Rapid Iron Reduction Rates Are Stimulated by High-Amplitude Redox Fluctuations in a Tropical Forest Soil

Active role of iron-dependent AOM in paddy fields under different long-term fertilizer management schemes

Iron-dependent anaerobic oxidation of methane (iron-AOM) has recently been reported to occur in paddy soils, but its actual role and regulation remain unclear. Here, we confirmed the occurrence of iron-AOM at different layers of paddy soils (0-10 cm, 10-20 cm, and 30-40 cm) across tillering, elongation, flowering, and ripening periods under three long-term fertilizer management schemes (CK-unamended, CF-chemical fertilizer, and CFS-chemical fertilizer with straw). The iron-AOM activity contributed 41 % to total AOM, which was greater than that of nitrite- (32 %) and nitrate-AOM (27 %). The iron-AOM activity varied significantly with soil layers, growth periods and fertilizer types, with layer being the most important variable. Soil moisture content and organic carbon content were most significant influencing factors on the AOM activity, and a Candidatus Methanoperedens ferrireducens-like lineage potentially catalyzed iron-AOM. Our results suggest that iron-AOM has an important potential for mitigating methane emissions from rice paddies.

Iron Oxyhydroxide Transformation in a Flooded Rice Paddy Field and the Effect of Adsorbed Phosphate

The mobility and bioavailability of phosphate in paddy soils are closely coupled to redox-driven Fe-mineral dynamics. However, the role of phosphate during Fe-mineral dissolution and transformations in soils remains unclear. Here, we investigated the transformations of ferrihydrite and lepidocrocite and the effects of phosphate pre-adsorbed to ferrihydrite during a 16-week field incubation in a flooded sandy rice paddy soil in Thailand. For the deployment of the synthetic Fe-minerals in the soil, the minerals were contained in mesh bags either in pure form or after mixing with soil material. In the latter case, the Fe-minerals were labeled with 57Fe to allow the tracing of minerals in the soil matrix with 57Fe Mössbauer spectroscopy. Porewater geochemical conditions were monitored, and changes in the Fe-mineral composition were analyzed using 57Fe Mössbauer spectroscopy and/or X-ray diffraction analysis. Reductive dissolution of ferrihydrite and lepidocrocite played a minor role in the pure mineral mesh bags, while in the 57Fe-mineral-soil mixes more than half of the minerals was dissolved. The pure ferrihydrite was transformed largely to goethite (82-85%), while ferrihydrite mixed with soil only resulted in 32% of all remaining 57Fe present as goethite after 16 weeks. In contrast, lepidocrocite was only transformed to 12% goethite when not mixed with soil, but 31% of all remaining 57Fe was found in goethite when it was mixed with soil. Adsorbed phosphate strongly hindered ferrihydrite transformation to other minerals, regardless of whether it was mixed with soil. Our results clearly demonstrate the influence of the complex soil matrix on Fe-mineral transformations in soils under field conditions and how phosphate can impact Fe oxyhydroxide dynamics under Fe reducing soil conditions.

Investigation on the role of ·OH for BPA removal in coastal sediments: The important mediation of low reactivity Fe(II)

Bisphenol A (BPA) in seawater tends to be deposited in coastal sediments. However, its degradation under tidal oscillations has not been explored comprehensively. Hydroxyl radicals (·OH) can be generated through Fe cycling under redox oscillations, which have a strong oxidizing capacity. This study focused on the contribution of Fe-mediated production of ·OH in BPA degradation under darkness. The removal of BPA was investigated by reoxygenating six natural coastal sediments, and three redox cycles were applied to prove the sustainability of the process. The importance of low reactivity Fe(II) in the production of ·OH was investigated, specifically, Fe(II) with carbonate and Fe(II) within goethite, hematite and magnetite. The degradation efficiency of BPA during reoxygenation of sediments was 76.78-94.82%, and the contribution of ·OH ranged from 36.74% to 74.51%. The path coefficient of ·OH on BPA degradation reached 0.6985 and the indirect effect of low reactivity Fe(II) on BPA degradation by mediating ·OH production reached 0.5240 obtained via partial least squares path modeling (PLS-PM). This study emphasizes the importance of low reactivity Fe(II) in ·OH production and provides a new perspective for the role of tidal-induced ·OH on the fate of refractory organic pollutants under darkness.

Fluctuating redox conditions accelerate the electron storage and transfer in magnetite and production of dark hydroxyl radicals

Magnetite (Fe3O4), known as a geo-battery that can store and transfer electrons, often co-occurs with sulfide in subsurface environments with fluctuating redox conditions. However, little is known about how fluctuating redox conditions (e.g., sulfidation-oxidation) affect the electron storage and transfer in Fe3O4 that was associated with the production of dark hydroxyl radicals (⋅OH) and the oxidation of dissolved organic matter (DOM). This study revealed that Fe3O4 sulfidated by sulfide (S-Fe3O4) at neutral pH exhibited higher ⋅OH production upon oxygenation than Fe3O4, in which the cumulative ⋅OH concentration increased with increasing initial S/Fe ratio (≤ 0.50), sulfidation duration and number of sulfidation-oxidation cycle. X-ray photoelectron spectroscopy and wet-chemical analyses of Fe and S species of S-Fe3O4 showed that sulfidation enables electron storage in Fe3O4 by increasing both structural and surface Fe(II). Sulfide was converted into S0, acid volatile sulfur (AVS), and chromium-reducible sulfur (CRS) during Fe3O4 sulfidation. S-Fe3O4 with lower AVS/CRS ratio exhibited higher reactivity to produce ⋅OH, indicating the important role of CRS in transferring electrons from Fe(II) to O2. Based on quenching experiments and electron paramagnetic resonance analysis, a one-step two-electron transfer mechanism was proposed for O2 reduction during S-Fe3O4 oxygenation, and surface-bound rather than free ⋅OH were identified as the primary reactive oxygen species. The ⋅OH from S-Fe3O4 oxygenation was shown to be efficient in degradation of DOM. Overall, these results suggested that sulfidation-oxidation can accelerate the electron storage and transfer in Fe3O4 for dark ⋅OH production, having an important impact on the carbon cycling in subsurface environments.

Reduced colloidal phosphorus release from paddy soils: A synergistic effect of micro-/nano-sized biochars and intermittent anoxic condition

Colloidal phosphorus (CP) has high mobility and great loss risk; their biogeochemical processes are influenced by agricultural management such as redox oscillation and biochar-amendment application. This study monitored CP concentration in pore-water, soil P species and P adsorption capacity, to investigate CP release from paddy soils as affected by the interactive effects of oxygen status (continuous anoxic/oxic for 12 days, CA/CO; intermittent anoxic for 2, 4, 6, 8, 10 days during the 12-day cycle, IA2-10) and management (soil only, CK; bulk/micro/nano-sized biochar with various properties: SBBulk, SBMicro, and SBNano). Compared to the control (0.25-0.84 mg L-1, CK-CA), the single intermittent anoxic treatment (CK-IA) reduced CP concentrations by 45 %, due to the rise of Eh and pH and the decline of the degree of P saturation along with the increased soil Fe/Al-P and organic-P. Longer anoxic duration under the CK-IA reduced CP release, probably donated from massive production of redox-stable amorphous Fe/Al-bound P. The single biochar treatment (SB-CA: SBBulk-CA > SBMicro-CA > SBNano-CA) decreased CP release by 37 % as compared to the CK-CA, ascribed to the increased soil pH, Eh, and P adsorption capacity. The combined treatment (SB-IA: SBBulk-IA2 > SBNano-IA10) synergistically reduced CP release by 68 % in comparison with the CK-CA, due to the increase of adsorption through interactions of soil Fe/Al/Ca- and organic-P. Therefore, nano-sized biochar and long intermittent anoxic duration are recommended for reducing CP release from paddy soils.

