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Organic fertilizers are essential for enhancing soil organic carbon (SOC) levels and promoting sustainable agriculture. The mechanisms of carbon sequestration in upland red soils using various organic materials are not well understood. Here, the impact of various organic fertilization strategies on SOC fractions, functional groups, iron oxides, and soil aggregates was investigated to clarify the protective mechanisms underlying SOC preservation. Six fertilization regimes were examined: no fertilization (CK), mineral fertilizer (CF), 60% mineral fertilizer (RF), RF + straw (RFS), RF + pig manure (RFP), and RF + vermicompost (RFV). The results demonstrated that SOC contents significantly increased by 62.5% in RFP and 63.1% in RFV compared to the CF treatment, with particulate organic carbon (POC) being the primary contributor. Scanning electron microscopy and X‐ray photoelectron spectroscopy demonstrated that RFP and RFV treatments induced the formation of denser flock‐like aggregates and increased aromatic C content, respectively. Fourier transform infrared spectroscopy analysis indicated that RFV treatment exhibited the highest abundance of functional groups among all treatments. SOC and POC were highly positively correlated with soil chemical properties, soil aggregates, poorly crystalline iron oxides, and aromatic rings. Structural equation models indicated that pig manure and vermicompost directly promoted POC accumulation, while straw provided physical protection for POC by enhancing aggregate formation. Furthermore, pig manure indirectly facilitated the development of POC and mineral‐associated organic carbon via soil biochemical properties and iron oxides, respectively. Overall, the application of organic fertilizers enhanced the physical protection, mineral immobilization, and recalcitrance (alkyl C, aromatic C) of organic carbon, facilitating C sequestration and stability. This study highlights the importance of organic management in upland red soils for increasing SOC storage and maintaining global C balance. [ABSTRACT FROM AUTHOR]

Soil iron (Fe)‐associated carbon (C) (Fe‐OC) plays a vital role in the soil C cycle due to its high stability, but vegetation restoration might alter the composition and quantity of Fe‐OC by introducing a large amount of plant‐derived C and affecting soil properties. However, how vegetation restoration affects soil Fe‐OC remains unclear. Herein, plant and topsoil samples from grasslands, shrublands, and forestlands across three soil types (loam, loess, and sandy soil) since cropland conversions were collected to address this issue. The results showed soil Fe‐OC content decreased in loam soil but increased in loess and sandy soil following vegetation restoration. Additionally, the Fe‐OC accumulation efficiency induced by vegetation restoration increased with the coarser soil texture. Vegetation restoration promoted the accumulation of Fe‐OC by increasing soil microbial biomass C, dissolved organic C, aromatic‐C, and citric acid, but also disrupted the combination of Fe oxides and C by introducing oxalic acid, reducing Fe oxide content and iron trivalent (Fe(III)). There were two‐sided effects of vegetation restoration on Fe‐OC, but the overall effect depends on the soil types. Moreover, isotopic evidence indicated that microbial source C is the main source of Fe‐OC, but Fe oxides preferentially adsorbed dissolved organic matter (DOM) and root deposits from plants rather than microbial residues and metabolites following vegetation restoration. In addition, Fe oxides preferentially adsorbed aromatic‐C compared to other functional group components. These findings indicated that vegetation restoration in coarser‐texture soils, coupled with selecting species that increase soil microbial biomass, produce more root deposits, and enhance DOM, contribute to the accumulation of soil Fe‐OC. Plain Language Summary: Iron (Fe)‐associated carbon (C) (Fe‐OC), formed by the combination of organic C and Fe oxides, plays a significant role in soil C cycling. However, the changes in soil Fe‐OC and their underlying mechanisms following vegetation restoration remain unclear. This gap hinders our understanding of soil C cycling and our ability to protect this important C pool. Here, croplands, grasslands, shrublands, and forestlands across three soil types (loam, loess, and sandy soil) were selected to address this issue. All three vegetation types had undergone 20 years of restoration from croplands. We found that soil Fe‐OC content decreased in loam soil but increased in loess and sandy soils after vegetation restoration. Furthermore, the effects of vegetation restoration on Fe‐OC accumulation and depletion were observed across all three soil types, but the overall effect varied depending on the soil type. Additionally, microbial‐derived C was identified as the primary source of Fe‐OC during vegetation restoration, while Fe oxides preferentially adsorbed plant‐derived dissolved organic matter and root deposits. These results deepen our understanding of soil C cycling and emphasize the potential for enhancing this stable C pool through human intervention. Key Points: Vegetation restoration has a two‐sided effect on Fe‐OC, and the overall effect depends on soil typeMicrobial‐derived C is the main source of Fe‐OC during vegetation restorationFe oxide preferentially adsorbs dissolved organic matter and root exudates from plants compared to microbial‐derived C [ABSTRACT FROM AUTHOR]

Potentially harmful element (PHE) bioavailability is important to environmental contamination and must be checked under several soil conditions. This study aimed to assess Fe, Mn, and PHE uptake by rice (Oryza sativa) grown on flooded and non-flooded Fe tailings collected from the Doce River basin after its collapse in Brazil. After 65 days of sowing, shoots and roots were harvested to determine PHE concentrations. The mean concentrations of Mn in shoots and Fe in the roots of rice grown on the flooded tailings were 2140 mg kg−1 and 15,219 mg kg−1, respectively. Mn was extensively translocated from roots to shoots (translocation factor (TF) = 2). Conversely, Fe accumulated in roots (TF = 0.015) and caused morphological damage to this rice organ. The application of macro and micronutrients lessened Fe toxicity in the roots of rice cultivated on the flooded tailings. The flooding of tailings influenced more Fe accumulation than Mn accumulation by rice plants. The PHE Ag, As, Cd, Ni, Hg, Pb, and Sb exhibited low total concentrations (maximum of 9 mg kg−1 for Ni and a minimum of 0.2 mg kg−1 for Cd, Hg, and Sb), and it was not observed an increase in their availability under tailings flooding conditions. [ABSTRACT FROM AUTHOR]

To further enhance the high-temperature corrosion resistance of boiler tubes for improved power generation efficiency of biomass and waste-to-energy plants, Ni-Al cladding layers with Al contents of 2, 4, 6 and 8 wt.% were prepared by laser cladding technique. The corrosion characteristics of Ni-Al cladding layers in two salts (NaCl-KCl and NaCl-KCl-K2SO4, at 650 °C) were investigated. The critical Al content of Ni-Al cladding layers with better corrosion resistance was determined, and the effect of Fe dilution from the substrate on the corrosion products was studied. In NaCl-KCl, the corrosion rates of all cladding layers were high at the initial stage. As the corrosion proceeded, a continuous and dense Al2O3 scale was gradually formed on the Ni-8Al surface, while the other three cladding layers failed to be generated. Therefore, Ni-8Al exhibited the best corrosion resistance. However, in NaCl-KCl- K2SO4, Al2O3 was continuously dissolved due to the "basic fluxing" mechanism and Ni-8Al underwent significantly severe corrosion. Fe oxides were detected in the corrosion products of all cladding layers. Since Fe ions diffuse more easily through the Al2O3 scale, Fe oxides were in the outermost layer and were prone to spalling, which affected the corrosion resistance. [ABSTRACT FROM AUTHOR]

Historical coal mining practices have caused various soil and water hazards, particularly through the dumping of mine waste. The primary environmental risk associated with this waste is the leaching of toxic metals from dumps of spoil or refuse into the subsurface soil or into nearby water resources. The extent of metal release is controlled via the oxidative dissolution of pyrite and potential re-sequestration through secondary Fe oxides. The characterization of the dominant Fe-bearing phase and the distribution of trace metals associated with these phases was determined via electron microscopy, synchrotron-based X-ray micro-fluorescence (μ-XRF) element and redox mapping from shallow mine soils from an impacted watershed in Appalachian Ohio. The dominant Fe-bearing phases were: (1) unweathered to partially weathered pyrite; (2) pseudomorphic replacement of pyrite with Fe(III) oxides; (3) fine-grained Fe oxide surface coatings; and (4) discrete Fe(III) oxide grains. Thicker secondary coatings and larger particles were sulfate rich, whereas smaller grains and thinner coatings were sulfate poor. The discrete Fe oxide grains exhibited the highest concentrations of Cr, Mn, Ni, and Cu, and sub-grain-scale concentration trends (Mn > Cr > Ni > Cu) were consistent with bulk soil properties. Predicting future metal transport requires an understanding of metal speciation and distribution from the sub-grain scale to the pedon scale. [ABSTRACT FROM AUTHOR]

Heat-induced changes in topsoil iron (Fe) species triggered by the relatively low temperatures (Ts) produced by wildfires (200–300 °C) have not been fully elucidated. Changes in the chemistry of Fe minerals can produce cascading impacts on the environment, and the resulting aftermath may be exacerbated in erosion-prone alpine soils. The aim of this study was to determine - at environmentally realistic conditions - changes in the composition and crystallinity of Fe species, and the dispersible organic matter (OM) pool (evaluated by the pyrophosphate extraction), as a function of rising Ts, native Fe phases and OM in alpine soils. Multiple techniques were employed, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), magnetic measurements and Fe K-edge X-ray absorption spectroscopy (XAS). The results demonstrated the dominance of poorly crystalline Fe phases at 300 °C. At the same T, we observed the partial conversion of maghemite to hematite (supporting the involvement of oxidative processes) and, in the presence of high organic carbon (OC) contents, an enrichment in magnetic minerals (suggesting the onset of reducing conditions). The heat-induced modifications in Fe species and organic compounds did not promote the stabilization of the remaining OM, highlighting the weak nature of soil organo-mineral associations in an after-fire scenario. These phenomena can be further aggravated in case of the steep terrain that characterize alpine soils.

