Ca2+ Effects on K+ Fluxes in Arabidopsis Seedlings Exposed to Al3+

Potassium transport was investigated in the root elongation zone of Arabidopsis seedlings during the first minutes of Al³⁺ exposure, using the non-invasive MIFE microelectrode technique. To prevent pH changes during Al³⁺ application, and to separate aluminium from acidic stress, plants were pre-treated with 5 mM homoPIPES before addition of AlCl₃ (pH 4.2). The 30-min treatment with 50 or 500 μM AlCl₃ led to a significant increase in K⁺ efflux in solutions containing 100 μM CaCl₂. This efflux was suppressed by high concentrations of Ca²⁺ (10 mM) in the bathing solution. Our results suggest that elevated external Ca²⁺ activities can sustain K⁺ influx in the root elongation zone during Al³⁺ exposure either by maintaining [Ca²⁺]_cyt or by affecting Al³⁺ uptake across the plasma membrane.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER. IV. ADSORPTION AND DESORPTION OF Pb2+, Cu2+, Cd2+, Zn2+ AND Ca2+ BY PEAT

The equilibria as well as the rates of adsorption and desorption of the ions Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺, and Ca²⁺ by soil organic matter were determined in batch experiments as a function of the amount of metal ions added to an aqueous suspension of HCl-washed peat. Simultaneous determination of the metal ions and hydrogen ions in the solution by atomic absorption spectrophotometry and pH-measurements showed that the adsorption of one divalent metal ion by peat was coupled with the release of two hydrogen ions. Since this equivalent ion-exchange process causes a corresponding increase of the electric conductivity of the solution, the rates of the adsorption and desorption processes were determined by an immersed conductivity electrode. The distribution coefficients show that the selective order for the metal adsorption by peat is Pb²⁺ > Cu²⁺ > Cd²⁺≌ Zn²⁺ > Ca²⁺ in the pH range of 3·5 to 4·5. The slope of -2, as observed in a double logarithmic plot of the distribution coefficients versus the total solution concentration confirms the equivalence of the ion-exchange process of divalent metal ions for monovalent H₃O⁺ -ions in peat. The absolute rates of adsorption, as well as the rates for the fractional attainment of the equilibrium, increase with increasing amounts of metal ions added. This behaviour is also observed for the subsequent desorption of the metal ions by H₃O⁺-ions. At a given amount of metal ions added, the absolute rates of adsorption decrease in the order Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Ca²⁺, while the rates for the fractional attainment of the equilibrium decrease in the order Ca²⁺ > Zn²⁺≌ Cd²⁺ > Pb²⁺ > Cu²⁺. The half times for adsorption and desorption were in the range of 5 to 15 sec.     

Complexation of Cu2+ and Pb2+ by peat and humic acid

The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu²⁺ and Pb²⁺) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb²⁺ than for Cu²⁺ (e.g. at pH = 5 HA gave 0·188 mmol Cu²⁺ g^?1 and 0·564 mmol Pb²⁺ g^?1: peat gave 0·111 mmol Cu²⁺ g^?1 and 0·391 mmol Pb²⁺ g^?1). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K₁) for Cu²⁺ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K₁= 2·63, and peat gave log K¹= 4·47 for Cu²⁺). The values for Pb²⁺ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K¹= 3·03 and peat gave log K¹= 3·00 for Pb²⁺). In the peat, Cu²⁺ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb²⁺ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions.     

The H+/M2+ exchange stoichiometry of calcium and zinc adsorption by ferrihydrite

The specific adsorption of Ca²⁺ and Zn²⁺ by ferrihydrite results in the net release of H⁺. The rate and H⁺/M²⁺ exchange stoichiometry of this reaction were monitored with a pH-stat. A rapid reaction of less than 6 min was followed by a slower reaction which continued at a diminishing rate for at least 2 days. Adsorption of Ca²⁺ at pH 7.8 and Zn²⁺ at pH 5.4 resulted in the net release of 0.92 and 1.70 mol H⁺/mol M²⁺ adsorbed, respectively. For Zn²⁺ adsorption, this stoichiometry was shown to be independent of pH. These estimates agree well with independent estimates based on the pH dependence of adsorption. The difference between the Ca²⁺ and Zn²⁺ stoichiometries was related to the differing acidity of the –OH₂ ligands attached to the adsorbed ions.     

