Considerations on cross‐linking by bivalent cations in soil organic matter with low exchange capacity

A soil's cation exchange capacity (CEC) is expected to be relatively inert against changes in cation loading. In this study, we treated a soil sample originating from the organic layer of a forest soil with various bivalent cations after removing the native cations. Sorption isotherms and cation exchange capacity were determined, the latter using the BaCl₂ method. Sorption showed Langmuir characteristics, with the maximum coverage (Q_max) increasing in the order Ba²⁺ < Ca²⁺ < Mg²⁺, but being clearly smaller than the initial load of native exchangeable cations. The Langmuir coefficient, k_Me, depended oppositely to the order obtained for Q_max. CEC increased upon cation treatment and it varied by a factor of almost two. The unexpected variation of CEC was explained by the low cation exchange capacity of the organic matter such that not all functional groups are close enough to be bridged and the second charge of a bivalent cation is not neutralized by the organic functional group. The Langmuir sorption type, and Q_max being smaller than the content of sorption sites and being largest for Mg, suggested that only a part of the sites can be cross‐linked and at least part of the cross‐links are formed by hydrated cations. Thermodynamic considerations allowed reconstruction of two contrasting processes during CEC determination by Ba²⁺: Case A: the disruption of cross‐links, which increases with the cationic strength and the cation load before CEC determination, but does not require structural re‐orientation in the SOM matrix, and Case B: the formation of new cross‐links during CEC determination, depending only on the content of unoccupied sites before CEC determination and requiring structural re‐organization of the matrix and thus a minimum matrix flexibility. The use of bivalent cations for CEC determination may thus result in an overestimation of CEC for organic matter with low CEC. This has, however, promising potential when comparing CEC determined with monovalent cations and bivalent cations. Using a set of bivalent cations, may allow probing distribution of distances between functional groups in the organic matter and even characterize the matrix rigidity of the cation‐cross‐linked network.     

Interactions between cations and water molecule bridges in soil organic matter

Purpose

Nutrient release, soil wettability, water binding, and matrix rigidity of soil organic matter (SOM) can be affected by cross-links between segments of SOM, cations, and water molecule bridges (WaMB). Not all cation effects on SOM can be explained with the currently accepted idea that multivalent cations cross-link organic matter segments via direct cation bridges (CaB). The objective was to understand these interactions and their effect on SOM matrix rigidity and wettability.

Materials and methods

We modified cation composition of two peats and an organic surface layer (OSL) using cation exchange resin to remove cations and solutions of Na⁺, Ca²⁺, or Al³⁺ to enrich samples with cations. SOM matrix rigidity was determined at 4 and >8 weeks after treatment via the WaMB transition temperature T*, using differential scanning calorimetry. Wettability was measured via sessile drop contact angle (CA).

Results and discussion

The effect of cation removal on T* depended on cation exchange capacity and initial cation content. Cation addition to OSL increased T*. This effect increased with increasing cation loading and valency, and T* correlated with CA. Classical cross-linking can neither explain the higher heterogeneous matrix of Ca-treated than Al-treated samples nor the aging-induced convergence of T* for different cations and concentrations. The latter is likely due to interaction between CaB and WaMB in SOM.

Conclusions

Associations of CaB and WaMB evolve slowly and form a supramolecular network in SOM. Those dynamic associations can fix molecular arrangements inducing water repellency and increase kinetic barriers for the release and uptake of water and nutrients from aged soil.     

Cation activation of pyrophosphatases from soil microorganisms

The intracellular inorganic pyrophosphatases obtained from mixed cultures of soil microorganisms were characterised in terms of the activating cation and pH-activity profile. Microorganisms were grown in an initial culture 10 mm in tetrasodium pyrophosphate, a nutritionally non-specific aqueous medium, buffered at pH 7.0 with an organic buffer and inoculated with soil. The intracellular pyrophosphatases in a desalted, cell-free enzyme preparation were examined for activity in assays using 1 mM tetrasodium pyrophosphate and nine cations (Mg²⁺, Zn²⁺, Co²⁺, Mn²⁺, Fe²⁺, Ca²⁺, Cu²⁺, Al³⁺ and Fe³⁺) over the pH range 2–10, with appropriate buffers. Only the first 5 cations activated the enzymes. The pH-activity profiles for each cation were different and each displayed only one pH optimum. The pH optima were not greatly affected by cation concentration or kind of buffer. The specific activities at the optimum pH and cation concentration for each culture decreased in order Mg²⁺ > Zn²⁺ > Co²⁺ > Mn²⁺ > Fe²⁺. The results indicate the production of functionally similar enzymes by diverse microorganisms.     

