Water-soluble Soil Organic Matter Research Articles

Rice Husk and Its Biochar Have Contrasting Effects on Water-Soluble Organic Matter and the Microbial Community in a Bamboo Forest Soil

Converting rice husk to biochar is one of the solutions to manage crop residues by transforming waste into a value-added material that has broad benefits to the environment when biochar is applied to the soil. This study investigated the impact of the application of rice husk and its biochar at different doses (i.e., 0, 10, and 30 t ha−1) on soil carbon stability, the property of water-soluble soil organic matter, and the abundance and diversity of microbial communities in a Lei bamboo (Phyllostachys praecox) forest soil 262 days after their application. The application of rice husk, especially at 30 t ha−1, increased dissolved organic carbon due to the high labile carbon (C) (e.g., cellulose, hemicellulose, polysaccharides) content in the rice husk. The biochar treatments stimulated the release of humic-like substances (e.g., (poly) phenols) into the soil solution, increased the aromatic C content by 412–557%, and increased the relative abundance of Chloroflexi, Planctomycetota, and Proteobacteria compared to the control. This study shows that biochar application, particularly at 30 t ha−1, enhanced the C stability by turning organic C into recalcitrant forms in the soil, demonstrating the merit of converting rice husk into biochar before its application to the soil.

Straw return in paddy field alters photodegradation of organic contaminants by changing the quantity rather than the quality of water-soluble soil organic matter

Straw return, an important agricultural management practice, is worldwide adopted to enhance soil carbon sequestration and soil fertility. Although water-soluble soil organic matter (WSOM) in paddy field is known to affect the photodegradation of organic contaminants, how straw return regulates the photosensitization of WSOM by changing its properties remain unclear. Here, we determined the temporal variations in the content, chemical characteristics, and photosensitizing ability of WSOM after wheat straw return in a wheat-rice rotation system using optical spectroscopy and steady-state photodegradation tests. After straw return, the WSOM content first increased to a maximum and then gradually decreased to pre-return level at day 90. Nevertheless, the relative abundance of humic-like components in WSOM was not shifted by straw return, and protein-like component in WSOM just showed a slight decrease at day 45. All the WSOM samples inhibited sulfamethoxazole (SMX) photodegradation by light filtering, reactive species quenching and other mechanisms, while promoted diuron (DIU) degradation via reacting with •OH, 1O2 and excited triplet WSOM. The photodegradation of SMX and DIU was little affected by straw return changing WSOM composition and photochemical activity. However, straw return could decelerate SMX and DIU photodegradation by elevating WSOM content in a relatively short-term. This study emphasizes that straw return may reduce the photodegradation of organic contaminants by increasing WSOM concentration instead of altering WSOM chemical characteristics.

From the litter layer to the saprolite: Chemical changes in water-soluble soil organic matter and their correlation to microbial community composition

Organic matter content and chemistry is vital to the structure and function of soil systems, but while organic matter is recognized as biogeochemically important, its chemical interaction with soil processes is not well understood. In this study we used fluorescence spectroscopy, which has been used extensively for understanding the role of organic matter in aquatic systems, to identify chemical changes in organic matter with depth in a soil system. Soil was collected from nine different pits in a first-order montane catchment in the Colorado Front Range. The water-soluble soil organic matter was extracted from each sample and fluorescence and UV–vis spectroscopy was used to analyze its chemical character. While organic matter chemistry had little correlation with landscape location and local vegetation, there were noticeable consistent trends between soil horizon and organic matter chemistry in each pit. Total organic matter decreased with depth and became less aromatic with increasing depth. This less aromatic material in the saprolite also had a greater microbial signature. The redox character of the organic matter accompanied this change in source and molecular structure, with more oxidized character corresponding with organic matter with more microbial input and more reduced character corresponding to organic matter with more plant input. A concurrent investigation of the microbial population of the same soil samples also showed microbial population composition varying more with soil depth than landscape position, and depth changes in microbial diversity occurred concomitantly with depth changes in organic matter chemistry.

