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Notes: The effect of pH and the addition of carboxylates (acetate, oxalate, tartrate, salicylate and citrate) on the solubility of soil phosphate has been investigated to assess the possible effects of root exudates on phosphate availability. The soil was a neutral calcic Luvisol with a large pH buffer capacity. Various concentrations of strong acid (0–20 mmol kg−1) and anion (0–2 mmol kg−1) were applied to soil in suspension (0.5 g soil cm−3). The effect of 2 mmol kg−1 oxalate on the adsorption isotherm of phosphate was also studied. The rate of isotopic exchange was largely unchanged by any of the treatments. Neither pH nor acetate had an effect on phosphate solubility. The addition of di– and trivalent anions increased phosphate solubility somewhat. The effect increased with increasing concentration of anion, and was generally independent of pH and the proportion of anion adsorbed. Oxalate was more efficient in limiting the adsorption of freshly added phosphate than in desorbing native phosphate. The results illustrate that rhizosphere acidification does not necessarily increase phosphate solubility. However, carboxylates, the conjugate bases of organic acids, may play an important role in improving the availability of soil phosphate.

Notes: A detailed study of the isotopic exchangeability of native nickel is presented for two contrasting soils. Non-sterile soils were compared with γ-sterilized soil to assess the role of microbial activity in modifying nickel speciation. The distribution coefficient, Kd, was measured in a range of suspensions and in moist soil. Solution and soil solids were separated by double centrifuging. Soils were wetted with either water or 0.01 m CaCl2 solution. The change in Kd was also recorded for the soils incubated moist over a 6-week period. Kd values decreased markedly as suspension was more concentrated, and data obtained in moist soil followed the same trend. Data obtained at a soil:solution ratio of 1:1 are closer to those obtained for soil incubated at field capacity (or drier) than those obtained at the more usual 1:10 ratio. Sterilization caused a marked increase in nickel solubility (about a threefold decrease in Kd for one soil and a 25% decrease for the other). Soil analysis showed that sterilization modified only water-extractable organic carbon and EDTA-extractable Mn. The effect of solution composition on Kd was less marked. These effects are attributed largely to dissolved organic carbon and easily extractable manganese oxides. There was relatively little change in Kd over the 6-week period, suggesting that except for strongly anoxic soils, the distribution of nickel measured after 24 hours is a good estimation of Kdin situ.〈section xml:id="abs1-2"〉〈title type="main"〉Effet de la stérilisation et des conditions expérimentales sur l'échange isotopique du nickel dans deux sols contrastés〈section xml:id="abs1-3"〉〈title type="main"〉RésuméUne étude détaillée de l'échangeabilité isotopique du nickel natif de deux sols contrastés est présentée. Les sols non-stériles ont été comparés aux sols stérilisés par rayonnement-γ pour évaluer le rôle de l'activité microbienne dans la modification de la spéciation du nickel. Le coefficient de distribution, Kd, a été mesuré dans une gamme de suspensions et dans le sol humide. La solution et la phase solide du sol ont été séparées par centrifugation en deux étapes. Les sols ont été humectés soit avec l'eau soit avec une solution de 0,01 m CaCl2. La modification du Kd a été suivie pour les sols humides sur une période de 6 semaines. Les valeurs de Kd diminuaient fortement avec l'augmentation de la teneur en solide de la suspension. Les données obtenues avec un rapport sol:solution de 1:1 sont plus proches de celles obtenues pour le sol incubéà la capacité au champ (ou plus sec) que celle obtenu au rapport 1:10, qui est plus souvent utilisé. La stérilisation a résulté dans une augmentation importante de la solubilité du nickel (une diminution du Kd d'environ un facteur de 3 pour un sol et une diminution de 25% pour l'autre). L'analyse du sol indiquait que la stérilisation modifiait seulement le carbone organique extractible à l'eau et le Mn extractible par l'EDTA. L'effet de la composition de la solution sur le Kdétait moins marqué. Ces effets sont attribués principalement au carbone organique dissout et les oxydes de manganèse facilement extractibles. Le Kdévoluait peu pendant la période de 6 semaines, ce qui suggère que la distribution du nickel mesuré après 24 heures soit une bonne estimation du Kdin situ, sauf éventuellement pour les sols fortement anoxiques.

