Methane and water soluble iron production under controlled soil pH and redox conditions

Abstract

When a soil is flooded, iron (Fe) reduction and methane (CH₄) production occurred in sequence as predicted by thermodynamics. The dissolution and precipitation of Fe reflected both soil pH and soil redox potential (Eh). The objective of our experiment was to determine both CH₄ production and Fe reduction as measured by Fe in solution in a flooded paddy soil over a wide range of closely controlled pH and Eh conditions. The greatest release of CH₄ gas occurred at neutral soil pH in combination with low soil redox potential (‐250 mV). Production of CH₄ decreased when soil pH was lowered in combination with an increase in the soil redox potential above ‐250 mV. Highest concentration of ferrous‐iron (Fe²⁺) under reducing conditions occurred when soil pH was lowered. Thus Fe reduction influenced CH4 formation in the flooded paddy soil. Results indicated that CH₄ production was inhibited by the process of ferric‐iron (Fe³⁺) reduction.     

Nitrate and nitrite reduction in flooded gamma-irradiated soil under controlled pH and redox potential conditions

Nitrate and nitrite reduction was studied in a waterlogged soil after γ-irradiation with 2.5 Mrad. Before irradiation and mineral-N addition the pH was controlled at 4.5, 6 and 8, and the redox potential controlled at 0, +200 and +400 mV Nitrate reduction rate increased with increasing pH as well as with decreasing redox potential. Nitrate reduction rate was doubled by decreasing the redox potential from +400 to 0 mV. At pH 4.5 almost no nitrite accumulated regardless of redox potential, while at pH 6 and 8 marked nitrite accumulation occurred at low redox potential. In relation to non-irradiation. γ-irradiation had a stimulating effect on nitrate reduction at pH 6 and 8 but a retarding effect at pH 4.5; nitrite reduction proceeded slower at pH 6 and 8 but at the same rate at pH 4.5. It has been demonstrated that nitrate and nitrite reduction can be carried out by radiation-resistant enzyme systems of non-proliferating cells of denitrifying organisms.     

Effects of flooding and redox conditions on solute diffusion in soil

Rates of solute diffusion fundamentally affect the properties of flooded soils, but the effects of flooding on solute diffusion have not previously been studied in detail. Four soils with widely differing chemical and physical properties were packed to a range of bulk densities, flooded for varying times, and the self‐diffusion of chloride through the soils measured. Diffusion impedance factors were derived from the results. In each soil the impedance factor decreased linearly with increase in bulk density, and between soils impedance factors increased with increasing clay content. The impedance factor decreased by up to 20% during the first 3–6 weeks following flooding, but with prolonged flooding it increased to at least its initial value. Concomitantly the cation exchange capacities of the soils increased by between 30 and 100%, there was reductive dissolution of soil iron, probably both structural iron in soil clays and iron oxyhydroxide coatings on clay surfaces, and subsequently there was re‐precipitation of ferrous iron, probably as mixed carbonates and hydroxides. The decreases in diffusion impedance factors were consistent with the increases in cation exchange capacity and changes in soil iron, and the subsequent increases were consistent with re‐crystallization of mixed ferrous–ferric compounds. We conclude that the effects of changes in redox on diffusion impedance will be important in some soils, although they are smaller than the effects of water content per se.     

Fate of isoxaflutole in soil under controlled conditions

Isoxaflutole (IFT, 5-cyclopropyl-1,2-oxazol-4yl-alpha,alpha,alpha-trifluoro-2-mesyl-p-tolyl ketone) is a new pre-emergence proherbicide used in maize and sugarcane. Its two main derivatives are a diketonitrile derivative, 2-cyano-3-cyclopropyl-1-(2-methanesulfonyl-4-trifluoromethylphenyl)propane-1,3-dione, called DKN, and a benzoic acid derivative, 2-methanesulfonyl-4-trifluoromethylbenzoic acid, called BA. Few data are available of the factors influencing the degradation of IFT in soil, and the purpose of the present work was to determine the relative importance of, and factors affecting, the degradation of IFT in soil. Experiments were conducted on five soils with distinct physicochemical characteristics, at different temperatures and moisture contents in biotic and abiotic conditions. The isomerization of IFT to DKN is rapid, increasing with higher moisture contents and higher temperatures. It depends strongly on pH and is governed by chemical processes. The degradation of DKN to BA appeared to be essentially due to the biological activity of the soil.     

