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.     

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.     

Einfluß von Bodenreaktion,Redoxbedingungen und organischer Substanz auf die Phosphatgehalte der Bodenlösung

Influence of soil reaction, redox conditions and organic matter on the phosphate content of soil solutions Samples of seven agriculturally used soils of different composition (A_p-horizons), one marine underwater soil and two garbage composts were adjusted in suspensions (soil-water-ratio 1:3) to different pH values between 3 and 8 by additions of NaOH or HCl. By a different degree of aeration the redox potential was kept constant at selected values between +600 and ?300 mV. After an incubation period of 22–24 days under controlled E_h-pH-conditions the content of total phosphate and orthophosphate was measured in the equilibrium solutions. At oxidizing conditions all soil and compost samples show the lowest phosphate content in solution at pH S6. At higher and lower pH values the phosphate content increases. The results indicate that the phosphate concentration is determined by adsorption/desorption processes – mainly connected with iron oxides – and not by dissolution or precipitation of definite phosphorus compounds. Under reducing conditions the phosphate content increases in the equilibrium solutions of all samples. Especially in samples with high content of sulphides a considerable increase of the phosphate concentration could be measured at Eh values below +300 mV at pH 5, +200 mV at pH 6, and ± 0 mV at pH7 and 8. Below these values phosphate containing iron(II1) oxides were reduced and – with further decreasing redox potentials – transformed to iron sulphides. In samples without sulphide formation the phosphate mobilization is much lower. With increasing amount of soluble organic matter the phosphate content of the solutions also increases because of phosphate desorption by organic anions or complexation of aluminium and iron from phosphate adsorbing compounds. But also the content of soluble organically bound phosphate increases and may amount to 70 % of the total phosphate content in solution.     

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     

Soil biological and physiochemical factors limiting the growth potential of tomato planted in waterlogged volcanic soil

Abstract

Many vegetable growers in Japan practice a unique waterlogged cultivation method with ample nitrogen (N) supply and microbial supplements, reporting vigorous plant growth, no soilborne diseases, and high yields. We simulated waterlogged soil conditions in greenhouse experiments to examine effects of soil pH and redox potential (Eh) as well as microbial influence on the growth of tomato seedlings. Soil pasteurization enhanced seedling growth whether the acidic, volcanic soil was waterlogged or well-drained. Among various antimicrobials, only soil treatment with polymyxin B improved shoot growth in nonpasteurized soil. The seedlings grew best in pasteurized acidic, waterlogged soil fertilized with ample potassium nitrate (KNO₃), which maintained soil Eh above zero. In nonpasteurized soil, growth was severely stunted by raising soil pH progressively to 8.5 while Eh dropped to –194?mV. The results suggested that heat-sensitive Gram-negative soil bacteria and low soil Eh were key factors limiting the growth potential of tomato plants in waterlogged soils.     

Saccharidic compounds as soil redox effectors and their influence on potential N2O production

Cellulose, xylan, and glucose were compared in waterlogged soil as modifying factors of the redox potential (Eh), of the quantity of reducing equivalents, and of the soil capacity to produce N₂O and CO₂. During the study period (168 h) soils supplied with glucose and xylan showed a higher Eh decrease than the control soil and the soil treated with cellulose. In samples taken after 0, 24, 48, and 168 h, the soils supplied with C showed a higher number of reducing equivalents than the control soil did. These quantities were not correlated with Eh values, nor with N₂O production. N₂O production was increased compared with the control soil over the entire experimental period in the glucose-amended soils but only after 48 h in the xylan-amended soils and not until 168 h in the cellulose-treated soils. The CO₂:N₂O ratio was consistently higher than the theoretical value of 2, suggesting that denitrification and CO₂ production via fermentation occurred simultaneously. Moreover, this ratio was highly correlated with the Eh values. We conclude that more research is needed to explain the role of soil redox intensity (Eh) and capacity (quantity of redox species undergoing reduction) in the expression of soil denitrification-fermentation pathways.     

Factors controlling carbon dioxide and methane production in acid sulfate soils

Methane and C02 production in flooded acid sulfate soils of Thailand were governed primarily by soil oxidation-reduction potential (Eh) and pH. The critical Eh and pH levels at which CH₄ emission began was Eh-150 mV, and pH 6.1. Low soil pH limited soil reduction and subsequently CH₄ production. Soil respiration (C02 production) was influenced by Eh-pH levels and organic matter content. Soils with higher C02 production rates produced greater amounts of CH₄. Soil pH, however, was the dominant variable which influenced organic matter decomposition, low soil Eh conditions and subsequent CH₄ and CO₂ production. Curvilinear or log transformations of pH, Eh and organic matter content (OM) were used in explaining variables controlling CH₄ and CO₂ production; CH₄ = ?2.359 ? 0.0001 Eh + 2.047 pH ? 3.019 (In pH)² CO₂ = ?5210 ? 1.6 Eh + 3144 (In pH) + 1011 (In OM).     

