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Prediction of the fate of metals in soil requires knowledge of their solid-liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid-liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or K d , which gives the ratio of the concentration in the solid phase to that in the solution phase. The K d value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The K d value can be derived from the solidliquid distribution of added metal or that of the soil-borne metal. Only part of the solid-phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this 'labile' phase, and K d values based on this phase often correlate better with soil properties than K d values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for K d determinations. Alternatively, water or dilute-salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal-complexing ligands such as dissolved organic matter. Multi-surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, K d values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The K d coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non-labile pools in the solid phase may invalidate this equilibrium assumption.

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Knowledge of the mechanistic basis of differential aluminum (Al) tolerance depends, in part, on an improved ability to quantify Al located in the apoplastic and symplastic compartments of the root apex. Using root tips excised from seedlings of an Al-tolerant wheat cultivar (Triticum aestivum L. cv Yecora Rojo) grown in Al solutions for 2 d, we established an operationally defined apoplastic Al fraction determined with six sequential 30-min washes using 5 mM CaC12 (pH 4.3). Soluble symplastic Al was eluted by freezing root tips to rupture cell membranes and performing four additional 30-min CaC12 washes, and a residual fraction was determined via digestion of root tips with HNO3. The three fractions were then determined in Yecora Rojo and a sensitive wheat cultivar (Tyler) grown at 18, 55, or 140 Mm total solution Al (AIT). When grown at equal AlT, Tyler contained more Al than Yecora Rojo in all fractions, but both total Al and fractional distribution were similar in the two cultivars grown at AIT levels effecting a 50% reduction in root growth. Residual Al was consistently 50 to 70% of the total, and its location was elucidated by staining root tips with the fluorophore morin and examining them using fluorescence and confocal laser scanning microscopy. Wall-associated Al was only observed in tips prior to any washing, and the residual fraction was manifested as distinct staining of the cytoplasm and nucleus but not of the apoplastic space. Accordingly, the residual fraction was allocated to the symplastic compartment for both cultivars, and recalculated apoplastic Al was consistently approximately 30 to 40% of the total. Distributions of Al in the two cultivars did not support a symplastic detoxification hypothesis, but the role of cytoplasmic exclusion remains unsettled.Root lesions caused by Al toxicity can cause disruption of membrane structure and function, disruption of DNA synthesis and mitosis, cell wall rigidification and reductions in cell elongation, and/or disturbance in mineral assimilation and metabolism, and these postulated toxicity mechanisms have recently been reviewed by Taylor (29). Despite vast quantities of published research, however, the principal physiological mechanism(s) of Al rhizotoxicity remains unresolved, and it remains unclear which are primary dysfunctions and which should more properly be considered secondary effects. There are broad, genetically determined differences in Al tolerance between plant species and genotypes (8). Intraspecfic differences in responses to Al may provide clues to mechanisms of toxicity and aid in plant breeding for superior Al tolerance (8).In general, theories concerning mechanisms of differential tolerance may be divided into three categories (3, 30): (a) the primary lesion of Al rhizotoxicity is cytoplasmic, and differential tolerance is the result of variation in the ability of a plant to detoxify or tolerate Al within the symplast; (b) the primary lesion is cytoplasmic, and differential tolerance is a result of genotypic variation in abili...

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practal interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing OA millimolar CaC12 at pH 4.3 variously supplemented with AlG3 and additionl amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 JAl3'J/(JAV3J + K, + alCQ') where 1A31+ is the activity of Al' expressed in micromolar, IC) is the activity of the added cation expressed in millimolar, and K. (= 1.2 micromolar) is the JAI'3+ required for 50% inhibition in the absence of added salt. For Ca2+, Mg2, and Na the values of a were 2.4, 1.6, and 0.011, respectively, and the values for v were 1.5, 1. (3,5, 25) describe high correlations between growth reductions and the activity of A13+ or total mononuclear species, but reports supporting a direct ameliorative effect of cations are few.Ca is a well known ameliorant that has been observed many times to relieve Al toxicity (2,3,7,18,26), but in only a few cases have the investigators attempted to demonstrate that some or part of the amelioration can be attributed to one or the other of the effects enumerated above (3, 18). Mg is also an effective ameliorant (2,10,18,26), and a recent study demonstrated that both Ca and Mg salts were much more effective ameliorants of Al toxicity than K and Na salts at comparable ionic strengths (18). Thus, Ca and Mg relieved toxicity beyond that attributable to ionic-strength effects, and a physiological interaction between Al and the cations was indicated. The only report ofamelioration by monovalent cations is the dissertation of Ali (2) who concluded that K and Na were equally ameliorative but less effective than Ca and Mg which were equal to each other.We have undertaken a study of cation amelioration that employs a recently published assay for Al phytotoxicity (18). The advantages of the assay are speed, simplicity, sensitivity to low Al levels, the elimination of nutrient requirements other than Ca, and a good correspondence to other assay procedures. The main advantage is that the simplicity of the basal medium (0.4 mM CaCl2 adjusted to pH 4.3 with HCI) permits a more precise computation of Al speciation than do more complete nutrient media. The objectives of the current study were (a)