Aims: Although phytate is the most abundant organic P compound in soil, its sorption makes it poorly bioavailable through enzymatic hydrolysis. The sustainable use of the P resource in agriculture will require improving the supply of P to plants from organic P forms. To this end, further knowledge on dynamics of phytates in the soil is necessary, particularly in alkaline soils. This work aimed to study the factors that control phytate sorption and hydrolysis by phytases in alkaline soils and how this sorption affects other P forms present in soil. Methods A phytate sorption experiment was carried out using six soils with basic pH. Subsequently, the hydrolysis by two phytases (from Aspergillus niger and Bacillus subtilis) of organic P remaining in solution and extracted from the soil was assessed. Results Phytate was rapidly sorbed by soils with an estimated maximum sorption of 603 mg kg−1 at 360 min. Phytate sorption was positively correlated with carbonates, poorly crystalline Fe oxides, organic C, pH, and electrical conductivity in soils. Phytate adsorption promoted the release of other organic and inorganic P forms with less affinity for sorbent surfaces. For inorganic P, this release was more evident in soils with a low poorly crystalline Fe oxide content, while for organic P, the release increased with carbonates and clay content. Most of the phytate adsorbed was not extracted by NaOH, particularly in soils with high carbonate and poorly crystalline oxide content. This reveals that a significant portion of adsorbed phytate can be bound to carbonates, precipitated as metal phytates, or was not desorbed from Fe oxides. The insights gained from this research contribute to increasing our understanding of organic P cycling in soils and how soil properties influence the dynamics of these P forms.

Cadmium (Cd) contamination in wheat fields has become a major environmental issue in many regions of the world. Mercapto-palygorskite (MPAL) is a high-performance amendment that can effectively immobilize Cd in alkaline wheat soil. However, MAPL as an in-situ Cd immobilization strategy for alkaline wheat soil remains to be evaluated on a field-scale and the underlying mechanisms requires further evaluation. Here, MPAL were used as soil amendment to evaluate their immobilization efficiency on Cd-contaminated alkaline soil in the field experiments. The field experiments showed that MPAL application significantly reduced wheat grain Cd concentration from 0.183 mg/kg to 0.056 mg/kg, with Cd concentration in wheat grain treated with MPAL all falling below the limit value of 0.1 mg/kg as defined in China's food safety standard (GB 2762–2022). The maximal immobilization efficiency of MPAL on soil Cd figured out by diethylenetriaminepentaacetic acid (DTPA) extraction was 61.5%. The mechanisms involved in Cd immobilization by MPAL were mainly related to the enhanced sorption of Cd onto Fe oxides, and the removal of amorphous or free Fe oxides from soil had a substantial impact on Cd immobilization efficiency by MPAL. Furthermore, the antagonistic effect between Mn and Cd uptake may also contribute to the reduction of wheat Cd accumulation after MPAL application. The current research can provide theoretical and technical support for the large-scale application of MPAL in Cd-contaminated wheat fields.

Sighter Induced Roll Magnetic Separation tests were conducted on a ferromanganese ore sample to determine the potential to significantly upgrade the Mn content and Mn:Fe ratio. The sample had a Mn:Fe ratio of 1.3 and was comprised of hematite, cryptomelane, pyrolusite, quartz, goethite and braunite. Tests at 4.7, 2.3 and 1 mm top sizes revealed better separation of Fe from Mn at the 2.3 and 1 mm top sizes. This reflected the better liberation and separation characteristics of Fe oxides at 2.3 mm and both Fe and Mn oxides at 1 mm. A test on the −2.3 + 0.6 mm size fraction improved the Mn:Fe ratio to 4.6, the Mn content to 41.5 wt% with 69.4% mass recovery and 84.2% Mn recovery. Particle magnetic susceptibility was influenced by particle microporosity, and its effect on apparent density, resulting in some Mn oxides reporting to the magnetic concentrate and Fe oxides to the tails. [ABSTRACT FROM AUTHOR]

Tidal channels are biogeochemical hotspots that horizontally exchange carbon (C) with marsh platforms, but the physiochemical drivers controlling these dynamics are poorly understood. We hypothesized that C‐bearing iron (Fe) oxides precipitate and immobilize dissolved organic carbon (DOC) during ebb tide as the soils oxygenate, and dissolve into the porewater during flood tide, promoting transport to the channel. The hydraulic gradient physically controls how these solutes are horizontally exchanged across the marsh platform‐tidal channel interface; we hypothesized that this gradient alters the concentration and source of C being exchanged. We further hypothesized that trace soil gases (i.e., CO2, CH4, dimethyl sulfide) are pushed out of the channel bank as the groundwater rises. To test these hypotheses, we measured porewater, surface water, and soil trace gases over two 24‐hr monitoring campaigns (i.e., summer and spring) in a mesohaline tidal marsh. We found that Fe2+ and DOC were positively related during flood tide but not during ebb tide in spring when soils were more oxidized. This finding shows evidence for the formation and dissolution of C‐bearing Fe oxides across a tidal cycle. In addition, the tidal channel contained significantly (p < 0.05) more terrestrial‐like DOC when the hydraulic gradient was driving flow toward the channel. In comparison, the channel water was saltier and contained significantly (p < 0.05) more marine‐like DOC when the hydraulic gradient reversed direction. Trace gas fluxes increased with rising groundwater levels, particularly dimethyl sulfide. These findings suggest multiple physiochemical mechanisms controlling the horizontal exchange of C at the marsh platform‐tidal channel interface. Plain Language Summary: Tidal salt marshes store large amounts of carbon belowground in soils, but there is also a significant amount of carbon flowing into and out of these ecosystems via tidal channels. We investigated the carbon flowing between the channel bank and surface water in a salt marsh in Delaware. We found that soil minerals (i.e., iron oxides) control the mobility of carbon as iron oxides retain carbon during ebb tides and release carbon during flood tides as the minerals dissolve. The gradient between the groundwater and surface water elevation (i.e., hydraulic gradient) controls the flow direction for dissolved carbon, altering the concentration and source of carbon found in the tidal channel across tidal cycles. In addition, gases trapped in channel banks are pushed out of the soils as the tide rises. These findings will improve our understanding of carbon cycles in these critical carbon sinks. Key Points: Physiochemical mechanisms control horizontal exchange of carbon across marsh‐tidal channel interfaces, affecting lateral carbon fluxDissolution and reprecipitation of carbon‐bearing Fe oxides during flood and ebb tides control the horizontal mobility of carbonHydraulic gradients control the carbon character in the tidal channel, and rising tides push greenhouse gases out of the channel bank [ABSTRACT FROM AUTHOR]

Soil parent material can strongly influence soil's physical and chemical properties, significantly affecting soil organic carbon (SOC) stabilization. However, the importance of different soil parent materials on the amount and composition of subsoil carbon in subtropical soils remains poorly qualified. Subsoil SOC (below A horizon) is chosen for this study because of its higher proportion of stable carbon than surface soils in A horizon. Here, we investigated the dependence of soil mineral‐associated organic carbon (MAOC) and its three mineral‐stabilized fractions on soil physical and chemical parameters (clay content, cation exchange capacity (CEC) and metal oxides) in the subsoil soil (B and C horizons) originating from three contrasting parent materials (sandshale, granite, limestone). The results showed that MAOC accounts for an average of 88% of total SOC, of which organo‐metal complexes (extracted with sodium pyrophosphate) are the dominant form at all three sites. MAOC and its three fractions were significantly correlated with soil clay, CEC, sodium pyrophosphate extractable Fe and oxalate extractable Fe concentrations (P < 0.05), which varied significantly with soil parent material. Since the sodium pyrophosphate plus oxalate‐ extracted mineral‐associated carbon accounted for more than 70% of MAOC, and the molar ratio between sodium pyrophosphate or oxalate‐ extracted mineral‐associated carbon and Fe (C:Fe) ranged from 2.7 to 17.1. The formation of Fe‐associated organic complex is dominated by co‐precipitation when C:Fe is >1. Therefore we conclude that non‐sorptive interactions with the mineral phase through co‐precipitation are the dominant mechanism for SOC stabilization in subtropical forest soils. Plain Language Summary: Identifying the mechanisms for the accumulation and stabilization of forest soil organic carbon (SOC) is particularly important for increasing land‐based carbon mitigation as a part of the solution toward carbon neutrality by 2060 in China. However, our understanding of the mechanism of SOC stabilization is still limited, particularly how soil carbon stabilization is influenced by soil parent material, which has been shown to have strong influences on soil's physical and chemical properties, therefore on SOC stabilization. By analyzing the carbon content and composition of the soils developed from the different parent materials, we showed that soil parent material indirectly affected SOC through soil physical and chemical properties, of which soil clay content, CEC and soil Fe oxides played an important role in SOC stabilization. Both sorptive and co‐precipitation interactions between Fe and SOC contributed to SOC stabilization in subtropical forest soil with co‐precipitation as the dominant mechanism. Key Points: Mineral‐associated organic carbon accounts for >80% of soil organic carbon (SOC) in southern China forest soilsCo‐precipitation with Fe is the dominant SOC stabilization mechanismParent material is a significant contributor to the observed SOC variations [ABSTRACT FROM AUTHOR]