Water-extractable soil organic matter characterization by chromophoric indicators: Effects of soil type and irrigation water quality

Use of treated wastewater (TWW) for irrigation may affect the concentration and composition of dissolved organic matter (DOM) in the soil solution. Our objectives were (i) to characterize the water-extractable fraction of soil organic matter (SOM) in terms of chromophoric DOM (CDOM) for different soil types and irrigation water qualities, and (ii) to examine the possible relations between DOM composition and various soil properties. Aqueous extracts from four different soil types irrigated with either secondary TWW or fresh water (FW) were characterized for dissolved organic carbon (DOC) concentration and absorbance at 254nm (Abs254). Excitation–emission matrices (EEM) of fluorescence were determined in these aqueous extracts and in extracts where DOM was separated into acid-soluble and acid-precipitated fractions. Parallel factor (PARAFAC) analysis of the EEMs of water-extractable DOM and their fractions identified two humic-like fluorescent components and one tryptophan-like component and provided their concentration scores. The concentration scores revealed that the acid-soluble DOM fraction contained the greater part of the humic-like fluorescent components present in the bulk DOM samples. Statistical examination demonstrated that the ratio of the concentration score of the humic-like substances that emit light at shorter wavelengths to that of humic-like substances that emit light at longer wavelengths was higher in the acid-soluble DOM fraction compared to the acid-precipitated DOM fraction. Among the FW-irrigated soils, the soil extracts varied in terms of concentration of DOM components. Irrigation of the coarse-textured soils with TWW generally increased the concentrations of DOC, fluorescent components and Abs254 in comparison to irrigation with FW. In the fine textured soils TWW application led to a decrease or had no impact on the concentration of DOC and the CDOM components in water-extractable SOM. Thus, the influence of TWW irrigation on water-soluble SOM relative to that of FW irrigation was also soil dependent. Strong linear relations (/r/>0.7) were found within CDOM indicators including concentration scores of fluorescent components and Abs254. Weak or no relations were found (i) between DOC concentration and CDOM indicators, and (ii) between attributes related to DOM properties and basic soil properties. Hence, nor DOC concentration neither SOM content can be used for estimating changes in concentrations of chromophoric components in water-extractable DOM.

Extractability of water-soluble soil organic matter as monitored by spectroscopic and chromatographic analyses

Cold and hot water processes have been intensively used to recover soil organic matter, but the effect of extraction conditions on the composition of the extracts were not well investigated. Our objective was to optimize the extraction conditions (time and temperature) to increase the extracted carbon efficiency while minimizing the possible alteration of water extractable organic matter of soil (WEOM). WEOM were extracted at 20°C, 60°C, or 80°C for 24 h, 10-60 min, and 20 min, respectively. The different processes were compared in terms of pH of suspensions, yield of organic carbon, spectroscopic properties (ultraviolet-visible absorption and fluorescence), and by chromatographic analyses. For extraction at 60°C, the time 30 min was optimal in terms of yield of organic carbon extracted and concentration of absorbing and fluorescent species. The comparison of WEOM 20°C, 24 h; 60°C, 30 min; and 80°C, 20 min highlighted significant differences. The content of total organic carbon, the value of specific ultraviolet absorbance (SUVA(254)), the absorbance ratio at 254 and 365 nm (E (2)/E (3)), and the humification index varied in the order: WEOM (20°C, 24 h) < WEOM (80°C, 20 min) < WEOM (60°C, 30 min). The three WEOM contained common fluorophores associated with simple aromatic structures and/or fulvic-like and common peaks of distinct polarity as detected by ultra performance liquid chromatography. For the soil chosen, extraction at 60°C for 30 min is the best procedure for enrichment in organic chemicals and minimal alteration of the organic matter.