Temperature effects on N mineralization: changes in soil solution composition and determination of temperature coefficients by TDR

We studied the changes in composition of the soil solution following mineralization of N at different temperatures, with a view to using TDR to calculate temperature coefficients for the mineralization of N. Mineralization from soil organic nitrogen was measured during aerobic incubation under controlled conditions at six temperatures ranging from 5.5 to 30°C, and at constant water content in a loamy sand soil. We also monitored during the incubation the concentrations of SO₄^2–, Cl^–, HCO₃^–, Ca²⁺, K⁺, Mg²⁺ and Na⁺, and the pH and the electrical conductivity in 1:2 soil:water extracts. Zero‐order N mineralization rates ranged between 0.164 at 5.5°C and 0.865 mg N kg^?1 soil day^?1 at 30°C. There was a significant decrease in soil pH during incubation, of up to 0.6 pH units at the end of the incubation at 30°C. The electrical conductivity of the soil extracts increased significantly at all temperatures (the increase between the start and the end of the incubation was 4‐fold at 30°C) and was strongly correlated with N mineralization. The ratio of bivalent to monovalent cations increased markedly during mineralization (from 2.2 to 5.9 at 30°C), and this increase influenced the evolution of the electrical conductivity of the soil solution through the differences in molar‐limiting ion conductivity between mainly Ca²⁺ and K⁺. Zero‐order mineralization rate constants, k, for NO₃^– concentrations calculated from TDR varied between 0.070 (at 5.5°C) and 0.734 mg N kg^?1 soil day^?1 (at 30°C), which were slightly smaller, but in the same range, as the measured rates. Underestimation of the measured N mineralization rates was due, at least in part, to differences in cation composition of the soil solution between calibration and mineralization experiments. A temperature‐dependence model for N mineralization from soil organic matter was fitted to both the measured and the TDR‐calculated mineralization rates, k and k_TDR, respectively. There were no significant differences between the model parameters from the two. Our results are promising for further use of TDR to monitor soil organic N mineralization. However, the influence of changing cation ratios will also have to be taken into account when trying to predict N mineralization from measured electrical conductivities.     

Uptake of cations by annual ryegrass as related to cations adsorbed onto root exchange sites

Abstract

Previously published results on exchange capacities for Ca²⁺, Mg²⁺, Mn²⁺, and K⁺ in the Donnan free space of roots of two ryegrass cultivars (Lolium multiflorum Lam. cv. Marshall and Wilo) grown at two Al levels in the nutrient solution (0 and 74 μM) were correlated with the average net uptakes of the same cations. For Al‐treated plants regressed separately, significant correlations r=0.906 and r=0.963 were found for Mn²⁺ and Ca²⁺/Mg²⁺, respectively. No significant correlations were observed for these cations in control plants. In contrast, when data of control and Al‐treated plants were combined, significant linear correlations r=0.805, r=0.924, and r=0.968 were found for Ca²⁺, K⁺, and K⁺/(Ca²⁺+Mg²⁺)^1/2, respectively. The influence of cations adsorbed onto the root exchange sites and the effect of Al on the cation uptake processes were discussed.     