Historical coal mining practices have caused various soil and water hazards, particularly through the dumping of mine waste. The primary environmental risk associated with this waste is the leaching of toxic metals from dumps of spoil or refuse into the subsurface soil or into nearby water resources. The extent of metal release is controlled via the oxidative dissolution of pyrite and potential re-sequestration through secondary Fe oxides. The characterization of the dominant Fe-bearing phase and the distribution of trace metals associated with these phases was determined via electron microscopy, synchrotron-based X-ray micro-fluorescence (μ-XRF) element and redox mapping from shallow mine soils from an impacted watershed in Appalachian Ohio. The dominant Fe-bearing phases were: (1) unweathered to partially weathered pyrite; (2) pseudomorphic replacement of pyrite with Fe(III) oxides; (3) fine-grained Fe oxide surface coatings; and (4) discrete Fe(III) oxide grains. Thicker secondary coatings and larger particles were sulfate rich, whereas smaller grains and thinner coatings were sulfate poor. The discrete Fe oxide grains exhibited the highest concentrations of Cr, Mn, Ni, and Cu, and sub-grain-scale concentration trends (Mn > Cr > Ni > Cu) were consistent with bulk soil properties. Predicting future metal transport requires an understanding of metal speciation and distribution from the sub-grain scale to the pedon scale.

The results of study of the Aleksinian–Venevian deposits of the Visean Stage exposed in a Mstikhino limestone open pit are presented. The samples for study were collected layer-by-layer and were studied using a complex of mineralogical–geochemical methods. Six unconformities are distinguished, which correspond to the main sedimentation breaks in a marine epicontinental basin. The deposits composing the unconformities are specified as paleosols and are divided into four pedotypes. Most profiles are thin and have a two-units structure: a variously subaerially transformed marine limestone (calcrete or karst) and a terrigenous layer also exogenically altered. The latter forms at the transgression stage and can represent the deposits of fresh temporally ephemeral lakes, marine shallow basins, or limestone eluvium. The following signatures of subaerial alteration of the top of limestones are revealed: the formation of platy and brecciated structures, a striking relief, the changes in physical properties (decrease in bulk density, the change in color and hardness), and the presence of rhizoliths. Carbonates exhibit dissolution and reprecipitation. The chemical composition is characterized by increased contents of Si, Al, Fe, K, and heavy metals. The terrigenous layers (carbonate or carbonate-free) in most cases are the coastal-marine pioneer soils. They contain organic matter; biota is presented by rhizoliths, plant detritus, and plant spores. Ferrugination is a morphologically striking and analytically supported feature. According to Mössbauer spectroscopy, the Fe-bearing phases include oxides: goethite and hematite in various proportions. The lower terrigenous level of the Mikhailovian Horizon (MHT1) is determined as a swamp paleosol (histosol) formed on a clastic carbonate-free substrate, which represents the bottom deposits of a freshwater lake, which was further shoaled and swamped. The diverse vegetation included bryophyte-like plants and lycopod trees. The complex of the characteristics indicated that the climate of the Mikhailovian period at the studied territory fluctuated, exhibiting dominant humid conditions and the amount of precipitation of 800–1000 mm/year. More arid periods with dominant Calcisol were also present. The comparison of our results with previously published data on a soil cover of the Moscow sedimentary basin in the Early Carboniferous showed its lateral diversity. At the same time, the paleosol of the MHT1 period is found in other open pits of Kaluga oblast. It is morphologically recognizable, has a specific mineral composition, and can be considered a significant stratigraphic unit (Geosol). [ABSTRACT FROM AUTHOR]

Fe oxides play a crucial role in the biogeochemical processes of nitrous oxide (N 2 O) emissions. However, the role of Fe in affecting the quantitative contribution of ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) to N 2 O production has not been well understood. To clarify the effects of goethite on relative contribution of AOB and AOA to N 2 O production, acetylene and 1-octyne inhibition culture experiment was conducted in two paddy soils with contrasting pH (pH 5.5 and pH 7.9). The results showed that goethite increased cumulative N 2 O emissions by 13.5 %–39.9 % and 2.7 %–27.2 % in acidic and alkaline paddy soils, respectively. In both paddy soils with nitrogen (N) application, AOB contributed more to N 2 O emissions than AOA, while the trend was opposite in the treatment without N application. Goethite increased the relative contribution of AOB to N 2 O production from 41 % to 56 % in acidic paddy soil and from 54 % to 59 % in alkaline paddy soil, respectively. The main genera involved in N 2 O emissions in the two paddy soils were Nitrosospira -AOB and Nitrososphaera -AOA, and goethite increased their abundance in alkaline paddy soil. Overall, goethite mainly increased the N 2 O emissions by promoting the autotrophic nitrification processes dominated by AOB. The findings of our study provide new insights into the development of strategies to reduce N 2 O emissions by steering the microbiome responsible for N 2 O emissions by regulating soil iron oxide in paddy soils. [Display omitted] • Goethite increased cumulative N 2 O emissions in paddy soils. • AOB, rather than AOA, was the main contributor to N 2 O emissions in both soils. • Goethite increased the abundance of AOB and its contribution to N 2 O emissions. • Goethite increased Nitrosospira -like AOB and Nitrososphaera -like AOA in alkaline paddy soil. [ABSTRACT FROM AUTHOR]

Both iron (Fe) oxide and soil organic carbon (SOC) content, among other factors, drive the development and subsequent stability of soil aggregates. However, the mechanism of these drivers in paddy‐upland rotation systems with straw residue incorporation is not well understood. In this multiyear (2011–2017) trial, we sought to study the factors which drive the stability of soil aggregates in such a rice‐rape (RR) agronomic system. In order to examine the effect of straw residue incorporation and potassium fertilization on soil aggregation dynamics, we divided our trial into four treatments: inorganic phosphorus (P) and nitrogen (N) fertilizer; inorganic NPK fertilizer; inorganic NP fertilizer plus straw residue; and inorganic NPK fertilizer plus straw residue. Treatments which incorporated straw residue significantly increased both the geometric mean diameter (GMD) and the mean weight diameter (MWD) of soil aggregates. Soil aggregate stability was greater after rape harvest than after rice harvest, while SOC shows the opposite trend. Straw residue incorporation increased the SOC and, specifically, concentrations of aliphatic‐C and aromatic‐C, particularly in aggregates greater than 0.25 mm. Straw residue addition significantly increased both the amorphous (Feo) and complex iron oxide (Fep) contents of soil. In both bulk soil and in aggregates greater than 5 mm, SOC and Fep were positively correlated with MWD, while in aggregates less than 5 mm, Feo was positively correlated with MWD. Overall, we suggest that the increased concentrations of SOC, alkane‐C, Feo, and Fep in soils after incorporation of straw residue were responsible for increasing soil aggregate stability. [ABSTRACT FROM AUTHOR]

Purpose: The poor soil structure and low stability of organic carbon limit the sustainable development of agriculture. The effect of organic fertilizer application on soil aggregates, soil organic carbon (SOC) functional groups, Fe oxides, SOC stability, and storage was assessed in saline-alkaline paddy soil. Methods: A 7-year field experiment was designed with organic fertilization in saline-alkaline soil of Yellow River Delta including (i) CK (no fertilizer), (ii) NPK (255, 28, and 190 kg ha−1 year−1 N, P, and K mineral fertilizers, respectively), (iii) NPKC1 (NPK + 450 kg C ha−1 year−1), and (iv) NPKC2 (NPK + 900 kg C ha−1 year−1). Results: Compared with NPK treatment, the mean weight diameter (MWD) of soil aggregates in NPKC1 and NPKC2 treatments was increased by 27.8% and 41.1%, respectively. Soil amorphous Fe oxide (Feo) content was significantly increased with organic fertilizer addition, especially in small macro-aggregates. Meanwhile, aromatic-C was primarily distributed in small macro-aggregates, which could be retained in aggregate interfaces by forming aromatic-Fe complexes with soil Feo. Furthermore, the specific C mineralization rate (SCMR) in NPKC1 treatment was lower than that in other treatments, in which the lowest SCMR was found in small macro-aggregate. Compared with NPK treatment, SOC storage in NPKC1 and NPKC2 treatments was increased by 14.4% and 20.5%, respectively, which was due to the improvement of soil structure, organo-Fe complex formation, and reduction of soil C mineralization. Conclusions: The combined organic and mineral fertilizer application was an optimized management practice to improve the C stability and sequestration in coastal saline alkaline paddy soil. [ABSTRACT FROM AUTHOR]

