Elisabetta Barberis

Interaction Of Inositol Hexaphosphate On Clays: Adsorption And Charging Phenomena

The interaction of myoinositol hexaphosphate (IHP) with goethite, and with phyllosilicates such as illite and kaolinite, was studied by assessing the adsorption mechanisms and the electrochemical modifications induced by adsorption of a molecule at such a high-charge density. In addition to quantitative studies, Fourier-Transform Infrared (FT-IR) spectroscopy was used to establish the mechanisms of interaction. Laser Doppler Velocimetry-Photon Correlation Spectroscopy (LDV-PCS) was employed to determine the electrophoretic mobility and the size of the particles. The experiments were also run with orthophosphate (Pi) for comparison. The quantity of adsorbed IHP reached 0.64 μmol m -2 on goethite, 0.38 μmol m -2 on illite, and 0.27 μmol m -2 on kaolinite. The mechanism of adsorption of IHP involved the phosphate groups, whereas the organic moiety affected the process only in terms of conformational hindrance. Thus, on goethite surfaces, because IHP occupied an area equivalent to four sites for Pi, it was supposed to be bound to the oxide by four of its six phosphate groups, whereas the other two were free. On the illite and kaolinite surfaces, the area occupied by IHP was equivalent to about two sites for Pi, suggesting that a lower number of phosphate groups are bound to phyllosilicates. The phosphate groups that did not react with the minerals caused a modification of the electrochemical properties. In particular, IHP adsorption caused dispersion of the particles and a net increase of the negative charge of the surface.

Phosphorus sorption in relation to soil properties in some cultivated Swedish soils

Phosphorus (P) sorption properties are poorly documented for Swedish soils. In this study, P sorption capacity and its relation to soil properties were determined and evaluated in 10 representative Swedish topsoils depleted in available P. P sorption indices were estimated from sorption isotherms using Langmuir and Freundlich equations (Xm and aF, respectively) and P buffering capacity (PBC). Xm ranged from 6.0 to 12.2 mmol kg−1. All indices obtained from sorption isotherms were significantly correlated with each other (r=0.96*** to r=0.99***). Two single-point sorption indices (PSI1 and PSI2) were also determined, with additions of 19.4 and 50 mmol P kg−1 soil, respectively. Both PSI indices were well correlated with Xm (r≥0.98***), with PSI1 giving the highest correlation. As isotherms for determining P sorption capacities involve laborious analytical operations, PSI1 would be preferable for routine analyses. Xm was significantly correlated with Fe extracted by sodium pyrophosphate and ammonium oxalate, to Al extracted by ammonium oxalate and dithionite-citrate-bicarbonate and to organic c. Xm was also significantly correlated with the sum of Fe and Al extracted by ammonium oxalate. The best prediction of Xm through multiple regression was obtained when Fe extracted in ammonium oxalate and Al extracted in dithionite-citrate-bicarbonate were used. Based on the results obtained, both PSI1 and oxalate-extractable Fe plus Al can be used for predicting P sorption capacity in Swedish soils.

Iron oxides and clay minerals within profiles as indicators of soil age in Northern Italy

Abstract Nine profiles representing the Alfisol, Inceptisol and Entisol orders were sampled on three terraces forming a chronosequence. Total iron, dithionite-extractable iron and oxalate-extractable iron were determined for all horizons of all profiles, and the clay mineralogy for horizons of three profiles. The percentage of total Fe (Fet) extracted by dithionite (Fed) increased with age of terraces, as did the difference between Fed and Feo (oxalate-extractable). Analysis of variance (ANOVA) of the horizon data showed that the ratios Fed/Fet and FedFeo/Fet were closely related to the ages of the terraces. Clay minerals were also related to terrace ages, with 2:1 minerals dominant in the profiles on the youngest and mixed-layer minerals and kaolinite more abundant in profiles on the older terraces.

European soils overfertilized with phosphorus. Part 1. Basic properties

Soils overfertilized with phosphorus (P) are widespread in the European Union and there is consensus among soil scientists to better explore their potential to release phosphate. In this work we report the principal physical and chemical properties of twelve overfertilized benchmark soils of contrasting agricultural areas in Italy, Germany, Great Britain, and Spain. The criterion used to consider them as overfertilized was that the available P amount, measured by the regional soil P test, was at least twice as large as the accepted critical level for an average crop. The soils could be usefully divided into three groups, calcareous, slightly acidic and acidic based upon their basic chemical properties and reactions and proportion of the major P fractions (NaOH- plus citrate-bicarbonate-, citrate-bicarbonate-dithionite-, and HCl-extractable P). Six extraction procedures commonly used to evaluate potentially plant-available P (Olsen, anion-exchange resin (resin-), anion plus cation-exchange resin (resin±), Ca acetatelactate (CAL), water, and Fe oxide-impregnated paper strips (strip) were compared. The extractable P values by each method were correlated but the amount of P extracted varied and differed in the order water-P< Olsen-P< CAL-P< strip-P< resin(±)-P<resin(−)-P. Olsen-P and strip-P extracted a proportion of the more available P fraction (NaOH- and citrate-bicarbonate-extractable P), which increased with increasing pH, and decreasing amounts of active Fe and Al forms in soil. Consequently, pH can be used in conjunction with simple soil P tests to provide a first evaluation of potentially releaseable P.