Notes: Understanding the movement of cations in soil, particularly trace metals, is required in many applications such as phytoremediation and pollution control. A dynamic mechanistic model has been developed to describe the long-term root uptake of a surface-applied, strongly adsorbed, pollutant metal cation, such as radiocaesium, from soil. It consists of two submodels. The first calculates uptake per unit root length at a local scale over a root's lifetime, for various initial conditions. The second calculates cumulative uptake at a whole-plant scale for the entire rooting depth as a function of time. The model takes into account the renewal of roots which are considered to have a limited lifetime. Root density may be a function of soil depth and a proportion of roots need not contribute to uptake. Recycling from decaying, or grazed, roots and shoots is considered.Simulations show that removal of cations from soil is exaggerated unless some recycling by roots or shoots is considered or the entire root length does not contribute to uptake. Because of root turnover, uptake is not rapidly limited by diffusive flux of the cation from the bulk soil solution to the solution–root interface. Uptake is very sensitive to root architecture and plant physiology.

Notes: The adsorption of a radioisotope of caesium, 137Cs, has been interpretation and ratio and solution measured on four soils with differing clay mineralogies. All measurements have been made using unwashed soils in suspensions shaken for 2 h at 20°C. The effects of concentration in potassium and caesium, the nature of the background electrolyte and the soil:solution ratio have been investigated. The results are expressed either as the distribution coefficient, KD or as Freundlich isotherms.The distribution coefficient of each of the soils decreases markedly as the concentration of caesium increases. The adsorption properties of the soils are not determined by the dominant clay mineralogy alone. Adsorption is always lower in 0.01 m CaCl2 solution than in water. The addition of potassium has relatively little effect on adsorption of trace amounts of caesium; however, KD decreases with increasing concentrations of stable caesium. The major reason for the dependence of KD on the soil: solution ratio is found to be the non-linear adsorption isotherm; the influence of the varying compositions of the solution and exchange complex is minor.The validity of the use of a single KD value as an indicator of adsorption capacity and the meaning of the relative values of the Freundlich parameters are considered. The implications of these findings for the use of radiotracers and the usefulness of KD as an indicator of bioavailability are discussed.

Notes: The consequences of slow diffusion of strongly adsorbed solutes into soil aggregates are not fully understood. The distribution of 32P after diffusing down a soil column cannot be explained from a consideration of liquid-phase impedance factor and isotopic exchange alone, as can that of 36CI.A model was developed that considers the soil to provide linked parallel intra-aggregate and inter-aggregate pathways. With this geometry, simulations agreed fairly well with experimental data when the intra-aggregate impedance factor was 0.001, using other model parameters that had been determined in independent equilibration studies. With this intra-aggregate impedance factor, the model yielded straight-line plots for chloride and the overall impedance factor derived agreed closely with the experimental one. The intra-aggregate impedance factor for 32P also agreed with the rate of reaction of the rapidly-exchanging P fraction previously determined. The slowly-exchanging P fraction has little influence on the P concentration profiles up to 10 d, but the small exchanged amount does have an effect at run times of 57 d.The slow intra-aggregate diffusion of strongly adsorbed solutes decreases the amount adsorbed from a surface source of supply, and it also decreases the amount that is taken up by a surface sink. However, the interaggregate solution concentration remote from a source of supply is considerably increased, which may be important if the solute is a pollutant or a nutrient.