A model for the impact of soil solution Ca: Al ratio,soil moisture and temperature on tree base cation uptake

A model for tree base cation uptake has been developed, dependent on the soil solution concentration of Al³⁺, divalent base cations such as Ca²⁺, Mg²⁺ and H⁺ ions, modelled with a Mikaelis-Menten type of expression based on the molar BC∶Al ratio, where BC is the sum of the divalent non-toxic base cations Ca²⁺ and Mg²⁺. The expression has the form of counteracting adsorption isoterms for BC and Al to the tree root. The effect of toxic levels of Hg and Pb is incorporated into the model, using root adsorption as the mechanism, and parameterization against experimental data. The expression is modified with an expression accounting for the effect of heavy metal toxicity and soil water content. The dependence of the uptake rate on soil moisture content can be shown to have the form of a Freundlich adsorption isotherm for water. The available data indicate an activation energy of 47 kJ^?1 mol for base cation uptake to trees. Data from the literature was used to estimate the rate coefficients and ion selectivity coefficients for typical coniferous and decideous trees in Sweden and Germany. The model indicates that Ca²⁺ and Mg²⁺ is effective in mitigating Al³⁺ toxicity to tree roots, and that increasing the Ca²⁺+Mg²⁺ soil solution concentration has a promotive effect on base cation uptake. Above a certain limit base cation uptake becomes independent of the solution base cation concentration. This is consistent with field observations, and may be developed to become a tool for assessing the impact of soil chemistry changes on forest growth rate and health status. Field data from the Swedish Forest Survey indicate that uptake depend on the square root of the soil solution base cation availability originating from weathering and deposition input, which is consistent with the BC∶Al expression of the model.     

Sorption of primisulfuron on soil, and inorganic and organic soil colloids

Inorganic and organic soil colloids are responsible for the sorption of many pesticides. We studied the sorption of the herbicide primisulfuron [methyl 2 N‐[[[[[4,6‐bis(difluoromethoxy)‐2‐pyrimidinyl]amino]carbonyl]amino]sulfonyl]benzoate] on Fe³⁺‐, Al³⁺‐, Ca²⁺‐ and Na⁺‐exchanged montmorillonite, soil organic matter (H⁺‐ and Ca²⁺‐saturated), amorphous iron oxide, and three soils in aqueous media. The sorption on soils was negatively correlated with pH. Ca²⁺‐ and Na⁺‐exchanged montmorillonites are ineffective in the sorption of primisulfuron. The sorption on Fe³⁺‐ and Al³⁺‐exchanged montmorillonite is rapid and follows the Freundlich equation. Fourier transform infrared (FT‐IR) and X‐ray powder diffraction studies of the Fe³⁺‐ and Al³⁺‐montmorillonite samples after the interaction with primisulfuron in chloroform solution suggest that primisulfuron is adsorbed and degraded in the interlayer. Humic acid is more effective in the sorption than is Ca humate, suggesting that the pH of the suspension (3.5 for humic acid and 6.0 for Ca humate) has a strong influence on the sorption of primisulfuron. Experiments on amorphous iron oxide indicate similar pH dependence. Infrared spectra indicate that the protonation of the pyrimidine nitrogen moiety of herbicide and subsequent hydrogen bonding with the surface hydroxyls of Fe oxide is the mechanism acting in the primisulfuron sorption.     

Predicting cation exchange equilibria in soil containing three heterovalent cations

Theoretical cation exchange equilibria relationships in a soil for a ternary system containing Na⁺, Ca²⁺ and Mg²⁺ are presented. The solid phase activity coefficients of cations for the three binary systems (i. e., Na⁺ – Ca²⁺, Na⁺ – Mg²⁺ and Ca²⁺ – Mg²⁺) were used to calculate the Wilson parameters. Adsorbed phase activity coefficients of Na⁺, Ca²⁺ and Mg²⁺ corresponding to their equivalent fractions on the adsorbed phase of A horizon of a Camborthid for the ternary system (i. e., Na⁺ – Ca²⁺ – Mg²⁺) were evaluated, using these Wilson parameters. Using the values of activity coefficients of adsorbed Na⁺, Ca²⁺ and Mg²⁺ in a ternary soil system and of thermodynamic equilibrium constants for the binary systems, the multiple linear regression relationships between the equivalent fractions of Na⁺, Ca²⁺ and Mg²⁺ on the adsorbed phase and their mole fractions in the equilibrium solution were obtained. The amounts of adsorbed phase cations so predicted showed a fair agreement with those observed experimentally in a ternary system.     

