Iron oxides in four Red Mediterranean soils on metarhyolite and metadolerite in Kilkis,Greece

The various iron fractions were quantified by selective dissolution (Fe_d, Fe_o, Fe_t) in four Red Mediterranean soils, developed on metarhyolite and metadolerite. They were similar in all profiles. A strong trend of iron removal from the surface horizon and of its subsequent illuvial translocation to the argillic horizons was observed. In all profiles, Fe_o was not related to the organic matter content indicating the Mediterranean xeric soil environment. The Fe_o/Fe_d ratio and the percentage of crystalline iron oxides (Fe_d-Fe_o) suggested that the pedoenvironment in which the profiles P1, P2 were formed, allowed the high crystallization of iron oxides. As indicated by the Fe_d/Fe_t values, the weathering process was more intense in the metarhyolite-developed soils. In contrast, the metadolerite-developed soils present conditions of poorly crystallized iron oxides and a lower degree of development.     

水稻根际中铁的形态转化

本文以熟化红壤性水稻土、淀浆白土、红壤和赤红壤为样品,研究了植稻后根际中铁的形态转化.结果表明,两种水稻土根际中无定形氧化铁、游离氧化铁、络合态铁、土壤中氧化铁的活化度及两种红壤上根际中的络合态铁均低于非根际土;而两种红壤上其余各项在根际内外分布趋势均与水稻土相反.用穆斯堡尔谱仪分析表明,所有根际土与非根际土相比,四极矩分裂增大,内磁场下降,说明根际中氧化铁被活化.同时,水稻土上根际中Fe²⁺增多,赤红壤中根际出现新矿物磁赤铁矿.根际中铁被活化,可能会影响根际中重金属等污染物的吸附和解吸特性、植物的铁素营养及对其它养分的吸收.     

EXTRACTABLE SESQUIOXIDES IN SIX MEDITERRANEAN SOILS DEVELOPED ON BASALT AND SCORIA

Iron, Al, and Mn were extracted by oxalate and dithionite from two Brown Mediterranean Soils, two Red Mediterranean Soils, one Vertisol and one Gley soil, all derived from basalt or scoria in the sub-humid and humid Mediterranean regions of the Golan Heights. Ratios of oxalate: dithionite extracted iron (Fe_o:Fe_d) were low in all soils, indicating that the predominant form of free iron is crystalline. Fe_o accumulates in the argillic B horizons of the Mediterranean soils, while Fe_d accumulates in the surface horizons. A large part of the free iron oxide in the surface horizons of Mediterranean soils is associated with non-clay fractions. While manganese behaves in a manner somewhat similar to that of iron, no definite trends could be discerned in the vertical distribution of free aluminium. In the Vertisol, Fe_o and Mn_o accumulate in the subsoil. Fe_d and Mn_d increase slightly with soil depth. In the Gley soil, amorphous iron accumulates in the surface horizon, total free iron in the bottom horizon. Both amorphous and total free Mn had been depleted from the upper horizons of the Gley soil.     

Disputable issues in interpreting the results of chemical extraction of iron compounds from soils

In Russia, iron is chemically fractionated according to a parallel scheme. Pyrophosphate-soluble iron (Fe_pyr) is considered to participate in organomineral complexes, oxalate-soluble iron (Fe_ox) is believed to enter amorphous + poorly crystallized compounds, and dithionite-soluble iron (Fe_dit) is meant to represent the free (nonsilicate) compounds. However, the investigations prove that the commonly used subtraction operations (Fe_ox ? Fe_pyr) and (Fe_dit ? Fe_ox) are invalid because of the nonadditive action of the reagents in the parallel scheme of extraction. The low selectivity of reagents requires a new interpretation of chemically extracted iron compounds. In automorphic soils, the content of oxalate-soluble iron should be interpreted as the amount of Fe(III) capable of complexing with organic ligands; in hydromorphic soils with a stagnant moisture regime, it should be interpreted as the amount of iron (III) capable of being reduced in a short time. The content of dithionite-soluble compounds should be regarded as the amount of iron (III) within both (hydr)oxides and silicates potentially prone to reduction.     

