Role Of Fe Oxides Research Articles

The stoichiometric C-Fe ratio regulates glucose mineralization and stabilization via microbial processes

The association of soil organic matter (SOM) with iron (Fe) oxides by adsorption and/or co-precipitation contributes to long term C stabilization in soil. While there is an understanding of the relationship between soil carbon (C) and the biogeochemical cycling of Fe, a lack of information exists on the role of Fe oxides on the accumulation of C in paddy soils. This study aimed to assess the role of Fe (oxyhydr)oxides on mineralization and stabilization processes following amendment of paddy soil with a labile C substrate (99 atom % 13C-glucose). The study utilized 4 paddy soils with a total Fe concentration ranging from 13.7 to 55.8 g kg−1. In soils with 42.7 and 55.8 g kg−1 Fe, the addition of glucose resulted in an Fe bound organic C: Fe molar ratio (C:Fe molar ratio) ≥6, suggesting the formation of Fe-OM complexes mainly via co-precipitation. The highest portion of 13C (13.8%) protected in Fe-OM complexes was found in soil containing 55.8 g kg−1 of Fe. The stabilization of the added labile C substrate was shown, using random forest analysis, to be controlled by the C: Fe molar ratio, while substrate mineralization was regulated by the core genera Sphingomonas and Devosa (r-strategists) (affiliated to Proteobacteria) and C:N ratio. Substrate mineralization was 47% lower in soil containing 55.8 g Fe kg−1 compared to 13.7 g Fe kg−1, with a concomitant reduction in SOM priming of 37%. This reduction in substrate mineralization and the priming effect was likely due to lower C substrate availability via the formation of Fe-OM complexes, thereby protecting the C from mineralization. In conclusion, the Fe concentration in paddy soils plays a central role in the abiotic stabilization of ‘new’ C through the formation of Fe-OM complexes via co-precipitation, thereby limiting the availability of this C substrate for microbial mineralization, and at the same time modulating the microbial community structure.

Contrasting Mg isotopic compositions between Fe-Mn nodules and surrounding soils: Accumulation of light Mg isotopes by Mg-depleted clay minerals and Fe oxides

Magnesium isotopic systematics has been increasingly used to trace the biogeochemical cycle of Mg in soil systems, and Fe oxides are the critical soil components that affect the geochemical behaviours of elements in soils. The role of Fe oxides in fractionating Mg isotopes, however, remains unclear. Here, Mg isotopic compositions are reported for typical Fe-Mn nodules (FMNs), surrounding soils, soil waters, and soil surface waters for a paddy soil profile, and stream waters, and rainwaters in southwestern China to improve our understanding of the processes that control the Mg isotopic compositions in soil systems. Further sequential extraction experiments are conducted to separate two pools of Mg in the FMNs and soils: structural Mg and exchange Mg. The FMNs (−1.39 to −1.58‰) are isotopically lighter than surrounding soils (−0.59 to −0.85‰) but heavier than soil waters (−1.59‰), and surrounding soils are isotopically lighter than parent granite (−0.25‰). The difference in Mg isotopic compositions between FMNs and surrounding soils reflects different sources of Mg in the mineral crystal structures. Structural Mg in surrounding soils is mainly from the chemical weathering of parent granite. By contrast, structural Mg in FMNs is from soil waters because of the frequently repeated dissolution and precipitation of Fe oxides under alternating redox conditions. Enrichment of heavy Mg isotopes in the FMNs relative to soil waters results from preferential incorporation of 26Mg via Mg2+ substitution for Fe3+ in goethite. Given that the exchangeable Mg (−1.62 to −1.91‰) is significantly enriched in light Mg isotopes, the lighter Mg isotopic compositions in surrounding soils relative to their parent granite can be explained by the retention of light Mg isotopes in exchangeable sites of Mg-depleted minerals (kaolinite). Exchangeable Mg in FMNs (−1.79 to −2.15‰) is also shown to be enriched in light Mg isotopes. These light isotopic compositions of exchangeable Mg can be explained by a combination of carbonate contribution and isotope fractionation processes on the soil exchange fractions. Ion-exchange processes preferentially remove heavy Mg isotopes from soil minerals, leaving soil exchange fractions hosting light Mg isotopes. Additionally, river waters draining carbonates contributes light Mg isotopes to soil exchangeable fractions. Our study demonstrates that the development of Mg-depleted clay minerals and Fe oxides can considerably lower the soil δ26Mg values, highlighting the major roles of these two soil minerals in controlling soil Mg isotopic compositions.