Response of labile soil organic matter to changes in forest vegetation in subtropical regions

Labile soil organic matter (SOM) can sensitively respond to changes in land use and management practices, and has been suggested as an early and sensitive indicator of SOM. However, knowledge of effects of forest vegetation type on labile SOM is still scarce, particularly in subtropical regions. Soil microbial biomass C and N, water-soluble soil organic C and N, and light SOM fraction in four subtropical forests were studied in subtropical China. Forest vegetation type significantly affected labile SOM. Secondary broadleaved forest (SBF) had the highest soil microbial biomass, basal respiration and water-soluble SOM, and the pure Cunninghamia lanceolata plantation (PC) the lowest. Soil microbial biomass C and N and respiration were on average 100%, 104% and 75%, respectively higher in the SBF than in the PC. The influence of vegetation on water-soluble SOM was generally larger in the 0–10 cm soil layer than in the 10–20 cm. Cold- and hot-water-soluble organic C and N were on average 33–70% higher in the SBF than in the PC. Cold- and hot-soluble soil organic C concentrations in the coniferous-broadleaved mixed plantations were on average 38.1 and 25.0% higher than in the pure coniferous plantation, and cold- and hot-soluble soil total N were 51.4 and 14.1% higher, respectively. Therefore, introducing native broadleaved trees into pure coniferous plantations increased water-soluble SOM. The light SOM fraction (free and occluded) in the 0–10 cm soil layer, which ranged from 11.7 to 29.2 g kg −1 dry weight of soil, was strongly affected by vegetation. The light fraction soil organic C, expressed as percent of total soil organic C, ranged from 18.3% in the mixed plantations of C. lanceolata and Kalopanax septemlobus to 26.3% in the SBF. In addition, there were strong correlations among soil organic C and labile fractions, suggesting that they were in close association and partly represented similar C pools in soils. Our results indicated that hot-water-soluble method could be a suitable measure for labile SOM in subtropical forest soils.

Influence of Tillage, Cropping, and Nitrogen Source on the Chemical Characteristics of Humic Acid, Fulvic Acid, and Water-Soluble Soil Organic Matter Fractions of a Long-Term Cropping System Study

The characterization of organic matter in agroecosystems is important because of its involvement in many important soil ecosystem processes. Humic acid, fulvic acid, and water-extractable organic matter from a 9-year agroecosystem study investigating the effects of tillage, cropping system, and N source was characterized using multidimensional fluorescence spectroscopy with parallel factor analysis (PARAFAC). The fluorescence spectra suite containing all three types of organic matter fractions was modeled by five PARAFAC components. The distribution of component concentrations was unique to each type of organic matter fraction, indicating that these operational extracted fractions reflect differing chemical pools of soil organic matter. The three treatment factors (tillage, cropping, and N source) investigated in this study did not affect the component distribution of the humic and fulvic acid fractions that are regarded as the refractory pools of soil organic matter. In contrast, the component distribution of the more labile water-extractable organic matter fraction was significantly affected by the N source treatment. The results of this study support the use of multidimensional fluorescence spectroscopy with PARAFAC as a method to investigate how management practices of agroecosystems affect the dynamics and chemical nature of soil organic matter pools.

Parallel Factor Analysis of Excitation–Emission Matrix Fluorescence Spectra of Water Soluble Soil Organic Matter as Basis for the Determination of Conditional Metal Binding Parameters

Organic matter-metal complexes in soil solution and aquatic systems are involved in important environmental and ecological processes such as plant nutrient availability and the solubilization and transport of metals. Our work presented here extends the use of fluorescence spectrometry for determining conditional stability constants for such complexes. We combine the use of excitation-emission matrix (EEM) fluorescence spectrometry and parallel factor analysis (PARAFAC) to determine the stability constants of the chemically meaningful components modeled by PARAFAC. Water-soluble organic matter (WSOM) from O-horizon soils of deciduous and coniferous forest stands were extracted and titrated at pH = 4.7 with iron(lll) (Fe) and aluminum (Al) which are important metals in acid soil systems. The EEM spectra were then recorded and PARAFAC analysis showed that the WSOM contained three humic-substance-like components. Fe titration led to fluorescence quenching of the three components, while Al titration enhanced fluorescence for two components and quenched one of the components. The average Ryan-Weber stability constants at pH 4.7 ranged from log K of 4.28 to 4.91 for Fe and 4.84 to 5.96 for Al. The conditional stability constants were similar for Fe binding for deciduous and coniferous stand-derived WSOM, while they were stronger for Al binding with coniferous stand-derived WSOM. This difference in binding strengths for Al may affect the chemical behavior of Al in soil and aquatic systems. Determining the individual binding parameters of organic matter components with metals represents a significant advance over current approaches that utilize fluorescence quenching at a single excitation-emission wavelength pair to characterize organic matter-metal interactions.

