Iron hydroxides in soils: A review of publications

Iron hydroxides are subdivided into thermodynamically unstable (ferrihydrite, feroxyhyte, and lepidocrocite) and stable (goethite) minerals. Hydroxides are formed either from Fe³⁺ (as ferrihydrite) or Fe²⁺ (as feroxyhyte and lepidocrocite). The high amount of feroxyhyte in ferromanganic concretions is proved, which points to the leading role of variable redox conditions in the synthesis of hydroxides. The structure of iron hydroxides is stabilized by inorganic elements, i.e., ferrihydrite, by silicon; feroxyhyte, by manganese; lepidocrocite, by phosphorus; and goethite, by aluminum. Ferrihydrite and feroxyhyte are formed with the participation of biota, whereas the abiotic formation of lepidocrocite and goethite is possible. The iron hydroxidogenesis is more pronounced in podzolic soils than in chernozems, and it is more pronounced in iron-manganic nodules than in the fine earth. Upon the dissolution of iron hydroxides, iron isotopes are fractioned with light-weight ⁵⁴Fe atoms being dissolved more readily. Unstable hydroxides are transformed into stable (hydr)oxides, i.e., feroxyhyte is spontaneously converted to goethite, and ferrihydrite, to hematite or goethite.     

Crystallization of Metal Substituted Ferrihydrites

The effects of a series of divalent, first row transition elements, i.e. Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ on the crystallization of ferrihydrite have been compared. With the exception of Mn²⁺, the metal ions considered, stabilized ferrihydrite and enhanced the amount of haematite in the reaction product. The stabilizing ability of these ions could be related to the increase in covalency of these metals along the series. With more than 15 mole% divalent metal ion present, ferrihydrite transformed to a spinel phase by a dissolution/reprecipitation mechanism. These metals can replace some Fe³⁺ in the structures of the crystallization products. Factors that influence the extent of isomorphous substitution are the match between the radii and charges of Fe³⁺ and the substituent ions and also, the congruency of dissolution of the M/ferrihydrite coprecipitate.     

Effect of Cadmium(Ii) and Anion Type on the Ageing of Ferrihydrite and its Subsequent Leaching under Neutral and Alkaline Conditions

The effect of cadmium(II) on the transformation of ferrihydrite[with Cd(II):Fe(III) ratios ranging from 0 to 5 mole %] in neutral and alkaline media (pH 7-11), combined with the effects of electrolyte type (NO₃ ^-, Cl^-, and SO₄ ^-2), was investigated at 20 °C over a period of 1 yr. The presence of Cd(II) strongly retards the conversion of ferrihydrite into hematite and/or goethite at pH 7–10, with decreases in the rate of transformation dependent on the amountof Cd(II). At a Cd(II):Fe(III) mole ratio of 1%, the transformation rate is NO₃ ^- > Cl^- > SO₄ ^-2, which correlates with the relative affinitiesof the anions for the ferrihydrite surface. The presence of Cd(II) promotes hematite formation at pH 9 and 10, whereas atpH 11 goethite is almost the sole product. With increasinginitial Cd(II) concentrations, increasing incorporationof Cd(II) into the products is observed. For 5 mole %Cd(II), ～ 2.5 mole % of Cd(II) is included in thetransformation products, principally hematite, while at pH 11, with 1 mole % Cd(II), all of the Cd(II) incorporates into thegoethite lattice. Transmission electron micrographs show that the presence of Cd(II) leads to a reduction in size and promotesthe twinning of goethite crystals, and can result in ellipsoidal-shaped hematite crystals. Leachability of Cd(II) fromfresh and aged coprecipitated Cd(II)-ferrihyrdite is dependent onthe extent of transformation of the ferrihydrite, with 70–90% of the Cd(II) leachable from ferrihydrite, while goethite is ableto incorporate and remove more Cd(II) than hematite.     