EFFECTS OF INTERACTION OF ORGANIC AND INORGANIC P WITH FERRIHYDRITE AND KAOLINITE-IRON OXIDE SYSTEMS ON IRON RELEASE

The solubility of iron oxides in soils is governed by crystal size, crystal order, isomorphous substitutions, and associations with other minerals. Their dissolution occurs by protonation, reduction, or complexation with ligands such as phosphate and inositol phosphates. In this work, the interaction of inositol hexaphosphate (IHP) and phosphate (Pi) with ferrihydrite (Fh) or ferrihydrite-kaolinite systems (Fh-KGa2) was studied to assess the effects on iron oxide dissolution. Adsorption of IHP and Pi on a two-line Fh and Fh-KGa2 was performed in the range 0 to 0.004 mol P L−1, and the release of P and Fe from samples of Fh or Fh-KGa2 saturated with IHP or Pi was evaluated at different pHs. The amount of P adsorbed on Fh increased, reaching a plateau at 2.12 μmol m−2 for IHP and 4.57 μmol m−2 for Pi. Sorption on KGa2 was lower. It increased to 2.24 μmol m−2 for IHP and to 2.96 μmol m−2 for Pi on Fh-KGa2. On the basis of the IHP/Pi ratios, it was hypothesised that IHP interacted with Fh through two of its six phosphate groups, whereas it interacted with Fh-KGa2 through one P group. Phosphate desorption from these complexes occurred only at pH ≥ 4.5 and was higher for Pi than for IHP and from the Fh-KGa2 system than from Fh. The desorption of IHP was followed by a consistent Fe release, higher at basic pHs. By contrast, Pi adsorption inhibited dissolution of both minerals, although the anion was desorbed in higher amounts compared with the P organic form.

Interaction of inositol phosphate with calcite.

The interaction of myo-inositol hexaphosphate with calcite was studied to evaluate the adsorption mechanisms and the electrochemical modifications induced by interaction of a molecule at such a high-charge density. In addition to quantitative information through the construction of adsorption isotherms, FT-IR and Laser Doppler Velocimetry - Photon Correlation Spectroscopy (LDV-PCS) were employed to investigate the nature of the adsorbent-adsorbate bonds and to determine the electrophoretic mobility and size of the particles before and after sorption. The experiments were also run with orthophosphate (Pi) for comparison. The amount of sorbed P increased to reach a plateau at 17.8 μmol m-2 for inositol hexaphosphate (IHP) while for Pi rose 1.4 μmol m-2 but at Ce > 6ċ10-4M it had a sharp increase reaching 155 μmol m-2. As expected, for Pi, adsorption predominated up Ce 6ċ10-4M by covering about 20% of total surface. The adsorption occurred at sites that behaved as nucleus of formation of the clustering of Ca- and PO4-ions with the ending formation of calcium phosphate precipitates at Ce higher than 6ċ10-4M. The reaction of inositol hexaphosphate with calcite involves, besides adsorption, precipitation of Ca salts and hence calcite dissolution also at the lowest added IHP concentrations, accounting for the large amount retained by calcite. Sorption of IHP on calcite caused aggregation of particles at low concentrations followed by an increase of their negative charge and hence re-dispersion at higher concentrations. These results indicate a great IHP-fixing capacity of calcite that can affect its accumulation in soils and P bioavailability, and a considerable change of calcite electrochemical properties and particle size distribution that can modify aggregate stability.

Impact of long-term inorganic phosphorus fertilization on accumulation, sorption and release of phosphorus in five Swedish soil profiles

The impact of long-term fertilization with inorganic P was studied in soil profiles (0–100 cm) from five sites in Sweden. Accumulation of P was studied by comparing P extracted with ammonium lactate/acetic acid (P-AL) and NaHCO3 (Olsen-P) in non-fertilized and fertilized soil profiles. The fertilized soils had received 42–49 kg P ha–1y–1 for more than 30 years. P-AL and Olsen-P were significantly higher in the fertilized than in the non-fertilized profiles down to 40 cm depth. The P sorption index (PSI2) based on a single-point P addition of 50 mmol P kg–1 soil was used to estimate P sorption capacity in the soils. The variation in PSI2 with depth was not consistent between the five soil profiles. PSI2 did not vary with depth in one soil, while it decreased in one and increased in the other three, and it was weakly but significantly correlated with the sum of Fe and Al extracted with ammonium oxalate (Feox +Alox) (r = 0.65**) and with clay content (r = 0.69***). To estimate P release in the soils, P was extracted with CaCl2 (CaCl2-P) and water (Pw). CaCl2-P and Pw were significantly higher in the fertilized treatment than in the non-fertilized treatment in the top 20 cm. Below 30 cm depth, CaCl2-P was very low in all soils, while Pw was relatively high in two soils and low in the other three soils. To estimate the degree of P saturation, the ratio of P-AL/PSI2 and Olsen-P/PSI2 was calculated. P-AL/PSI2 was significantly higher in the fertilized treatment in the 0–20 cm layer, while Olsen-P/PSI2 was significantly higher in the fertilized treatment in the 0–40 cm layer. P-AL/PSI2 was correlated with CaCl2-P and Pw when all soils and horizons were included (r≥0.78***), but the correlation increased markedly when only 0–40 cm was included (r≥0.94***). Olsen-P/PSI2 was well correlated with CaCl2-P and Pw (r≥0.94***) for all soils and depths. Thus the two indices, P-AL/PSI2 and Olsen-P/PSI2, were comparable in their ability to predict P release in the top 40 cm, whereas Olsen-P/PSI2 was better when all depths were included. The overall conclusion was that P fertilization had an impact on P properties down to 40 cm depth, while the effects were small below this depth.