Constraints on biotic and abiotic role in the formation of Fe-Si oxides from the PACMANUS hydrothermal field

Fe-Si oxide deposits were recovered from the PACMANUS (Papua New Guinea-Australia-Canada-Manus) hydrothermal field in Eastern Manus basin. Samples were loose and fragile. Optical and scanning electron microscopy showed that the samples had abundant rod-like or twisted filamentous and granular structures. Electron probe microanalysis revealed that these filaments and grains were mainly composed of Fe and Si. The presence of spherical grains on the surface of the filaments suggests the intergrowth of biotic and abiotic reactions. Biotic and abiotic kinetics competition always exists in the redox gradient. Based on the physico-chemical conditions of PACMANUS hydrothermal fluids, we calculated a strict abiotic oxidation rate of Fe2+ to Fe3+, which is approximately 0.0123 g/min. If the fluids had been erupting consistently and the concentration of Fe2+ was constant, 3.232 kg per year of Fe would be deposited in this vent. The amount of Fe oxides around the studied vent was larger than the amount determined by strict abiotic kinetic calculation. Bacteria may also play an important role in Fe oxidation. A mesh-like microenvironment constructed by biogenic filaments ensured adequate Fe2+ and low oxygen content for the growth of bacteria. Moreover, this structure promoted the deposition of abiotic Fe-Si oxides.

FeIIaq–FeIIIoxide electron transfer and Fe exchange: effect of organic carbon

Environmental context Concerns about carbon-driven climate change make it critical to better understand how carbon interacts with soils. We examined whether carbon compounds commonly found in soils affect the behaviour of soil iron minerals, and found that iron minerals remain dynamic and mix with the surrounding water, despite the presence of carbon in the water. Our findings suggest that both carbon and trace elements in the minerals may be more mobile and less stable than we previously thought. Abstract The close association of organic carbon and Fe oxides has been recognised for decades and recently interest in the role of Fe oxides in organic C sequestration has increased. Here we explore the effect of natural organic C on electron transfer and exchange of aqueous FeII and Fe in the bulk structure of goethite and magnetite. Using 57Fe isotope experiments coupled with 57Fe Mössbauer spectroscopy it is found that electron transfer occurs between aqueous FeII and structural FeIII in both goethite and magnetite in the presence of most organic C compounds, including natural organic matter (NOM), extracellular polysaccharides (EPS), and cell materials. Only a long-chain (39-C) phospholipid containing organic C significantly inhibited electron transfer. Despite evidence for FeII–FeIIIoxide electron transfer in the presence of NOM, exchange of Fe between the aqueous phase and bulk Fe was less than observed in the absence of NOM. Our findings suggest that electron transfer between aqueous FeII and bulk structural Fe in goethite and magnetite is a robust process likely to occur in a variety of organic-rich biogeochemical environments but that Fe exchange may be inhibited by the presence of some C compounds.

Role of Fe oxides in corrosion of pipeline steel in a red clay soil

Red clay soils show especially corrosive towards metallic materials, but no satisfactory interpretation exists to explain the phenomenon. Soil Fe oxides (goethite, hematite, etc.) appear to play a key role in soil corrosion by functioning as acathodic depolarizer. A series of experiments were therefore conducted to evaluate the effect of Fe oxides on corrosion of pipeline steel in a red clay soil. The results indicate that corrosivity of the red clay soil greatly relieves after Fe oxides are removed from the soil. A Fe oxide-catalyzed kinetic mechanism is proposed to clarify high corrosivity of red clay soils.