Fertilization is an effective approach to improve aggregation of red soil (Ultisol). However, the effect of soil aggregation after long-term organic or mineral fertilization on red soil drought degree during intermittent seasonal drought remains unclear. The objective of this study was to examine the effect of long-term fertilization on soil aggregation (aggregate size distribution and stability) and aggregate-binding agents, and to determine the relationship between aggregation and soil drought degree (D) during wet and dry years. Red soil was continuously applied with manure, NPK + straw, or NPK for 21 years. Surface soil water content (SWC), D, aggregate size distribution, wet aggregate stability, and aggregate-binding agents contents (Fe oxides and soil organic carbon (SOC)) were determined in 2017 (normal wet year), 2019 (dry year), and 2020 (above average wet year). The results showed that manure and NPK + straw treatments resulted in greater SWC variation than the control, especially a sharper SWC decline than control under dry conditions. The results indicated that D values varied with climatic conditions, and that manure and NPK + straw resulted in higher D values than NPK in the normal wet year and dry year, while an opposite trend was observed in the above-average wet year. The D value peaked to 1 in a 70-day dry spell in 2019, while it peaked to 0.7 in < 10-day dry spell in other years. The variation in D can be explained by aggregation due to a significant negative correlation between D and the 2–0.25 mm aggregate size fraction (− 0.58 < r < − 0.64, p < 0.05). In the dry year (2019), the low proportion of 2- to 0.25-mm size aggregates under manure and NPK + straw treatments was significantly correlated with a lower SOC and amorphous Fe oxide content, which might explain their greater D values. Generally, long-term organic fertilization aggravated D during drought periods. The results of this study will contribute to the sustainable management of red soils. [ABSTRACT FROM AUTHOR]

Long‐term management with lime and/or phosphogypsum (PG) can change phosphorus (P) forms across the soil profile. In this study, we used synchrotron‐based spectroscopy and sequential chemical fractionation to investigate P chemical species after long‐term (18 years) application of amendments in a subtropical Oxisol. The availability of nutrients increased with the combined use of lime and PG and a stratification of attributes was identified due to long‐term broadcast applications. X‐ray absorption near edge structure (XANES) spectra profiles revealed the predominance of P bounded to haematite (Hm) (48% on average) and also P bound to kaolinite (32% on average) species. Low proportions (16%) of calcium‐phosphates in the uppermost soil layer (0–5 cm) were found in the treatment that received high calcium (Ca) inputs by lime and PG broadcast. Few correlations were identified between the proportions of P species obtained by XANES and P fractions operationally defined by sequential chemical fractionation (SF). Our results suggest that P‐species bound to kaolinite can store stable P forms, which correlated (p < 0.05) with the residual P fractions obtained by SF, highlighting the importance of kaolinite as a legacy P store in Oxisols. Highlights: Lime and phosphogypsum broadcast under no‐till improve soil fertility and P availability.Labile and moderately labile P fractions were the only ones influenced by lime and/or phosphogypsum.Proportions among P bounded to Fe‐ and Al‐(hydr)oxides with different structural may be affected by long‐term Al‐suppressors.P‐species bounded to kaolinite may store stable P forms in Oxisols. [ABSTRACT FROM AUTHOR]

Abstract Iron is the most abundant redox‐sensitive element on the Earth's surface, and the oxidation state, mineral host, and crystallinity of Fe‐rich phases in sedimentary systems can record details of water‐rock interactions and environmental conditions. However, we lack a complete understanding of how these Fe‐rich materials are created, maintained, and oxidized or reduced in sedimentary environments, particularly those with mafic sources. The catchment of Lake Towuti, Indonesia, is known to contain a wide range of abundant crystalline Fe oxide, and the lake has a long sedimentary history. Here, we study a ∼100 m long drill core from the lake to understand patterns of sedimentation and how young iron‐rich sediments are affected by diagenesis through geologic time. We use visible/near infrared and Mössbauer spectroscopy, X‐ray diffraction, bulk chemistry measurements, and statistical cluster analysis to characterize the core sediment. We find that the core sediment can be divided into three statistically different zones dominated by Mg serpentine, Al clay minerals, and Fe2+ carbonate, respectively. The entire core is rich in nanophase Fe, and elemental correlations and Fe mineralogy vary between these zones. The nanophase Fe is highly complex with both ferrous and ferric components, and contributes to, but does not dictate, variations in sediment color. We propose that the distinctive zones are the result of structural basin changes (notably river capture and shifting drainage patterns), and diagenetic overprinting caused by deep burial of reactive Fe. This complex record has implications for disentangling depositional and diagenetic trends in other mafic lacustrine systems.

Interfacial chemistry of phosphorus (P) is important for understanding P sequestration and bioavailability in nature. Polyphosphate is a group of important P species in aquatic environments. The geochemical behaviors of polyphosphate at the mineral-water interface play critical roles in mediating aquatic P transformation, yet remain poorly constrained. This study investigates the hydrolysis of polyphosphate in the presence of four common iron (Fe) oxide minerals (ferrihydrite, hematite, goethite, lepidocrocite) and the subsequent precipitation of calcium phosphate minerals. Batch studies are combined with microscopic and spectroscopic characterizations to reveal P speciation and complexation state under varied solution chemistry (pH 6–9, 1 mM Ca2+, and artificial seawater). All four Fe oxides can hydrolyze polyphosphate and the hydrolysis rate and extent are both enhanced in the presence of Ca2+. In the presence of 1 mM Ca2+, the apparent hydrolysis rate fitted by first-order kinetic model follows the order of lepidocrocite > hematite > ferrihydrite > goethite. After normalization by specific surface area of Fe oxides, the hydrolysis rate is in the order of lepidocrocite ≈ goethite > hematite > ferrihydrite at pH 6, regardless of Ca2+ presence. At pH 7.5 and 9, the order of area-normalized apparent hydrolysis rate is lepidocrocite > goethite ≈ hematite > ferrihydrite. A terminal-only pathway via one-by-one cleavage of terminal phosphate groups is the dominant hydrolysis mechanism. Under alkaline conditions, amorphous calcium phosphate forms in the presence of Ca2+, which transforms to crystalline hydroxyapatite after long-term aging of 70 or 150 days. This study demonstrates the importance of natural minerals in controlling polyphosphate transformation into crystalline calcium phosphate minerals, and provides new insights for understanding P cycling and sequestration in the environment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Chemistry of iron (Fe) minerals in fire-affected soils is unclear. • Changes in Fe speciation were systematically addressed in alpine soils. • Fe phases dominate as poorly crystalline structures up to 300 °C. • Heating induced enrichment in hematite and ferromagnetic minerals. • Mineral and organic phases were less associated at 300 °C than at room temperature. Heat-induced changes in topsoil iron (Fe) species triggered by the relatively low temperatures (Ts) produced by wildfires (200–300 °C) have not been fully elucidated. Changes in the chemistry of Fe minerals can produce cascading impacts on the environment, and the resulting aftermath may be exacerbated in erosion-prone alpine soils. The aim of this study was to determine - at environmentally realistic conditions - changes in the composition and crystallinity of Fe species, and the dispersible organic matter (OM) pool (evaluated by the pyrophosphate extraction), as a function of rising Ts, native Fe phases and OM in alpine soils. Multiple techniques were employed, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), magnetic measurements and Fe K-edge X-ray absorption spectroscopy (XAS). The results demonstrated the dominance of poorly crystalline Fe phases at 300 °C. At the same T, we observed the partial conversion of maghemite to hematite (supporting the involvement of oxidative processes) and, in the presence of high organic carbon (OC) contents, an enrichment in magnetic minerals (suggesting the onset of reducing conditions). The heat-induced modifications in Fe species and organic compounds did not promote the stabilization of the remaining OM, highlighting the weak nature of soil organo-mineral associations in an after-fire scenario. These phenomena can be further aggravated in case of the steep terrain that characterize alpine soils. [ABSTRACT FROM AUTHOR]

• Phytate sorption was a fast process controlled by poorly crystalline Fe oxides and carbonates. • Poorly crystalline Fe oxides are more relevant in phytate sorption than the more abundant crystalline oxides. • Precipitation of metal phytates contribute to its sorption in allkaline soils. • Phytate adsorption promoted the release of organic and inorganic P with less affinity for sorbent surfaces. Although phytate is the most abundant organic P compound in soil, its sorption makes it poorly bioavailable through enzymatic hydrolysis. The sustainable use of the P resource in agriculture will require improving the supply of P to plants from organic P forms. To this end, further knowledge on dynamics of phytates in the soil is necessary, particularly in alkaline soils. This work aimed to study the factors that control phytate sorption and hydrolysis by phytases in alkaline soils and how this sorption affects other P forms present in soil. Methods A phytate sorption experiment was carried out using six soils with basic pH. Subsequently, the hydrolysis by two phytases (from Aspergillus niger and Bacillus subtilis) of organic P remaining in solution and extracted from the soil was assessed. Results Phytate was rapidly sorbed by soils with an estimated maximum sorption of 603 mg kg−1 at 360 min. Phytate sorption was positively correlated with carbonates, poorly crystalline Fe oxides, organic C, pH, and electrical conductivity in soils. Phytate adsorption promoted the release of other organic and inorganic P forms with less affinity for sorbent surfaces. For inorganic P, this release was more evident in soils with a low poorly crystalline Fe oxide content, while for organic P, the release increased with carbonates and clay content. Most of the phytate adsorbed was not extracted by NaOH, particularly in soils with high carbonate and poorly crystalline oxide content. This reveals that a significant portion of adsorbed phytate can be bound to carbonates, precipitated as metal phytates, or was not desorbed from Fe oxides. The insights gained from this research contribute to increasing our understanding of organic P cycling in soils and how soil properties influence the dynamics of these P forms. [ABSTRACT FROM AUTHOR]