Effects of soil acidification and forest type on water soluble soil organic matter properties

Dissolved organic matter (DOM) plays an important role in ecosystem processes such as nutrient release and utilization, mobilization and transport of metals, and carbon sequestration. We investigated the chemical properties of soils from the Bear Brook Watershed in Maine (BBWM) which is the site of a long-term paired-watershed experimental acidification study that includes both deciduous and coniferous stands. Multi-dimensional fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) of the spectra, nuclear magnetic resonance spectroscopy, and size-exclusion chromatography, were used to characterize the DOM extracted from soils sampled from BBWM at four soil depth increments in 2003. Principal component analysis of the base cation, metal, and total C content of the mineral horizons indicated a strong negative relationship between pH and soil Al, Fe, P, and C. The close clustering of Al, Fe, P, and C loadings for the mineral soils suggests that organic matter is important for Al and Fe mobilization and that P solubility is influenced by the metal mobilization process. PARAFAC modeled the fluorescence spectra with three fluorescing components that were in the landscape regions typical of humic substances: (339 nm excitation/470 nm emission), (324 nm/418 nm), and (< 240/465 nm) and the relative distributions of the three components were similar amongst the four treatments and four soil depth increments. The size exclusion chromatograms also revealed a high degree of similarity between all the extracted DOM. Our data suggests that litter quality as influenced by forest composition and ecosystem acidification exert a minor influence on the chemical composition of the water-soluble soil DOM fraction.

Sulfur K-edge XANES spectroscopy reveals differences in sulfur speciation of bulk soils, humic acid, fulvic acid, and particle size separates

X-ray absorption near edge structure (XANES) spectra at the sulfur (S) K-edge ( E=2472 eV) were compared for bulk soil material, humic and fulvic acid fractions, and different particle size separates from Ah horizons of two arable Luvisols, from an O and a Bs horizon of a Podzol under Norway spruce forest, and from an H horizon of a Histosol (peat bog). In the bulk soil samples, the contribution of reduced organic S (organic mono- and disulfides) to total sulfur increased from 27% to 52%, and the contribution of ester sulfate and SO 4 2−-S decreased from 39% to 14% of total S in the following order: arable Luvisols Ah—forested Podzol O—Histosol H. This sequence reflects the increasing organic carbon content and the decreasing O 2 availability in that order. Neither sulfonate nor inorganic sulfide was detected in any of the bulk soil samples. For all samples except the Podzol Bs, the XANES spectra of the bulk soils differed considerably from the spectra of the humic and acid fractions of the respective soils, with the latter containing less reduced S (16–44% of total S) and more oxidized S (sulfone S: 19–35%; ester sulfate S: 14–38% of total S). Also the S speciation of most particle size fractions extracted from the Ah horizon of the Viehhausen Luvisol and the Bs horizon of the Podzol was different from that of the bulk soil. For both soils, the contribution of oxidized S species to total S increased and the contribution of sulfoxides and organic mono- and disulfides decreased with decreasing particle size. Thus, sulfur K-edge XANES spectra of alkaline soil extracts, including humic and fulvic acids or of particle size separates are not representative for the S speciation of the original soil sample they are derived from. The differences can be attributed to (i) artificial changes of the sulfur speciation during alkaline extraction (conversion of reduced S into oxidized S, loss of SO 4 2− during purification of the extracts by dialysis) or particle size separation (carry-over of water-soluble S, such as SO 4 2−), but also to (ii) preferential enrichment of oxidized S in hydrophilic water-soluble soil organic matter (ester sulfate) and in the clay fraction of soils (ester sulfate, adsorbed SO 4 2−).