Blocking the release of potassium from clay interlayers by small concentrations of NH4+ and Cs+

To understand the process and the kinetics of potassium release from the clay interlayer in natural and arable soils in more detail, I tested the hypotheses that large, monovalent cations, especially NH₄⁺ and Cs⁺, can reduce the release rates of K⁺ which is exchanged by Ca²⁺, even if these monovalent cations are present in concentrations of only a few μm . Percolation experiments were carried out with different illitic soil materials, some containing vermiculite, with 5 m m CaCl₂ at pH 5.8 and 20°C, in some cases for over 7000 h. NH₄⁺ and Cs⁺ both caused a large decrease in the rate at which K⁺ was released, Cs⁺ especially. Suppression began at 5 μm NH₄⁺ Blocking by 20 μm NH₄⁺ was easily reversible: the release rates readily increased when NH₄⁺ was omitted from the exchange solution. Blocking by 2 μm Cs⁺ was equal to approximately 90% of that at 10 μm Cs^+. Larger concentrations of Cs⁺ than 10 μm did not further reduce release but rather caused a slight increase, probably because of enhanced exchange of K⁺ by Cs⁺ without exfoliation of the interlayer space. Blocking by Cs⁺ was not reversible within > 7000 h of percolation by 5 m m CaCl₂. The blocking effect was reproduced in several different soil materials using 10 μm Cs⁺ but was most pronounced in vermiculite-rich samples. As NH₄⁺ is present in most arable soils, at least in concentrations of a few μm , I conclude that the observed effects are of significance in the K dynamics processes in soils, for example near the roots of plants. Further, very small concentrations of Cs⁺ in exchange solutions containing a large background of Ca²⁺ appear to be useful for suppressing K⁺ release from the interlayer in laboratory studies, probably without significantly altering the exchange at outer mineral surfaces.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Interactions of aluminium and fulvic acid in moderately acid solutions: stoichiometry of the H+/Al3+ exchange

Complexation with organic matter controls the activity of dissolved Al³⁺ in many soils. The buffering intensity of these soils is largely dependent on the H⁺/Al³⁺ exchange ratio, i.e. the number of protons consumed by the solid phase when one Al³⁺ is released. Here, the H⁺/Al³⁺ exchange ratio was determined from batch titrations using solutions of fulvic acid (FA) as a model for soil organic matter. Aluminium was added, from 1.04 to 6.29 mmol Al per g FA, which is within the range of humus‐bound Al found in the upper B horizon of podzolized soils. Furthermore, pH was varied with NaOH to give values between 3.5 and 5.0. The H⁺/Al³⁺ exchange ratio ranged between 1.49 and 2.23 with a mean of 1.94. It correlated positively with pH and the total concentration of Al present. Theoretically, this can be explained with a partial hydrolysis of bound Al. The slope of logAl (log₁₀ of Al³⁺ activity) against pH generally underestimated the actual exchange ratio, which can partly be attributed to the systems being diluted (100 mg FA l^?1). However, where 4 mmol Al or more had been added per g FA, the logAl slope gradually approached ?3 between pH 4.5 and 5.0. This might be the result of a shift from Al³⁺ activity control by humus complexation to control by Al(OH)₃(s).     

Aluminosilicate phases during initial clay formation: H+-Al3+–oxalic acid–silicic acid–Na+ system