Light hydrocarbon production in freshwater marsh soil as influenced by soil redox conditions

Water, Air, & Soil Pollution - The potential role of wetland soil redox condition to global atmospheric light hydrocarbon budget was evaluated. The effect of soil redox condition on gaseous...     

Effect of redox conditions on phosphate exchangeability and iron forms in a soil amended with ferrous iron

Application of iron (Fe) -rich amendments to soils has been proposed as a means of decreasing phosphorus (P) losses from soils. However, anoxic conditions following soil saturation are known to increase Fe and P solubility in soils, thus cancelling out the potential benefits. Our aim was to evaluate the effects of continuous oxic, continuous anoxic and alternating anoxic/oxic conditions on P exchangeability and Fe forms in soil amended with Ca(OH)₂ and FeSO₄. We incubated amended and unamended soils under these conditions for 8 weeks and measured Fe forms and P exchangeability. Under oxic conditions, addition of Ca(OH)₂ and FeSO₄ resulted in a strong decrease in P exchangeability and an increase in oxalate-extractable Fe. Mössbauer analyses suggested that an unidentified Fe oxide (D1oxide) with a strong sorbing capacity for P was precipitated. Under continuously anoxic conditions, P exchangeability and oxalate-extractable Fe increased with or without the amendments. Mössbauer analyses suggested that there was a partial dissolution of the D1oxide phase, precipitation of another unidentified Fe oxide (S3) and a reduction of structural Fe³⁺ in phyllosilicate, thereby increasing soil negative charge. These transformations resulted in a strong increase in rapidly exchangeable P. Alternating anoxic and oxic periods induced the dissolution and precipitation of iron oxides and the increase and decrease in P exchangeability. Implications of the results for limiting P losses from grassland soils are discussed.     

Impact of experimental drought and rewetting on redox transformations and methanogenesis in mesocosms of a northern fen soil

The impact of climate change on the greenhouse gas balance of peatlands is debated as they function both as sinks of carbon and significant sources of methane. To study redox transformations influencing methane production, we incubated two intact soil monoliths from a northern temperate fen and compared a permanently wet treatment to a treatment undergoing an experimentally induced drought for 50 days. Net turnover of dissolved inorganic carbon (DIC), methane (CH₄) and electron acceptors in the saturated zone was calculated using a mass balance approach, and sulfate gross reduction rates were determined using a ³⁵S radiotracer. Thermodynamic energy yield of different electron accepting processes was calculated and related to the observed respiration patterns. Permanently wet conditions lead to a depletion of electron acceptors within 50 days and onset of methanogenic conditions. During drought, electron acceptors were renewed and methanogenesis was temporarily suppressed in most of the peat for another 20-50 days after rewetting. Methanogenesis began, however, apparently locally before electron acceptors were fully depleted in the remainder of the peat, and iron and sulfate reduction occurred simultaneously. Anaerobic production of DIC could mostly but not fully be explained by reduction of nitrate, sulfate and ferric iron. Sulfate gross reduction rates of up to ∼450 nmol cm⁻³ d⁻¹ determined with ³⁵S-SO₄ and potentially explained the surplus of 50-60 mmol m⁻² of DIC production in one treatment; however, the sulfate pools were too small to sustain such rates beyond some hours to days. Furthermore, anaerobic DIC production proceeded at constant rates after depletion of dissolved inorganic electron acceptors, although not being balanced by methane production. An unknown electron acceptor was thus consumed, and sulfate and potentially other electron acceptors recycled, either by humic substances, by aerenchymatic oxygen transport, or by oxygen in the capillary fringe at low levels of air filled porosity.     

Microbial responses to fluctuation of soil aeration status and redox conditions

 The response of the microbial community to changes in aeration status, from oxic to anoxic and from anoxic to oxic, was determined in arable soil incubated in a continuous flow incubation apparatus. Soil incubated in permanently oxic (air) and/or anoxic (O₂-free N₂) conditions was used as the control. Before experiments soil was preincubated for 6 days, then aeration status was changed and glucose added. Glucose concentration, extractable C, CO₂ production, microbial biomass, pH and redox potential were determined 0, 4, 8, 12, 16, 24, 36 and 48 h after change of aeration status. If oxic conditions were changed to anoxic, the amount of glucose consumed was reduced by about 60%, and CO₂ production was 10 times lower at the end of incubation compared to the control (permanently oxic conditions). Microbial biomass increased by 114% in glucose-amended soil but did not change in unamended soil. C immobilization prevailed over C mineralization. Redox potential decreased from +627 mV to –306 mV. If anoxic conditions were changed to oxic, consumption of glucose and CO₂ evolution significantly increased, compared to permanently anoxic conditions. Microbial biomass did not change in glucose-amended soil, but decreased by 78% in unamended soil. C mineralization was accelerated. Redox potential increased from +238 to +541 mV. The rate of glucose consumption was low in anoxic conditions if soil was incubated in pure N₂ but increased significantly when incubation was carried out in a CO₂/N₂ mixture. Received: 6 January 1999     