• Sb(V) associated with ferrihydrite was rapidly released after addition of sulfide. • A considerable proportion of the released Sb was in the colloidal fraction. • Antimonate was the primary Sb aqueous phase species; Sb thiolation was unimportant. • The initial sulfide-driven Sb release was followed by Sb immobilization. • Sb was immobilized via sorption to secondary Fe oxides and newly-formed Fe sulfides. Iron (Fe) oxides are important host phases for antimony (Sb), a toxic metalloid of environmental concern. In wetland soils and sediments, poorly ordered Fe oxides such as ferrihydrite may undergo reductive dissolution and mineralogical transformation upon reaction with dissolved sulfide (S(-II)). The consequences of these processes for the mobility of associated Sb have not been investigated to date. Here, we allowed Sb(V)-bearing ferrihydrite (molar ratio of Fe:Sb = 400) to react with varying S(-II) concentrations (Fe(III):S(-II) = 0.2, 0.5, and 1) at pH 6 and 8 over 32 days. Changes in speciation and concentration of Fe, S and Sb in the aqueous, colloidal and solid phase were examined through a combination of aqueous-phase analyses, X-ray diffraction and synchrotron X-ray absorption spectroscopy. Addition of S(-II) caused rapid reduction of Fe(III), thereby producing elemental S and Fe(II). X-ray diffractometry and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that S(-II) addition resulted in the precipitation of Fe(II) sulfides (mackinawite (FeS) and pyrite (FeS 2)) and formation of secondary Fe(III) oxides (goethite (FeOOH) and hematite (Fe 2 O 3)). The formation of mackinawite and pyrite was further confirmed by S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and was found to occur to the greatest extent in the high-sulfide treatments. The initial reductive dissolution of ferrihydrite was paralleled by a fast increase in dissolved Sb concentrations, with ∼25% of total Sb being released in the high-sulfide treatments. The Sb release was followed by Sb immobilization within ∼1–7 days. Since ion-chromatography ICP-MS revealed antimonate (Sb(OH) 6 -) as the primary Sb aqueous phase species throughout the experiment, with only negligible concentrations of antimonite (Sb(OH) 3) and only very minor amounts (<4% of total Sb) of tri- and tetrathioantimonate (H x SbS 3 Ox−3/H x SbS 4 x−3), the decrease in Sb concentrations was attributed to surface-based sorption and structural incorporation of primarily Sb(V) by the secondary Fe oxides. In accordance with the dominance of aqueous Sb(V), Sb K-edge XANES spectroscopy showed that Sb(V) was also the dominant Sb species in the solid phase, comprising up to 90% of solid-phase Sb in the low-sulfide treatments. However, higher S(-II) addition and lower pH favored production of Sb(III) and resulted in up to 40% and 20% of solid-phase Sb(V) being reduced to Sb(III) at pH 6 and 8, respectively, with this Sb(III) comprising a mixture of O- and S-coordinated species. Around 15% of < 0.45-µm Sb occurred in the colloidal (>3 kDa) size fraction at pH 8 under medium and high S(-II) conditions, while no colloidal Sb was found in other treatments. Together, these results show that Fe oxide sulfidization can have opposing effects on Sb mobility. On the one hand, the initial sulfide-promoted Fe oxide dissolution triggers Sb release into the aqueous phase. On the other hand, Sb can subsequently be immobilized via sorption to secondary Fe oxides and newly-formed Fe sulfides during the later stages of sulfidization. Sulfidization reactions, and the complex opposing impacts on Sb mobility, should therefore be considered for the risk assessment and derivation of adequate management strategies at Sb-impacted sites which experience sulfidic conditions. [ABSTRACT FROM AUTHOR]

Scandium is often associated with iron oxides in the environment. Despite the use of scandium as a geochemical tracer and the existence of world-class supergene deposits, uncertainties on speciation obscure the processes governing its sequestration and concentration. Here, we use first-principles approaches to interpret experimental K-edge X‑ray absorption near-edge structure spectra of scandium either incorporated in or adsorbed on goethite and hematite, at concentrations relevant for the environment. This modeling helps to interpret the characteristic spectral features, providing key information to determine scandium speciation when associated with iron oxides. We show that scandium is substituted into iron oxides at low concentrations without modifying the crystal structure. When scandium is adsorbed onto iron oxide surfaces, the process occurs through outer-sphere complexation with a reduction in the coordination number of the hydration shell. Considering available X‑ray absorption spectra from laterites, the present results confirm that scandium adsorption onto iron oxides is the dominant mechanism of sequestration in these geochemical conditions. This speciation explains efficient scandium recovery through mild metallurgical treatments of supergene lateritic ores. The specificities of scandium sorption mechanisms are related to the preservation of adsorbed scandium in million-years old laterites. These results demonstrate the emerging ability to precisely model fine X‑ray absorption spectral features of trace metals associated with mineral phases relevant to the environment. It opens new perspectives to accurately determine trace metals speciation from high-resolution spatially resolved X‑ray absorption near-edge structure spectroscopy in order to constrain the molecular mechanisms controlling their dynamics. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) contamination in wheat fields has become a major environmental issue in many regions of the world. Mercapto-palygorskite (MPAL) is a high-performance amendment that can effectively immobilize Cd in alkaline wheat soil. However, MAPL as an in-situ Cd immobilization strategy for alkaline wheat soil remains to be evaluated on a field-scale and the underlying mechanisms requires further evaluation. Here, MPAL were used as soil amendment to evaluate their immobilization efficiency on Cd-contaminated alkaline soil in the field experiments. The field experiments showed that MPAL application significantly reduced wheat grain Cd concentration from 0.183 mg/kg to 0.056 mg/kg, with Cd concentration in wheat grain treated with MPAL all falling below the limit value of 0.1 mg/kg as defined in China's food safety standard (GB 2762–2022). The maximal immobilization efficiency of MPAL on soil Cd figured out by diethylenetriaminepentaacetic acid (DTPA) extraction was 61.5%. The mechanisms involved in Cd immobilization by MPAL were mainly related to the enhanced sorption of Cd onto Fe oxides, and the removal of amorphous or free Fe oxides from soil had a substantial impact on Cd immobilization efficiency by MPAL. Furthermore, the antagonistic effect between Mn and Cd uptake may also contribute to the reduction of wheat Cd accumulation after MPAL application. The current research can provide theoretical and technical support for the large-scale application of MPAL in Cd-contaminated wheat fields. [Display omitted] • MPAL demonstrated great immobilization efficiency on Cd-contaminated alkaline soil under field conditions. • Wheat grain Cd with MPAL treatment met the national standard. • MPAL significantly increased soil-available Mn and Mn concentration in wheat plants • MPAL enhanced the sorption of soil Cd onto Fe oxides. [ABSTRACT FROM AUTHOR]

The removal of chemical species from seawater during the precipitation of authigenic minerals is difficult to constrain but may play a major role in the global biogeochemical cycles of some elements, including silicon (Si) and germanium (Ge). Here, we present Ge/Si, δ74Ge, and supporting chemical data of pore waters and core incubations at three continental margin sites in California and the Gulf of Mexico. We used these data to partition Ge release and uptake by the various allogenic (delivered via sedimentation) and authigenic (formed in situ) phases in these sediments. About half of the pore water Ge (δ74Gepw = 1.3–2.4‰) is supplied by biogenic silica dissolution (δ74Ge ~ 3‰), with the other half contributed by lithogenic particulates (δ74Ge ~ 0.6‰). The highest Ge/Si (~3μmol/mol) and lowest δ74Ge (1.3–1.9‰) are observed at the Fe redox horizon, suggesting a supply from detrital Ge-rich Fe oxides. The precipitation of authigenic phases (most likely aluminosilicate clays) in deeper sediments preferentially incorporates Ge over Si, resulting in low pore water Ge/Si (~0.3μmol/mol). The lack of corresponding δ74Gepw trend indicates negligible Ge isotope fractionation during this process. Ge fluxes measured via core incubations were variable and appeared strongly controlled by Fe redox behavior near the sediment-water interface. In some cases, reductive Fe oxide dissolution appeared to enhance the benthic Ge flux by over 100% and released fractionated low δ74Ge of ~−0.7‰, resulting in overall benthic δ74Geinc between –0.2 and 3.6‰, depending on Fe oxide contribution to Ge flux. We estimate that detrital inputs supply 12–31% of total dissolved Ge to continental margin pore fluids globally, resulting in an average pore water and benthic flux δ74Ge between 2.2 and 2.7‰. Assuming 10-60% of pore water Ge is captured by the authigenic aluminosilicate sink, the dissolved Ge flux to the ocean derived from terrigenous inputs should be roughly 2.5–6.6 Mmol/y, much higher than previously estimated. Our results imply that authigenic Si burial in continental margins should be in the range of 1–8 Tmol/y (best estimate 3.1 Tmol/y), sufficient to close the global marine Si budget.