Lysimeter study with a cambic arenosol exposed to artificial acid rain: II. Input-output budgets and soil chemical properties

The effects of artificial precipitation with different pH levels on soil chemical properties and element flux were studied in a lysimeter experiment. Cambic Arenosol (Typic Udipsamment) in monolith lysimeters was treated for 6 1/2 yr with 125 mm yr^?1 artificial rain in addition to natural precipitation. Artificial acid rain was produced from groundwater with H₂SO₄ added. pH levels of 6.1, 4 and 3 were used. ‘Rain’ acidity was buffered, mainly due to cation exchange with Ca²⁺ and Mg²⁺, which were increasingly leached due to the acid input. The H⁺ retention was not accompanied by a similar increase in the output of Al ions, but a slight increase in the leaching of Al ions was observed in the most acidic treatment. The net flux of SO₄ ^2? from the lysimeters increased with increasing input of H₂SO₄, but in the most acidified lysimeters significant sorption of SO₄ ^2? was observed. The sorption was, however, most likely a concentration effect. The ‘long-term’ acidification effects on soil were mainly seen in the upper O and Ah-horizons, where an impoverishment of exchangeable Ca²⁺ and Mg²⁺ was observed. An increased proportion of Al ions on exchange sites in the organic layer was observed in the pH 3-treated soil. By means of budget calculations the annual release of base cations due to weathering was estimated to be between 33 and 77 mmol_c m^?2.     

EFFECTS OF IONIC STRENGTH ON CHEMISORPTION AND POTENTIAL-DETERMINING SORPTION OF PHOSPHATE BY SOILS

Amounts of inorganic phosphate (P) sorbed by two unfertilized soils, during times less than required to reach equilibrium, were affected by the ionic strength and cation species of the matrix solution. For non-equilibrium conditions the amounts of P sorbed increased with increasing ionic strength and were greater with Ca²⁺ than Na⁺. For higher P additions, resulting in equilibrium solution P concentrations greater than 30 to 40μrnole 1^?1, the effects of the matrix solution on P sorption were maintained at equilibrium, whereas at lower P additions the dependence of sorption on matrix solution composition was eliminated at equilibrium. Equilibrium sorption isotherms for each soil and matrix solution were described by three Langmuir equations, which corresponded to distinct concentration ranges or regions (I, II, and III) on the overall isotherm. The free energies of sorption (ΔG) for each region, were essentially independent of the soil matrix solution. The sorption maxima for regions I and II of the isotherm for a particular soil were also virtually independent of the matrix solution used. The sorption maximum for region III, however, was markedly dependent on the matrix solution, implying a potential-determining (p.d.) sorption mechanism.     

Zur Bestimmung der Kationenaustauschkapazität von Böden mittels Barium

Regarding the Determination of Cation Exchange Capacity of Soils with Barium Very often the cation exchange capacity (CEC) of soils is determined using Ba⁺⁺ as exchange cation. In soils with high contents of organic matter, Ba⁺⁺ cannot be desorbed completely by Mg⁺⁺ or H⁺ but by Ca⁺⁺. Soils containing vermiculite adsorb Ba⁺⁺ in non exchangeable form. The fixed Barium can only be determined by total digestion. Therefore in soils containing vermiculite the exchange with Ba⁺⁺ leads to an underestimation of the CEC.     

Aluminium release in relation to the determination of cation exchange capacity of some podzolized New Zealand soils

In upper mineral horizons, CEC by compulsive and isotopic exchange methods, using Ba²⁺ as the saturating cation, gave higher values than the effective CEC at natural soil pH, and much higher values than CEC determined with m NH₄OAc at pH 7. Cumulative Al release during leaching was considerably higher using Mg²⁺ and Ba²⁺ chlorides than K⁺ and NH₄⁺ chlorides, and gave a different shape extraction curve. Basal spacing of the dominant dioctahedral vermiculite in the soil clays contracted from 14.5Å to 10.0–10.9 Å when saturated with NH₄⁺ and K⁺, restricting release of interlayer Al. Lower horizons, containing a large proportion of Al-chlorite in the clay fraction, which did not contract with any of the cations, showed more normal exchange behaviour. On leaching, Al release was slightly greater with K⁺ and NH₄⁺, than with Mg²⁺ and Ba²⁺, chlorides. The implication of the results for CEC measurements is discussed.     