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.     

Iodine desorption from rice paddy soil

Laboratory experiments on the desorption phenomena of iodine from rice paddy soil under waterlogged conditions, with a special reference to soil redox potential (Eh) and pH, have been conducted. Radioiodine tracer (1251), added to the soil, was readily sorbed on it. At the beginning of the waterlogging, the iodine desorption was low. However, iodine was desorbed into soil solution with time. The iodine desorption was enhanced markedly by the addition of organic substances such as straw pieces and glucose to the soil. Cultivation of rice plants in soil also affected the iodine desorption, suggesting root exudates and/or root autolysis might be participating in the desorption process. Eh dropped considerably after soil was waterlogged due to microbial metabolisms. Particularly low Eh values were observed in soils with plants and also with added organic substances. A negative correlation was seen between the desorption and soil Eh. High desorption was frequently observed when the Eh dropped to about -100 mV or below. Due to the reducing conditions (low Eh) by waterlogging, iodine in soil was leached into the soil solution; consequently total iodine concentration in paddy soil was considerably lower than forest and upland field soils. These iodine desorption phenomena under anaerobic conditions should be considered in assessing transfer of the long-lived radioiodine (129I) in the environment, especially in rice fields and marshland.     

Effects of soil redox potential (Eh) and pH on growth of sunflower and wheat

ABSTRACT

Soil redox potential (Eh) and pH are fundamentals parameters for plants growth. Measuring soil Eh is essential but complex due to the lack of measurement reliability resulting from high temporal variability and metrological challenges. This paper proposes practical advancements for measuring Eh in aerobic soils using combined electrodes (improvements in methodology specific to cleaning electrodes, measurement time, cleaning the dataset). The study of soil Eh and pH on sunflower and wheat in pot experiments has highlighted the relationship between Eh_cumul (Eh associated with a dimension of time) and the portion of porosity that is accessible to air. For reduced soil conditions, sunflower positively reacts to better aeration. Strong correlations exist between the duration of each potential range and the growth of sunflower. The study of sunflower growth in soil reveals extremely harmful impact resulting from too high and/or too low Eh values.     

Arsenic biogeochemistry as affected by phosphorus fertilizer addition,redox potential and pH in a west Bengal (India) soil

This laboratory experiment systematically examines arsenic, iron, and phosphorus solubilities in soil suspensions as affected by addition of phosphorus fertilizer under different redox potential (Eh) and pH conditions. Under aerobic conditions, As solubility was low, however, under moderately reducing conditions (0, − 150 mV), As solubility significantly increased due to dissolution of iron oxy-hydroxides. Upon reduction to − 250 mV, As solubility was controlled by the formation of insoluble sulfides, and as a result soluble As contents significantly decreased. Soluble Fe concentration increased from moderate to highly anaerobic conditions; however, it decreased under aerobic conditions likely due to formation of insoluble oxy-hydroxides. A low pH, 5.5, led to increased soluble concentrations of As, Fe, and P. Finally, addition of P-fertilizers resulted in higher soluble P and As, even though the concentration of As did not increased after an addition rate of 600 mg P kg^− 1 soil.     

Nitrate stability in loess soils under anaerobic conditions—laboratory studies

The objective of this laboratory study with six loess soils (three Eutric CambisoIs and three Haplic Phaeozems) incubated under flooded conditions was to examine the effect of a wide range of NO doses under anaerobic conditions on soil redox potential and N₂O emission or absorption. Due to the fact that loess soils are usually well‐drained and are expected to be absorbers during prevailing part of the season, the study aimed at determination of the conditions decisive for the transition from emission to absorption process. On the basis of the response to soil nitrate level, the two groups of soils were distinguished with high and low denitrification capacity. The soil denitrification activity showed Michaelis‐Menten kinetics with respect to soil nitrate content with K_M in the range 50–100 mg NO ‐N kg^–1. Percentage of nitrates converted to N₂O increased linearly with nitrate concentration in the range from 25 to 100 mg NO ‐N kg^–1 up to 43% and decreased linearly at higher concentrations reaching practically zero at concentrations about 600 mg NO ‐N kg^–1. No denitrification was observed below 25 mg NO ‐N kg^–1. Nitrous oxide absorption in soil occurred only at nitrate concentrations to 100 mg NO ‐N kg^–1 and in this concentration range was proportional to the denitrification rate. Nitrous oxide was formed at redox potentials below +200 mV and started to disappear at negative E_h values.     