Iron inefficient and efficient oat cultivars. II. Characterization of phytosiderophore released in response to Fe deficiency stress

Abstract

Iron‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats (Strategy II plants) differ in their release of phytosiderophore in response to iron‐deficiency stress—the Fe‐efficient Coker 227 releases a phytosiderophore whereas the Fe‐inefficient TAM 0–312 does not. The phytosiderophore released by Coker 227 oats in response to Fe‐deficiency stress does not appear to transport Fe into the plant as Fe phytosiderophore. When the Fe‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats were subjected to Fe supplied as Fe²⁺(BPDS)₃, Fe³⁺HEDTA, as Fe³⁺EDDHA, Coker 227 utilized the Fe more efficiently than TAM 0–312 in every case. Both cultivars reduced Fe³⁺ as FeCl₃ to form Fe²⁺(BPOS)₃ and responded better to this form of Fe than Fe supplied as the ferric chelate. Reduction of Fe³⁺ at the root appears to be a factor that facilitates iron uptake by Coker 227 oats and the release of a phytosiderophore appears to make more Fe available at the root that can be reduced and transported to plant tops.     

电解质对酸性土壤铝的溶解及铝形态在土壤溶液中的分布的影响

KCl, CaCl₂, NH₄Cl, NaCl, K₂SO₄ and KF solutions were used for studying the effects of cations and anions on the dissolution of aluminum and the distribution of aluminum forms respectively. Power of exchanging and releasing aluminum of four kinds of cations was in the decreasing order Ca²⁺ >K⁺ >NH₄⁺ >Na⁺. The dissolution of aluminum increased with the cation concentration. The adsorption affinity of various soils for aluminum was different. The aluminum in the soil with a stronger adsorption affinity was difficult to be exchanged and released by cations. The Al-F complexes were main species of inorganic aluminum at a low concentration of cations, while Al³⁺ became major species of inorganic aluminum at a high concentration of cations. The results on the effect of anions indicated that the concentrations of total aluminum, three kinds of inorganic aluminum (Al³⁺, Al-F and Al-OH complexes) and organic aluminum complexes (Al-OM) when SO₄^2- was added into soil suspension were lower than those when Cl^- was added. The dissolution of aluminum from soils and the distribution of aluminum forms in solution were affected by the adsorption of F^- on the soil. For soils with strong affinity for F^-, the concentrations of the three inorganic aluminum species in soil solution after addition of F^- were lower than those after addition of Cl^-; but for soils with weak affinity for F^-, the concentrations of Al³⁺ and Al-OM were lower and the concentrations of Al-F complexes and total inorganic aluminum after addition of F^- were higher than those after addition of Cl^-. The increase of F^- concentration in soil solution accelerated the dissolution of aluminum from soils.     

Swelling and mineralogy of smectites in paddy soils derived from marine alluvium,Japan

A low-swelling smectite exhibiting little intra-crystalline swelling even after saturation with Na was found in paddy soils derived from marine alluvium of Ariake Bay in Kyushu, Japan. The low-swelling smectite was considered to be beidellite-nontronite mineral containing as much as 10% Fe₂O₃. The low-swelling characteristics of this smectite are ascribed to the considerable substitution of Fe²⁺ for Al³⁺ in the ochtahedral layer which depresses the dissociation of unit layers of smectite. Transformation of low-swelling to high-swelling smectite is believed to be due to the oxidation of Fe²⁺ to Fe³⁺ in the octahedral layer.     

Mechanisms of fixation and release of ammonium in paddy soils after flooding III. Effect of the oxidation state of octahedral Fe on ammonium fixation

An experiment with two typical paddy soils from China and two clay minerals was conducted to study the effect of reduction of octahedral Fe^III on fixation of NH₄⁺ ions. Reduction of octahedral Fe^III was achieved by treating soils and clay minerals with dithionite‐citrate‐bicarbonate (DCB) followed by dialyzing the samples under oxygen free conditions. Reduction of Fe^III increased the negative charge of interlayers and resulted in a significantly higher ammonium fixation. Close positive correlations were found between the Fe²⁺ concentration or the ratio of Fe²⁺/Fe³⁺ and non‐exchangeable NH₄⁺‐N. Therefore, it is concluded that the reduction of octahedral Fe induced by flooding is one of the important prerequisites for the pronounced ammonium fixation in flooded soils. However, the relation between ΔFe²⁺ and Δfix‐N was not stoichiometric.     