The solid phases and the precipitation boundary characterizing the system H⁺-Al³⁺-oxalic acid-silicic acid-Na⁺ are discussed. Model experiments have been used to throw more light on two environmental problems: the formation of sparingly soluble aluminium silicates in oceans and alkaline lakes, which could be determining aluminium and silicate concentrations in pore waters of sediments, and the validity of inorganic and organic mechanisms of podzolization and their significance for soil science. pH and Tyndallometric measurements were performed at constant ionic strength of 0.6 M NaCl at 25°C. Three phases Al(OH)₄, H₄SiO₄ (phase Vi^a), Al₂, (OH)₆.H₄SiO₄ (phase VI^b) and NaAl(OH)₄.(H₄SiO₄), (phase VIII) determine the precipitation boundary. Phase NaAl(OH)₄.H₄SiO₄ (phase VII precipitates at 0.4pH units above NaAl(OH)₄.(H₄SiO₄)₂. Using a set of previously determined binary and ternary complexes, and phases of the subsystems, the following formation constants were deduced: Phases VI^a and VI^b are described as end-members of the allophane series with Si: Al ratios of 1:1 and 1.2. Phase VI^b was identified with protoimogolite allophane. These two phases are good model clays for podzolic soils and are extremely soluble at pH < 4. Sodium phases could be hydrous feldspathoids. These phases are possible in sediments of seawater or saline lakes. It is suggested that organic and inorganic mechanisms of podzolization operate sequentially and that neither of them alone can completely describe the process.     

Coupled diffusion and oxidation of ferrous iron in soils. II. A model of the diffusion and reaction of O2, Fe2+, H+ and HCO3− in soils and a sensitivity analysis of the model

Kinetic equations are developed for a system in which a column of reduced soil is exposed to oxygen at one end. The equations are combined in a simulation model in which they are solved by finite-difference methods. The model predicts the consequent diffusion of oxygen into the column; the diffusion of ferrous iron towards the oxidation zone; the rate of formation and concentration profile of the ferric hydroxide formed; and the diffusion by acid-base transfer of the acidity produced in the oxidation reaction. A sensitivity analysis of the model, in which runs were made for a wide range of input parameters, showed that for most combinations of parameters, in water-saturated soil, substantial amounts of iron are transferred towards the air-exposed surface, leading to a well-defined zone of ferric hydroxide accumulation. The profile of total iron in this zone is often banded. The pH in the zone falls by at least two units. A small amount of air-filled pore space increases the depth of the oxidation front dramatically. The model indicates that coupled iron oxidation and diffusion reactions, which are very widespread in natural soils, may be understood quantitatively.     

COUNTER DIFFUSION OF Rb86 AND Sr89 IN COMPACTED SOIL

Counter-diffusion coefficients of Rb⁸⁶ and Sr⁸⁹ counter diffusing against H⁺ ions were measured in Dundee silt loam and Sharkey clay soils at differing soil bulk-densities. The cation exchange complex of each soil was saturated with either Rb⁺, Sr⁺⁺, or H⁺ and washed free of salts before making diffusion measurements. The water content of the soil on an oven-dry weight basis was maintained at a constant value for all bulk-densities; 14.2 and 28.0 per cent for the Dundee and Sharkey soils respectively. These moisture contents correspond to a tension of 2/3 bar for sieved soil. The diffusion coefficients were dependent upon concentration. Average counter-diffusion coefficients were calculated and related to soil bulk-density. Soil compaction of Dundee silt loam had little or no effect upon the counter diffusion of Rb⁸⁶. The average counter-diffusion coefficients of Sr⁸⁹ in Dundee silt loam and Sharkey clay were significantly and linearly related to bulk-density; as bulk-density increased the average counter-diffusion coefficients increased. The average counter-diffusion coefficients were approximately 0.5–0.75 of the corresponding self-diffusion coefficients measured previously in these soils. The applicability of counter- and self-diffusion data to practical field problems are discussed.     

THERMODYNAMICS AND THERMOCHEMISTRY OF THE EXCHANGE REACTION BETWEEN NH4+ AND Mn2+ IN A MONTMORILLONITE CLAY

The exchange reaction between NH₄⁺ and Mn³⁺ was studied on a montmorillonite clay at several temperatures and different ionic strengths. Manganese was preferred to ammonium; this preference increased with the temperature and dilution of the dialysate. Comparison with published data concerning exchanges involving NH₄⁺ and the alkaline-earths showed that in the sequence of increasing selectivity: Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺, Mn²+ lies between Mg²⁺ and Ca²⁺. The enthalpy change was measured calorimetrically and calculated by application of the van't Hoff law to the temperature coefficient of the equilibrium constants. Both values were in good agreement. The excellent recoveries of Mn²⁺ at the end of the exchange reaction and the constancy of the cation exchange capacity over the whole range of surface composition ruled out the possibility of significant adsorption in the MnOH⁺ form. The behaviour of manganese was very similar to that of the alkaline-earth cations.     