Soil disturbance and elemental dynamics in a Northern Hardwood forest soil,USA

Loss of soil nutrients due to disturbance may serve as an index of the homeostasis of biogeochemical cycling and ecosystem stability. Soil and the surrounding root system were disturbed during the installation of Soil Containment Systems (SCSs) in the hill slope at the Bear Brook Watershed in Maine (BBWM). The SCSs were constructed from high density PVC pipe (24 cm i.d. and 30 cm height) implanted at field. Leachate cations and anions, soil organic matter and exchangeable cations were analyzed. Leachate NO₃ ^? was higher by an order of magnitude compared to undisturbed soils from the same research site and other hardwood forest soils in the northeast U.S. The concentrations of cations in the leachate from SCSs were also higher and loss of NO₃ ^? was positively correlated with the loss of most cations. Calcium was the dominant cation representing 55% of the base cation composition of soil leachate. Monthly losses of Ca²⁺, Mg²⁺ and K⁺ were 1.8, 1.6 and 1.2% of total exchangeable pools, respectively. Disturbance of the BBWM soil ecosystem caused high rates of NO₃ ^? leaching which markedly changed the soil biogeochemistry. These results and other supporting data from watershed mass balances and experimental chemical additions suggest that BBWM may be N saturated.     

Cation exchange capacity measurements

Abstract

Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, vaste disposal on soils, and soil taxonomy. The objective of this research was to compare CEC values for arable Ultisols from the humid region of the United States as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of two of the CEC procedures were also evaluated. The six procedures evaluated were: (1) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity; (2) N Ca(OAc)₂ (pH 7.0) saturation with Mg(OAc)₂ (pH 7.0) displacement of Ca²⁺; (3) N NH₄OAc (pH 7.0) saturation with NaCl displacement of NH₄ ⁺; (4) N MgCl₂ saturation with N KCl displacement of Mg²⁺; (5) compulsive exchange of Mg²⁺ for Ba²⁺; and (6) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable AJ. The unbuffered procedures reflect the pH dependent CEC component to a greater degree than the buffered methods. The compulsive exchange and the summation of N NH₄OAc exchangeable cations plus N KCl exchangeable Al procedures gave CEC estimates of the same magnitude that reflect differences in soil pH and texture. The buffered procedures, particularly the summation of N NH₄OAc exchangeable cations plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity, indicated inflated CEC values for these acid Ultisols that are seldom limed above pH 6.5. Exchangeable soil Ca and Mg levels determined from extraction with 0.1 M BaCl₂ were consistently greater than values for the N NH₄Oac (pH 7.0) extractions. The Ba²⁺ ion is apparently a more efficient displacing agent than the NH₄ ⁺ ion. Also, the potential for dissolving unreacted limestone is greater for the Ba₂ ⁺ procedures than in the NH₄ ⁺ extraction.     

A new index of cation saturation state in soil

Abstract

Proposed is a new index for chemical properties of soil which designates the cation saturation state of soil colloids. It has the formula (K⁺+ l/2Ca²⁺+ 1/2Mg²⁺ + Na⁺)/H⁺. In a model experiment, three levels of soil moisture tension and four combinations of calcium (Ca) and magnesium (Mg) were applied. These factors significantly affected the variation in the number of cations adsorbed, which allowed a comparison of the base saturation index and the cation saturation state index over a wide range of values. It was determined that a new index of cation saturation state might be useful in describing the chemical characteristics of a soil.     

Base Cation Fluxes in Mountain Landscapes of Lake Baikal Southern Shore

Abstract

Assessing base cation [calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺)] fluxes is necessary for determining ecosystem stability. This study was conducted in the Osinovka River catchment located on the Khamar‐Daban Ridge, South Baikal, which is characterized by high precipitation (more than 1700 mm per annum). The contributions of different ecosystem components such as atmospheric deposition, soil exchange pool, minerals, vegetation, surface water, and groundwater into the total base cations flux were evaluated, combining the strontium isotope approach and budget study. Results show the leading role of atmosphere in ecosystem supply with base cations. The atmosphere contributes 48% of total base cations flux, and its contributions to plant‐available nutrition pools of both organic and mineral horizons are equal to 50%. This makes the vitality of vegetation, to a great extent, dependent on the stability of atmospheric chemistry.     