Temperature-dependent oxide removal from manganese- and iron oxide-coated soil redox bars

Purpose

Soil temperature is a fundamental parameter affecting not only microbial activity but also manganese (Mn^III,IV) and iron (Fe^III) oxide reduction rates. The relationship between Mn^III,IV oxide removal from oxide-coated redox bars is missing at present. This study investigated the effect of variable soil temperatures on oxide removal by Mn^III,IV and Fe^III oxide-coated redox bars in water-saturated soil columns in the laboratory.

Materials and methods

The Mn coatings contained the mineral birnessite, whereas the Fe coatings contained a mixture of ferrihydrite and goethite. Additionally, platinum (Pt) electrodes designed to measure the redox potential (E_H) were installed in the soil columns, which were filled with either a humic topsoil with an organic carbon (C_org) content of 85 g kg^?1 (pH 5.8) or a subsoil containing 2 g C_org kg^?1 (pH 7.5). Experiments were performed at 5, 15, and 25 °C.

Results and discussion

Although elevated soil temperatures accelerated the decrease in E_H after water saturation in the topsoil, no E_H decreases regardless of soil temperature occurred in the subsoil. Besides soil temperature, the importance of soil organic matter as an electron donor is highlighted in this case. Complete removal of the Mn^III,IV oxide coating was observed after 28, 14, and 7 days in the soil columns filled with topsoil at 5, 15, and 25 °C, respectively. Along the Fe redox bars, Fe^III reducing conditions first appeared at 15 °C and oxide removal was enhanced at 25 °C because of lower E_H, with the preferential dissolution of ferrihydrite over goethite as revealed by visual differences in the Fe^III oxide coating. Oxide removal along redox bars followed the thermodynamics of the applied minerals in the order birnessite > ferrihydrite > goethite.

Conclusions

In line with Van’t Hoff’s rule, turnover rates of Mn^III,IV and Fe^III oxide reduction increased as a result of increased soil temperatures. Taking into account the stability lines of the designated minerals, E_H-pH conditions were in accordance with oxide removal. Soil temperature must therefore be considered a master variable when evaluating the oxide removal of redox bars employed for the monitoring of soil redox status.

     

Standard potentials (Eo) of iron(III) oxides under reducing conditions

Partial reduction of iron(III) oxides with hydrogen in the presence of a platinum catalyst leads to an equilibrium state after 4–20 h. From the measured Eh, pH, and Fe²⁺ concentration conditional standard potentials can be calculated using the formula E_o (volt) = Eh + 0.059 lg(Fe²⁺) + 0.18 pH which indicate the stability of Fe oxides against reduction. The reduceability decreases following the order ferrihydrite > lepidocrocite > hematite > goethite. The difference between hematite and goethite was more pronounced than that predicted from thermodynamic data.     

螃蟹对闽江河口互花米草湿地土壤活性养分变化的影响

为阐明螃蟹活动对湿地土壤活性养分含量变化的影响,对闽江河口湿地不同潮滩螃蟹干扰下的土壤DOC、MBC、NH_4~+-N、NO_3~--N、Fe及其价态含量特征进行测定和分析。结果表明:有螃蟹组土壤DOC和MBC含量分别为95.98,11.13mg/kg,对照组含量分别为106.99,7.54mg/kg,螃蟹组土壤DOC含量略低于对照组(P0.05),螃蟹组土壤MBC含量高于对照组(P0.05);两者含量最高值和最低值分别出现在夏季和冬季,且夏季显著高于其他季节(P0.05)。螃蟹组土壤NH_4~+-N和土壤NO_3~--N含量分别为22.45,1.08mg/kg,对照组含量分别为23.65,1.44mg/kg,螃蟹组土壤NH_4~+-N含量低于对照组(P0.05),螃蟹组土壤NO_3~--N含量低于对照组(P0.05);不同季节,土壤NH_4~+-N和NO_3~--N含量存在显著差异(P0.01)。螃蟹组土壤总铁含量略高于对照组(P0.05);螃蟹组土壤Fe2+含量显著高于对照组(P0.05);螃蟹组和对照组土壤Fe3+含量差异不明显(P0.05)。不同季节,土壤总Fe、Fe~(2+)和Fe~(3+)含量存在显著差异(P0.01)。     