Na~+、Ca~(2+)及Na~+-Ca~(2+)混合离子对胡敏酸胶体稳定性的影响

以胡敏酸为研究对象,通过光散射技术比较土壤中常见盐基离子(Na~+、Ca~(2+)和Na~+-Ca~(2+)混合三种电解质)不同浓度条件下胡敏酸胶体凝聚动力学过程,明确不同价态离子界面行为和陪补离子效应对胡敏酸分散稳定性和凝聚体结构的影响。结果发现,Na~+和Ca~(2+)对胡敏酸胶体聚沉能力的差异远远大于舒尔策-哈迪(Schulze-Hardy)规则中因其化合价不同所引起的差异;Na~+-Ca~(2+)混合体系中Ca~(2+)主导凝聚过程,且Na~+-Ca~(2+)混合与Ca~(2+)两种离子体系中凝聚现象的差异随离子浓度的降低而增大;混合离子体系中Na~+作为陪补离子,其陪补离子效应对临界聚沉浓度、颗粒间活化能和胡敏酸凝聚体结构均有一定影响,尤以对凝聚体的结构特征影响显著;Na~+-Ca~(2+)混合体系中胡敏酸凝聚体的结构紧实程度介于单纯离子体系之间,因此可通过调节溶液中的离子组成实现对凝聚体结构紧实程度的调控。上述结果表明,Ca~(2+)对胡敏酸的聚沉不仅依赖于静电作用,还有Ca~(2+)在强电场中的极化诱导其与胡敏酸表面含氧官能团之间发生的共价键和桥键的贡献;此外,陪补离子Na~+与Ca~(2+)在胡敏酸胶体表面的竞争吸附抑制了Ca~(2+)对胡敏酸的聚沉作用,从而形成紧实程度适中的结构体。研究结果为探究胶体界面反应及土壤胶体凝聚机制提供新的思路和理论。     

Infrared spectra of Cu2+ Pb2+ and Ca2+ complexes of soil humic substances

An infrared spectroscopic investigation of the complexes of Cu²⁺, Pb²⁺, and Ca²⁺ with humic and fulvic acids demonstrated the participation of OH and CO groups in addition to COOH in the binding of heavy-metal cations. The degree to which metal-carboxylate linkages are ionic or covalent cannot be accurately determined from the positions of antisymmetric and symmetric carboxylate stretching vibrations due to interference from covalent bonding with other groups. The apparent order of the reaction of three divalent cations with humic and fulvic acids was Cu²⁺ > Pb²⁺ > Ca²⁺.     

Determination of free Al3+ in soil solutions by capillary electrophoresis

Colorimetric and ion exchange methods are commonly used to distinguish and measure Al species in natural waters. Unfortunately they also include weakly complexed Al species in their ‘reactive' or ‘labile' Al fractions and thus are of limited value for the estimation of free Al³⁺. Capillary electrophoresis (CE) has the potential for direct measurement of Al³⁺, and its performance has been verified experimentally. The method also detected the stable and positively charged AlOx⁺ complex formed with oxalic acid. It was compared with a colorimetric and an ion exchange method by analysing artificial solutions containing low molecular weight organic acids as well as soil extracts and seepage waters and was found to be the only method closely matching the theoretically calculated values of free Al³⁺. In samples from the upper soil horizons of an acid forest soil less than 14% of total Al was present as free Al³⁺, whereas the colorimetric method found more than 65%, and the ion exchange method more than 80% of total Al in a ‘reactive' or ‘labile' form. The latter methods thus would seriously overestimate Al toxicity, whereas using CE Al toxicity is likely to be only slightly underestimated.     