Resistance of mineral soils to Fe(III) reduction

Gleying and enhancement of hydromorphism in wetland soils due to Fe(III) reduction entail a series of degradation processes. The resistance of wetlands to degradation can be calculated from the content of potentially reducible iron, Fe(III)_pr, which is found from the van Bodegom equation taking into account the contents of oxalate-soluble iron Fe_ox and dithionite-soluble iron Fe_dit in the soil. In addition, this makes it possible to distinguish relict and actual gleysols. The van Bodegom equation is applicable to soils from which the oxalate solution extracts only amorphous and poorly crystallized iron compounds, which are quickly reduced by Fe-reducing bacteria. These soils have a low proportion of Fe(II) (no more that 15% of the total iron), as well as an accumulative profile distribution of Fe_ox. The van Bodegom equation is unsuitable for calculating the Fe(III)_pr content in soils with a high proportion of Fe(II) and a nonaccumulative profile distribution of Fe_ox.     

中国下蜀黄土发育的土壤表层及典型剖面的硫状况

Studies were conducted to examine factors which might influence the status and distribution of S in some surface horizons and typical profiles of soils derived from Xiashu loess on the upper slope (US), middle slope (MS) and lower slope (LS) of Nanjing-Zhenjiang-Yangzhou hilly zone. The total S contents varied from 70.30 to 350.21 mg/kg, and the average for all surface soils was 218.3 mg/kg. The average S contents in the profiles followed the sequence: USo) and the ratio of amorphous iron oxide to free iron oxide (Fe_o/Fe_a), but no significant relationship was found between total S and the ratio of free iron oxide to total iron (Fe_d/Fe_t). Inorganic sulphate in paddy soils (MS and LS) was nearly higher in surface soil than in subsurface soil and subsoil, it, however, remained relatively unchanged with increasing depth for the original soil profile (US). The average organic S accounted for 94% of the total S in the surface soils, but the percentage decreased with depth in the profiles. Like the total S, the organic and inorganic S contents were highly significantly correlated with organic matter, total N, Fe_o and Fe_o/Fe_d ratio, but they were insignificantly related to Fe_d/Fe_t ratio. The C/S and N/S ratios in this study were somewhat lower than the results reported by others. The C/N/S ratios varied considerably within the same profile and among different soils but they fell within the range of values reported worldwide.     

Iron minerals in urban soils

Technogenically contaminated urban soils contain a substantial amount of magnetite Fe₃O₄, whereas another ferrimagnetic, i.e., maghemite αFe₂O₃, more often prevails in unpolluted soils. The content of magnetite may exceed the content of another iron oxide, hematite, in contaminated soils. In the town of Chusovoi, where emissions from a single enterprise, a metallurgical plant, predominate among pollutants, the upper soil horizons are contaminated with magnetite of one type. In the much larger city of Perm, the polluting sources are diverse, which results in a wide variation of magnetic susceptibility of technogenic magnetite. The difference in magnetite properties may depend on the composition and the content of heavy metals associated with this mineral. A considerable amount of oxalate-soluble magnetite in technogenically contaminated soils produces two important consequences. Schwertmann’s criterion Fe_ox: Fe_dit as a gleying index turns out to be overestimated and, therefore, does not work in technogenically contaminated soils. The second consequence is that Tamm’s reagent is inapplicable to extracting heavy metals bound to amorphous iron compounds from contaminated soils. On the other hand, a high solubility (30–60%) of technogenic magnetite by oxalate favors the use of Tamm’s reagent for the complete extraction of iron (hydr)oxides and heavy metals bound to them.     

Distribution of iron oxides forms on a transect of calcareous soils,north-west of Iran

The iron oxides fractions of four major physiographic units obtained from a transect of calcareous materials were studied to assess the effects of key pedogenic processes and local hydrology conditions as well as physiographic units in controlling iron oxides forms in the north-west of Iran. Samples from different horizons belonging to six pedons were selected and analyzed for soil physicochemical properties, clay minerals, and Fe oxides forms (Fe_d, Fe_o, Fe_p). In general, the soils indicated some variation in the concentration of iron oxides that could be related to rate of weathering, pedogenic accumulations, geomorphologic conditions (as results of different in physiographic units), wet and dry cycle, and organic matter. A wide relative variation in mean values of Fe_d (6.4–9.9 g kg^?1), Fe_o (2.9–4 g kg^?1), and Fe_p (0.68–1.3 g kg^?1) was observed among physiographic units. On the plateau unit, the presence of the most stable geomorphologic conditions and high rate in situ weathering (reflected in clay content), coupled with minor deposition of sediment suggest that the soils have more dynamic conditions than other units, reflecting in the greatest amount Fe_d and the lowest Fe_o/Fe_d ratio. Fe_d content of the soils containing less clay content (15–25%) was significantly different from those with greater clay content (25–35%).     