The primacy of temporal dynamics in driving spatial self-organization of soil iron redox patterns

This study investigates mechanisms that generate regularly spaced iron-rich bands in upland soils. These striking features appear in soils worldwide, but beyond a generalized association with changing redox, their genesis is yet to be explained. Upland soils exhibit significant redox fluctuations driven by rainfall, groundwater changes, or irrigation. Pattern formation in such systems provides an opportunity to investigate the temporal aspects of spatial self-organization, which have been heretofore understudied. By comparing multiple alternative mechanisms, we found that regular iron banding in upland soils is explained by coupling two sets of scale-dependent feedbacks, the general principle of Turing morphogenesis. First, clay dispersion and coagulation in iron redox fluctuations amplify soil Fe(III) aggregation and crystal growth to a level that negatively affects root growth. Second, the activation of this negative root response to highly crystalline Fe(III) leads to the formation of rhythmic iron bands. In forming iron bands, environmental variability plays a critical role. It creates alternating anoxic and oxic conditions for required pattern-forming processes to occur in distinctly separated times and determines durations of anoxic and oxic episodes, thereby controlling relative rates of processes accompanying oxidation and reduction reactions. As Turing morphogenesis requires ratios of certain process rates to be within a specific range, environmental variability thus modifies the likelihood that pattern formation will occur. Projected changes of climatic regime could significantly alter many spatially self-organized systems, as well as the ecological functioning associated with the striking patterns they present. This temporal dimension of pattern formation merits close attention in the future.

New Insight into the Natural Detoxification of Cr(VI) in Fe-Rich Surface Soil: Crucial Role of Photogenerated Silicate-Bound Fe(II)

Photoexcitation of natural semiconductor Fe(III) minerals has been proven to generate Fe(II), but the photogeneration of Fe(II) in Fe-rich surface soil as well as its role in the redox biogeochemistry of Cr(VI) remains poorly understood. In this work, we confirmed the generation of Fe(II) in soil by solar irradiation and proposed a new mechanism for the natural reductive detoxification of Cr(VI) to Cr(III) in surface soil. The kinetic results showed that solar irradiation promoted the reduction of Cr(VI) in Fe-rich soils, while a negligible Cr(VI) reduction was observed in the dark. Fe(II), mainly in the form of silicate-bound Fe(II), was generated under solar irradiation and responsible for the reduction of Cr(VI) in soils, which was evidenced by sequential extraction, transmission electron microscopy with electron energy loss spectroscopy, and electron transfer calculation. Photogenerated silicate-bound Fe(II) resulted from the massive clay-iron (hydr)oxide associations, consisting of iron (hydr)oxides (e.g., hematite and goethite) and kaolinite. These associations could generate Fe(II) under solar irradiation either via intrinsic excitation to produce photoelectrons or via the ligand-to-metal charge transfer process after the formation of clay-iron (hydr)oxide-organic matter complexes, which was proven by photoluminescence spectroscopy and X-ray photoelectron spectroscopy. These findings highlight the important role of photogenerated Fe(II) in Cr(VI) reduction in surface soil, which advances a fundamental understanding of the natural detoxification of Cr(VI) as well as the redox biogeochemistry of Cr(VI) in soil.

Contact with soil impacts ferrihydrite and lepidocrocite transformations during redox cycling in a paddy soil

Iron (Fe) oxyhydroxides can be reductively dissolved or transformed under Fe reducing conditions, affecting mineral crystallinity and the sorption capacity for other elements. However, the pathways and rates at which these processes occur under natural soil conditions are still poorly understood. Here, we studied Fe oxyhydroxide transformations during reduction-oxidation cycles by incubating mesh bags containing ferrihydrite or lepidocrocite in paddy soil mesocosms for up to 12 weeks. To investigate the influence of close contact with the soil matrix, mesh bags were either filled with pure Fe minerals or with soil mixed with 57Fe-labeled Fe minerals. Three cycles of flooding (3 weeks) and drainage (1 week) were applied to induce soil redox cycles. The Fe mineral composition was analyzed with Fe K-edge X-ray absorption fine structure spectroscopy, X-ray diffraction analysis and/or 57Fe Mössbauer spectroscopy. Ferrihydrite and lepidocrocite in mesh bags without soil transformed to magnetite and/or goethite, likely catalyzed by Fe(II) released to the pore water by microbial Fe reduction in the surrounding soil. In contrast, 57Fe-ferrihydrite in mineral-soil mixes transformed to a highly disordered mixed-valence Fe(II)-Fe(III) phase, suggesting hindered transformation to crystalline Fe minerals. The 57Fe-lepidocrocite transformed to goethite and small amounts of the highly disordered Fe phase. The extent of reductive dissolution of minerals in 57Fe-mineral-soil mixes during anoxic periods increased with every redox cycle, while ferrihydrite and lepidocrocite precipitated during oxic periods. The results demonstrate that the soil matrix strongly impacts Fe oxyhydroxide transformations when minerals are in close spatial association or direct contact with other soil components. This can lead to highly disordered and reactive Fe phases from ferrihydrite rather than crystalline mineral products and promoted goethite formation from lepidocrocite.

Cultivation and biogeochemical analyses reveal insights into biomineralization caused by piezotolerant iron-reducing bacteria from petroleum reservoirs and their application in MEOR

Interactions between minerals and iron-reducing bacteria under in-situ pressure and temperature conditions play important roles in oil extraction, residual oil methanation, and CO2 storage in petroleum reservoirs. However, the impacts of pressure on dissimilatory iron-reducing bacteria (DIRB) are poorly understood. Herein, the interactions between clay minerals and microbes under elevated hydrostatic pressure conditions were elucidated through enrichment experiments. Bioreduction experiments were performed under hydrostatic pressures of 0.1-40 MPa. Microbial diversity analysis revealed that high pressures significantly increased microbial diversity in petroleum reservoirs, which is helpful for restoring underground ecosystems in situ. The key piezotolerant iron-reducing bacteria in the samples were Shewanella and Flaviflexus. These two genera were isolated for the first time from petroleum reservoirs and identified as piezophiles. The SEM results clearly showed mineral surface dissolution. Moreover, nanoscale secondary minerals were produced during biomineralization. XRD analysis revealed that illite, albite, and clinoptilolite were present after bioreduction. The isolates showed the capacity to inhibit hydro-swelling and prevent plugging-related damage in reservoirs.

Geochemical processes of phosphorus‑iron on sediment-water interface during discharge of groundwater to freshwater lakes: Kinetic and mechanistic insights

Groundwater is widely recognized as a source of lake materials. When it discharges into lakes, phosphorus(P)‑iron(Fe) geochemical reactions occur due to environmental changes, affecting P discharge from groundwater. However, redox kinetics of Fe and associated P geochemical processes at the sediment-water interface are not fully understood. Taking Dongting Lake as an example, this study explored Fe and P geochemical processes at the sediment-water interface under groundwater discharge with high Fe and P concentrations. We incubated sediments from Dongting Lake under anoxic-oxic conditions with different initial aqueous P/Fe ratios and pH. Aqueous PO43--P and Fe2+, and solid P and Fe phases in sediments were analyzed, and experimental data were further simulated using numerical reactive models. At the beginning of the experiment, aqueous P and Fe were adsorbed rapidly on sediments. Under anoxic conditions, the Fe reduction rate decreased with decreasing content of poorly crystalline ferric (oxyhydr)oxides, and the addition of aqueous P and Fe at neutral pH enhanced the reduction rate. The increased aqueous P was dominated by desorption caused by sediment Fe reduction and then fixed by gibbsite adsorption and hydroxyapatite precipitation. Under oxic conditions, Fe(II) oxidation under was pH- and (P:Fe)ini-independent, with a sharp rate decline. Furthermore, the final sediment Fe(II) content was higher than the initial content, indicating the formation of a low-oxidizability Fe(II) phase. The P dynamics were dominated by adsorption on the produced Fe-oxides. The numerical models also suggested that heterogeneity in natural sediments promotes hydroxyapatite formation at low pH, but restricts it at high pH. The findings reveal that although aqueous P concentration decreased during groundwater discharge to lakes, PO43--P concentration remained much higher than that in natural lake water, increasing the risk of lake eutrophication. The paper provides references for further understanding of P loading from groundwater discharge into lakes.

Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Elemental sulfur (S0) plays a vital role in the coupled cycling of sulfur and iron, which in turn affects the transformation of carbon and various pollutants. These processes have been well characterized under static anoxic or oxic conditions, however, how the natural redox fluctuations affect the bio-mediated sulfur cycling and coupled iron cycling remain enigmatic. The present work examined S0 disproportionation as driven by natural microbial communities under fluctuating redox conditions and the contribution of S0 disproportionation to ferrihydrite transformation. Samples were incubated at either neutral or alkaline pH values, applying sequential anaerobic, aerobic and anaerobic conditions over 60 days. Under anaerobic conditions, S0 was found to undergo disproportionation to sulfate and sulfide, which subsequently reduced ferrihydrite at both pH 7.4 and 9.5. Ferrihydrite promoted S0 disproportionation by scavenging biogenic sulfide and maintaining a suitable degree of sulfate formation. After an oxic period, during the subsequent anoxic incubation, bioreduction of sulfate occurred and the biogenic sulfide reduced iron (hydr)oxides at a rate approximately 25 % lower than that observed during the former anoxic period. A 16S rDNA-based microbial community analysis revealed changes in the microbial community in response to the redox fluctuations, implying an intimate association with the coupled cycling of sulfur and iron. Microscopic and spectroscopic analyses confirmed the S0-mediated transformation of ferrihydrite to crystalline iron (hydr)oxide minerals such as lepidocrocite and magnetite and the formation of iron sulfides precipitated under fluctuating redox conditions. Finally, a reaction mechanism based on mass balance was proposed, demonstrating that bio-mediated sulfur transformation maintained a sustainable redox reaction with iron (hydr)oxides under fluctuating anaerobic-aerobic-anaerobic conditions tested in this study. Altogether, the finding of our study is critical for obtaining a more complete understanding of the dynamics of iron redox reactions and pollutant transformation in sulfur-rich aquatic environments.

Pyrolysis temperature and biochar redox activity on arsenic availability and speciation in a sediment

Biochar is widely used for water and soil remediation in part because of its local availability and low production cost. However, its effectiveness depends on physicochemical properties related to its feedstock and pyrolysis temperature, as well as the environmental conditions of its use site. Furthermore, biochar is susceptible to natural aging caused by changes in soil or sediment moisture, which can alter its redox properties and interactions with contaminants such as arsenic (As). In this study, we investigated the effect of pyrolysis temperature and biochar application on the release and transformations of As in contaminated sediments subjected to redox fluctuations. Biochar application and pyrolysis temperature played an important role in As species availability, As methylation, and dissolved organic carbon concentration. Furthermore, successive flooding cycles that induced reductive conditions in sediments increased the As content in the solution by up to seven times. In the solid phase, the application of biochar and the flooding cycle altered the spatial distribution and speciation of carbon, iron (Fe) and As. In general, the application of biochar decreased the reduction of Fe(III) and As(V) after the first cycle of flooding. Our results demonstrate that the flooding cycle plays an important role in the reoxidation of biochar to the point of enhancing the immobilization of As.

Predicting laterite redox potential with iron activity and electron transfer term

Predicting the redox behavior of organic contaminants and heavy metals in soils is challenging because there are few soil redox potential (E_h) models. In particular, current aqueous and suspension models usually show a significant deviation for complex laterites with few Fe(II). Here, we measured the E_h of simulated laterites over a range of soil conditions (2450 tests). The impacts of soil pH, organic carbon, and Fe speciation on the Fe activity were quantified as Fe activity coefficients, respectively, using a two-step Universal Global Optimization method. Integrating these Fe activity coefficients and electron transfer terms into the formula significantly improved the correlation of measured and modeled E_h values (R² = 0.92), and the estimated E_h values closely matched the relevant measured E_h values (accuracy R² = 0.93). The developed model was further verified with natural laterites, presenting a linear fit and accuracy R² of 0.89 and 0.86, respectively. These findings provide compelling evidence that integrating Fe activity into the Nernst formula could accurately calculate the E_h if the Fe(III)/Fe(II) couple does not work. The developed model could help to predict the soil E_h toward controllable and selective oxidation-reduction of contaminants for soil remediation.

Carbon sequestration in paddy soils: Contribution and mechanisms of mineral-associated SOC formation

In this work, comparative study of paddy and upland soils were carried out to unravel mechanisms of enhanced soil organic carbon (SOC) sequestration in paddy soils using fractionation methods, ¹³C NMR and Nano-SIMS analysis, as well as organic layer thickness calculations (Core-Shell model). The results showed that although there is a strong increase in particulate SOC in paddy soils compared to that in the upland soils, the increase in mineral-associated SOC is more important, explaining 60-75% of SOC increase in the paddy soils. In the wet and dry alternate cycles of paddy soil, iron (hydr)oxides adsorb relatively small and soluble organic molecules (fulvic acid-like), promote catalytic oxidation and polymerization, thus accelerating formation of larger organic molecules. Upon reductive iron dissolution, these molecules are released and incorporated into existing less soluble organic compounds (humic acid or humin-like), which are coagulated and associated with clay minerals, becoming part of the mineral-associated SOC. The functioning of this "iron wheel" process stimulates accumulation of relatively young SOC into mineral-associated organic carbon pool, and reduces the difference in chemical structure between oxides-bound and clay-bound SOC. Further, the faster turnover of oxides and soil aggregates in paddy soil also facilities interaction between SOC and minerals. The formation of mineral-associated SOC may delay degradation of organic matter during both wet and dry period in the paddy field, therefore enhancing carbon sequestration in paddy soils.

Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41-59% and 55-77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63-78% of Cd was associated with Fe minerals, while 3-12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

Fe(III) stabilizing soil organic matter and reducing methane emissions in paddy fields under varying flooding conditions

The role of iron (Fe) in soil organic matter (SOM) stabilization and decomposition in paddy soils has recently gained attention, but the underlying mechanisms during flooding and drying periods remain elusive. As the depth water layer is maintained in the fallow season, there will be more soluble Fe than during the wet and drainage seasons and the availability of oxygen (O₂) will be different. To assess the influence of soluble Fe on SOM mineralization during flooding, an incubation experiment was designed under oxic and anoxic flooding conditions, with and without Fe(III) addition. The results showed that Fe(III) addition significantly (p < 0.05) decreased SOM mineralization by 14.4 % under oxic flooding conditions over 16 days. Under anoxic flooding incubation, Fe(III) addition significantly (p < 0.05) decreased 10.8 % SOM decomposition, mainly by 43.6 % methane (CH₄) emission, while no difference in carbon dioxide (CO₂) emission was noticed. These findings suggest that implementing appropriate water management strategies in paddy soils, considering the roles of Fe under both oxic and anoxic flooding conditions, can contribute to SOM preservation and mitigation of CH₄ emissions.

Acquiring Iron-Reducing Enrichment Cultures: Environments, Methods and Quality Assessments

Lateritic duricrusts cover iron ore deposits and form spatially restricted, unique canga ecosystems endangered by mining. Iron cycling, i.e., the dissolution and subsequent precipitation of iron, is able to restitute canga duricrusts, generating new habitats for endangered biota in post-mining landscapes. As iron-reducing bacteria can accelerate this iron cycling, we aim to retrieve microbial enrichment cultures suitable to mediate the large-scale restoration of cangas. For that, we collected water and sediment samples from the Carajás National Forest and cultivated the iron-reducing microorganisms therein using a specific medium. We measured the potential to reduce iron using ferrozine assays, growth rate and metabolic activity. Six out of seven enrichment cultures effectively reduced iron, showing that different environments harbor iron-reducing bacteria. The most promising enrichment cultures were obtained from environments with repeated flooding and drying cycles, i.e., periodically inundated grasslands and a plateau of an iron mining waste pile characterized by frequent soaking. Selected enrichment cultures contained iron-reducing and fermenting bacteria, such as Serratia and Enterobacter. We found higher iron-reducing potential in enrichment cultures with a higher cell density and microorganism diversity. The obtained enrichment cultures should be tested for canga restoration to generate benefits for biodiversity and contribute to more sustainable iron mining in the region.

Combined application of chemical fertilizer with green manure increased the stabilization of organic carbon in the organo-mineral complexes of paddy soil

The influence of the combined application of chemical fertilizer with green manure on the stabilization of organic carbon (C) was explored in the organo-mineral complexes of paddy soil. The organo-mineral complexes were isolated from paddy soil treated with no fertilizer, chemical fertilizer alone, and chemical fertilizer combined with increasing amounts of Chinese milk vetch (CMV). The stability (reflected by mineralizable carbon proportion), the content and chemical composition of organic C, the Fe/Al oxides and their associated organic C in the organo-mineral complexes were investigated. The application of chemical fertilizer in combination with CMV significantly improved the stability of organic C in the organo-mineral complexes. The combined application of chemical fertilizer with CMV slightly decreased the proportion of O-alkyl C (easily decomposed) yet somewhat increased the proportions of carbonyl C and aromatic C (difficultly decomposed) and aromaticity index in the organo-mineral complexes. The treatments of chemical fertilizer combined with CMV showed more Fe oxides and Fe/Al-associated organic C and higher proportion of Fe/Al-associated organic C in the total organic C of the organo-mineral complexes. The mineralizable carbon proportion displayed significantly negative correlations with carbonyl C and Fe/Al oxide-associated organic C in the organo-mineral complexes. The Fe/Al oxides were likely to be preferentially bound with the aromatic C and carbonyl C in the organo-mineral complexes. Overall, the combined application of chemical fertilizer with CMV facilitated the association of difficultly decomposed carbon and Fe/Al oxides, which significantly improved the stabilization of organic C in the organo-mineral complexes of paddy soil.