Anaerobic reoxidation of Mn2+, Fe2+, S0 and S2– in submerged paddy soils

Anaerobic reoxidation of reduced products in paddy soils was investigated. Ferrous iron (Fe²⁺) and monosulfide ion (S^2–) added to the soil chemically reduced MnO₂ to Mn²⁺, and MnO₂ and Fe(OH)₃ to Mn²⁺ and Fe²⁺, respectively, where Fe²⁺ and S^2– were considered to be oxidized to Fe³⁺ and S⁰. Elemental sulfur was oxidized to sulfate by anaerobic incubation with NO₃ ^– MnO₂ and Fe(OH)₃. A new conceptual model for the reduction processes in submerged paddy soil including the reoxidation processes of reduced products, in which soil heterogeneity in paddy fields was taken into consideration, was proposed based on the results. Received: 20 October 1996     

Effects of land use and mineral characteristics on the organic carbon content,and the amount and composition of Na‐pyrophosphate‐soluble organic matter,in subsurface soils

Land use and mineral characteristics affect the ability of surface as well as subsurface soils to sequester organic carbon and their contribution to mitigation of the greenhouse effect. There is less information about the effects of land use and soil properties on the amount and composition of organic matter (OM) for subsurface soils as compared with surface soils. Here we aimed to analyse the long‐term (≥ 100 years) impact of arable and forest land use and soil mineral characteristics on subsurface soil organic carbon (SOC) contents, as well as on amount and composition of OM sequentially separated by Na pyrophosphate solution (OM(PY)) from subsurface soil samples. Seven soils with different mineral characteristics (Albic and Haplic Luvisol, Colluvic and Haplic Regosol, Haplic and Vertic Cambisol, Haplic Stagnosol) were selected from within Germany. Soil samples were taken from subsurface horizons of forest and adjacent arable sites continuously used for >100 years. The OM(PY) fractions were analysed for their OC content (OC_PY) and characterized by Fourier transform infrared spectroscopy. Multiple regression analyses for the arable subsurface soils indicated significant positive relationships between the SOC contents and combined effects of the (i) exchangeable Ca (Ca_ex) and oxalate‐soluble Fe (Fe_ox) and (ii) the Ca_ex and Al_ox contents. For these soils the increase in OC (OC_PY multiplied by the relative C=O content of OM(PY)) and increasing contents of Ca_ex indicated that OM(PY) mainly interacts with Ca²⁺. For the forest subsurface soils (pH < 5), the OC_PY contents were related to the contents of Na‐pyrophosphate‐soluble Fe and Al. The long‐term arable and forest land use seems to result in different OM(PY)‐mineral interactions in subsurface soils. On the basis of this, we hypothesize that a long‐term land‐use change from arable to forest may lead to a shift from mainly OM(PY)‐Ca²⁺ to mainly OM(PY)‐Fe³⁺ and ‐Al³⁺ interactions if the pH of subsurface soils significantly decreases to <5.     