Selectivity of reagents used to extract iron from soil

The selectivity of the Tamm and Mehra-Jackson reagents to iron (hydr)oxides was verified chemically by comparing the efficiencies of the two methods of iron extraction: parallel (commonly adopted) and sequential, i.e., by calculating the difference ΔFe = [Fe_dit ? (Fe_ox(paral) + Fe_dit(seq))]. The expected equality of the extracted iron (i.e., ΔFe ～ 0) is rarely observed. A positive balance reaching ΔFe = (0.5?0.8)% predominates upon iron extraction from forest soils of the Cis-Ural region. This is probably due to the stable Fe(II)-oxalate forming in the course of the Tamm extraction, which is incompletely dissolved by the dithionite-citrate-bicarbonate (DCB). On the contrary, a negative balance with ΔFe reaching ?0.4% predominates in the steppe soils of the Stavropol region. This may be caused by Fe(II) minerals (pyrite and siderite) acting as catalyzers in Tamm’s extract, while being weakly soluble in the DCB. To follow the additivity principle for the Tamm and Mehra-Jackson extracts, we suggest a modification of the Schwertmann criterion K_Sch = (Fe_ox: Fe_dit(paral)), which sometimes exceeds 1, to K _Sch ^m = Fe_ox: (Fe_ox + Fe_dit(seq)), which is always less than 1. The values of the modified Schwertmann criterion better agree with the color of the Cis-Ural forest soils expressed in the CIE-L*a*b* optical system as compared with the initial Schwertmann criterion. In the steppe soils, the use of the modified criterion makes it possible to replace the illogical values of K_Sch = 1.11?1.37 by the quite acceptable K _Sch ^m = 0.68?0.83.     

Potential Iron and Phosphate Mobilization During Flooding of Soil Material

This study shows that mobilization of phosphate from soils under anaerobic conditions can be intimately coupled with reductive dissolution of iron from iron oxides. Among four soil samples from the reclaimed Skjernå estuary in Denmark incubated anaerobically and amended with glucose, 28–39% of the dithionite-citrate-bicarbonate-extractable iron and 10–25% of the oxalate-extractable phosphorus (P_ox) were released to the soil solution after 31 days. Significant correlation (r = 0.992**) between the molar ratio P_ox/(Fe_ox + Al_ox) for the aerobic samples and (P_{P sol}/Fe_sol) (the molar ratio between phosphate and iron in solution during anaerobic incubation), indicates that the phosphate saturation status of the soil is an important determinant of the amount of phosphate released during flooding of moderately acid soils.     

Iron oxides as weathering indicator and the origin of Luvisols from the Vistula glaciation region in Poland

Purpose

The aim of the research was to determine the effect of lithogenic and pedogenic processes on the formation of Luvisols from the area of Vistula glaciation on the base of profile distribution of iron oxides and total iron in relation to texture and physicochemical properties. The indices of weathering of the soil material in genetic horizons were calculated, and changes in the content and forms of iron oxides were evaluated.

Materials and methods

The predominant type of soil in the study area is Luvisols under agricultural use, formed from silt formations on loam. The analyses were made applying the following methods: grain size composition using the sieve method and hydrometer method, the interpretation of the results was performed according to the World Reference Base for Soil Resources classification, the pH of soils was measured with the potentiometric method, C-organic with the Walkley-Black dichromate method, the content of the following iron forms was determined (total iron (Fe_t) after the mineralization of soils in the mixture of HF and HClO₄ acids), free iron oxides were extracted using dithionite-citrate-bicarbonate method, and amorphous iron oxides after the ammonium oxalate extraction (using the Philips 9100PU apparatus). The clay mineralogy was estimated by X-ray diffraction analysis.

Results and discussion

It was observed that total iron enrichment occurs in argic horizons accompanied by iron depletion in luvic horizons, while the profile distribution of iron is similar to the distribution of clay. The (Fe_d/Fe_t) ratio indicates a low degree of weathering; the highest values were observed in argic (Bt) horizons, which confirms the effect of the process of pedogenesis on the value of that index. In the soils investigated, crystalline iron oxides generally dominate over the amorphous forms. The mineralogical composition of clay fraction separated from the upper part of soils was different as compared to the underlying material.

Conclusions

The results of the study showed that iron contents (together with the other indicators) and its forms can be used to distinguish soil layers of different origin. The depth distribution of Fe_d, Fe_o and Fe_t within soil profiles indicates that the soil material may be of different lithogenic origin in the studied pedons.

     

THE OXIDATION OF IRON SULPHIDES IN SOILS IN RELATION TO THE FORMATION OF ACID SULPHATE SOILS, AND OF OCHRE DEPOSITS IN FIELD DRAINS

Aerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat-clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe²⁺ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe²⁺ and SO²₄^?. Under these conditions iron enters the drains as Fe²⁺ and is there oxidized by T. ferrooicidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. ferrooxidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of Oxidation.     