Iron oxides supposedly provide physicochemical protection for soil organic carbon (SOC) under anoxic conditions. Likewise, biochar can modulate the composition of soil microbial communities. However, how Fe oxides and microbial communities influence the fate of SOC with biochar amendment remains unresolved, especially the effect of the bacteria-virus interaction on SOC dynamics. Here, we performed a four-month pot experiment using rice seedlings with a biochar amendment under waterlogged conditions. Then, soil aggregate sizes were examined to explore the factors influencing the SOC patterns and the underlying mechanisms. We found that biochar altered soil enzyme activities, especially in macroaggregates. Fe oxides and necromass exhibited significant negative relationships with SOC. Bacterial communities were notably associated with viral communities. Here, the keystone ecological cluster (module 1) and keystone taxa in the bacteria-virus network showed significant negative correlations with SOC. However, Fe oxides exhibited substantial positive relationships with module 1. In contrast to the prevailing view, the SOC increase was not primarily driven by Fe oxides but strongly influenced by bacteria-virus interactions and keystone taxa. These findings indicate that biochar governs microbial-mediated SOC accumulation in paddy soil and ascertains the role of viruses in regulating the bacterial community, thus predicting SOC stock.

Zinc (Zn) deficiency constrains crop yield and quality, but soil factors influencing Zn availability to plants and reactions of applied Zn fertilizer are not fully understood. This work is aimed at studying Zn availability in soil and the use efficiency of Zn fertilizers by plants as affected by soil properties and particularly by soil available P. We performed a pot experiment involving four consecutive crops fertilized with Zn sulfate using 36 soils. The cumulative Zn uptake and dry matter yield in the four crops increased with increased initial diethylenetriamine pentaacetic acid extraction of Zn (DTPA-Zn) (R2 = 0.75 and R2 = 0.61; p < 0.001). The initial DTPA-Zn increased with increased Olsen P (R2 = 0.41; p < 0.001) and with increased ratio of Fe in poorly crystalline to Fe in crystalline oxides (R2 = 0.58; p < 0.001). DTPA-Zn decreased with increased cumulative Zn uptake, but not in soils with DTPA-Zn < 0.5 mg kg−1. Overall, the available Zn is more relevant in explaining Zn uptake by plants than applied Zn sulfate. However, in Zn-deficient soils, Zn fertilizer explained most of the Zn uptake by crops. Poorly crystalline Fe oxides and P availability exerted a positive role on Zn availability to plants in soil.

Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales. [ABSTRACT FROM AUTHOR]

Field observations show that subtropical forests can accumulate soil organic carbon (SOC) at high rate; however, the key mechanisms of SOC accumulation in subtropical forests remain unclear. Here we selected three typical subtropical forests with soil originated from same parent material under same climate condition in Dinghushan Biosphere Reserve, southern China: coniferous forest, mixed forest, and broadleaved forest with very different rates of SOC accumulation over the last three decades. To quantify the relative importance of physical versus chemical process on SOC stabilization in these forests, we used regression analysis to evaluate the relationship between the soil physical (clay, silt, and sand) and chemical (pH, Fe, and Al oxides) properties and SOC, and explored their respective contributions to SOC stabilization. Results showed that SOC stabilization was more strongly influenced by soil clay fraction through sorptive protection or microaggregate formation with SOC in the coniferous forest (r2 = 0.89, p < 0.05), and by chemical protection through forming organo‐mineral complexes in the mixed or broadleaved forest. Increasing carbon input was likely the main cause for SOC accumulation in the coniferous forest, but was not in the broadleaved forest. The strong linear relationships between SOC and Fe/Al oxides indicate that both the mixed (r2 = 0.63 and 0.57 with Feo and Alo, p < 0.05) and broadleaved forests (r2 = 0.96 and 0.78 with Fed and Alo, p < 0.05) on acid soils in southern China still have a great potential for further carbon sequestration in the future. Key Points: Stable fraction of soil organic carbon was strongly influenced by soil clay in the coniferous forestSoil Fe and Al oxides contributed more to organic carbon stability in the mixed and broadleaved forestsThe mixed and broadleaved forests still have a great potential for sequestering more carbon by soil minerals in the future [ABSTRACT FROM AUTHOR]

Long-term soil chronosequences are valuable model systems for investigating pedogenesis and investigating the process of element coupling. Here, we assessed the coupling relationships among C, Fe, and Fe-reducing bacteria (Anaeromyxobacter, Geobacter, and Shewanella) in a paddy soil chronosequence of approximately 50 to 1000 years. Soils of the chronosequence originated from tidal marsh under nearly identical landscape and climate conditions. During 1000 years of rice cultivation, soil organic carbon (SOC) contents in surface horizons (0–20 cm) increased from 10.4 to 21.8 g kg−1. In contrast, total Fe contents declined from 59.6 to 45.1 g kg−1 during the initial 50 years of paddy rice cultivation and then further decreased at a low rate of 0.004 g kg−1 soil year−1 (equivalent to 10 kg ha−1 soil year−1). Organically complexed Fe oxides (Fep) increased from 219 to 642 mg g−1 with increasing time of pedogenesis, but free total Fe oxides (Fed) and amorphous Fe oxides (Feo) declined at early stage of soil development, followed by a slow accumulation at later stages of the chronosequence. Gene copy numbers of Anaeromyxobacter and Geobacter increased from 4.6 × 105 and 3.6 × 106 copies g−1 to 3.8 × 107 and 3.6 × 107 copies g−1 dry soil with continuous paddy rice cultivation, while concurrently Shewanella gene abundance decreased gradually from 4.5 × 105 to 9.3 × 104 copies g−1 dry soil. Using structural equation modeling (SEM), different coupling relationships were observed among C, Fe, and Fe-reducing bacteria for the first 300 years of paddy chronosequence and thereafter. Overall, all Fe-reducing bacteria did not show consistent variation. With the stable microbial community and iron oxide fractions, the microbially mediated dissimilatory coupling relationship between C and Fe becomes simple during 1000 years of paddy soil development. [ABSTRACT FROM AUTHOR]

The accumulation of soil organic matter (SOM), poorly crystalline Fe oxides and metal-humus complexes is a trait of non-allophanic Andosols. The process of andosolization, which may occur in ferrallitic soils with high organic matter content, can involve transformation from well crystallized Fe oxides to poorly crystallized Fe oxides and Fe-humus complexes. This study investigates such changes in pedogenic Fe mineral associations for a soil toposequence between 1500 and 2260 m altitude along the southern flank of the volcanic Bambouto Mountains, Western Cameroon. The soils consist of highly weathered material, dominated by kaolinite, gibbsite and Fe oxides, grading to Protoandic Umbrisols at high altitude, recording an increase in SOM content with increasing altitude. As revealed by selective extraction analysis, the relative amount of poorly crystalline Fe oxides is low in the Bt and Bo horizons of low-altitude pedons, as well as in deep subsurface horizons of the high-altitude pedons. In contrast, it is significantly higher in the A and Bw horizons of the high-altitude pedons, with a clear increase with increasing altitude. Mössbauer spectroscopy analysis of B horizon samples identifies goethite as the dominant Fe oxide phase in nearly all pedons, with higher hematite contents in a mid-altitude zone marked by lower annual rainfall than in other parts of the toposequence. The Mössbauer spectra also reveal the presence of dissolved organic matter (DOM)-ferrihydrite, whose abundance is greatest in the Bw horizon of the high-altitude pedons, with an increase in relative abundance with increasing altitude. The observed patterns are attributed to dissolution-reprecipitation of Fe oxides that initially formed through ferrallitic weathering of volcanic parent materials that were roughly uniform along the toposequence. At high altitude, coupled hematite dissolution and DOM-ferrihydrite formation are favoured by high organic matter contents and low pH, related to cool humid environmental conditions and their effect on the vegetation and organic matter cycling. • Altitude-dependent variations in relative abundance of Fe oxide minerals • Fe oxide transformations through dissolution-reprecipitation reactions • Soil organic matter as major factor in Fe oxide stability • DOM-ferrihydrite formation at the expense of more crystalline Fe oxides • Hematite stability in lower-rainfall zone [ABSTRACT FROM AUTHOR]