恒电荷土壤与可变电荷土壤对Cl~-和NO_3~-吸附的差异及其机理

对 3种可变电荷土壤和 4种恒电荷土壤在不同 pH、不同浓度、不同相伴阴阳离子下混合体系中Cl-和NO3-的吸附进行了测定。结果表明 ,在Cl-和NO3- 共存体系中 ,Cl-比例增大使可变电荷土壤Na+吸附量及OH-释放量增加 ,而对恒电荷土壤影响不大。Cl-和NO3-吸附量随平衡Cl-和NO3-浓度增加而增大 ,随pH升高而减少。但恒电荷土壤在上述各种条件下对Cl-和NO3-吸附均相同 ,而可变电荷土壤对Cl-吸附量大于NO3-的吸附量 ;NO3-、Cl-的选择系数为 0.51～0.78,Cl-和NO3-的相对吸附量分别为56.9%和 43.1%。在不同相伴阳离子下 ,可变电荷土壤平衡溶液Cl-/NO3-比值均小于 1,且为Na+K+Ca2+Mg2+Fe3+;而恒电荷土壤Cl-/NO3-比值为 1左右 ,且不受阳离子类型的影响。由此认为 ,Cl-和NO3-在两类土壤中均以电性吸附为主 ,恒电荷土壤对Cl-和NO3-的亲合力及吸附机理相同 ;而可变电荷土壤对Cl-的亲合力 NO3- ,Cl-存在着专性吸附     

Soil solution aluminium activity related to theoretical A1 mineral solubilities in four Australian soils

Four soils were treated with HNO₃, CaCO₃ and K₂SO₄ to enable observation of the response of the soil solution composition and the solution A1 ion activity (Al³⁺) to the treatments and to time. The clay fraction of three of the soils was dominated by illite, kaolinite and quartz. The fourth was minated by kaolinite and iron oxides. The initial pH in 0.01 M CaCl₂ varied between 4.0 and 5.0 and the organic carbon content from 0.7 to 1.1%. The soil solutions from soils dominated by kaolinite, illite and quartz were generally supersaturated with respect to quartz and well ordered kaolinite, and unsaturated with respect to illite. The soil solutions from the soil dominated by kaolin and iron oxide were generally unsaturated with respect to quartz but still saturated with respect to ell crystallized kaolin. Within mineral groups such as Al₂SiO₅ compounds, A1₂Si₂O₅(OH)₄ (kaolinite group), and Al(OH)₃ (A1 oxide) minerals, the more soluble forms became less supersaturated or unsaturated with time for many treatments. Lime treatment usually increased the ion activity product of AI(OH)₃ in all soils, and of minerals with the composition, Al₂SiO₅, in the illite/kaolinite soils. Acid treatment reduced the apparent solubility of Al(OH)₃, and the A1 silicates in the Al₂SiO₅, and Al₂, Si₂, O₅,(OH)₄, mineral groups on all soils. These results are interpreted to indicate that lime treatment led to the formation of trace quantities of more soluble A1 minerals that subsequently controlled (Al³⁺), whereas acid treatment dissolved trace quantities of such minerals leaving less soluble minerals to control (Al³⁺). The results suggest that, in mineral soils such as these, (Al³⁺) is under the control of inorganic dissolution and precipitation processes. These processes conform to expectations given the free energy of various inorganic aluminium compounds. Furthermore the sequence of dissolution and formation processes appears to be governed by the Gay-Lussac—Ostwald step rule.     

Cyanide in paper de‐inking sludge used as a soil amendment

Paper de‐inking sludge is processed during the recycling of paper, and is sometimes used as a soil amendment. In this study the effect of a compost application on the cyanide (CN) status in soils of a public park was investigated. The compost was a mixture of chipped limbs and paper de‐inking sludge. Furthermore, the cyanide solubility was studied by conducting batch experiments with different pH levels. Total cyanide in the amended soils ranged from 540 to 740 mg CN kg^—1, and water soluble cyanide from 170 to 370 μg CN l^—1 as determined by means of an aqueous extract. Easily‐liberatable cyanides, which include the toxic free cyanide (HCN and CN^—) and weak metal‐cyanide complexes, were not present in the soil. From this result and the fact that iron blue pigments are used during paper printing, it can be inferred that cyanides occurring here were exclusively stable iron‐cyanide complexes [Fe(CN)₆]. With increasing pH the solubility of cyanide increased. In contrast to soils of coking plants, in which cyanide occur as Berlin blue, Fe₄[Fe(CN)₆]₃, the cyanide solubility in the paper de‐inking sludge amended soils was substantially lower, especially in the neutral and alkaline range. Thus, cyanides in paper de‐inking sludge could be present as sparingly soluble metal‐cyanide compounds with the general formula A₂B[Fe^II(CN)₆] with A = K⁺, Na⁺ and B = Ca²⁺ or divalent transition metals and B₂[Fe^II(CN)₆] with B = divalent transition metals. Pollution exposure by the pathways soil → human, and soil → air → human can be neglected. However, since leaching of iron‐cyanide complexes into the ground water cannot be excluded, and since they are decomposed to HCN when exposed to day light, environmental hazards by the pathway soil → ground water → surface water are possible. This is the risk arising from paper de‐inking sludge applications to soils.     