Clay Mineralogy of Central Victorian (Creswick) Soils: Clay Mineral Contents as a Possible Tool of Environmental Indicator

The clay mineralogy and heavy metal/metalloid (As, Pb and Cu) contents of soils developed on the various rock units in a central highlands environment in Victoria (Creswick, Australia) have been investigated. The clay minerals identified showed an order of abundance as: kaolinite ≫ illite > smectite > mixed-layer (ML) ≈ vermiculite. The soil clay mineralogy did not change systematically with depth (0∼ 10, 10∼ 20 and 20∼ 30 cm) and showed large variations spatially. The high proportion of kaolinite was probably due to the removal of 2:1 phyllosilicates by the formation of 1:1 kaolinite through weathering, which also reduced the cation exchange capacity (CEC) and electrical conductivity (EC, soil: water ratio of 1:5) of soils by aging. Soils were classified as silty loam to loam with a low clay size (≤ 2μ m) fraction. The soils were acidic to moderately acidic with pH ranging from 4.5 to 7.1, averaging 5.7. Concentrations of As, Pb, and Cu (average values 24.3, 16.7 and 11.0 mg/kg, respectively) did not show an association with the clay mineral contents except vermiculite. The occurrence of smectite and mixed-layer clay contents, although far lower than kaolinite and illite, contributed significantly to CEC of soils. The study area was affected by mining, high natural background As values dominate the area and the role of clay minerals in fixation of metalloid/metals was found to be less significant. Low organic matter content (average ∼ 6.5%), low soil surface area (average ∼ 1.0 m2/g) and the high proportion of kaolinite mineral content result in a limited ability to fix heavy metals. The role of Fe oxides appeared to be a key influence in the fixation of As and other potentially toxic metals, rather than the clay minerals, and therefore requires further research. This work highlighted the importance of the determination of types and amounts of clay minerals of natural soils in environmental management.

Neutrophilic Fe-oxidizing bacteria are abundant at the Loihi Seamount hydrothermal vents and play a major role in Fe oxide deposition.

A number of hydrothermal vent sites exist on the summit of the Loihi Seamount, a shield volcano that is part of the Hawaiian archipelago. The vents are 1,100 to 1,325 m below the surface and range in temperature from slightly above ambient (10 degrees C) to high temperature (167 degrees C). The vent fluid is characterized by high concentrations of CO2 (up to 17 mM) and Fe(II) (up to 268 microM), but there is a general paucity of H2S. Most of the vents are surrounded by microbial mats that have a gelatinous texture and are heavily encrusted with rust-colored Fe oxides. Visually, the Fe oxides appeared homogeneous. However, light microscopy revealed that the oxides had different morphologies, which fell into three classes: (i) sheaths, (ii) twisted or irregular filaments, and (iii) amorphous oxides. A morphological analysis of eight different samples indicated that the amorphous oxides were overall the most abundant; however, five sites had >50% sheaths and filamentous oxides. These latter morphologies are most likely the direct result of microbial deposition. Direct cell counts revealed that all of the oxides had abundant microbial populations associated with them, from 6.9 x 10(7) to 5.3 x 10(8) cells per ml of mat material. At most sites, end point dilution series for lithotrophic Fe oxidizers were successful out to dilutions of 10(-6) and 10(-7). A pure culture was obtained from a 10(-7) dilution tube; this strain, JV-1, was an obligate, microaerophilic Fe oxidizer that grew at 25 to 30 degrees C. A non-cultivation-based molecular approach with terminal-restriction fragment length polymorphism also indicated the common presence of Fe-oxidizing bacteria at Loihi. Together, these results indicate that Fe-oxidizing bacteria are common at the Loihi Seamount and probably play a major role in Fe oxidation. A review of the literature suggests that microbially mediated Fe oxidation at hydrothermal vents may be important globally.