不同土壤的还原状况对铁镉形态转化和水稻吸收的影响

采用土壤-蛭石联合培养,以填充蛭石的网袋模拟根际,置于红壤、水稻土、盐土中后淹水栽培水稻13 d.试验结果表明,水稻栽培期问,红壤、水稻土、盐土pH变化范围分别为6.05 ～6.78、6.47 ～7.33、6.42 ～7.44;有机质处理下,除红壤根际pH明显升高外,其余土壤根际和非根际pH均有所下降.各土壤对照根际Eh保持在233 ～ 385 mV;有机质处理使根际Eh下降,同时也导致除盐土外的非根际Eh上升.土壤还原溶解Fe与蛭石吸附Fe的90％以上均米自铁锰氧化物结合态铁(Oxide-Fe)组分,与溶液Eh、pe+ pH均有显著相关性,表明两表面同为Fe的氧化还原反应,但方向相反.水稻根表Fe膜的形成与根际氧化还原状况有关,在对照根际(高Eh)环境下,根表Fe含量随pH升高而降低,在有机质处理根际(低Eh)环境下则随pH升高而升高;在红壤中,根表Fe膜阻碍Fe的吸收,在水稻土和盐土中,根表Fe膜促进Fe吸收.根表Cd含量与根内Cd、地上部Cd有显著正相关;在红壤中,根表Fe膜阻碍了水稻Cd的吸附和吸收;水稻土和盐土中,根表Fe膜促进了水稻Cd的吸附和吸收.     

Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O₂) supply and the related redox potential (E_H) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O₂ concentration and E_H is poorly documented under aerobic soil conditions. We investigated how far O₂ consumption of roots and microorganisms in the rhizosphere is replenished by O₂ diffusion as a function of water/air‐filled porosity. Oxygen concentration and E_H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E_H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O₂ profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O₂ concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O₂ transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O₂ transport was limited, causing a decline in oxygen partial pressures (pO₂) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O₂ increased considerably. This was confirmed by E_H around 300 mV under aerated conditions, while E_H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO₂ and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E_H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO₂) and is strongly dependent on soil moisture.     

Influence of soil redox potential on nitrogen uptake and growth of wetland Oak seedlings

A study was conducted to determine ammonium nitrogen uptake by cherrybark oak and overcup oak as affected by soil redox conditions. Seedlings of the two species were grown in laboratory microcosms for 60 days in soil suspensions incubated at three separate Eh levels (+560, +340, and + 175 mV). In both species, uptake of added ¹⁵N labeled nitrogen (N) (NH₄Cl) and photosynthesis activity decreased with decreasing soil redox potentials. Net photosynthesis in cherrybark oak decreased by 72.9 and 100% in response to the +340 and + 175 mV treatments as compared to control plants, respectively. Similarly, net photosynthetic rates were reduced by 53.7 and 65.8% in overcup oak under + 340 and + 175 mV treatments, respectively. A significant correlation between net photosynthesis and leaf ¹⁵N content was found in cherrybark oak (r=0.866, P=0.01) but not in overcup oak (r=0.648, p>0.05). However, in overcup oak, the correlation between net photosynthesis and total leaf N content was significant (r=0.911, P=0.01). Maximum ¹⁵N uptake was measured at the highest soil redox level (+560 mV). Uptake of ¹⁵N in cherrybark oak decreased from a maximum of 1.83 mg under aerated (+560 mV) treatment to 0.11 mg under reduced conditions (+175 mV). Similarly, ¹⁵N uptake in overcup oak decreased from 2.37 mg under the + 560 mV treatment to 0.28 mg under + 175 mV treatment. Greater uptake of soil and fertilizer nitrogen was measured under the soil redox conditions in which adequate plant growth was recorded. Moderately reducing soil redox conditions (+340 and + 175 mV) adversely affected both plant nitrogen uptake and net photosynthetic activities of the two species.     

Noncontinuous Development of Reducing Conditions in Wetland Soils

Abstract

Soil aeration status in relation to water table was analyzed by contour mapping in a forested wetland, an emergent marsh, and an irrigated rice field. Two soil aeration status indicators, oxygen (O₂) and redox potential (Eh), showed a significant correlation (P<0.01). Soil O₂ and Eh levels generally decreased with the water table rise in the forested wetland and marsh, but with a noncontinuous pattern. The results indicate that soil aeration status could be temporally improved at an optimum water table level, probably due to O₂ transport by wetland plants. The soil Eh in the rice fields clearly showed a seasonal pattern regardless of the water tables.     

