Changes In Soil Redox Potential Research Articles

Reducing conditions increased the mobilisation and hazardous effects of arsenic in a highly contaminated gold mine spoil

Arsenic (As) redox-induced mobilisation and speciation in polluted gold mine sites in tropical climates largely remains unknown. Here, we investigated the impact of changes in soil redox potential (EH) (−54 mV to +429 mV) on mobilisation of As and its dominant species in an abandoned spoil (total As = 4283 mg/kg) using an automated biogeochemical microcosm set-up. Arsenic mobilisation increased (85–137 mg/L) at moderately reducing conditions (−54 mV to + 200 mV)), while its reduced (6–35 mg/L) under oxic conditions (+200 to +400 mV). This indicates the high risk of As potential loss under reducing conditions. The mobilisation of As was governed by the redox chemistry of Fe. XANES and EXAFS analyses showed that sorbed-As(V)-goethite, sorbed-As(III)-ferrihydrite, scorodite and arsenopyrite were the predominant As species in the mine spoil. As(V) dominated at oxic conditions and As(III) predominated at moderately reducing conditions, which may be attributed to either inability of arsenate bacteria to reduce As or incomplete reduction. Lower Fe/As molar ratios during moderately reducing conditions show that the mine spoil may migrate As to watercourses during flooding, which may increase the hazardous effects of this toxic element. Therefore, encouraging aerobic conditions may mitigate As release and potential loss from the mine field.

Stable-Isotope-Aided Investigation of the Effect of Redox Potential on Nitrous Oxide Emissions as Affected by Water Status and N Fertilization

Soils are the dominant source of atmospheric nitrous oxide (N2O), especially agricultural soils that experience both waterlogging and intensive nitrogen fertilization. However, soil heterogeneity and the irregular occurrence of hydrological events hamper the prediction of the temporal and spatial dynamics of N2O production and transport in soils. Because soil moisture influences soil redox potential, and as soil N cycling processes are redox-sensitive, redox potential measurements could help us to better understand and predict soil N cycling and N2O emissions. Despite its importance, only a few studies have investigated the control of redox potential on N2Oemission from soils in detail. This study aimed to partition the different microbial processes involved in N2O production (nitrification and denitrification) by using redox measurements combined with isotope analysis at natural abundance and 15N-enriched. To this end, we performed long-term laboratory lysimeter experiments to mimic common agricultural irrigation and fertilization procedures. In addition, we used isotope analysis to characterize the distribution and partitioning of N2O sources and explored the 15N-N2O site preference to further constrain N2O microbial processes. We found that irrigation, saturation, and drainage induced changes in soil redox potential, which were closely related to changes in N2O emission from the soil as well as to changes in the vertical concentration profiles of dissolved N2O, nitrate (NO3−) and ammonium (NH4+). The results showed that the redox potential could be used as an indicator for NH4+, NO3−, and N2O production and consumption processes along the soil profile. For example, after a longer saturation period of unfertilized soil, the NO3− concentration was linearly correlated with the average redox values at the different depths (R2 = 0.81). During the transition from saturation to drainage, but before fertilization, the soil showed an increase in N2O emissions, which originated mainly from nitrification as indicated by the isotopic signatures of N2O (δ15N bulk, δ18O and 15N-N2O site preference). After fertilization, N2O still mainly originated from nitrification at the beginning, also indicated by high redox potential and the increase of dissolved NO3−. Denitrification mainly occurred during the last saturation period, deduced from the simultaneous 15N isotope analysis of NO3− and N2O. Our findings suggest that redox potential measurements provide suitable information for improving the prediction of soil N2O emissions and the distribution of mineral N species along the soil profile under different hydrological and fertilization regimes.

Interpreting redox potential (Eh) and diffusive fluxes of phosphorus (P) and nitrate (NO3−) from commercial rice grown on histosols