Effects ofIn SituOzonation on Structural Change of Soil Organic Matter

Four different soils were collected from the field and oxidized using gaseous ozone. The soil organic matter (SOM) was extracted from test soils and analyzed to examine the structural changes of SOM due to the chemical oxidation. Various characteristics of the water-soluble SOM, such as the pH, dissolved organic carbon (DOC), specific UV absorbance (SUVA), carboxylic acidity, average molecular weight, and structure, were strongly affected by SOM content and ozonation time. The pH values of the extracts were found to decrease with longer ozonation times, and this was due to the formation of carboxylic acids originating from the oxidation of the SOM. The DOC concentration and SUVA increased with a greater oxidation time due to the increased dissolution of humic or fulvic acids into the extracts. As well, the ozonation of SOM enhanced the dissolution of hydrophilic fraction of SOM into the soil water phase. Hence, the biologically degradable fraction of the water-soluble SOM increased proportional with ozone...

土壌環境鉱物学への道

This article describes a series of research done on the mineralogy of Canadian soils by the author and his colleagues over the past 35 years. During the period, 1962-77, the research was devoted to the characterization of minerals in soils and sediments with continuing effort to improve analytical methods for mineral identification and characterization. Of the research, a major difference in the clay mineral composition of the western prairie soil and eastern maritime soils was highlighted here. The prairie soils are smectitic, whereas the maritime soils are micaceous. It was noted, however, that potassium available to plants is always sufficient in the prairie soils but not in the maritime soils, although both soils have similar K2O content (∼3%). This is obviously reflected in the smectitic composition of the prairie soils. The composition includes smectite and vermiculite component layers as interstratified minerals, all of which can release K relatively easily than illites in the maritime soils as discrete or/and interstratified minerals. In the following 10 years from 1978-87, the research focused on various interactions of clay minerals with soil organic matter and inorganic compounds, as well as on the detection of poorly crystalline inorganic components in soils. Expandable clay minerals such as smectite can adsorb water-soluble soil organic matter between the layers only under low pH conditions, because under these conditions the majority of soil organic matter molecules are relatively undissociated and mostly remain neutral. At higher pH, the molecules dissociate negatively. This illustrates the importance of pH-dependent charge in the soil system. Oxide, oxyhydroxide, and hydroxide minerals occurring in soils also possess pH-dependent charge, thus they react as positively or negatively charged particles depending upon pH, unlike clay minerals which are permanently negatively charged. In the soil system, recognizing the presence of these charges is very important to understand reactions between minerals, organic matter, metal ions, and agrochemicals. The direction of research further shifted to make mineralogy interact with other domains of science in the last 10 years. This produced research work on mineralogy in a microenvironment rhizosphere, mineral grains as architectural materials for housing soil living habitats and a mineralogical database of Canadian soils. The role of minerals in soils is very diverse. Minerals release major and minor elements, adsorb water, and react with organic and inorganic compounds, and these actions are environmentally favourable and adverse as well. Mineral particles of appropriate sizes are apparently effective for protecting microbes by retarding decomposition of organic matter. Clay minerals, in particular, provide with large surfaces to promote reactions and interactions that play an important role in soil forming processes. Undoubtedly mineralogy will make a significant contribution to a better understanding of very complicated soil environments that have a great impact on the well-being of humankind.

Partitioning of polycyclic aromatic hydrocarbons (PAH) to water‐soluble soil organic matter

SummaryThe mobility of polycyclic aromatic hydrocarbons (PAH) in soils can be influenced by the presence of dissolved organic matter. Partition coefficients of selected polycyclic aromatic hydrocarbons, ranging from 3‐ring to 6‐ring compounds, to water‐soluble soil organic matter (WSSOM) were determined. Partition coefficients were determined for WSSOM obtained from two soils under agricultural use and forest and for commercially available humic acid (Aldrich), taking advantage of a reversed phase (C18) separation method. The WSSOM was characterised with regard to charge and hydrophilic/hydrophobic properties with a dissolved organic matter (DOM) fractionation method. No sorption to WSSOM was found for the tri‐ and tetracyclic PAH, whereas the penta‐ and hexacyclic PAH showed a significant binding to both types of WSSOM and to Aldrich humic acid. The affinity of penta‐ and hexacyclic PAH to WSSOM was considerably lower compared to the affinity to Aldrich humic acid. This is suggested to be due to the lower amount of hydrophobic fractions, c. 30%, in the natural WSSOM as compared to Aldrich humic acid. Effective partition coefficients (Koceff) for the sorption of PAH to bulk soil calculated from KDOC and DOM in the naturally occurring concentration range were only 60–70% of the Koc values in pure water. The impact of DOM on pollutant transport is further influenced by non‐equilibrium behaviour of PAH in soils and by sorption of DOM to the solid‐soil matrix. Several scenarios are described in which the effect of DOM on pollutant transport may become important.