不同氧化还原条件下土壤性质对非石灰性土壤中锌溶解动力学的影响

Zinc(Zn) deficiency in paddy soils is often a problem for rice production.Flooding can decrease metal availability in some noncalcareous soils through different mechanisms associated with soil redox status.Laboratory experiments were performed in order to better understand the processes that governed the dynamics of Zn in non-calcareous paddy soils at varying redox potentials(Eh).Airdried non-calcareous soil samples collected from four different paddy field sites in the Philippines were submerged and incubated in a reaction cell with continuous stirring and nitrogen purging for 4 weeks,and then purged with compressed air for another week to reoxidize the system.The Eh of the four soils started at 120 to 300 mV,decreased to —220 to —300 mV after 100 to 250 h of reduction,and was maintained at this low plateau for about 2 weeks before increasing again upon reoxidation.Zinc solubility showed contrasting patterns in the four soils,with two of the soils showing a decrease in soluble Zn as the Eh became low,probably due to zinc sulfide(ZnS) precipitation.In contrast,the other two soils showed that Zn solubility was maintained during the reduced phase which could be due to the competition with iron(Fe) for precipitation with sulfide.Differences in the relative amounts of S,Fe,and manganese(Mn) oxides in the four soils apparently influenced the pattern of Zn solubility after flooding.     

Mineralisation of crop residues on the soil surface or incorporated in the soil under controlled conditions

In the present work, we compare the effect of mature crop residues mixed into a ferralitic soil or placed as a single layer on soil surface on the mineralisation of C and N over 55 days. As residues, we used dry stems of rice, soybean, sorghum, brachiaria and wheat. There were no significant effects of residue placement on C mineralisation kinetics. Decomposition of the residues on the soil surface slightly increased net N mineralisation for residues having the smallest C/N ratio.     

Isolating the effect of soil properties on agricultural soil greenhouse gas emissions under controlled conditions

Agricultural soils are important sources of greenhouse gases (GHGs). Soil properties and environmental factors have complex interactions which influence the dynamics of these GHG fluxes. Four arable and five grassland soils which represent the range of soil textures and climatic conditions of the main agricultural areas in the UK were incubated at two different moisture contents (50 or 80% water holding capacity) and with or without inorganic fertiliser application (70 kg N ha⁻¹ ammonium nitrate) over 22 days. Emissions of N₂O, CO₂ and CH₄ were measured twice per week by headspace gas sampling, and cumulative fluxes were calculated. Multiple regression modelling was carried out to determine which factors (soil mineral N, organic carbon and total nitrogen contents, C:N ratios, clay contents and pH) that best explained the variation in GHG fluxes. Clay, mineral N and soil C contents were found to be the most important explanatory variables controlling GHG fluxes in this study. However, none of the measured variables explained a significant amount of variation in CO₂ fluxes from the arable soils. The results were generally consistent with previously published work. However, N₂O emissions from the two Scottish soils were substantially more sensitive to inorganic N fertiliser application at 80% water holding capacity than the other soils, with the N₂O emissions being up to 107 times higher than the other studied soils.     

Changes of redox and pH conditions in a flooded soil amended with glucose and manganese oxide or iron oxide under laboratory conditions

The changes of Eh and pH in soil suspension (Ah-horizon of a Mollic Gleysol) and Mn²⁺ or Fe²⁺ concentrations in the equilibrium soil solution at different levels of glucose (0%, 0.5% and 1%) and MnO₂ (0%, 0.025%, 0.05% and 0.1%) or Fe₂O₃ (0%, 0.025%, 0.05% and 0.1%) were examined. It was found that the degree of Mn- and Fe-reduction in soil depends mainly on the presence and the amount of an easily decomposable carbon source and to a minor degree on the content of native or added forms of MnO_O2 or Fe₂O₃ in the soil. Theoretical relationships between the water soluble manganese and iron and the Eh and pH values have been verified, when the observed initial drop of Eh was eliminated. It was found that the water soluble manganese content was described best by the Mn₂O₃/Mn²⁺ redox system, and that of iron by the Fe₃ (OH)₃/Fe²⁺ system.     