Effect of Palygorskite Clay,Fertilizers, and Lime on the Degradation of Oil Products in Oligotrophic Peat Soil under Laboratory Experimental Conditions

The effect of native palygorskite clay and that modified with dodecyltrimethylammonium chloride on the degradation of oil products in an oligotrophic peat soil under complete flooding at the application of lime and mineral fertilizers has been studied under laboratory conditions. It has been shown that the incubation of oil-contaminated soil with unmodified clay and fertilizers at the application of lime under complete flooding with water affects the dynamics of pH and Eh and slows the development of reducing conditions compared to the use of clay without fertilizers. The addition of organoclay under similar conditions favors the formation of potential-determining system with a high redox capacity, which is capable of retaining the potential on a level of 100–200 mV at pH ～ 7 for two months. It has been found that, under the experimental conditions, unmodified and modified clay, which has no toxic effect on the bacterial complex, does not increase the biodegradation efficiency of oil products in the oligotrophic peat soil compared to the experimental treatments without clay addition. Possible reasons for no positive effect of palygorskite clay on the biodegradation rate of oil products under experimental conditions have been analyzed.     

Production of dinitrogen and nitrous oxide in soil suspensions as affected by redox potential

The effect of soil redox potential on N₂O and N₂ emission from soil suspensions was studied under laboratory conditions. Crowley silt loam soil suspensions were equilibrated under controlled (?200, ?100, 0, +100, +200, +300, and +400 mV) redox levels, and the amounts of N₂ and N₂O evolved quantified. At higher redox levels (+300, and +400 mV) nitrification was the dominant soil biological process controlling N chemistry. A small amount of N₂O evolved during nitrification. A redox value between +300 and +200 mV was found critical for denitrification to occur. Both N₂ and N₂O were produced during denitrification. The maximum amount of N₂O evolved at a redox value of 0 mV. Dinitrogen emission increased at lower redox levels. The highest N₂/N₂O evolution ratio was observed at ?200 mV and the ratio decreased with increasing redox. A lack of N-balance during denitrification at redox levels of +100, and +200 mV is also reported.     

Influence of pH on the redox chemistry of metal (hydr)oxides and organic matter in paddy soils

Purpose

The primary purpose of this study was to determine how flooding and draining cycles affect the redox chemistry of metal (hydr)oxides and organic matter in paddy soils and how the pH influences these processes. Our secondary purpose was to determine to what extent a geochemical thermodynamic equilibrium model can be used to predict the solubility of Mn and Fe during flooding and draining cycles in paddy soils.

Material and methods

We performed a carefully designed column experiment with two paddy soils with similar soil properties but contrasting pH. We monitored the redox potential (Eh) continuously and took soil solution samples regularly at four depths along the soil profile during two successive flooding and drainage cycles. To determine dominant mineral phases of Mn and Fe under equilibrium conditions, stability diagrams of Mn and Fe were constructed as a function of Eh and pH. Geochemical equilibrium model calculations were performed to identify Mn and Fe solubility-controlling minerals and to compare predicted total dissolved concentrations with their measured values.

Results and discussion

Flooding led to strong Eh gradients in the columns of both soils. In the acidic soil, pH increased with decreasing Eh and vice versa, whereas pH in the alkaline soil was buffered by CaCO₃. In the acidic soil, Mn and Fe solubility increased during flooding due to reductive dissolution of their (hydr)oxides and decreased during drainage because of re-oxidation. In the alkaline soil, Mn and Fe solubility did not increase during flooding due to Mn(II) and Fe(II) precipitation as MnCO₃, FeCO₃, and FeS. The predicted levels of soluble Mn and Fe in the acidic soil were much higher than their measured values, but predictions and measurements were rather similar in the alkaline soil. This difference is likely due to kinetically limited reductive dissolution of Mn and Fe (hydr)oxides in the acidic soil. During flooding, the solubility of dissolved organic matter increased in both soils, probably because of reductive dissolution of Fe (hydr)oxides and the observed increase in pH.

Conclusions

Under alternating flooding and draining conditions, the pH greatly affected Mn and Fe solubility via influencing either reductive dissolution or carbonate formation. Comparison between measurements and geochemical equilibrium model predictions revealed that reductive dissolution of Mn and Fe (hydr)oxides was kinetically limited in the acidic soil. Therefore, when applying such models to systems with changing redox conditions, such rate-limiting reactions should be parameterized and implemented to enable more accurate predictions of Mn and Fe solubility.     

Influence of nitrate on methane production and oxidation in flooded soil

Abstract

Methane (CH₄) production in paddy soils and sediments as influenced by nitrate (NO₃) addition was studied under a closely controlled soil pH and oxidation‐reduction (redox) potential (Eh) conditions. CH₄ production was affected by soil pH and NO₃ ^‐. Added NO₃ ^‐ reduced the amount of CH₄ produced of each pH level studied. Nitrate addition primary effect in reducing CH₄ production was through the resultant increase in soil redox potential (Eh). Using Methyl Fluoride (MF), a CH₄ oxidation inhibitor we found that added NO₃ ^‐ was not used in CH₄ oxidation by methanotrophic bacteria.