A process-based model for describing redox kinetics of Cr(VI) in natural sediments containing variable reactive Fe(II) species

Sediment-associated Fe(II) is a critical reductant for immobilizing groundwater contaminants, such as Cr(VI). The reduction reactivity of sediment-associated Fe(II) is dependent on its binding environment and influenced by the biogeochemical transformation of other elements (i.e., C, N and Mn), challenging the description and prediction of the reactivity of Fe(II) in natural sediments. Here, anaerobic batch experiments were conducted to study the variation in sediment-associated Fe(II) reactivity toward Cr(VI) in natural sediments collected from an intensive agricultural area located in Guangxi, China, where nitrate is a common surface water and groundwater contaminant. Then, a process-based model was developed to describe the coupled biogeochemical processes of C, N, Mn, Fe, and Cr. In the process-based model, Cr(VI) reduction by sediment-associated Fe(II) was described using a previously developed multirate model, which categorized the reactive Fe(II) into three fractions based on their extractabilities in sodium acetate and HCl solutions. The experimental results showed that Fe(II) generation was inhibited by NO₃^- and/or NO₂^-. After NO₃^- and NO₂^- were exhausted, the Fe(II) content and its reduction rate toward Cr(VI) increased rapidly. As the Fe(II) content increased, the three reactive Fe(II) fractions exhibited approximately linear correlations with aqueous Fe(II) concentrations ( [Formula: see text] ), which was probably driven by sorptive equilibrium and redox equilibrium between aqueous and solid phases. The model results indicated that the reaction rate constants of the three Fe(II) fractions (k_n) significantly increased with incubation time, and log(k_n) correlated well with [Formula: see text] [ [Formula: see text] , [Formula: see text] and [Formula: see text] ]. The numerical model developed in this study provides an applicable method to describe and predict Cr(VI) removal from groundwater under dynamic redox conditions.

Effect of organic substrate and Fe oxides transformation on the mobility of arsenic by biotic reductive dissolution under repetitive redox conditions

The mobility of arsenic (As) in soil is highly affected by the change in the form of iron oxides present in the soil, which has a strong correlation with the change in redox potential. In this study, the altered mobility of As under repetitive redox conditions and the effect of organic substrates (i.e., glucose) on such change during four anoxic-oxic cycles were studied. During the 1st anoxic period, 37.1% of soil As was released into the soil solution, but the As in the soil solution decreased to 25.2% after the 1st oxic period. Moreover, the As in the soil solution further decreased during the 2nd to 4th oxic periods, indicating further re-adsorption of aqueous As. The analysis of As speciation revealed that inorganic arsenate (As(V)) increased under the redox-oscillating conditions, probably due to the depletion of electron donors. When glucose was re-spiked at the beginning of the 4th cycle, aqueous As increased to 47.3% again in the anoxic period and decreased to 27.6% in the subsequent oxic period, indicating inhibition of As re-adsorption. During the same period, the amount of highly sorptive As(V) in the solution decreased sharply to less than 3.3%. The X-ray absorption near edge structure analysis with linear combination fitting confirmed that the transformation of Fe oxides to poorly crystalline structures such as ferrihydrite occurred during repetitive cycles. These results imply that the mobility of As can be increased in As-contaminated redox transition zones by the introduction of rainfall with labile organics or by the fluctuation of organic-rich groundwater.

Rejuvenation of iron oxides enhances carbon sequestration by the 'iron gate' and 'enzyme latch' mechanisms in a rice-wheat cropping system

The 'enzyme latch' theory believes that oxygen constraints on phenol oxidase can restrain the activity of hydrolytic enzymes responsible for decomposition, while the 'iron (Fe) gate' theory suggests that Fe oxidation can decrease phenol oxidase activity and enhance Fe-lignin complexation under oxygen exposure. The objective of this study was to explore the roles of the 'enzyme latch' and 'Fe gate' mechanisms in regulating soil organic carbon (SOC) sequestration in a rice-wheat cropping system subjected to six fertilization treatments: control (CT), chemical fertilizer (CF), CF plus manure (CFM), CF plus straw (CFS), CF plus manure and straw (CFMS), and CF plus organic-inorganic compound fertilizer (OICF). Soil samples were collected after the rice and wheat harvests and wet sieved into large macroaggregates, small macroaggregates, microaggregates, and silt and clay particles. Variations in amorphous and free Fe oxides, Fe-bound organic carbon and phenol oxidase activity were examined. After nine years, compared with the initial soil, the activation degree of free Fe oxides increased by 1.3- to 1.6-fold and the topsoil SOC stock increased by 13-61% across all treatments. Amorphous Fe oxide content, phenol oxidase activity and aggregate mean-weight diameter were higher after the wheat harvest than after the rice harvest. Amorphous Fe oxide content was positively correlated with Fe-bound organic carbon content (P < 0.001) but negatively correlated with phenol oxidase activity (P < 0.001). Therefore, seasonal alternation of wetting and drying can progressively drive the rejuvenation of Fe oxides and simultaneously affect the activity of phenol oxidase. Oxidative precipitation of amorphous Fe oxides promoted the formation of organo-Fe complexes and macroaggregates, while flooding of the paddies decreased the activity of phenol oxidase, thereby resulting in year-round hindered decomposition. Organic fertilization strengthened the roles of the 'Fe gate' and 'enzyme latch' mechanisms, and thus accelerated SOC sequestration in the rice-wheat cropping system.

Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils

We report an unrecognized, tidal source of reactive oxygen species (ROS). Using a newly developed ROS-trapping gel film, we observed hot spots for ROS generation within ∼2.5 mm of coastal surface soil. Kinetic analyses showed rapid production of hydroxyl radicals (^•OH), superoxide (O₂^•-), and hydrogen peroxide (H₂O₂) upon a shift from high tide to low tide. The ROS production exhibited a distinct rhythmic fluctuation. The oscillations of the redox potential and dissolved oxygen concentration followed the same pattern as the ^•OH production, suggesting the alternating oxic-anoxic conditions as the main geochemical drive for ROS production. Nationwide coastal field investigations confirmed the widespread and sustainable production of ROS via tidal processes (22.1-117.4 μmol/m²/day), which was 5- to 36-fold more efficient than those via classical photochemical routes (1.5-7.6 μmol/m²/day). Analyses of soil physicochemical properties demonstrated that soil redox-metastable components such as redox-active iron minerals and organic matter played a key role in storing electrons at high tide and shuttling electrons to infiltrated oxygen at low tide for ROS production. Our work sheds light on a ubiquitous but previously overlooked tidal source of ROS, which may accelerate carbon and metal cycles as well as pollutant degradation in coastal soils.

Co-response of Fe-reducing/oxidizing bacteria and Fe species to the dynamic redox cycles of natural sediment

Fe(III)-reducing bacteria (FRB) and Fe(II)-oxidizing bacteria (FOB) play essential roles in the biogeochemical cycling of iron (Fe). Although the redox transformation of Fe species mediated by FRB/FOB has been extensively studied, the co-responses of FRB and FOB and Fe species transformation in natural sediment under dynamic redox conditions are poorly known. This study explored the variations of potential FRB and FOB abundances and Fe species transformation in natural sediment during successive anoxic-oxic-anoxic-oxic-anoxic cycles. Compared with the pristine sediment sample, the FRB abundance increased 121-793% (initial: (2.6 ± 0.6) × 10⁷ copies/g) in the anoxic stages, while it decreased by 38-64% in the oxic stages. The increase in FRB abundance was ascribed to energy gain of FRB from the reduction of the amorphous Fe(III) (Fe(III)_am) and the crystalline Fe(III) (Fe(III)_cry) to the aqueous Fe(II) (Fe(II)_aq), the adsorbed Fe(II) (Fe(II)_ad) and the amorphous Fe(II) (Fe(II)_am), while the decrease was attributed to the oxidative stress caused by the reactive oxidant produced from the abiotic oxidation of Fe(II)_aq, Fe(II)_ad and Fe(II)_am to Fe(III)_am and Fe(III)_cry. The FOB abundance decreased 38-44% (initial: (5 ± 1.8) × 10⁷ copies/g) in the second and third anoxic stages, while slightly fluctuated in the oxic periods. This observation was contrary to the variation of FRB, which might be attributed to the strong resistance to oxidative stress of FOB and its ability to obtain energy under oxic conditions. Although the functions of FRB and FOB were impaired during anoxic-oxic cycles, the transformation of Fe(II)/Fe(III) was not immediately affected, which may be related to the residual reactivity of dead bacteria and the bio-availability of Fe(II)/Fe(III) species. In the anoxic-oxic alternation process, the iron cycle is mainly the mutual transformation between Fe(II)_aq, Fe(II)_ad, Fe(II)_am and Fe(III)_am, Fe(III)_cry. This finding deepens our understanding of the biogeochemical cycling of Fe in the redox-dynamic environments.