Iron oxides and particle aggregation in B horizons of some Italian soils

The relative importance of iron oxides to the aggregation status of soil has been evaluated for thirteen samples collected from B horizons of Inceptisols and Alfisols. The particle size distributions before and after selective dissolution treatments were performed to assess the capacity of various iron oxides to aggregate soil particles. Iron extractable with NH4-oxalate was more effective in aggregation than goethite and hematite. The capacity of iron oxides to aggregate clay-sized particles was not significantly different in samples containing goethite and hematite from those having goethite only. Goethite and hematite in clay and in sand fractions had similar Al substitution and mean crystallite dimensions but different reductive dissolution rates in dithionite.

Desorption And Plant Availability Of Myo-inositol Hexaphosphate Adsorbed On Goethite

The specific adsorption and high affinity of inorganic phosphate for Fe oxides limit its desorption and, hence, its availability to plants. Organic P compounds such as inositol phosphate show even greater affinity for soil oxides. This may result in low desorption and, hence, poor bioavailabilty, explaining the accumulation of these compounds in soils, although inositol phosphate desorption from Fe oxides has never been quantified. Desorption is controlled by several parameters such as soil pH, percentage of P saturation, ligand competition, and the characteristics of the P-containing compounds. In this work the desorption of myo-inositol hexaphosphate (IHP) and inorganic phosphate (Pi) from goethite (Gt) was studied while assessing the effect of pH, citrate and bicarbonate extractability and the number of desorption cycles. P availability to Lolium perenne L. was also studied in the presence of IHP or Pi added in solution or sorbed on goethite. The amount of desorbed P, as IHP, increased with pH and the number of desorption cycles, but in all cases it did not exceed one-fifth of the desorbed Pi, suggesting a limited availability of IHP when adsorbed on goethite, especially at low pH. Citrate and bicarbonate solutions extracted a small amount of IHP with respect to Pi, probably the result of the inability of their mechanisms to detach the four P groups involved in the IHP bonding with the goethite surface and of the high negative charge of the IHP-Gt complex that hampers the approach of ligands to the surface. The ryegrass that received P as IHP showed limited growth, signs of P deficiency, and a low shoot/root ratio as a consequence of the poor nutrient availability, particularly when IHP was supplied in solution and Gt was present in the pots. In contrast, significantly larger biomass productions were always obtained when P was supplied in inorganic form, and no significant differences among the tests were observed.

Factors Affecting the Formation, Nature, and Properties of Iron Precipitation Products at the Soil–Root Interface

Abstract A number of iron oxides (hematite, goethite, lepidocrocite, maghemite, and magnetite) or short‐range ordered precipitates (ferrihydrite) may be found in soil environments, but in the rhizosphere the presence of organic ligands released by plants (exudates) or microorganisms promote the formation of ferrihydrite. Iron ions are liberated into soil solution by acidic weathering of minerals and then precipitated either locally or after translocation in soil environments. Humic and fulvic acids as well as organic substances produced by plants and microorganisms are involved in the weathering of primary minerals. Organic compounds play a very important role in the hydrolytic reactions of iron and on the formation, nature, surface properties, reactivity, and transformation of Fe oxides. Organic substances present in the rhizosphere interact with Fe promoting the formation of ferrihydrite and organo‐mineral complexes. The solubility of Fe precipitation products is usually low. However, the formation of soluble complexes of Fe(II) or Fe(III) with organic ligands, usually present in the rhizosphere increases the solubility of Fe‐oxides. Mobilization of Fe from Fe oxides by siderophores is of great importance in natural systems. They can form stable Fe(III) complexes (pK up to 32) and thus mobilize Fe from Fe(III) compounds. These higher Fe concentrations are important for the supply of Fe to plant roots which excrete organic acids at the soil–root interface. Iron oxides adsorb a wide variety of organic and inorganic anions and cations, which include natural organics, nutrients, and xenobiotics. There is competition between anions and cations for the surfaces of Fe‐oxides. Root exudates suppress phosphate or sulfate adsorption on Fe‐oxides. This is a mechanism by which plant roots mobilize adsorbed phosphate and improve their phosphate supply. Anions adsorption on iron oxides modify their dispersion/flocculation behavior and thus their mobility in the soil system. That can increase or decrease the possibility of contact between Fe‐oxides and organics or organisms able to dissolve them.