Extraction efficiency, reagent specificity and selectivity, and element redistribution are potential problems with trace element fractionation by sequential extraction. As part of a larger study of Cd reactivity in soils, we optimized a sequential extraction procedure for accurate, reproducible Cd fractionation using four soil samples and two soil standard reference materials diverse in Cd source, physicochemical properties, and total extractable Cd (CdT, varying from 22 to 42 mg kg−1). Cadmium was partitioned into five operationally defined fractions: 0.1 M Sr(NO3)2 (F1, soluble–exchangeable); 1 M Na acetate, pH 5.0 (F2, sorbed–carbonate); 5% NaOCl, pH 8.5 (F3, oxidizable); 0.4 M oxalate + 0.1 M ascorbate (F4, reducible); and 3 HNO3:1 HCl (F5, residual). By repeating treatments at F1, F3, F4, and F5, we maximized the amount of Cd released for these respective steps. Supernatant pH was used to evaluate carbonate dissolution at F2. Multi‐element analyses were used to assess reagent specificity/selectivity. Cd redistribution was estimated by extraction with Pb acetate. Reagent specificity and selectivity were good, suggesting the dissolution of major components at targeted phases (e.g., high Ca in F2). In general, redistribution was minimal (≤3%), but reached 12% for F3 of the sludge‐amended soil. Quantitative, reproducible recovery of Cd (96.5 ± 2.1%) was obtained across all samples and averaged 11, 32, 40, 8, and 6% CdT in the respective five fractions. Fractionation trends reflect the Cd sources and physicochemical properties of the samples with Cd being dominant in F3 for soils high in organic matter or contaminated by metal sulfides.

Given growing numbers of college students with attention deficit/hyperactivity disorder (ADHD) and/or learning disabilities (LD), it is important to understand why these students choose ADD ("executive function") coaching to enhance their academic success when more traditional forms of campus support already offer this help. Fifty-four undergraduates with ADHD and/or LD participated in a study of their experiences with coaching. To better understand students' perspectives on the manner in which coaching helped them minimize executive function challenges while addressing academic goals, a purposive sample of seven of these students participated in two interviews. All seven described highly self-determined approaches to goal attainment that they associated with coaching. These students also reported that, in contrast to traditional campus services, coaching focused primarily on supporting their emerging autonomy, helping them develop and manage their executive function skills and promoting their self-efficacy and confidence about future success. Findings are linked to recommendations for additional research and service delivery options.

In a previous study we demonstrated the phytotoxicity of polynuclear hydroxy‐Al, a finding that disagrees with a number of existing reports. The objectives of this study were to examine how toxicity of polynuclear Al might vary with experimental conditions or with choice of test plants, and to identify and characterize the toxic polynuclear species. Wheat (Triticum aestivum L. cv. Tyler or Seneca) or soybean (Glycine max (L.) Merr. cv. Stafford) seedlings were cultured for 2 d in dilute CaCl2 solutions containing 15 µM total Al at a basicity (initial molar OH/Al ratio) of 2.0. Increasing rate of base addition, solution age, or levels of added phosphate decreased the fraction of reactive polymers (Alb) and increased that of precipitated Al (Alc) as determined by the ferron method. Mononuclear Al was consistently ≤3 µM and did not contribute to toxicity. Inhibition of root growth was well correlated with [Alb] for both plant species. That only 3 to 4 µM [Alb] was required to fully inhibit wheat root growth suggested that detection of toxic polynuclear Al may at times be difficult in chemically complex media. All plants tested were considerably more tolerant of Al3+ than of Alb, suggesting fundamental physiological differences in the toxicity of the two types of Al. This conclusion was further supported by the finding that the two wheat cultivars exhibited differential tolerance to Al3+, but not to Alb. Several analyses suggested that Alb was composed primarily of the so‐called Al13 polymer, which may form as a consequence of synthesis conditions. Given uncertainties in the occurrence of this polymeric species, both in the laboratory and in nature, several lines of future investigation are suggested.