The influence of increased NH4+ deposition rates on aluminium chemistry in the percolate of acid soils

The influence of different NH₄⁺ loads on aluminium speciation in percolating water was studied on acid luvisols with similar soil-pH values but with different soil adsorption surface characteristics. All of the applied NH₄⁺ was nitrified and therefore led to proton production. Thus, Al displacement in the percolate was increased. The extent of this enhanced translocation was found to be chiefly a function of the amount of exchangeable base cations which buffered the protons produced by nitrification. The proportion of the different Al species in the percolate was altered at large NH₄⁺ loads. Very large A1³⁺ concentrations eliminated Al species bound to dissolved organic matter and particles, presumably because of flocculation and precipitation of organic ligands. Furthermore, increased NO₃^? and A1³⁺ concentrations enhanced the desorption of F^? giving rise to an additional increase in Al displacement by the formation of Al fluoro complexes in the aqueous phase.     

电位计滴定法对Co2+, Ni2+, Cu2+ and Zn2+与胡敏酸和富里酸形成复合物时的结合常数的确定

The formation constants of Co^2 ,Ni^2 ,Cu^2 and Zn^2 complexes with humic acid(HA) and fulvic acid(FA) in red soil wrer determined by the potentiometric titration method.The constants as a function of composition of the complexation solutions were obtained by two graphical approaches respetively,The formation constants decreased with increasing concentration of metal in the solution,The results provide unambiguous evidence for the heterogeneity of the function groups of humic substances,the formation constants of FA were much smaller than those of HA,and the formation constants of Cu^2 were much greater than those of Co^2 ,Ni^2 and Zn^2 ,The potentiometric titration methon for determining formation constants are also discussed in the article.     

K+浓度对潮土和褐土钾吸附动力学的影响

研究了潮土、褐土在五种Ｋ十浓度下吸附Ｋ＋的动力学，结果表明：不同浓度下的表观吸附过程均可用一级反应动力学方程描述；表观吸附速率常数ｋａ及其与表观平衡吸附量ｑ∞的乘积与淋洗液Ｋ＋浓度之间均表现为极显著的直线正相关；表观平衡吸附量与浓度之间的关系符合Ｌａｎｇｍｕｉｒ或Ｆｒｅｕｎｄｌｉｃｈ等温吸附公式。依据试验结果对土壤钾吸附动力学进行了讨论与推导，找到四个动力学参数ｎ、ｋ１、Ｋ２、ｑｍａｘ，通过这些动力学参数可以计算表观吸附动力学参数Ｋａ、ｑ∞和平衡常数Ｋ及吸附过程中的自由能变△Ｇ，在一定条件下，还可以用来判断等温吸附的类型、估算等温吸附参数。     

一个改进的土壤铵、磷和钾等温吸附新模型

土壤对养分的吸附特性是土壤化学和植物营养学的重要研究内容。9个供试土壤的NH4+、PO43-和K+吸附试验表明,Langmuir、Freundlish和Temkin等温吸附方程对其有较好的拟合效果,但亦有部分供试土壤的拟合精度不良。本文引进种群生态学的"形状因子d"概念,在Langmuir方程基础上构造了一个新的吸附模型。理论分析表明,新吸附模型综合反映了3个等温吸附方程所描述的吸附特征。模拟结果表明,等温吸附方程模拟不佳的供试土壤,用新模型模拟均能达到显著水平;新吸附模型的拟合精度(R2)高于3个等温吸附方程,且标准差最小。形状因子d的变化幅度在0.608 0～2.929 0,体现了土壤理化性质对NH4+、PO43-和K+等温吸附曲线形状的影响。新吸附模型和Langmuir方程的NH4+、PO43-和K+的qm值之间的线性相关系数分别为0.851 5**、0.825 8**和0.912 8**(n=9)。土壤理化性质分析表明,新吸附模型的PO43-和K+的qm值与土壤有机质之间、土壤NH4+和K+的qm值与CEC之间均有显著水平的线性正相关。NH4+的qm值与土壤碱解氮含量亦呈显著水平的线性正相关,而PO34-的qm值与土壤有效磷含量则呈显著水平的线性负相关。数学形式简洁的新等温吸附模型为采用统一模式定量处理土壤对NH4+、PO43-和K+的吸附特性提供了一条新途径。