Insights into the effects of serpentine soil conditions on the community composition of fungal symbionts in the roots of Onosma echioides

In this study, we investigated the impact of the extreme edaphic conditions of serpentine soils on the community structure of fungal symbionts in the roots of the perennial herb Onosma echioides (Boraginaceae). Sixteen root systems of O. echioides were obtained from two serpentine and two non-serpentine sites in the region of Tuscany (Italy) and the associated fungal communities were characterized by 454 pyrosequencing of fungal internal transcribed spacer 1 (ITS1) amplicon libraries. Clustering analysis of 173,639 sequence reads yielded a total of 699 non-singleton operational taxonomic units (OTUs), with the number of non-singleton OTUs per fungal community varying between 65 and 225. The richness of the fungal communities did not appear to be affected by the edaphic conditions of serpentine soils but the compositions of the serpentine and non-serpentine communities differed significantly, even though community structures were mainly influenced by strong spatial effects at low-to-medium distance scales. Whereas non-serpentine communities were dominated, at least in terms of sequence read abundance, by OTUs assigned to the genera Rhizophagus and Glomus (Glomeromycota), serpentine communities were dominated by OTUs assigned to the genera Ceratobasidium and Coprinellus (Basidiomycota). The functional roles of these basidiomycetous species in the roots of serpentine O. echioides specimens are, however, unclear and although their presence may affect the serpentine tolerance of the host plant, they may also be opportunistic symbionts that benefited from the reduced abundance of arbuscular mycorrhizal fungi to colonize the roots of O. echioides in serpentine environments.     

The impact of redox conditions on the rare earth element signature of redoximorphic features in a soil sequence developed from limestone

Redox processes, which are widespread in soils, need to be quantified for an improved comprehension of the dynamics of Fe- and Mn-oxides and their associated trace elements. The classical methodology used to study these redox processes generally relies on the quantification of all mineral species in the various pedological features that can be related to different redox stages. However, this approach usually encounters the difficulty of precisely quantifying the different forms of poorly crystallised Fe- and Mn-oxides.In this study, we use the signature of rare earth elements (REEs) to visualise and, eventually, quantify the importance of redox processes in soils. Our approach relies on that developed by Laveuf et al. (2008) and the idea that the relative contribution to the mobilisation of REEs that is made by the primary minerals reactive to redox conditions depends on the following factors: (i) their initial proportion in the different pedological features that can be related to various redox processes, (ii) their relative mobilisation during the redox process in question, and (iii) their initial REE signatures.The catena studied is characterised by two stages of redox conditions: the first is related to the formation and subsequent dissolution of Fe–Mn concretions, and the second is related to the bleaching of the soil matrix due to morphological degradation. In this soil, the main minerals reactive to redox conditions are Mn-oxides, ferrihydrite, goethite and (fluor)apatite. The results indicate that the primary redox conditions can be characterised by a positive Ce anomaly on the REE pattern, which has been attributed to a preferential immobilisation of this element, due to its association with Mn-oxides. The results also indicate that the secondary redox conditions can be characterised by depletion in medium REEs (MREEs) in the REE pattern, which has been attributed to a preferential release of these elements during the dissolution of (fluor)apatite and, to a lesser extent, of ferrihydrite.These results emphasise the potential of REE signatures of the visualisation of the various redox processes that have been active in a soil. Additionally, REE signatures are a proxy of the frequency and intensity of the redox conditions.     

Iron-cyanide complexes in soil under varying redox conditions: speciation, solubility and modelling

The distribution of iron‐cyanide complexes between ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, in soils on contaminated sites depends on the redox potential, E_H. We carried out microcosm experiments in which ferrocyanide (20 mg l^?1) was added to an uncontaminated moderately acidic subsoil (pH 5.2), and varied the E_H of the soil suspension between 200 and 700 mV over up to 109 days. Ferrocyanide and ferricyanide were analysed by capillary isotachophoresis. At redox potentials ranging from 400 to 700 mV, small amounts of iron‐cyanide complexes were adsorbed, and ferrocyanide was almost completely oxidized to ferricyanide. Decreasing E_H to 200 mV led to nearly complete removal of iron‐cyanide complexes from solution, and the complexes were not mobilized after subsequent aeration (E_H > 350 mV). Under weakly to moderately reducing conditions (E_H ≈ 200 mV), iron‐cyanide complexes were removed from solution by precipitation, which occurred, presumably in the form of e.g. Fe₂[Fe^II(CN)₆], Fe₄[Fe^II(CN)₆]₃ or Mn₂[Fe^II(CN)₆], after reductive dissolution of Mn and Fe oxides. Four different sets of geochemical model calculations were carried out. The species distribution between ferrocyanide and ferricyanide in solution was predicted reliably under varying pH and redox conditions when iron‐cyanide complex concentrations and Fe concentrations, excluding Fe bound in iron‐cyanide complexes, were used in model calculations. In model calculations on the fate of iron‐cyanide complexes in soil, adsorption reactions must be considered, especially under oxidizing conditions. Otherwise, the calculated iron‐cyanide complex concentrations are larger than those actually measured.     