Theoretische Betrachtungen zur reduktiven Auflösung von Eisen(III)-oxiden

Theoretical Considerations of the reductive dissolution of iron(III) oxides At the case of equilibrium, the extent of reductive dissolution of iron(III) oxides can be obtained by the Nernst equation. At a given pH, the maximum Fe²⁺ concentration depends on the standard potential E_o of the system, which decreases with increasing stability of the oxide. Crystal imperfections as well as increasing surface area lead to an increase of the equilibrium Fe²⁺ concentration, whereas the influence of ionic replacement cannot be determined in general. The formation of stable Fe²⁺ complexes or of solid Fe²⁺ compounds results in a higher extent of reduction. From theoretical calculations it is concluded, that, for the bacterial reduction of iron oxides, the electron transfer between donor and Fe(III) precedes the protolytic dissolution of the oxide.     

Factors affecting phosphate sorption along a Mediterranean, dolomitic soil and vegetation chronosequence

Phosphate sorption by calcareous soils has been studied mainly on heavily fertilized agricultural soils and soils with calcite as the main carbonate mineral. We examined factors affecting phosphate adsorption in the soils of a semi-arid, mediterranean, dolomitic, soil and vegetation chrono-sequence in southeastern Spain. The youngest soils are highly eroded, Sandy Regosols (Typic Xerorthents) under gorse-scrubland vegetation. These have small P sorption capacities, large Mg-Ca carbonate contents but small amounts of Fe and Al oxides. Small total P (HNO₃/HClO₄ digestion) concentrations (30–130 μg P g^?1), of which up to 90% is Ca-bound (HCl-extractable), are typical of these young soils. P sorption markedly increased when Ca²⁺ was added to the solution. The fractionation of previously sorbed P indicates that the fate of most of this extra-sorbed P is the labile-P fraction sorbed on to (carbonate) surfaces and the apatite-like fraction (NaHCO₃-extractable and HCl-extractable fractions). At the other extreme, older more-intensively weathered, sandy-clay-loam rendzinas (Entic Haploxerolls), supporting dense mature garrigue, have a much greater P adsorption capacity and larger clay and Fe and Al oxide concentrations. They have more total P (ca 400 μg P g^?1), much of it in occluded form (residual fraction). These soils show no significant differences in P sorption whether or not CaCl₂ was used as a background electrolyte. Considering the overall variations within the chronosequence, dithionite extractable Fe and Al are the properties best correlated with P sorption. This support the general finding that crystalline Fe-oxides (e.g. goethite and haematite) appear to be the most important P-sorbing component for soils in the Mediterranean region, rather than amorphous Fe-oxides (e.g. ferrihydrite) as is reported for more mesic areas. Stepwise multiple regression and fractionation data, however, suggest that, provided the soil solution is rich in Ca²⁺, carbonate may also be a significant contributing factor to P sorption, especially in the youngest of these dolomitic soils.     