In South Florida, approximately 11,000 ha of rice is grown every summer on highly organic histosols. With no added fertilizer inputs of phosphorus (P) or nitrogen (N) as part of the cultivation program, growing flooded rice has the potential to reduce nutrient fluxes from soils to surface water and limit its losses to the water column. Changes in soil redox potential (Eh) will play an important role in the availability of P and nitrate (NO3−) concentration within the groundwater, which ultimately drives the direction of diffusive fluxes across the soil–water interface. The objective of this study was to measure vertical Eh and groundwater concentrations of P and NO3− from commercial flooded rice fields and estimate diffusive fluxes across the soil–water interface. Results indicate that Eh was depth dependent, where deeper soils (30 cm deep) were slightly more anaerobic (mean − 162 mV) than shallow soils (15 cm deep) (mean − 144 mV). Groundwater P concentrations ranged between 0.38 and 1.09 mg L−1 in deep soils (up to 60 cm) and 0.018–0.608 mg L−1 in shallow soils (up to 40 cm). Groundwater NO3− concentrations ranged between 0.065 and 0.64 mg L−1 in deep soils (up to 60 cm) and 0.055–0.499 mg L−1 in shallow soils (up to 40 cm). Diffusive fluxes were mostly positive, which meant they flowed from shallow groundwater to the water column. Phosphorus fluxes were as high as 0.134 mg m−2 d−1 observed in 60-cm deep soils, whereas NO3− fluxes were as high as 0.12 mg m−2 d−1 observed in 40-cm shallow soils. Diffusive fluxes of P and NO3− from flooded rice fields were relatively lower compared to other hydrologic systems.

The effect of the ferrihydrite dissolution/transformation process on mobility of arsenic in soils: Investigated by coupling a two-step sequential extraction with the diffusive gradient in the thin films (DGT) technique

Ferrihydrite has been prevalently used in soil remediation as an effective amendment for in situ immobilization of arsenic (As). However, its poorly crystalline structure is unstable and can pose the risk of re-releasing As into soil. In this study, sequential extraction was coupled with a newly-established method, chelex-metsorb diffusive gradient in thin films (DGT), to study the ferrihydrite transformation/dissolution process and its effect on the mobility of arsenic in soil. Experiments in this work found that high soil moisture (70% SWHC) with a low soil redox potential (Eh) can significantly increase the rate of the ferrihydrite transformation/dissolution process compared to 30% SWHC. Soils with low pH and high available iron (Fe) content may also accelerate ferrihydrite transformation/dissolution, while soils with high clay fraction and high soil total organic matter (STOM) may inhibit the process. The amount of arsenic adsorption can also affect ferrihydrite transformation, even exceeding the effect of soil pH and dissolved Fe. Arsenic release was clearly observed in all three soils across all treatments, and it was also affected by changes in soil redox potential. More arsenic was released at high soil moisture (70% SWHC) roughly 7–15 d after the release of Fe. In addition, a partial arsenic fraction was transformed, along with ferrihydrite, from a combined As (F1As) amorphous phase to combined As (F2As) well-crystallized phase. These results suggested that ferrihydrite transformation/dissolution can affect the mobility of arsenic and that this phenomenon is more extreme at higher soil moisture levels.

Changes in soil redox potential in response to flood irrigation with waste water in central Mexico

SummaryIrrigation with untreated sewage water adds fresh organic matter to the soil. When it is applied by flooding, as in the Mezquital Valley of Mexico, many of the pores in the soil become temporarily waterlogged and depleted of oxygen, and reduction generates nitrous oxide (N2O) and methane (CH4). We monitored the redox potential, Eh, in the soil at two sites in the Mezquital Valley to discover whether the short‐term gaseous emissions matched the changes in Eh. One site is irrigated periodically by flooding with waste water and has alfalfa, rye grass and maize grown in succession; the other site grows maize with water from summer rain only. Each electrode buried in the soil for the purpose produced sequences of measurements that were correlated in time. We modelled the correlation and took it into account to analyse the effects of the irrigation by residual maximum likelihood (reml). After each flooding, the redox potential under alfalfa and rye grass decreased by 150–200 mV from a norm of about 450 mV for 2 days, after which it returned to its norm. The short‐term response to flooding under maize was similar, but the redox potential did not recover completely; instead there was a decrease from one irrigation event to another, in particular as a result of heavy rain in September that saturated the soil. The soil under rain‐fed maize was slower to respond, partly, we believe, because infiltration into the less aggregated clay soil and drainage from it were also slow. Irrigation with untreated sewage water caused a sharp decrease in Eh lasting 1–2 days. It seems to have depleted the soil of oxygen, causing reduction in microhabitats rich in carbon and nitrogen and generation of N2O, which was captured in the closed chambers that we had installed.Highlights Land in the Mezquital Valley of Mexico emits much greenhouse gas when irrigated with sewage water. The soil's redox potential was monitored in situ as crops were irrigated or depended solely on rain. Each irrigation caused a decrease in the soil's redox potential of 150–200 mV for 2 days. The decrease in redox potential corresponded in time with emission of nitrous oxide.