土壤对镉的吸附与解吸——Ⅰ.土壤组份对镉的吸附和解吸的影响

采用选择溶解法研究了有机质、游离铁、无定型硅、铝等土壤组份对青黑土、黄棕壤、红壤和砖红壤胶体吸附和解吸Cd的影响。结果表明,去除有机质后胶体吸附Cd减少,这可能是由于交换吸附的减少所致;游离铁的去除使得黄棕壤、红壤和砖红壤的吸附量显著减少,显示了在这些土壤中游离氧化铁专性吸附的重要性;随着无定形铝含量的上升,吸附量下降,这是因为铝离子占据了高能量的吸附位。经不同处理后的土壤胶体,其Cd的解吸顺序(解吸%)大致为:去无定型硅、铝者>去游离铁者>去有机质者>原胶体,但在不同土壤和不同pH条件下该顺序略有差别。研究结果为控制和改造土壤Cd污染提供了理论依据。     

Comparison of different models for phosphate sorption as a function of the iron and aluminium oxides of soils

Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into zccount. Oxalate-extractable P, (P_ox), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Fe_ox) and aluminium (Al_ox). A significant relation between crystalline iron (Fe_d– Fe_ox) and total P sorption was not found. Reversibly adsorbed phosphate (P_i), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.     

A magnetic investigation along a NE–SW transect of the Yasouj Plain,southwestern Iran

The magnetic susceptibility (χ) of soils varies with the slope position due to some factors such as texture, drainage class, and land use. Limited information is available about the magnetic susceptibility properties of semi arid regions of southwestern Iran. This study attempts to link χ and Fe oxides of the soils to landforms, soil characteristics, and land use (paddy and dryland soils) on the same parent materials. Ten representative pedons were taken along a NE–SW transect in different physiographic units in the Yasouj Plain. The results indicated that χ is determined mainly by topography and land use. In most pedons, the highest concentrations of poorly crystalline Fe (Fe_o) were observed in the top soil with different types of land use. Aquic condition in paddy soils seems the main cause for a significant decrease in the amount of Fe_d in the studied soils. Maximum concentration of pedogenic Fe (Fe_d) was noticed in more stable landforms with non-aquic moisture regime. Paddy soils exhibited higher Fe_o/Fe_d ratio values, suggesting that they have more poorly crystalline iron oxide phases than dryland soils. Magnetic measurements showed that the χ values of paddy soils were much lower (3.8 times) than those of dryland soils. The highest value of χ was found in the pedons located on plateau and piedmont plains and the lowest value of χ belongs to those that are located on hill and river lower terraces. A positive correlation was found between the frequency dependence of χ (χ_fd) and χ in paddy and dryland soils. Higher values of χ_fd were observed at soil surface than at deeper levels, suggesting a greater proportion of ultrafine grains.     

High gradient magnetic separation of some soil clays from Nigeria, Brazil and Colombia.

High gradient magnetic separation was used to fractionate the clay from some tropical soils. Acid-oxalate-extractable iron (Fe_ox) and aluminium (Al_ox) and total carbon were measured in the whole clay, the magnetic fraction and the tailings. The magnetic separation resulted in a wider range of concentrations of these elements than in the whole clays. In each of the clays Fe_ox was greater in the magnetic fraction than in the tailings; Al_ox was more variable. Carbon was also concentrated in the magnetic fraction suggesting that it is associated more with Fe_ox than Al_ox. The relationships between Fe_ox, Al_ox and carbon depend on soil classification and soil age.     

Redoxpotentiale und redoxprozesse von mangan-, eisenund schwefelverbindungen in hydromorphen böden und sedimenten