Water-soluble cations in shoot dry weight of common bean and balance of ions in the soil in response to the magnesium application

The increased magnesium (Mg) concentration in vegetables may be reduced due to inter-ionic inhibition caused by the concentration of other water-soluble cations that are mainly associated to low molecular weight organic anions. However, it is not known whether the levels of these compounds in crop residues are modified by increasing the Mg soil application. This study aimed to assess the effects of the Mg application on the contents of water soluble cations [potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), cooper (Cu²⁺), iron (Fe²⁺), manganese (Mn²⁺), and zinc (Zn²⁺)] on common bean plant residues. The experiment was conducted under greenhouse conditions with Ustoxix Quatzipsamment in completely randomized design in 4×5 factorial scheme, with three replicates. The treatments consisted of four rates of Mg [0, 50, 100 and 200 mg kg^?1, source magnesium chloride (MgCl₂)] and five varieties of common bean of the carioca group [BRS Estilo, BRS Ametista, IPR Campos Gerais (CG), IPR Tangará and IAPAR 81]. The Mg rates affected the contents of water soluble Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺ and Mn²⁺ in the extracts of bean residue. The soluble Mg²⁺ showed a significant correlation with foliar Mg content, indicating the need for further research on the method used to assess nutrient availability in vegetables. The bean varieties showed different responses regarding balance of ions in cation exchange capacity (CEC) and in the Ca/Mg, Ca/K and Mg/K ratios in the soil.     

Cation treatment and drying‐temperature effects on nonylphenol and phenanthrene sorption to a sandy soil

The objective of this study was to investigate the effects of mono‐ and polyvalent cations on sorption of the two hydrophobic compounds nonylphenol (NP) and phenanthrene (Phe). To this end, exchange sites of a sandy soil were saturated with either Na⁺, Ca²⁺, or Al³⁺ and excess salts were removed by washing. The samples were then sterilized and either stored moist, dried at room temperature, or at 20°C, 60°C, or 105°C in a vented oven. Saturation with Na⁺ led to an increase of dissolved organic C (DOC) concentration in the soil water extracts, whereas the polyvalent cations Ca²⁺ and Al³⁺ decreased it. The ¹H‐NMR relaxometry analyses showed that Al³⁺ restricted the mobility of water molecules that are confined within the SOM structure to a higher extent than Ca²⁺ or Na⁺. According to contact‐angle (CA) analyses, cation treatment did not significantly change the wetting properties of the samples. Batch sorption–desorption experiments showed no clear salt‐treatment effects on the sorption and desorption equilibria or kinetics of NP and Phe. Instead, the sorption coefficients and sorption hysteresis of NP and Phe increased in dry soil. With increasing drying temperature the CA of the soils and the sorption of both xenobiotics increased significantly. We conclude that structural modifications of SOM due to incorporation of polyvalent cations into the interphase structure do not modify the sorption characteristics of the soil for hydrophobic compounds. Instead, increasing hydrophobization of organic soil constituents due to heat treatment significantly increased the accessible sorption sites for nonpolar organic compounds in this soil.     

Phosphate adsorption by Na−, Mg− and Ca−saturated soil clays

Phosphate adsorption by the homoionic clay (Na, Ca, Mg) of three soils with widely varying mineralogy obeyed the Langmuir isotherm. The Langmuir constants b and k were a function both of the nature of the dominant clay mineral present and of the saturating cations following the order: Ca⁺⁺ ? Mg⁺⁺ ? Na⁺. This order was in agreement with those predicted by the diffuse double layer theory. The sample having montmorillonite when saturated with sodium, exhibited a negative adsorption of phosphorus which changed to a large positive adsorption in the presence of 0.1 N NaCl; It was concluded that the exchangeable cations influenced the extent of P adsorption by controlling the accessibility of the edge clay surfaces to phosphate ions.