Bacterial community response to changes in soil redox potential along a moisture gradient in restored wetlands

Hydrology is greatly modified during wetland restoration, especially when restoration sites were previously under agricultural management. Fluctuations in hydrology affect soil redox potential, causing a shift in microbial metabolic pathways due to changes in nutrient and oxygen availability. We hypothesize that hydrologic variability influences microbial community composition. To evaluate the relationship between hydrology and bacterial community composition, we characterized soil redox potential and local soil factors along a hydrologic gradient at two floodplain wetland restoration sites. We collected soil samples along a gradient from upland to wetland, and monitored redox potential (in situ redox probes) and bacterial community composition (DNA fingerprinting) bi-weekly (June–October 2011). In addition, we measured soil organic matter, ammonium, nitrate, temperature, and pH along the moisture gradient. During these observations, both sites shifted from reducing to more oxidizing environments, based on changes in soil redox potential. At both sites, distinct bacterial communities were observed at each location along the moisture gradient, corresponding to spatial shifts in redox conditions. Additionally, we experimentally tested if hydrologic history constrained bacterial response to contemporary soil moisture conditions by exposing field soil collected from upland (dry) to wetland (saturated) plots at one wetland site to a range of moisture levels. Where soils originated from hydrologically variable field conditions, the experimental moisture additions resulted in distinct bacterial assemblages among moisture treatments. Our results revealed that bacterial communities originating from fluctuating hydrologic conditions in the field were more diverse and capable of greater changes in community composition in response to changes in soil moisture than bacterial communities shaped by stable, less dynamic, hydrologic conditions (i.e., more permanently wet or dry conditions). As a consequence, land use changes that alter hydrologic conditions may impact soil microbial communities more when environmental conditions were historically more stable compared to fluctuating. Wetland management aimed at restoring microbial functions should consider ways in which management can be adapted to overcome biological, physical, and chemical legacies from prior land use that may constrain restoration of microbial ecosystem services.

Barley responses to combined waterlogging and salinity stress: separating effects of oxygen deprivation and elemental toxicity.

Salinity and waterlogging are two major factors affecting crop production around the world and often occur together (e.g., salt brought to the surface by rising water tables). While the physiological and molecular mechanisms of plant responses to each of these environmental constraints are studied in detail, the mechanisms underlying plant tolerance to their combined stress are much less understood. In this study, whole-plant physiological responses to individual/combined salinity and waterlogging stresses were studied using two barley varieties grown in either vermiculite (semi-hydroponics) or sandy loam. Two weeks of combined salinity and waterlogging treatment significantly decreased plant biomass, chlorophyll content, maximal quantum efficiency of PSII and water content (WC) in both varieties, while the percentage of chlorotic and necrotic leaves and leaf sap osmolality increased. The adverse effects of the combined stresses were much stronger in the waterlogging-sensitive variety Naso Nijo. Compared with salinity stress alone, the combined stress resulted in a 2-fold increase in leaf Na+, but a 40% decrease in leaf K+ content. Importantly, the effects of the combined stress were more pronounced in sandy loam compared with vermiculite and correlated with changes in the soil redox potential and accumulation of Mn and Fe in the waterlogged soils. It is concluded that hypoxia alone is not a major factor determining differential plant growth under adverse stress conditions, and that elemental toxicities resulting from changes in soil redox potential have a major impact on genotypic differences in plant physiological and agronomical responses. These results are further discussed in the context of plant breeding for waterlogging stress tolerance.

Application of membrane technology for water treatment: Ultrasound-associated cleaning of fouled microfiltration membranes

Lead (Pb) is normally considered as a trace element in soils and sediments for geochemical study. However, the concentration of Pb in firing range soils is generally so high that it should be considered as a major element during the evaluation of the soil geochemical properties. Soil organic matter (SOM) has been reported as one of the major factors to expedite the corrosion of metallic lead (Pb) in acidic and organic-rich soils. The main impacts of SOM on the fate and transport of Pb in firing range soils lie in the following two aspects; (1) the complexation of organic matter with Pb, which has received lots of attention, and; (2) changes in soil redox potential due to the transformation of SOM and its subsequent impact on Pb speciation, which has rarely been investigated. Soils from 6 different f a n g ranges are selected for this study. These samples have been stored under a closed condition for more than 3 years. The soil moisture contents were well-retained, as all the samples were kept in closed plastic buckets. The analytical data showed that the summation of the soil total organic carbon content (TOC) and inorganic carbon contents (TIC) were consistent with soil total carbon contents (TC) measured in previous years, although the TOC and TIC contents have changed respectively after years of storage. In general, it is observed that the soil TOC decreased against an increase of TIC. The mass balance on such a transformation suggested a major conversion of organic carbon (Corg) to inorganic carbon (CO32) in the stored soils. This kind of conversion could result in a significant change in soil redox potential (oxidation), which favoured Pb carbonation. At least, the presence of SOM could greatly affect the Pb weathering process (carbonation) in f~ing range soils under anaerobic conditions.