Results of laboratory experiments with soil material saturated with sea water indicate that, as predicted by thermodynamics, manganese (III, IV)-oxides are first reduced to Mn²⁺-ions (beginning at about +450 mV at pH 6.1.; E₇ ≈ +400 mV), next amorphous iron (III)-oxides are reduced to Fe²⁺-ions (beginning at about +220 mV at pH 6.0; E₇ ≈ +160 mV), and finally sulphates are reduced to sulphides (beginning at about +10 mV at pH 6.0; E₇ ≈ -50 mV). Direct quantitative relations between redox potentials, pH-values and Mn²⁺- (or Fe²⁺-) contents of water-saturated soils and sediments and calculated redox reactions of known manganese and iron systems could not be established.The influence of organic redox systems produced by microbial fermentation processes on the measured potentials and on the reduction of manganese and iron oxides is discussed.A reduction of the oxides by microbially formed sulphides, which themselves are oxidized by this process, seems also to be possible. Therefore, sulphides do not occur as stable sulphur phase in higher amounts before all available Fe-oxides are reduced to Fe²⁺-ions. Then formation of iron monosulphides takes place by precipitation of Fe²⁺- ions by sulphides (H₂S, HS). In a sulphide-stabilized environment redox reactions of sulphur — especially the reaction H₂S_aq = S₀ + 2 H⁺ + 2 e^? — may determined the measured potentials.The results show that the dynamics and morphology of hydromorphic soils and sediments are strongly dependent on microbial processes.     

Mineralogical properties of andisols of the Kitakami mountain range,Northeastern Japan

We studied the mineralogical properties of Andisols of the Kitakami mountain range. Soils of the northern and central parts of the mountain range contained higher levels of acid oxalate extractable silicon (Si_o), aluminum (A1_o), and iron (Fe_o) but smaller amounts of sodium pyrophosphate-extractable aluminum (Al_p), iron (Fe_p), and carbon (C_p) than those of the southern part. Consequently, the soils of the northern and central parts of the mountain range contained large amounts of allophane and ferrihydrite whereas in the soils of the southern part A1 (Fe)-humus complexes and 2:1 and 2:1:1 clays predominated. The amount of sand (20-2,000 μm) in the soils tended to decrease sharply from the north to the south of the mountain range. The soils of the northern and central parts of the mountain range contained larger amounts of heavy minerals in their fine sand fraction (20-200 μm). The soils of the southern part, on the other hand, contained larger amounts of fine-grained quartz (2-20 μm). All the soils of the mountain range contained substantial amounts of volcanic glass in their sand fraction. However, on a total soil basis, the amount of volcanic glass in the soils decreased from the north to the south of the mountain range and the trend was parallel to that of the sand content of the soils. From these results, we concluded that (i) the soils of the northern and central parts of the mountain range were derived mainly from tephras and (ii) the soils of the southern part were strongly influenced by long-range eolian dust.     

Forms and pedogenic distribution of extractable iron in selected wetland soils in Nigeria

Abstract

The content of various forms of iron (Fe) (free, reducible, and organic) were determined by selective extraction methods in three wetland profiles between 1993 and 1995 seasons. The result showed that Fe distribution was in the order: dithionite (Fe_d) > hydroxylamine (Fe_H) > pyrophosphate (Fe_p) iron in the three pedons. The hydroxylamine‐Fe constituted between 10–42% (1993), 20–47% (1994), and 10–12% (1995) of the total free Fe oxides. The pyrophosphate‐Fe, on the other hand, constituted between 0.2–1.0% (1993), 19–52% (1994), and 3–9% (1995) of the total free Fe oxides. Dithionite‐Fe (total free iron oxides) content increases with the increasing depth, while hydroxylamine‐Fe decreases, suggesting that larger proportions of Fe oxides are present as crystalline forms in the lower horizons. The active Fe ratios were generally high in the top soils and low in the subsoil. It ranged between 0.03 and 0.69 (1993), 0.05 and 68 (1994), 0.05 and 0.53 (1995) in all pedons. This suggests that poor drainage slowed down soil development. Highly significant correlations (0.1%) were evident between phosphorus (P) and organic carbon; ECEC and base saturation; Fe_H and active Fe ratio. Significant correlations (1%) were also evident between Fe²⁺ and organic carbon; P and Fe_H; ECEC and clay. Furthermore, significant correlations (5%) were also obtained between clay and Fe_d; pH and Fe_d; active Fe ratio and P; Fe_H and clay; active Fe ratio and Fe_d.     

The cause of redness in some buried and non-buried soils in eastern England

Relationships were examined between colours and the amounts of extractable-iron or of individual iron oxides present in <2μm fractions of samples from buried and non-buried soils in eastern England. Colour was expressed in terms of a redness rating (RR). Oxalate-extractable iron (Fe_o), dithionite-extractable iron (Fe_d) and goethite contents were not significantly correlated with RR. There was, however, a strong correlation between RR and hematite content. These preliminary results provide some justification for the assumptions concerning relationships between reddish colours and hematite content, which are implicit in the paleo-argillic concept of the Soil Survey of England and Wales. However, reddish colours (particularly within the 7.5 YR and possibly the 5 YR hue range) should not be used in isolation as an indiscriminate indicator of pre-Devensian pedogenesis in Britain.