Methanogen Productivity and Microbial Community Composition Varies With Iron Oxide Mineralogy

Quantifying the flux of methane from terrestrial environments remains challenging, owing to considerable spatial and temporal variability in emissions. Amongst a myriad of factors, variation in the composition of electron acceptors, including metal (oxyhydr)oxides, may impart controls on methane emission. The purpose of this research is to understand how iron (oxyhydr)oxide minerals with varied physicochemical properties influence microbial methane production and subsequent microbial community development. Incubation experiments, using lake sediment as an inoculum and acetate as a carbon source, were used to understand the influence of one poorly crystalline iron oxide mineral, ferrihydrite, and two well-crystalline minerals, hematite and goethite, on methane production. Iron speciation, headspace methane, and 16S-rRNA sequencing microbial community data were measured over time. Substantial iron reduction only occurred in the presence of ferrihydrite while hematite and goethite had little effect on methane production throughout the incubations. In ferrihydrite experiments the time taken to reach the maximum methane production rate was slower than under other conditions, but methane production, eventually occurred in the presence of ferrihydrite. We suggest that this is due to ferrihydrite transformation into more stable minerals like magnetite and goethite or surface passivation by Fe(II). While all experimental conditions enriched for Methanosarcina, only the presence of ferrihydrite enriched for iron reducing bacteria Geobacter. Additionally, the presence of ferrihydrite continued to influence microbial community development after the onset of methanogenesis, with the dissimilarity between communities growing in ferrihydrite compared to no-Fe-added controls increasing over time. This work improves our understanding of how the presence of different iron oxides influences microbial community composition and methane production in soils and sediments.

Soil carbon stocks in temperate grasslands differ strongly across sites but are insensitive to decade-long fertilization

Enhancing soil carbon (C) storage has the potential to offset human-caused increases in atmospheric CO₂ . Rising CO₂ has occurred concurrently with increasing supply rates of biologically limiting nutrients such as nitrogen (N) and phosphorus (P). However, it is unclear how increased supplies of N and P will alter soil C sequestration, particularly in grasslands, which make up nearly a third of non-agricultural land worldwide. Here, we leverage a globally distributed nutrient addition experiment (the Nutrient Network) to examine how a decade of N and P fertilization (alone and in combination) influenced soil C and N stocks at nine grassland sites spanning the continental United States. We measured changes in bulk soil C and N stocks and in three soil C fractions (light and heavy particulate organic matter, and mineral-associated organic matter fractions). Nutrient amendment had variable effects on soil C and N pools that ranged from strongly positive to strongly negative, while soil C and N pool sizes varied by more than an order of magnitude across sites. Piecewise SEM clarified that small increases in plant C inputs with fertilization did not translate to greater soil C storage. Nevertheless, peak season aboveground plant biomass (but not root biomass or production) was strongly positively related to soil C storage at seven of the nine sites, and across all nine sites, soil C covaried with moisture index and soil mineralogy, regardless of fertilization. Overall, we show that site factors such as moisture index, plant productivity, soil texture, and mineralogy were key predictors of cross-site soil C, while nutrient amendment had weaker and site-specific effects on C sequestration. This suggests that prioritizing the protection of highly productive temperate grasslands is critical for reducing future greenhouse gas losses arising from land use change.

Nitrogen fixation: a poorly understood process along the freshwater-marine continuum

Although N₂ fixation is a major component of the global N cycle and has been extensively studied in open-ocean and terrestrial ecosystems, rates and ecological dynamics remain virtually unknown for the inland and coastal aquatic ecosystems (lakes, wetlands, rivers, streams, estuaries) that connect terrestrial and marine biomes. This is due to the diversity of these habitats, as well as the traditional paradigm that N₂ fixation rates were low to nonexistent, and therefore not important, in these ecosystems. We identify three major research themes to advance understanding of aquatic N₂ fixation: 1) the biological diversity of diazotrophs and variability of N₂ fixation rates, 2) the ecological stoichiometry of N₂ fixation, and 3) the upscaling of N₂ fixation rates from genes to ecosystems. Coordinating research across these areas will advance limnology and oceanography by fully integrating N₂ fixation into ecological dynamics of aquatic ecosystems from local to global scales.

Active Iron Phases Regulate the Abiotic Transformation of Organic Carbon during Redox Fluctuation Cycles of Paddy Soil

Iron (Fe) phases are tightly linked to the preservation rather than the loss of organic carbon (OC) in soil; however, during redox fluctuations, OC may be lost due to Fe phase-mediated abiotic processes. This study examined the role of Fe phases in driving hydroxyl radical (^•OH) formation and OC transformation during redox cycles in paddy soils. Chemical probes, sequential extraction, and Mössbauer analyses showed that the active Fe species, such as exchangeable and surface-bound Fe and Fe in low-crystalline minerals (e.g., green rust-like Fe phases), predominantly regulated ^•OH formation during redox cycles. The ^•OH oxidation strongly induced the oxidative transformation of OC, which accounted for 15.1-30.8% of CO₂ production during oxygenation. Microbial processes contributed 7.3-12.1% of CO₂ production, as estimated by chemical quenching and γ-irradiation experiments. After five redox cycles, 30.1-71.9% of the OC associated with active Fe species was released, whereas 5.2-7.1% was stabilized by high-crystalline Fe phases due to the irreversible transformation of these active Fe species during redox cycles. Collectively, our findings might unveil the under-appreciated role of active Fe phases in driving more loss than conservation of OC in soil redox fluctuation events.

Redox Heterogeneity Entangles Soil and Climate Interactions

Interactions between soils and climate impact wider environmental sustainability. Soil heterogeneity intricately regulates these interactions over short spatiotemporal scales and therefore needs to be more finely examined. This paper examines how redox heterogeneity at the level of minerals, microbial cells, organic matter, and the rhizosphere entangles biogeochemical cycles in soil with climate change. Redox heterogeneity is used to develop a conceptual framework that encompasses soil microsites (anaerobic and aerobic) and cryptic biogeochemical cycling, helping to explain poorly understood processes such as methanogenesis in oxygenated soils. This framework is further shown to disentangle global carbon (C) and nitrogen (N) pathways that include CO2, CH4, and N2O. Climate-driven redox perturbations are discussed using wetlands and tropical forests as model systems. Powerful analytical methods are proposed to be combined and used more extensively to study coupled abiotic and biotic reactions that are affected by redox heterogeneity. A core view is that emerging and future research will benefit substantially from developing multifaceted analyses of redox heterogeneity over short spatiotemporal scales in soil. Taking a leap in our understanding of soil and climate interactions and their evolving influence on environmental sustainability then depends on greater collaborative efforts to comprehensively investigate redox heterogeneity spanning the domain of microscopic soil interfaces.