Methane and Nitrous Oxide Emissions from a Rice Field in Relation to Soil Redox and Microbiological Processes

Paddy rice fields provide an environment for production of two important greenhouse gases, CH4 and N2O, because of variations in soil characteristics, moisture content, and microbial activity during the cropping season. Emissions of CH4 and N2O from a paddy rice field in northern China were measured in situ by static chamber technique during March to December in 1995 and 1996. Factors affecting gas emission, including soil temperature, pH, and redox potential (Eh), were measured as well. Emissions of CH4 and N2O were strongly correlated with changes in soil redox potential. Significant CH4 emission occurred only at soil redox potential lower than approximately −100 mV, while the emission of N2O was not significant below +200 mV. A significant inverse relationship between CH4 and N2O emissions was observed (r = −0.49, n = 16, 5% confidence level). The results suggest the possibility of using management practices to maintain the redox potential in a range where both N2O and CH4 emissions are low. The activities of six related bacteria groups (zymogenic bacteria, acetic acid and hydrogen‐producers, methanogens, CH4 oxidizers, and nitrifiers and denitrifiers) in the soil were also measured in an effort to explain the relationship between gas emission and soil microbiological processes. Methane emission was significantly related to the logarithm number of zymogenic bacteria (r = 0.76, n = 12, 1% confidence level), as well as to soil redox potential (r = −0.72, n = 12, 1% confidence level). Both zymogenic bacteria number and soil redox potential appear to be predicators of CH4 emission potential.

The influence of soil oxygen deficiency on alcohol dehydrogenase activity, root porosity, ethylene production and photosynthesis in Spartina patens

Laboratory experiments evaluated root-shoot responses of Spartina patens (Ait) Muhl. to changes in soil redox potential ( Eh). Three levels of soil redox potential, +460, +230 and −110 mV were imposed in microcosms where plants were grown. Leaf chlorophyll content, gas exchange, root aerenchyma formation, alcohol dehydrogenase (ADH) activity in the roots and ethylene production of leaf and roots in response to the redox treatments were measured. Root responses to hypoxia included a significantly increased porosity and greater ADH activity in hypoxic roots as compared to roots of control (aerated) plants. Ethylene production was significantly greater in leaves and roots of plants under hypoxic treatment compared to control plants. Leaf chlorophyll content was not affected by the treatments; however, stomatal conductance and net carbon assimilation were reduced significantly in response to hypoxia, by 46 and 18%, respectively. Results show a close relationship between root hypoxia, increase in ADH activity, ethylene production and aerenchyma tissue development in S. patens. The enhanced ADH activity and ethylene production found in plants subjected to hypoxia support the postulate that these metabolites have adaptive significance for plants under hypoxic conditions.

Influence of Redox Potential and Plant Species on the Uptake of Nickel and Cadmium from Soils

AbstractThe influence of soil redox potential on the uptake of nickel and cadmium from three soils was studied in a pot experiment with oat (Avena sativa L. cv. Pirol) and rye‐grass (Lolium multiflorum L. cv. Lema). The soils used were a Luvisol (naturally low in Ni and Cd), a sewage‐sludge amended Luvisol (anthropogenic enrichment of Ni and Cd) and a Cambisol (naturally high in Ni and Cd).Nickel and Cd supplied in sewage‐sludge amendments were several times more available for plant uptake than those present naturally in the soil. Short term decreases in soil redox potential, as a result of soil flooding, had little effect on the availability of Ni and Cd from sludge amended soils, but resulted in a substantial increase in Ni and Cd availability in the soil naturally high in Ni and Cd. In general, increases in soil solution concentrations of Ni and Cd, due to sludge amendment or changes in soil redox potential, resulted in increased Ni and Cd concentrations in the aerial parts of plants. Nickel concentrations in the grain of oat increased with increasing Ni concentration in the soil solution, whereas grain Cd concentrations did not. In contrast, Cd concentrations in vegetative parts of both oat and rye‐grass were strongly affected by soil type and sludge‐amendment. In all treatments Ni concentrations in the grain were much higher than in the vegetative parts, whereas Cd concentrations were always higher in vegetative parts than in the grain.Tissue Ni concentrations correlated well with DTPA extractable soil Ni but only poorly with total soil Ni. Differences in the availability and uptake of Ni and Cd in response to soil type, environmental conditions and plant species suggest that the use of total soil‐metal content to regulate the use of sewage sludge on agricultural land is inappropriate.