Notes: Radiocaesium is more available to biological systems in organic soils than in mineral soils. Part of the reason is that the adsorption of caesium on minerals, particularly illites, is very specific, whereas its adsorption on organic matter is non-specific. However, increasing evidence shows that even organic upland soils contain enough illitic material to immobilize caesium effectively. The presence of organic matter may reduce the affinity of soil minerals for caesium.We have studied adsorption of radiocaesium on the clay fractions of a mineral soil obtained from a surface, and the corresponding C, horizon. The soil clay fractions were treated to remove organic matter and Fe oxides progressively.Adsorption isotherms of caesium were obtained in dilute suspension using trace amounts of 137Cs and stable caesium. In general, adsorption was greater when the organic matter content was less, namely on the soil from the C horizon rather than the surface horizon, or when organic matter had been removed. However, adsorption is not a simple function of organic matter content: Cs adsorption was greater on the topsoil fraction after organic matter removal than on the subsoil fraction. X-ray diffraction analysis indicated that the surface soil clay contained more weathered minerals and less illite than the subsoil clay. The effect of organic matter removal was more marked at large Cs concentrations than when only radiocaesium was added, suggesting that very selective Cs adsorption sites are less sensitive to the presence of organic macromolecules. Iron oxides do not play a role in the affinity of soil clays for Cs. The form of the Freundlich isotherm was unchanged by any of the chemical treatments.

Notes: Organic matter can be removed from soil to varying extents using many chemical methods, but little information may be obtained on the nature of the organic matter thus removed and its chemical interactions with soil mineral components. We selected two surface soils, with similar organic carbon contents. Chemical analysis showed the presence of a range of amino acids and amino sugars. Quantitative mineralogical analysis revealed some differences between the soils, particularly in the poorly crystallized components. The clay-sized fractions were submitted to various chemical treatments, then the organic carbon remaining was investigated using infrared spectroscopy. We also studied the adsorption of purified soil polysaccharide to some of these residues. The nature and proportion of organic matter removed by each of the treatments was broadly similar for both soils. Residual organic matter remaining was enriched in amide groups. The infrared spectrum after maximum removal of organic matter was very similar to that of the clay-sized fraction of soil from the equivalent C horizon. Differences in the ability of the clay residues from the two surface horizons to adsorb polysaccharide correlated with their original organic matter content and is postulated to be related to poorly ordered amorphous mineral matter. Oxidation of added polysaccharide showed no enhancement of nitrogen in the residue, indicating that it was not bonded to the clay fraction of soil by absorption through the amino groups, in addition to the conventionally postulated hydrogen bonding involving hydroxyl groups.〈section xml:id="abs1-2"〉〈title type="main"〉L'interaction entre la matière organique et les minéraux argileux du sol par l'extraction sélective et l'addition contrôlée de la matière organique〈section xml:id="abs1-3"〉〈title type="main"〉RésuméLa matière organique du sol peut être détruite avec une efficiacité variable par diverses techniques chimiques, par contre peu d'information est ainsi obtenue sur la nature de la matière organique et ses interactions avec les composés minéraux du sol. Nous avons choisi deux sols de surface, ayant des teneurs en matière organique similaires. L'analyse chimique a mis en évidence une gamme importante d'acides aminés et de sucres aminés. Une analyse minéralogique quantitative a montré quelques différences entre les sols, notamment pour les composés faiblement cristallisés. Les fractions argileuses ont subi divers traitements chimiques, puis le carbone organique restant a été analysé par spectroscopie infrarouge. Nous avons également étudié l'adsorption de polysaccharide de sol purifié sur certains de ces sols traités. La nature et la proportion de matière organique détruite par chacun de ces traitements étaientassez similaires pour les deux sols. La matière organique restant était enrichie en groupements amide. Les spectres infrarouges après destruction maximale de la matière organique étaient très similaires à celui de la fraction argileuse du sol de l'horizon C correspondant. Les différences dans la capacité des deux sols de surface après traitement à adsorber un polysaccharide était corrélé avec la teneur en matière organique initiale et nous postulons qu'elles sont liées à la phase minérale amorphe. L'oxydation du polysaccharide ajouté n'était pas accompagnée d'un enrichissement d'azote dans le solide restant, ce qui indique que la liaison avec la fraction minérale du sol n'implique pas les groupements aminés, en plus des liaisons hydrogène des groupements hydroxyles, postulées classiquement.