pH和磷的交互作用对稳定化土壤砷释放的影响

以3种含Fe材料稳定化土壤(FeSO_4、FeS和Fe~0稳定化土壤)为研究对象,研究了不同pH和P的交互作用对As释放量的影响及其作用机理。结果表明,在pH=3条件下,随着时间的延长能够促进释放的As重新趋于稳定化,而pH=11时3种稳定化土壤中As的释放量显著增加,144 h后分别约为pH=3时的10.5、16和10倍。添加P时,在3种pH条件下都促进了稳定化土壤中As的释放,尤其在酸性条件下,相对于无P体系As的释放量增加最为明显。FeSO_4稳定化土壤中As的释放特征能用Elovich方程较好地描述,而双常数方程拟合FeS和Fe~0稳定化土壤效果更优。在pH=3条件下,3种稳定化土壤中As的释放量与Ca、Mg、Mn的溶出量呈(极)显著相关,而pH=11时FeSO_4和FeS稳定化土壤中As的释放量与SO_4~(2–)的溶出量呈(极)显著相关,表明在酸性条件下,稳定化土壤As的释放主要受Ca、Mg、Mn氧化物结合态As溶解的影响,而碱性条件下,FeSO_4和FeS稳定化土壤中As的释放可能与硫化物的溶解有关。     

The influence of vegetation and soil type on the speciation of iron in soil water

Soluble organic matter derived from exotic Pinus species has been shown to form stronger complexes with iron (Fe) than that derived from most native Australian species. It has also been proposed that the establishment of exotic Pinus plantations in coastal southeast Queensland may have enhanced the solubility of Fe in soils by increasing the amount of organically complexed Fe, but this remains inconclusive. In this study we test whether the concentration and speciation of Fe in soil water from Pinus plantations differs significantly from soil water from native vegetation areas. Both Fe redox speciation and the interaction between Fe and dissolved organic matter (DOM) were considered; Fe – DOM interaction was assessed using the Stockholm Humic Model. Iron concentrations (mainly Fe²⁺) were greatest in the soil waters with the greatest DOM content collected from sandy podosols (Podzols), where they are largely controlled by redox potential. Iron concentrations were small in soil waters from clay and iron oxide‐rich soils, in spite of similar redox potentials. This condition is related to stronger sorption on to the reactive clay and iron oxide mineral surfaces in these soils, which reduces the amount of DOM available for electron shuttling and microbial metabolism, restricting reductive dissolution of Fe. Vegetation type had no significant influence on the concentration and speciation of iron in soil waters, although DOM from Pinus sites had greater acidic functional group site densities than DOM from native vegetation sites. This is because Fe is mainly in the ferrous form, even in samples from the relatively well‐drained podosols. However, modelling suggests that Pinus DOM can significantly increase the amount of truly dissolved ferric iron remaining in solution in oxic conditions. Therefore, the input of ferrous iron together with Pinus DOM to surface waters may reduce precipitation of hydrous ferric oxides (ferrihydrite) and increase the flux of dissolved Fe out of the catchment. Such inputs of iron are most probably derived from podosols planted with Pinus.     

Coupled diffusion and oxidation of ferrous iron in soils: III. Further development of the model and experimental testing

Equations are developed to predict the distribution of Fe²⁺ between solid and solution phases in a reduced soil undergoing oxidation at different pHs, based on cation-exchange equilibria and electrical neutrality in the solid and solution. The equations satisfactorily explained experimental results. They are incorporated in the model of Fe²⁺ diffusion and oxidation developed in Part II, and the model is also extended to allow for O₂ consumption in processes other than Fe²⁺ oxidation. The resultant predictions are tested against measured profiles of Fe(II), Fe(III) and pH in cylinders of reduced soil exposed to O₂ at one end. When oxidation rate constants measured in stirred soil suspensions were used to run the model, the predicted rates of O₂ consumption were too great and the spread of the oxidation front too small. Satisfactory agreement was achieved for oxidation rate constant values about one-eighth of those measured in the stirred suspensions. The findings are consistent with the rate of Fe²⁺ oxidation in soil being controlled by access of O₂ to Fe²⁺ sorption sites, as suggested in Part I. The revised model allows a study of the effects of Fe²⁺ oxidation on the mobility of other cations in reduced soils, e.g. nutrient cations in the rice rhizosphere. Fez+ oxidation and the accompanying acidification may greatly impede cation mobility in reduced soils.