Abstract Recent studies have shown the feasibility of using of sewage sludge for the remediation of heavy metal-contaminated soils. However, there are no researches to check the influence of clay mineralogy on the efficiency of the sewage sludge to remediation of contaminated soils with heavy metals. For this purpose, we use two contrasting soils: Oxisol rich in hematite and gibbsite and Inceptisol rich in kaolinite. Thermal-treated sludge was applied to Pb-contaminated soil samples and incubated for 40 days. The soil samples were submitted to seven sequential extractions: soluble-Pb, exchangeable-Pb, precipitated-Pb, organic matter-Pb, Fe and Mn oxide-Pb, gibbsite and kaolinite-Pb, and residual-Pb. The reduction of soluble Pb forms by thermal sludge application was more pronounced in the Oxisol than in the Inceptisol because of the conversion of soluble-Pb into more stable forms, such as precipitated-Pb and oxides-Pb. For Inceptisol was necessary to apply high rates of thermal sludge to reach a significant reduction in soluble-Pb contents. The addition of humic fractions in the form of thermal sludge increased the concentration of organic matter-Pb. In confined area, the use of sewage sludge to reduce the heavy metals levels in soils must be better considered, mainly in more weathered soils. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Purpose: The concentrations and distribution of arsenic (As) in two different soil types (Vertisols and Entisols) of Central Mexico impacted by mine activities and irrigation with As-rich groundwater are analyzed in order to determine their impact on the soil quality, and to contribute reliable data that may help to assess the environmental risk that represents the progressive accumulation of As in the arable soils of Guanajuato. Materials and methods: Two Entisol and two Vertisol profiles located in the Guanajuato state (Mexico) were described and sampled from ~ 1.20-m-deep pits. Soils are irrigated with As-rich deep and shallow groundwaters that were sampled from irrigation boreholes. Additionally, a Vertisol profile located in a parcel not impacted by irrigation was sampled and used as a control soil. Minerals were identified by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with dispersive X-ray spectrometry (EDS). Geoaccumulation indexes (Igeo) were calculated to evaluate As enrichment with respect to a control soil and the Upper Continental Crust (UCC). Anions and cations of groundwater were analyzed by high-performance liquid chromatography (HPLC) and by inductively coupled plasma atomic emission spectroscopy (ICP-AES), respectively. As in soils was determined by ICP-AES. Results and discussion: Near total As concentrations are higher in Entisols (mean As value = 7.20 mg/kg) than in Vertisols (mean As = 1.02 mg/kg). As concentrations in the control soil are lower (0.34 to 0.70 mg/kg). The in-depth distribution of As in Vertisol profiles reveals that the higher As concentrations are found in the uppermost horizons (10 cm) and they tend to decrease with depth. In Entisols, As concentrations do not follow a vertical trend. Igeo values of As indicate moderate to heavy As contamination in Vertisols and moderate contamination in Entisols. SEM-EDS analyses revealed the presence of some potential As-bearing minerals such as magnetite and abundant Fe oxides and Ti-Fe coatings precipitated onto feldspar grains, particularly in Entisols. Conclusions: Irrigation of Vertisols with As-rich groundwater determines As concentrations in the uppermost horizons that exceed the natural background of the region (0.4 mg/kg). In depth, clay grain-sized particles inhibit the downward migration of As, while Fe oxides and organic matter scavenge As by adsorption. As concentrations in Entisols are higher, and the in-depth distribution of this element is controlled by periodic contributions of As-bearing minerals delivered from mine prospects located at the river's catchments. [ABSTRACT FROM AUTHOR]

Abstract Sediments collected from Lake Towuti, an ultramafic-hosted lake in Indonesia, preserve a visible alternating pattern of red and green sediments due to variations in clay mineral and Fe-oxide composition and abundance consistent with changes in iron oxidation state through time. Spectral, mineralogical, and chemical analyses on soils, river, and sediment samples from across the lake and its catchment were carried out to better understand the starting composition of these sediments and the processes that affected them before and after deposition. Despite high Fe abundances in all samples and abundant Fe oxides in lateritic source regions, mineralogical analyses (X-ray diffraction (XRD) and Mössbauer spectroscopy) of the modern lake sediment show almost no well-crystalline iron oxides. In addition, sequential Fe extractions suggest an increasing proportion of easily extractable, poorly crystalline (X-ray amorphous) material with burial depth. XRD, bulk chemistry, and visible-near infrared (VNIR) spectral reflectance measurements demonstrate that clay mineralogy and bulk chemistry can be inferred from VNIR data. These results provide evidence for variations in Fe mineralogy and crystallinity based on location in this source to sink system. Understanding how the mineralogy and chemistry of sediments within a ferruginous lake basin are affected by transport, chemical alteration, physical alteration, and deposition from source to sink on Earth, and the degree to which these trends and underlying processes can be inferred from chemical and spectral properties, may provide useful direction in assessing paleoenvironmental conditions in other terrestrial lakes as well as ancient lacustrine environments preserved in the stratigraphic record of Mars. [ABSTRACT FROM AUTHOR]

Abstract Fe(III) oxides stabilize soil organic matter by forming Fe organo-mineral associations (Fe-OMA). Under anoxic conditions, Fe-OMA may be destabilized by microbial dissimilatory iron reduction that releases aqueous Fe(II) and also possibly adsorbed or co-precipitated organic matter. Soil spodic horizons that accumulate Fe-OMA are ideal natural materials to study such impact. Here, we study three spodic horizons from pedons (P) of increasing age: P-270 yrs, P-330 yrs and P-530 yrs. Their contents of total carbon and short range ordered (SRO) Fe oxides increase with soil age from, respectively, 1486 to 3618 and 13 to 249 μmol g−1. The samples were incubated for 96 h under anoxic conditions, with and without Shewanella putrefaciens (a model dissimilatory Fe(III)-reducing bacteria) in a minimal medium devoid of any pH buffer and external electron donor. The concentration of dissolved Fe(II), total Fe and organic C was monitored at 9 time steps. With increasing age, both the rate and extent of microbial Fe(III) reduction increased after S. putrefaciens addition. In P-270 yrs, P-330 yrs and P-530 yrs, respectively, the microbial reduction rates were 0.004, 0.026 and 0.114 fmol Fe(II) h−1 cell−1 while the amounts of released Fe(II) were 0.23, 0.32 and 1.98 μmol Fe(II) g−1, both being strongly correlated with SRO Fe oxides content. Dissolved organic carbon (DOC) was released with or without S. putrefaciens in all samples (up to 73 μmol OC g−1 after 96 h in P-530 yrs). Adding S. putrefaciens significantly increased DOC release, but only in P-270 yrs. Podzol development thus increases the impact of anoxia on Fe(II) release since organic matter accumulation impedes Fe oxide crystallization, thereby amplifying Fe availability for Fe reducing microbes. The evolution of Fe oxide content and crystallinity thus affects the fate of both C and Fe in soils. Highlights • Microbial Fe reduction correlates with short range ordered Fe oxides content. • Microbial Fe reduction did not lead to a significant OM release in solution. • OM impact on Fe oxide crystallinity probably affected microbial Fe reduction. [ABSTRACT FROM AUTHOR]

Abstract The Yinma River is a typical river located in the core region of the Phaeozem zone in Northeast China. The adsorption of trace metals including Pb(II), Cu(II), Cd(II), and Ni(II) onto surficial river and reservoir sediments from Yinma River and their components was investigated. The adsorption capacity of sediment components i.e., organic matter (OM), Fe oxides, Mn oxides and residual materials was compared via selective extraction followed by adsorption experiments and statistical analysis. Adsorption data were analyzed via nonlinear least-squares fitting to estimate the adsorption parameters of each component based on Langmuir adsorption isotherm model. The results indicated that Pb(II) and Cu(II) demonstrated a greater affinity for sediment adsorption than Cd(II) and Ni(II). The reservoir sediment exhibited a higher adsorption capacity for trace metals than that for the river sediment. Unlike results from previous studies, Fe/Mn oxides represented a significant contribution to the adsorption of trace metals rather than OM and residual materials. Additionally, the adsorption capacity of Fe/Mn oxides in different sediments differed due to the diversity of Fe/Mn oxides, whereas for OM, the adsorption capacity to different trace metals showed limited variation. Overall, the results from this study indicated that both Fe oxides and Mn oxides exerted a vital influence on the adsorption of trace metals in sediments from natural aquatic environments in the Phaeozem zone, Northeast China. Highlights • Trace metal adsorption onto sediments in Phaeozem zone of China was investigated. • Reservoir sediment showed higher adsorption capacity compared with river sediment. • Contribution was revealed by selective extraction-adsorption-statistical analysis. • Fe and Mn oxides exhibited a significant contribution to trace metal adsorption. [ABSTRACT FROM AUTHOR]