The Structure of Natural Biogenic Iron (Oxyhydr)oxides Formed in Circumneutral pH Environments

Biogenic iron (Fe) (oxyhydr)oxides (BIOS) partially control the cycling of organic matter, nutrients, and pollutants in soils and water via sorption and redox reactions. Although recent studies have shown that the structure of BIOS resembles that of two-line ferrihydrite (2LFh), we lack detailed knowledge of the BIOS local coordination environment and structure required to understand the drivers of BIOS reactivity in redox active environments. Therefore, we used a combination of microscopy, scattering, and spectroscopic methods to elucidate the structure of BIOS sampled from a groundwater seep in North Carolina and compare them to 2LFh. We also simulated the effects of wet-dry cycles by varying sample preparation (e.g., freezing, flash freezing with freeze drying, freezing with freeze drying and oven drying). In general, the results show that both the long- and short-range ordering in BIOS are structurally distinct and notably more disordered than 2LFh. Our structure analysis, which utilized Fe K-edge X-ray absorption spectroscopy, Mössbauer spectroscopy, X-ray diffraction, and pair distribution function analyses, showed that the BIOS samples were more poorly ordered than 2LFh and intimately mixed with organic matter. Furthermore, pair distribution function analyses resulted in coherent scattering domains for the BIOS samples ranging from 12-18 Å, smaller than those of 2LFh (21-27 Å), consistent with reduced ordering. Additionally, Fe L-edge XAS indicated that the local coordination environment of 2LFh samples consisted of minor amounts of tetrahedral Fe(III), whereas BIOS were dominated by octahedral Fe(III), consistent with depletion of the sites due to small domain size and incorporation of impurities (e.g., organic C, Al, Si, P). Within sample sets, the frozen freeze dried and oven dried sample preparation increased the crystallinity of the 2LFh samples when compared to the frozen treatment, whereas the BIOS samples remained more poorly crystalline under all sample preparations. This research shows that BIOS formed in circumneutral pH waters are poorly ordered and more environmentally stable than 2LFh.

Organic Matter from Redoximorphic Soils Accelerates and Sustains Microbial Fe(III) Reduction

Microbial reduction of Fe(III) minerals is a prominent process in redoximorphic soils and is strongly affected by organic matter (OM). We herein determined the rate and extent of microbial reduction of ferrihydrite (Fh) with either adsorbed or coprecipitated OM by Geobacter sulfurreducens. We focused on OM-mediated effects on electron uptake and alterations in Fh crystallinity. The OM was obtained from anoxic soil columns (effluent OM, efOM) and included-unlike water-extractable OM-compounds released by microbial activity under anoxic conditions. We found that organic molecules in efOM had generally no or only very low electron-accepting capacity and were incorporated into the Fh aggregates when coprecipitated with Fh. Compared to OM-free Fh, adsorption of efOM to Fh decelerated the microbial Fe(III) reduction by passivating the Fh surface toward electron uptake. In contrast, coprecipitation of Fh with efOM accelerated the microbial reduction, likely because efOM disrupted the Fh structure, as noted by Mössbauer spectroscopy. Additionally, the adsorbed and coprecipitated efOM resulted in a more sustained Fe(III) reduction, potentially because efOM could have effectively scavenged biogenic Fe(II) and prevented the passivation of the Fh surface by the adsorbed Fe(II). Fe(III)-OM coprecipitates forming at anoxic-oxic interfaces are thus likely readily reducible by Fe(III)-reducing bacteria in redoximorphic soils.

Role of Fe dynamic in release of metals at Rio Doce estuary: Unfolding of a mining disaster

The role of Fe oxyhydroxides dynamic on metal bioavailability was studied in the Rio Doce estuary after the largest mining disaster in the world. Soon after the disaster in 2015, metals were associated with Fe oxyhydroxides under a redox-active estuarine environment. Our results indicate that organic matter inputs from plant colonization on deposited tailings over estuarine soils led to a reductive dissolution of Fe oxyhydroxides within two years. Soil pseudo-total Fe content decreased by 70% between 2015 and 2017, while the total metal contents (Cr, Cu, Ni, Pb, and Zn) decreased by 79% in the soil. The losses of Fe and metals coupled to changes in Fe oxides crystallinity reveal a future ephemeral control of Fe oxyhydroxides over metal immobilization. Our results suggest a potential chronic contamination at the estuary and points to an aggravating scenario for the following years due to the increasing dominance of poorly crystalline Fe oxyhydroxides.

Coprecipitation of Fe/Cr Hydroxides with Organics: Roles of Organic Properties in Composition and Stability of the Coprecipitates

Iron hydroxides are important scavengers for dissolved chromium (Cr) via coprecipitation processes; however, the influences of organic matter (OM) on Cr sequestration in Fe/Cr-OM ternary systems and the stability of the coprecipitates are not well understood. Here, Fe/Cr-OM coprecipitation was conducted at pH 3, and Cr hydroxide was undersaturated. Acetic acid (HAc), poly(acrylic acid) (PAA), and Suwannee River natural organic matter (SRNOM) were selected as model OMs, which showed different complexation capabilities with Fe/Cr ions and Fe/Cr hydroxide particles. HAc had no significant effect on the coprecipitation, as the monodentate carboxyl ligand in HAc did not favor complexation with dissolved Fe/Cr ions or Fe/Cr hydroxide nanoparticles. Contrarily, PAA and SRNOM with polydentate carboxyl ligand had strong complexation with Fe/Cr ions and Fe/Cr hydroxide nanoparticles, leading to significant amounts of PAA/SRNOM sequestered in the coprecipitates, which caused the structural disorder and fast aggregation of the coprecipitates. In comparison with that of PAA, preferential complexation of Cr ions with SRNOM resulted in higher Cr/Fe ratios in the coprecipitates. This study advances the fundamental understanding of Fe/Cr-OM coprecipitation and mechanisms controlling the composition and stability of the coprecipitates, which is essential for successful Cr remediation and removal in both natural and engineered settings.

Interaction between hexavalent chromium and biologically formed iron mineral-biochar composites: Kinetics, products and mechanisms

Biogenic Fe(II) is a dominant natural reductant to convert carcinogenic Cr(VI) to less toxic Cr(III). Field-applied biochar could promote microbial production of Fe(II) and form iron-biochar composites. Although there have been mounting research on the interactions of biochar or Fe(II) with Cr(VI), their coupling effects on Cr(VI) immobilization have been largely neglected. Here, iron mineral-biochar composite (IMBC) was prepared via biochar-mediated dissimilatory reduction of ferrihydrite or goethite by Shewanella oneidensis MR-1, and its reaction with Cr(VI) was investigated. IMBC was able to effectively remove aqueous Cr(VI) via reductive transformation by adsorbed Fe(II). The removal process nicely followed pseudo-second-order kinetics and Langmuir isotherm model. The removal ability of IMBC decreased with increasing pH (5.5-8.0) but was independent of ionic strength changes (0-100 mM). After reaction, the Fe-Cr coprecipitates formed on IMBC exhibited slightly higher Fe/Cr ratios (0.93-0.96) than those on corresponding iron mineral controls (0.88-0.94). For IMBC, while the presence of biochar decreased the reactivity of adsorbed Fe(II), their removal capacities were ~30% higher than those of iron minerals alone, due to the enhanced yields of adsorbed Fe(II). These findings improved our knowledge of interactions among biochar, iron mineral and iron-reducing bacteria and their contribution to chromium immobilization.

An evolving view on biogeochemical cycling of iron

Biogeochemical cycling of iron is crucial to many environmental processes, such as ocean productivity, carbon storage, greenhouse gas emissions and the fate of nutrients, toxic metals and metalloids. Knowledge of the underlying processes involved in iron cycling has accelerated in recent years along with appreciation of the complex network of biotic and abiotic reactions dictating the speciation, mobility and reactivity of iron in the environment. Recent studies have provided insights into novel processes in the biogeochemical iron cycle such as microbial ammonium oxidation and methane oxidation coupled to Fe(III) reduction. They have also revealed that processes in the biogeochemical iron cycle spatially overlap and may compete with each other, and that oxidation and reduction of iron occur cyclically or simultaneously in many environments. This Review discusses these advances with particular focus on their environmental consequences, including the formation of greenhouse gases and the fate of nutrients and contaminants.

Iron Transformation and Its Role in Phosphorus Immobilization in a UCT-MBR with Vivianite Formation Enhancement

The formation of vivianite (Fe₃(PO₄)₂·8H₂O) in iron (Fe)-dosed wastewater treatment facilities has the potential to develop into an economically feasible method of phosphorus (P) recovery. In this work, a long-term steady Fe^III-dosed University of Cape Town process-membrane bioreactor (UCT-MBR) system was investigated to evaluate the role of Fe transformations in immobilizing P via vivianite crystallization. The highest fraction of Fe^II, to total Fe (Fe_tot), was observed in the anaerobic chamber, revealing that a redox condition suitable for Fe^III reduction was established by improving operational and configurational conditions. The supersaturation index for vivianite in the anaerobic chamber varied but averaged ∼4, which is within the metastable zone and appropriate for its crystallization. Vivianite accounted for over 50% of the Fe_tot in the anaerobic chamber, and its oxidation as it passed through the aerobic chambers was slow, even in the presence of high dissolved oxygen concentrations at circumneutral pH. This study has shown that the high stability and growth of vivianite crystals in oxygenated activated sludge can allow for the subsequent separation of vivianite as a P recovery product.