Influence of soil water content and soil amendments on trace metal release and seedling growth in serpentine soil

Journal of Soils and Sediments - This study was conducted to evaluate the synergistic effects of organic amendments and soil water status on trace metal release from serpentine soil. Two organic...     

Effect of alternate aerobic and anaerobic conditions on redox potential,organic matter decomposition and nitrogen loss in a flooded soil

The effect of several cycles of varying length of alternate aerobic and anaerobic conditions on redox potential, organic matter decomposition and loss of added and native nitrogen was investigated under laboratory conditions in flooded soil incubated for 128 days. Redox potential decreased rapidly when air was replaced with argon for the short-time cycles, but decreased more slowly where the aerobic period was long enough to permit build-up of nitrate. The minimum redox potential reached during the anaerobic period was generally lower for the longer cycles, but in all cases was low enough for denitrification to occur. Rate of decomposition of organic matter was faster in the treatments with a greater number of alternate aerobic and anaerobic periods. Total N (native and applied) losses as high as 24.3 per cent occurred in the treatment with the maximum number of cycles and with alternate aerobic and anaerobic periods of 2 and 2 days. Increasing the durations of the aerobic-anaerobic periods decreased the loss of N. A maximum loss of 63.0 per cent of applied ¹⁵NH₄-N resulted from the shortest (2 and 2 day) aerobic and anaerobic incubation. For soil undergoing frequent changes in aeration status the only labelled N that remained at the end of incubation was found in the organic fraction. Loss of N may have been even greater if labelled inorganic N had not been immobilized by microorganisms decomposing the added rice straw. The greater loss of N resulting from the 2 and 2 day aerobic-anaerobic incubation shows that, in soils where the redox potential falls low enough for denitrification to occur, increasing the frequency of changing from aerobic to anaerobic conditions will increase the loss of N.     

Hydrological controls on soil redox dynamics in a peat-based,restored wetland

Increasing areas of altered wetland are being restored by re-flooding the soil. Evidence in the literature indicates that this practice can induce the redox-mediated release of soil nutrients, thereby increasing the risk of diffuse water pollution. However, for the sake of improving wetland management decisions, there is a need for more detailed studies of the underlying relationship between the hydrological and redox dynamics that explain this risk; this is particularly the case in agricultural peatlands that are commonly targeted for the creation of lowland wet grassland. A 12-month field study was conducted to evaluate the relationship between hydrological fluctuations and soil redox potential (Eh) in a nutrient-rich peat field (32 g N kg^− 1 and 1100 mg P kg^− 1 in the surface 0–30 cm soil) that had been restored as lowland wet grassland from intensive arable production. Field tensiometers were installed at the 30-, 60- and 90-cm soil depths, and Pt electrodes at the 10-, 30-, 60- and 90-cm depths, for daily logging of soil water tension and Eh, respectively. The values for soil water tension displayed a strong negative relationship (P < 0.001) with monthly dip well observations of water table height. Calculations of soil water potential from the logged tension values were used, therefore, to provide a detailed profile of field water level and, together with precipitation data, explained some of the variation in Eh. For example, during the summer, alternating periods of aerobism (Eh > 330 mV) in the surface, 0–10 cm layer of peat coincided with intense precipitation events. Redox potential throughout the 30–100 cm profile also fluctuated seasonally; indeed, at all depths Eh displayed a strong, negative relationship (P < 0.001) with water table height over the 12-month study period. However, Eh throughout the 30–100 cm profile remained relatively low (< 230 mV), indicating permanently reduced conditions that are associated with denitrification and reductive dissolution of Fe-bound P. The implications of these processes in the N- and P-rich peat for wetland plant diversity and water quality are discussed.