Kinetics of copper complexation with dissolved organic matter using stopped‐flow fluorescence technique

This study provided direct observations on reaction kinetics of Cu and dissolved organic matter (DOM). Stopped‐flow method and fluorescence technique were used to measure Cu(II)‐DOM complexation over time. Suwannee River reference fulvic acid and water soluble soil organic matter were used as model DOM. Experimental conditions of pH 6, 5 × 10‐5 M Cu(II), and 5 mg C/L of DOM were used and the results were to simulate the relevant natural conditions. Both organic ligands reacted fairly rapidly with Cu(II). Reaction half‐life of DOM was in the millisecond range. Two types of metal binding sites of organic ligands were identified. Fulvic acid yield forward rate constants: 1 x 106 M‐1 s‐1 and 5.15 × 103 M‐1 s‐1, based on second‐order reaction rate model. Water soluble soil organic matter gave forward rate constants: 1.34 × 105 M‐1 s‐1 and 2.2 × 103 M‐1 s‐1. From comparison of kinetic natures, the fast‐reacting ligands were comparable to nitrilotriacetic acid (NTA) and slow‐reacting ligands were more like salcy...

Interactions of Herbicides with Water-Soluble Soil Organic Matter

Interactions of water-soluble soil organic matter (WSSOM) with the herbicides bromacil, metribuzin, alachlor, diquat, and paraquat were examined to determine if these interactions can improve predictions of herbicide leaching potential. A high-performance liquid chromatography gel filtration column was used to separate WSSOM extracts from four mineral agricultural soils into fractions with approximate log molecular weights of 4.96 to 6.82. WSSOM fractions were predominantly anionic in nature with some hydrophobic character. The amount of paraquat bound by WSSOM ranged from 1.1 to 2.1 mmol g−1DOC−1 with KDOC (partition coefficient for dissolved organic carbon) values from 0.050 to 0.187 L kg−1. Diquat was bound at 0.9 to 1.5 mmol g−1DOC−1 by the extracts with KDOC values from 0.044 to 0.143 L kg−1. Bromacil, metribuzin, and alachlor did not bind to the extracts tested. WSSOM did not increase paraquat solubility in the presence of soil. Binding of these herbicides to WSSOM in the soils used in this study would not be a significant mechanism for increased mobility and groundwater contamination potential.

Constantes de stabilité de complexes organo-minéraux. Interactions des ions plombeux avec les composés organiques hydrosolubles des eaux gravitaires de podzol

The complexation of lead(II) ions by water-soluble soil organic matter as ligand (recovered by percolating water through a A2 horizon of a podzolic soil) has been studied by potentiometric methods at 25°C in 0.1 M NaClO 4 medium. The total acidity of these solutions was resolved into strong, weak and very weak acidity. During the alkali titration, the dissociation of humic samples was characterized by an α coefficient that may be determined according to a procedure derived from Irving-Rossotti's method. The so-called Henderson-Hasselbalch equation was used to obtain pK A values for each of the two weak acidities ( pK A = 4.80 (COOH) and pK A = 8.85 (OH)). For the determination of stability constants of metal-humate complexes (HA) in the acidic range, three methods have been investigated: • —an approach using only pH measurements. • —direct estimation of the free metal ion concentrations from ISE determinations. • —application of Marinsky's method accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A]. In these experiments typical values used for total lead(II) ion concentrations and total ligand concentrations are respectively in the range 4.10 −5 M to 4.10 −4 M and 0,33 to 0,57 meq·1 −1. All results agree with the existence of mono and bis-complexes PbA and PbA 2 with stability constants (protometric determinations): log K 1 = 4.2 and log K 2 = 3.7. The values obtained by direct estimation of free lead(II) concentration are slightly lower. An increase in stability constants was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH). Ligand conformational properties are expected to be of great importance in the complexation phenomena.