Redox-dependent effects of phosphate on arsenic speciation in paddy soils

Evaluating speciation of arsenic (As) is essential to assess its risk in paddy soils. In this study, effects of phosphate on speciation of As in six paddy soils differing in redox status were studied over a range of pH (pH 3-9) and different background calcium (Ca) levels by batch adsorption experiments and speciation modeling. Contrasting effects of phosphate on As speciation were observed in suboxic and anoxic soils. Under suboxic conditions, phosphate inhibited Fe and As reduction probably due to stabilization of Fe-(hydr)oxides, but increased soluble As(V) concentration as a result of competitive adsorption between As(V) and phosphate. In anoxic soils, phosphate stimulated Fe and As reduction and caused increases of As(III) in soil solution under both acidic and neutral/alkaline pH. The LCD (Ligand and Charge Distribution) and NOM-CD (Natural Organic Matter-Charge Distribution) model can describe effects of pH, calcium and phosphate on As speciation in these paddy soils. The results suggest that phosphate fertilization may decrease (at low pH) or increase (at neutral/alkaline pH) As mobility in paddy soils under (sub)oxic conditions, but under anoxic conditions and in phosphorus deficient soils phosphate fertilization may strongly mobilize As by promoting microbial activities.

Role of Pyrogenic Carbon in Parallel Microbial Reduction of Nitrobenzene in the Liquid and Sorbed Phases

Surface functional groups and graphitic carbons make up the electroactive components of pyrogenic carbon. The role of pyrogenic carbon with different contents of electroactive components in mediating electron transfer in biochemical reactions has not been systematically studied. Here, we determined the electron exchange capacity (EEC) of pyrogenic carbon to be 0.067-0.120 mmol e^-·(g of pyrogenic carbon)^-1, and the maximum electrical conductivity (EC) was 4.85 S·cm^-1. Nitrobenzene was simultaneously reduced in both the liquid and sorbed phases by Shewanella oneidensis MR-1 in the presence of pyrogenic carbon. Pyrogenic carbon did not affect the aqueous nitrobenzene reduction, and the reduction of sorbed nitrobenzene was much slower than that of the aqueous species. Enhancing contents of oxygenated functional moieties in pyrogenic carbon with HNO₃ oxidation elevated bioreduction rates of the aqueous and sorbed species. Anthraquinone groups were deemed as the most likely oxygenated functional redox compounds on the basis of both voltammetric curve tests and spectroscopic analysis. The reactivity of pyrogenic carbon in mediating the reduction of sorbed nitrobenzene was positively correlated with its EC, which was demonstrated to be related to condensed aromatic structures. This work elucidates the mechanism for pyrogenic carbon-mediated biotransformation of nitrobenzene and helps properly evaluate the role of pyrogenic carbon in biogeochemical redox processes happening in nature.

Quantifying Microbially Mediated Kinetics of Ferrihydrite Transformation and Arsenic Reduction: Role of the Arsenate-Reducing Gene Expression Pattern

The behavior of arsenic (As) is usually coupled with iron (Fe) oxide transformation and mediated by both abiotic reactions and microbial processes in the environment. However, quantitative models for the coupled kinetic processes, which specifically consider the arsenate-reducing gene expression correspondent to different reaction conditions, are lacking. In this study, based on the pure cultured Shewanella putrefaciens incubation experiments, extended X-ray absorption fine structure spectroscopy, high resolution transmission electron microscopy, and a suite of microbial analyses, we developed a coupled kinetics model for microbially mediated As reduction and Fe oxide transformation and specifically quantified the As(V) reduction rate coefficients based on the expression patterns of arrA genes. The model reasonably described the temporal changes of As speciation and distribution. The microbial reduction rates of As(V) varied dramatically during the reactions, which were well represented by the varying transcript abundances of arrA genes at different As concentrations. The contributions of biotic and abiotic reactions to the overall reaction rates were assessed. The results improved our quantitative understanding on the key role of As(V)-reducing genes in regulating the speciation and distribution of As. The kinetic modeling approaches based on microbial gene expression patterns are promising for developing comprehensive biogeochemical models of As involving multiple coupled reactions.

Iron-mediated organic matter decomposition in humid soils can counteract protection

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O₂) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding ¹³C dissolved organic matter (DOM) and ⁵⁷Fe^II to soil slurries incubated under static or fluctuating O₂. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive Fe^III phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding ⁵⁷Fe^II alone increases SOM mineralization by 8% following oxidation to ⁵⁷Fe^III. Under O₂ limitation, de novo reactive ⁵⁷Fe^III phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O₂ limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.

Reactive iron minerals protect vast amounts of terrestrial carbon from decomposition and release as CO₂. Here the authors show that reactive iron alone does not provide sufficient protection except under strict oxic conditions—instead, iron itself promotes carbon decomposition.

Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage

Mineral availability for carbon (C) binding is a key regulator of soil C storage, yet little is known about the highly reactive nanomineral assembly in the paddy soil colloids. Here, using high-resolution transmission electron microscopy (HRTEM), solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we investigated the coordination nature of short-range-ordered (SRO) minerals in water-dispersible colloids that were isolated from the paddy soil under different six-year fertilization regimes. Our results showed that organic fertilization not only promoted the transformation of crystalline minerals to SRO phases in the bulk soils but also increased the concentrations of Fe, Al and Si in the soil colloids compared to chemical fertilization alone, and thus enhanced the accumulation of organic C in both the bulk soils and the soil colloids. The HRTEM images revealed that water-dispersible colloids in all soils, regardless of treatment, were composed of crystalline Fe nanominerals (with some Al/Si) and SRO-Al/Si nanominerals (with some Fe) associated with organic C. Furthermore, the combined results from the ²⁷Al and ²⁹Si NMR spectroscopy and XPS not only confirmed the presence of SRO-Al/Si nanoparticles as Si-rich allophane and phytolith but also demonstrated that organic fertilization promoted the transformation of aluminosilicates to SRO-Al/Si nanominerals in soil colloids. Together, these findings indicate that six-year organic fertilization promotes the formation of SRO minerals (e.g., ferrihydrite, Si-rich allophane and Fe-substituted allophane, as well as phytolith) in soils and modulates the assembly of organo-mineral complexes possibly by driving the biogeochemical cycles of Fe, Al, Si and specific organic ligands, thus contributing to the long-term storage of C in paddy soils.

Phenazine-1-Carboxylic Acid-Producing Bacteria Enhance the Reactivity of Iron Minerals in Dryland and Irrigated Wheat Rhizospheres

Phenazine-1-carboxylic acid (PCA) is a broad-spectrum antibiotic produced by rhizobacteria in the dryland wheat fields of the Columbia Plateau. PCA and other phenazines reductively dissolve Fe and Mn oxyhydroxides in bacterial culture systems, but the impact of PCA upon Fe and Mn cycling in the rhizosphere is unknown. Here, concentrations of dithionite-extractable and poorly crystalline Fe were approximately 10% and 30-40% higher, respectively, in dryland and irrigated rhizospheres inoculated with the PCA-producing (PCA⁺) strain Pseudomonas synxantha 2-79 than in rhizospheres inoculated with a PCA-deficient mutant. However, rhizosphere concentrations of Fe(II) and Mn did not differ significantly, indicating that PCA-mediated redox transformations of Fe and Mn were transient or were masked by competing processes. Total Fe and Mn uptake into wheat biomass also did not differ significantly, but the PCA⁺ strain significantly altered Fe translocation into shoots. X-ray absorption near edge spectroscopy revealed an abundance of Fe-bearing oxyhydroxides and phyllosilicates in all rhizospheres. These results indicate that the PCA⁺ strain enhanced the reactivity and mobility of Fe derived from soil minerals without producing parallel changes in plant Fe uptake. This is the first report that directly links significant alterations of Fe-bearing minerals in the rhizosphere to a single bacterial trait.