Abstract Inhibition of reductive transformation of arsenic (As) in flooded paddy soils is of fundamental importance for mitigating As transfer into food chain. Anaerobic arsenite (As(III)) oxidizers maintain As in less mobile fraction under nitrate-reducing conditions. In this study, we explored the dynamic profile of As speciation in porewater and As distribution among the pools of differential bioavailability in soil solid phase with and without nitrate treatment. In parallel, the abundance and diversity of As(III) oxidase gene (aioA) in flooded paddy soil with nitrate amendment was examined by quantitative PCR and aioA gene clone library. Furthermore, the impact of nitrate on As accumulation and speciation in rice seedlings was unraveled. With nitrate addition (25 mmol NO 3 − kg−1 soil), porewater As(III) was maintained at a consistently negligible concentration in the flooded paddy soil and the reductive dissolution of As-bearing Fe oxides/hydroxides was significantly restrained. Specifically, nitrate amendment kept 81% of total soil As in the nonlabile fraction with arsenate (As(V)) dominating after 30 days of flooding, compared to only 61% in the unamended control. Nitrate treatment induced 4-fold higher abundance of aioA gene, which belonged to domains of bacteria and archaea under the classes α-Proteobacteria (6%), ß-Proteobacteria (90%), ɣ-Proteobacteria (2%), and Thermoprotei (2%). By nitrate addition, As accumulation in rice seedlings was decreased by 85% with simultaneously elevated As(V) ratio in rice plant relative to control after 22 days of growth under flooded conditions. These results highlight that nitrate application can serve an efficient method to inhibit reductive dissolution of As in flooded paddy soils, and hence diminish As uptake by rice under anaerobic growing conditions. Graphical abstract Image 1 Highlights • Nitrate inhibited reductive dissolution of crystallized Fe oxides in flooded paddy soil. • Nitrate stimulated anaerobic As(III) oxidation to As(V), resulting in negligible porewater As(III). • 4-fold higher abundance of aioA gene was induced with nitrate amendment. • Nitrate decreased As accumulation in rice seedlings by 85%. Nitrate amendment inhibited reductive dissolution of crystallized Fe oxides and stimulated microbial As(III) oxidation to As(V) in flooded paddy soil, resulting in largely decreased rice As uptake. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) contamination of soils is a global problem, particularly in paddy soils. Fe oxides, as a key fraction of paddy soils, can significantly affect the environmental behavior of Cd, which is controlled by complicated environmental factors. Therefore, it is necessary to systematically collect and generalize relevant knowledge, which can provide more insight into the migration mechanism of Cd and a theoretical basis for future remediation of Cd contaminated paddy soils. This paper summarized that (1) Fe oxides influence Cd activity through adsorption, complexation, and coprecipitation during transformation; (2) compared with the flooded period, the activity of Cd during the drainage period is stronger in paddy soils, and the affinity of different Fe components for Cd was distinct; (3) Fe plaque reduced Cd activity but was associated with plant Fe2+ nutritional status; (4) the physicochemical properties of paddy soils have the greatest impact on the interaction between Fe oxides and Cd, especially with pH and water fluctuations. [Display omitted] • Fe2+ catalysis and microbial mediation can regulate the rate and pathway of Fe transformation. • Fe oxides can immobilize Cd through adsorption, complexation, and coprecipitation. • The activity of Cd was stronger under oxidizing conditions than under reducing conditions in paddy fields. • The availability of Cd in root IP is related to plant Fe nutritional status. • Paddy soil physicochemical properties are the primary influencing factor. [ABSTRACT FROM AUTHOR]

While the contribution of iron (Fe)-bearing minerals to organic carbon (C) stabilization in terrestrial systems is well-described, the influence of Fe solid-phase speciation on organomineral associations is unclear in highly dynamic, oxidation-reduction (redox)-active soils. In humid tropic forest soils, fluctuations in redox state accelerate weathering of Fe-bearing mineral phases, producing a spectrum of mineral sizes and bonding environments available for C stabilization, and confounding our understanding of C stability. Characterizing these Fe-bearing phases can improve predictions of the response of redox-active soil systems to climatic changes that may alter Fe mineral crystallinity and solubility, such as precipitation intensity, storm event frequency and temperature. Leveraging inorganic selective dissolution techniques, 57 Fe Mössbauer spectroscopy (MBS), specific surface area (SSA) analyses and X-ray diffraction (XRD), we investigated mineral speciation in surface soils of contrasting lithologies from the Luquillo Critical Zone Observatory (LCZO), Puerto Rico. The LCZO provides a model investigatory framework in which high C inputs to surface horizons by similar vegetation, topography and climatic forcings are intercepted by highly-weathered, volcaniclastic Oxisols or quartz diorite-derived Inceptisols, producing a gradient of Fe content and speciation. Strong correlations observed between Fe concentrations and extraction-induced changes in SSA indicated target Fe phases contribute substantially to SSA of the bulk mineral matrix. MBS analysis of untreated soils reveal both Oxisol and Inceptisol soils are largely composed of Fe III -oxyhydroxides, accompanied by substantial Fe II and silicate Fe III contributions in Inceptisol soils. Fe III -oxyhydroxides in the Oxisol soils were largely short-range-ordered (SRO), and notably, a fraction of particularly low-crystallinity Fe III -oxyhydroxide mineral phases in these soils appear protected against harsh reductive dissolution, whereas the overall higher crystallinity Fe phases in the Inceptisol soils do not. These findings suggest that some high-SSA, SRO Fe III phases, which likely also have high C sorption capacities, may be immobilized against reduction in these Oxisol soils. Consequently, C associated with these Fe III phases may be preferentially stabilized in Oxisol soils, potentially driving disparate C mineralization and CO 2 production rates across contrasting lithologies. [ABSTRACT FROM AUTHOR]

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 26 Mg via Mg 2+ substitution for Fe 3+ 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 δ 26 Mg values, highlighting the major roles of these two soil minerals in controlling soil Mg isotopic compositions. [ABSTRACT FROM AUTHOR]

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Ferric iron (Fe III ) solid phases serve many functions in soils and sediments, which include providing sorption sites for soil organic matter, nutrients, and pollutants. The reactivity of Fe solid phases depends on the mineral structure, including the overall crystallinity. In redox-active soils and sediments, repeated reductive dissolution with subsequent exposure to aqueous ferrous iron (Fe 2+ ) and oxidative re-precipitation can alter Fe phase crystallinity and reactivity. However, the trajectory of Fe mineral transformation under redox fluctuations is unclear and has been reported to result in both increases and decreases in Fe phase crystallinity. Several factors such as water budget, organic matter input, redox dynamics as well as the initial Fe phase composition might play a role. The objective of our study was to examine if Fe minerals in soils that differ in porosity-dependent water leaching rate and initial Fe phase crystallinity, demonstrate distinct mineral transformations when subjected to redox fluctuations. We sampled paired plots of two soil types under similar management but with different water leaching rates and contrasting Fe oxide crystallinity — an Alisol rich in crystalline Fe phases and an Andosol rich in short-range-ordered (SRO) Fe phases. The two soils were either exposed to several decades of redox fluctuations during rice paddy cultivation (paddy) or to predominantly oxic conditions in neighboring vegetable gardens (non-paddy). Paddy soils are uniquely suited for this type of study because they are regularly submerged and develop regular redox fluctuations. We also incubated the non-paddy soils in the laboratory for one year through eight anoxic/oxic cycles and monitored the aqueous soil geochemistry. Mössbauer spectroscopy was then used to evaluate Fe mineral speciation in field soils (paddy and non-paddy) and laboratory incubations. In the field soils, we found that redox fluctuation had contrasting effects on Fe oxide crystallinity, with crystallinity being lower in the Alisol paddy soil and higher in the Andosol paddy soil than in their corresponding non-paddy controls. In the laboratory incubation experiment, Eh, pH and dissolved Fe 2+ responded as anticipated, with elevated Fe 2+ concentrations during the anoxic periods as well as low Eh and high pH. Mössbauer measurements suggest the fluctuating redox incubation was beginning to alter Fe oxide crystallinity along the same trajectory as observed in the field, but the changes were within the range of fitting errors. We propose that reductive dissolution of crystalline Fe oxides prevails in the soil rich in crystalline Fe oxides (Alisol) and that re-precipitation as SRO Fe oxides is favored by constrained leaching, which leads to the observed decrease in Fe oxide crystallinity. In the soil rich in SRO Fe phases (Andosol), preferential reductive dissolution of SRO Fe oxides coupled with stronger leaching of dissolved Fe 2+ causes the observed relative increase in crystallinity of the remaining Fe oxides. The observed increase in Fe oxide crystallinity may further be a result of Fe(II)-catalyzed re-crystallization of SRO Fe oxides. These findings indicate that, besides other factors, the Fe mineral composition of the initial soil or sediment as well as the leaching rate likely influence the trajectory of Fe oxide evolution under alternating redox-conditions. [ABSTRACT FROM AUTHOR]

The spatial distribution of soil constituents at the micrometer scale is of great importance to understand processes controlling the formation of micro-aggregates and the stabilization of organic carbon. Here, the spatial distribution of organic and mineral constituents in Podzol horizons is studied by concerted measurements of (i) the content of various forms of Fe, Al, Si and C determined by selective extraction in the fine earth fraction of soil (f < 2 mm); (ii) the elemental composition of the clay fraction (f < 2 μm) with lateral resolution using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and with surface selectivity using X-ray photoelectron spectroscopy (XPS); (iii) the specific surface area (SSA) of fine earth and clay fractions by krypton physisorption. The SSA of the fine earth in illuvial horizons is predominantly due to finely divided Fe oxides, including goethite, characterized by an equivalent particle size of about 10 nm. Kaolinite platelets of about 2 μm size account for a large volume proportion in the clay fraction but have a minor contribution to SSA. Fe oxides and organic matter (OM) are intimately associated. Heterogeneity at the μm scale is created by local variations in the relative amounts of kaolinite and Fe-OM associations. These two kinds of physical entities are in random mixture. Moreover, variation of C/Fe atomic ratios reveals sub-μm scale heterogeneity. The latter is due to variation in the relative proportion of organic compounds and Fe oxides, indicating that aggregation of nanoparticles, and not only mere adsorption or pore filling, plays a role in these associations. In this regard, our results highlight that OM associated with Fe protects Fe oxides against physical displacement and that part of this associated OM is oxidizable by NaOCl treatment. These findings demonstrate that the concept of OM stabilization through association with Fe must be revisited when considering the sub-μm scale level because fine Fe oxide particles can be easily dispersed during oxidation of associated carbon. Combination of physical fractionation and microanalysis (e.g. SEM-EDS, vibrational spectroscopy) offer promising perspectives to clarify the relationship between chemical composition and sub-μm scale architecture, and to better understand soil processes. [ABSTRACT FROM AUTHOR]