New approach for the determination of stability constants of metal ions-soil organic matter

The binding of metal ions to organic materials in soil and water is an area of considerable interest. A better understanding of the equilibria that determine the behaviour of the chemical elements in the superficial environment is clearly essential for effective geochemical research. Naturally occurring organic compounds are an integral part of soils and aquatic systems. These substances play a major role in the retention and deactivation of anthropogenic organic and mineral chemicals and are involved in the geotransport of metals. Humic substances make up the bulk of organic matter and they probably control the abundance and distribution of trace metals in the environment by complexation or chelation processes through functional groups including carboxylic, phenolic, alcoholic and enolic OH, carbonyl and NH 2 sites at which binding may occur [1]. The determination of stability constants of metal-humic and fulvic acid complexes permits the prediction of the chemical speciation of many metals and the elucidation of their dispersion cycles. The present work deals with the interactions of lead(II), cadmium(II) and copper(II) ions with water soluble soil organic matter. Cu(II) like Zn(II), Mn(II)…, is a metal which is essential to plants while Cd(II) and Pb(II) are without known function in plants but are of considerable interest in view of their toxicity and accumulation in the environment. A variety of methods have been applied for determining stability constants of metalhumate complexes, as reviewed by Stevenson [1]. Considerable progress has been made, but agreement has not yet been reached as to how the data can best be analyzed and interpreted. One of the most popular methods is base titration, which uses the competition of metal ions and protons for complexing sites on the ligand. It is such a modified approach that has been developed in this work. Humic substance samples (in this context, this term will be used to designate a system of organic water soluble compounds including polymers) were recovered by percolating rain-water through a A2 horizon of a podzolic soil and purified by filtration, ultracentrifugation, and treatment by passage through ion exchange resins [2]. Stock solutions of metallic perchlorates were prepared from commercial products and standardized against EDTA. Samples containing constant quantities of humic substances and perchloric acid were titrated by sodium hydroxide in the absence and in the presence of metal ion. Protometric measurements were performed at 25 °C in 0.1 NaClO 4 medium under nitrogen atmosphere. An automatic titrator ‘Tacussel TT Processor’ fitted with extension glass and calomel electrodes was used for recording direct and differential titration curves; pH readings were combined with determinations of free metal ion concentrations by means of ion selective electrodes (ISE). Titration and Protonation Constants of Humic Samples For humic substances which contain COOH and OH groups of various acid strengths we shall define the ligand concentration in terms of titratable acidity. The total acidity content was resolved into a strong, weak and weak acidity by applying linear titration plots derived by McCallum and Midgley [3] in combination with Gran's plots as previously used by Takamatsu and Yoshida [4]. In order to obtain pK a values for each of the two weak acidities, the so-called extended Henderson-Hasselbalch equation was used pH = pK a + n log α/(1 − α) During the alkali titration, the dissociation of humic samples was characterized by a coefficient that may be determined from titration curves of perchloric acid alone and in the presence of humic substances, according to a procedure derived from Irving-Rossotti's method [5]. Figure 1 shows typical Henderson-Hasselbalch plots leading for instance to results such as: • strong acidity: 0.32 meq 1 −1 • weak acidity groups (COOH): 0.33 meq 1 −1; pK a = 4.54; n = 2.15. • very weak acidity groups (OH): 0.34 meq 1 −1; pK a = 9.33; n = 2.59. ▪ Stability Constants of MetalHumate Complexes (HA) Three different methods have been investigated in the acidic range: • an approach closely related to that of Gregor . [6] but the experimental values of [HA] and [A −] are determined using an original derivation of equations resulting from the application of Irving and Rossotti's concepts [5]. • direct estimation of the free metal ion concentrations from ISE measurements. • application of Marinsky's method [7] accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A −]. Reasonable agreement is observed. The existence of mono-complexes MA is expected for the three metal ions, while bis-complexes MA 2 are detected with Pb(II) and Cu(II) ions. Stability of the complexes followed the order Cu > Pb 》Cd. An increase in stability constant was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH), as exemplified by Fig. 2.