Redox transformation of soil minerals and arsenic in arsenic-contaminated soil under cycling redox conditions

Changes in the saturation degree of aquifers control the geochemical reactions of redox-sensitive elements such as iron (Fe), sulfur (S), and arsenic (As). In this study, the effects of redox conditions and the presence of Fe and S on the behavior of As in a soil environment were investigated by observation in a batch experimental system. Arsenic was stable on Fe(III) solid surface in an oxidizing environment but was easily released into the aqueous phase following the reductive dissolution of Fe during an anoxic period. The alternating redox cycles led to a change in the concentrations of Fe, S, and As in both the aqueous and solid phases. The composition of Fe minerals changed to a less crystalline phase while that of solid phase As changed to a more reduced phase in both the As-contaminated natural soil and FeS-amended soil batch systems. This tendency was more prominent in the batch containing higher amounts of total Fe and S. These results show that a redox cycle can increase the possibility of As contamination of groundwater during dissolution and reprecipitation of Fe minerals and simultaneous microbial reduction of S and/or As species.

Iron Redox Reactions Can Drive Microtopographic Variation in Upland Soil Carbon Dioxide and Nitrous Oxide Emissions

Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO2) and nitrous oxide (N2O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO2 and N2O production, it is unclear what their cumulative effect is on CO2 and N2O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO2 and N2O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO2 and N2O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO2 emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N2O production, suggesting that chemodenitrification may be an important pathway of N2O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO2 and N2O production may become increasingly important pathways of soil greenhouse gas emissions.

Enrichment of Lignin-Derived Carbon in Mineral-Associated Soil Organic Matter

A modern paradigm of soil organic matter proposes that persistent carbon (C) derives primarily from microbial residues interacting with minerals, challenging older ideas that lignin moieties contribute to soil C because of inherent recalcitrance. We proposed that aspects of these old and new paradigms can be partially reconciled by considering interactions between lignin decomposition products and redox-sensitive iron (Fe) minerals. An Fe-rich tropical soil (with C₄ litter and either ¹³C-labeled or unlabeled lignin) was pretreated with different durations of anaerobiosis (0-12 days) and incubated aerobically for 317 days. Only 5.7 ± 0.2% of lignin ¹³C was mineralized to CO₂ versus 51.2 ± 0.4% of litter C. More added lignin-derived C (48.2 ± 0.9%) than bulk litter-derived C (30.6 ± 0.7%) was retained in mineral-associated organic matter (MAOM; density >1.8 g cm^-3), and 12.2 ± 0.3% of lignin-derived C vs 6.4 ± 0.1% of litter C accrued in clay-sized (<2 μm) MAOM. Longer anaerobic pretreatments increased added lignin-derived C associated with Fe, according to extractions and nanoscale secondary ion mass spectrometry (NanoSIMS). Microbial residues are important, but lignin-derived C may also contribute disproportionately to MAOM relative to bulk litter-derived C, especially following redox-sensitive biogeochemical interactions.

Biogeochemical fate of ferrihydrite-model organic compound complexes during anaerobic microbial reduction

Associations of organic carbon (OC) with iron (Fe) oxide minerals play an important role in regulating the stability of OC in soil environments. Knowledge about the fate and stability of Fe-OC complexes is impaired by the heterogeneity of OC. Additional biogeochemical variables in soil environments, such as redox conditions and microbes, further increase complexity in understanding the stability of mineral-associated soil OC. This study investigated the fate and stability of model organic compounds, including glucose (GL), glucosamine (GN), tyrosine (TN), benzoquinone (BQ), amylose (AM), and alginate (AL), complexed with an Fe oxide mineral, ferrihydrite (Fh), during microbial reduction. During a 25-d anaerobic incubation with Shewanella putrefaciens CN32, the reduction of Fe followed the order of Fh-BQ > Fh-GL > Fh-GN > Fh-TN > Fh-AL > Fh-AM. In terms of OC released during the anaerobic incubation, Fh-GN complexes released the highest amount of OC while Fh-AM complexes released the lowest. Organic carbon regulated the reduction of Fe by acting as an electron shuttle, affecting microbial activities, and associating with Fh. Benzoquinone had the highest electron accepting capacity, but potentially can inhibit microbial activity. These findings provide insights into the roles of different organic functional groups in regulating Fe reduction and the stability of Fh-bound OC under anaerobic conditions.

Underlying lithology controls trace metal mobilization during redox fluctuations

Redox state fluctuations are a primary mechanism controlling the mobilization of trace metals in soils. However, underlying lithology may modulate the effect that redox fluctuations have on trace metal mobility by influencing soil particle size and mineral composition. To investigate the relationships among trace metal behavior, lithology, and redox state, we subjected surface soils from two intensely weathered soil profiles formed on contrasting lithologies to consecutive, 8-day redox cycles. A suite of metals (Al, Mn, Fe, Ti, Rb, Zr, Nb, Mo, REEs, Pb, Th, U) were quantified in the aqueous phase (<10 nm) and solution (<415 nm, including colloids) from soil slurries. In soil formed on volcaniclastic bedrock with high clay content and a high abundance of short-range-ordered Fe-(oxyhydr)oxides phases (e.g. nano-goethite; quantified by Mössbauer spectroscopy), reducing events and colloidal dynamics drove trace metal mobilization. In contrast, in soil formed on granite bedrock with lower clay content and a low abundance of short-range-ordered Fe-(oxyhydr)oxides phases (nano-goethite and lepidocrocite), overall trace metal mobilization was lower, and mobilization was not predictable from redox state. Molybdenum isotopes were also measured through redox cycles but did not exhibit redox-dependent behavior. This study provides direct evidence that lithology remains an overarching factor governing the characteristics of metal mobility in soils, even after extended and intense chemical weathering and soil development processes.

Phosphorus enriched effluents increase eutrophication risks for mangrove systems in northeastern Brazil

Discharge of effluents loaded with phosphorus (P) from anthropogenic activities constitutes serious eutrophication risks in marine and terrestrial ecosystems, including mangroves. Three mangroves in NE-Brazil were studied to evaluate the impact of P-rich-effluents from shrimp farming and domestic sewage, in relation to a control area (natural mangrove). Soil phosphorus fractionation and water chemical analysis were performed to assess potential pollution. We observed the most labile P forms increased gradually and significantly from control to sewage to shrimp farm impacted mangroves as observed by increasingly dissolved orthophosphate (PO₄^3-) content in water and the exchangeable/soluble P (Exch-P) extracted from soils, which is supported by the discriminant analysis. Exch-P results were correlated to Humic-Acid-P, which can release more labile P forms when mineralized. Our results demonstrate a substantial impact of aquiculture and sewage effluents in mangroves at both organic and inorganic P fractions, raising important concerns regarding pollution for these marine ecosystems.

Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to pollutant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides toward reduction under controlled thermodynamic boundary conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-Fe^III) were conducted over a range of solution conditions (pH_trans = 6.50 to 7.50 at 5 mM Fe²⁺ and for pH_trans = 7.00 also at 1 mM Fe²⁺) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite or magnetite. The changes in iron oxide mineralogy during the transformations were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension aliquots collected during the transformations were performed over a range of pH_MER at constant applied reduction potential. The extents and rates of iron oxide reduction in MER decreased with decreasing reaction driving force resulting from both increasing pH_MER and increasing transformation of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases in iron oxide reduction extents and rates during ferrihydrite transformations can be linked to the concurrent changes in iron oxide mineralogy.

Microbial kinetics and thermodynamic (MKT) processes for soil organic matter decomposition and dynamic oxidation-reduction potential: Model descriptions and applications to soil N2O emissions

A conversion of the global terrestrial carbon sink to a source is critically dependent on the microbially mediated decomposition of soil organic matter (SOM). We have developed a detailed, process-based, mechanistic model for simulating SOM decomposition and its associated processes, based on Microbial Kinetics and Thermodynamics, called the MKT model. We formulated the sequential oxidation-reduction potential (ORP) and chemical reactions undergoing at the soil-water zone using dual Michaelis-Menten kinetics. Soil environmental variables, as required in the MKT model, are simulated using one of the most widely used watershed-scale models - the soil water assessment tool (SWAT). The MKT model was calibrated and validated using field-scale data of soil temperature, soil moisture, and N₂O emissions from three locations in the province of Saskatchewan, Canada. The model evaluation statistics show good performance of the MKT model for daily soil N₂O simulations. The results show that the proposed MKT model can perform better than the more widely used process-based and SWAT-based models for soil N₂O simulations. This is because the multiple processes of microbial activities and environmental constraints, which govern the availability of substrates to enzymes were explicitly represented. Most importantly, the MKT model represents a step forward from conceptual carbon pools at varying rates.