PHOSPHORUS DYNAMICS AND UPTAKE BY WHEAT IN A MODEL CALCITE-FERRIHYDRITE SYSTEM

The role of Fe oxides on the availability and dynamics of phosphorus (P) is poorly understood in calcareous soils. We ascertained its role in laboratory and plant growth experiments on substrates prepared by mixing calcium carbonate (calcite) sand (CCS, 0.2-0.5 mm) and Fe oxide (ferrihydrite)-coated quartz sand (FOCS, 0.2-0.5mm). Distribution of P into the solid and liquid phases was studied after one, three, six, and ten wetting-drying cycles (WDC) after adding P as KH2PO4 at the rate of 0, 5, 10, 20, 50, and 100 μg P g−1 to substrates containing 0, 10, 30, 50, 70, 80, and 90% FOCS. Different P levels, WDC and substrate composition had a significant (P < 0.05) main and interactive effect on P distribution between solution and solid phases. There was a significant (P < 0.05) 1- to 8-fold increase in the distribution ratio of P between solution and solid phases with increasing FOCS in the substrate at 10 and 20 μg g−1 P. The increase in water-soluble P with FOCS was sharper at 10 μg P g−1 than at 20 μg P g−1. In the plant growth experiment, durum wheat (Triticum durum cv. Don Pedro) was grown in substrates containing 0, 10, 20, 30, 50, and 90% FOCS, which received P as KH2PO4 at the rate of 0, 6, 12, 18, and 72 μg P g−1. Substrates containing higher proportion of CCS tested higher for Olsen P for the same rate of P application. The same trend was observed for Olsen P after cropping. Substrate composition and P application rate had a significant (P < 0.05) main and interactive effect on growth and P uptake by plants. Phosphorus-Zn interaction due to increased P supply in the soil-plant system depressed plant growth with increasing P application rate at higher proportion of FOCS in the substrate. Concentration of P in plants increased significantly with increasing proportion of FOCS in the substrate. The relationship between plant P concentration and concentration of P in the substrate solution could be described by a Michaelis-Menten equation. At a certain level of Olsen P, plant P concentration increased with increasing FOCS proportion in the substrate. This suggests that Olsen P might overestimate plant-available P in Fe oxide-poor relative to Fe oxide-rich calcareous soils.

Pb Adsorption and Surface Area of Bacterially Oxidized Mn

Trace metal cycling in aquatic environments is controlled largely by adsorption to suspended particulate material (SPM) (1), and this process is governed predominantly by Fe and Mn oxides contained in the SPM (2). Initial estimates of the relative contributions of Fe and Mn oxides to Pb adsorption indicated that the role of Fe oxides far overshadowed that of Mn oxides based on measurement of the Fe and Mn content of natural surface coatings and adsorption of Pb by abiotically produced, laboratory surrogate Fe and Mn oxides (3). However, Mn oxides in circumneutral aquatic environments are likely to be of biogenic origin (4), and biologically oxidized Mn may exhibit a greater specific surface area and thus greater trace metal adsorption than abiotic Mn oxides. To test this hypothesis, in this work Mn was oxidized by the bacterium Leptothrix discophora SS-1 and the surface area and Pb adsorption of the resulting Mn oxides were determined and compared to those properties of abiotic Mn oxides. Methods

Evaluation of phosphorus in savannah soils of western Nigeria under bush fallow systems

SUMMARYYield calibration of soil tests in western Nigeria savannah has been difficult owing to low response to P and response to only low P rates despite low available P in the soils. A study on the distribution of P forms in 60 surface soils of the savannah derived from igneous and metamorphic rocks and grouped into 11 classes was made. Average total P was 0·018% and organic P, which varied between 44 and 114 mg/kg, constituted 41% of total P. Active P components made up 40% of total inorganic P, the bulk of which was in the Fe-P fraction. All P forms except non-extractable P correlated significantly with organic matter.The amounts of P needed to increase available P by 1 mg/kg in the soils, an index of P fixation, was correlated with pH and the free oxides of Fe and Al. The direct role of Fe-oxides and probable indirect effect of organic matter were indicated by regression analysis.There was minimal maize yield response to P application and this was obtained only at low P rates from which an optimum rate of 20 kg P/ha was suggested. An attempt to use phosphorus sorption capacity (PSC) for P recommendation was not successful probably because of the low PSC values. A quick P recommendation based on available P status, fertilizer factor of 3·0 mg/1 and an optimum available P level derived from the relative yield v. soil P calibration curve was therefore suggested.