment,whereas FA-like decreased by 13%and increased by 94%under the same condition.These results indicate that microbial effects could result in rapid mineralization of soil humic substances or DOM into CO₂ and other end products.In water extraction,after 30 days of photo irradiation,HA-like identified at shorter wavelength region and FA-like at longer wavelength region along with simultaneous increase of approximately 90%of DOC in water extraction.Such increased DOC could be originated from SOC that were weathered through sunlight irradiation.The rate of DOC production was higher over30-days may be attributed to more water content at the preliminary stage of experiment than later.During 75 to 150 days of photo-irradiation new fluorescence components were detected,which could be originated from the new SOC.In alkaline extraction,over 150-days,FA-like peak M was completely degraded as well as HA-like component shifted to shorter wavelength region that was depicted after 30 days in water extraction.This result provide an effective evidence that photo-irradiation could significantly degrade the labile fraction of DOM very quickly than complexed state of DOM.Along with rapid transformation of soil humic substances or DOM during photo degradation experiments,metals contents were significantly changed with increasing their contents in water extraction over 150-days.For example,Alkaline earth（Mg,Sr）and trace metals（As,Cu,Mn,Ni,Sb,Pb,U,Zn）concentrations were increased by 2%to 4-folds over 150-days in labile fraction of DOM.During alkaline extraction,only Mn concentrations was increased significantly（2-fold）under photo-irradiation whereas other metals concentrations changed insignificantly and all the metals concentrations were decreased after 75 days except Ca,Sr,Sb and U.Aluminium,Si and Ni were gradually increased up to 75 days whereas Fe,As,Pb decreased concurrently due to sunlit effect.Dark-induced process reduced metal concentration up to 3-66%.The occurrence of increasing DOC concentrations and alkaline earth metals in water extraction along with degradation of humic substances posit the significant transformation by photo-irradiation.Overall,it was found that both photo and microbial effects are more efficient than control mineralization and altering of soil humic substances or overall DOM.Finally,these results highlight the crucial importance of sunlight and temperature impacts on soil humic substances and metals under upcoming global warming on soil organic matter.From these above results and findings it could be certainly evidenced that soil HSand metals exist as labile and complexed state in soils which could leach/transfer to the surrounding surface water that substantially influenced by the climatic conditions.Therefore,upcoming global warming,particularly temperature and sunlight will accelerate the dynamics/transportation of terrestrial metals and soil humic substances into ambient surface water that will subsequently modify their overall biogeochemical processes and functions in the soil-water paradigm.