Construction and evaluation of a reference electrode assembly for use in monitoring in situ soil redox potentials

Abstract

A simple procedure for the construction of inexpensive Ag,AgCl reference electrode assemblies is described. Disposable plastic syringes (60‐cm) were used to form the bodies of the electrode assemblies and small Ag,AgCl internal reference elements were made from silver wire. The internal reference elements were immersed in a solution of AgCl‐saturated KCl, which also served as the salt bridge. Contact between the salt bridge and test solution or soil was made through a ceramic microtensiometer cup. This procedure yields reference electrode assemblies that perform as well as commercially available reference electrodes in terms of their stability and reproducibility. Because of their rugged construction and ease of maintenance, these reference electrode assemblies are ideal for use in the field. Moreover, they proved to be particularly useful when in situ soil redox potentials were monitored for 84 h.     

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.     

Salt Tolerance of Seedling and Adult Bread Wheat Plants Based on Ion Contents and Agronomic Traits

Abstract

A greenhouse experiment was conducted on two salt‐tolerant, two moderately tolerant, and two sensitive Iranian and exotic bread wheat cultivars and their F₁ generations to investigate the effect of salt stress on ion contents of young leaves, biomass yield, and salt stress tolerance index. The materials were evaluated in gravel culture under high salinity (EC=22.5 dSm^?1) and nonstress (EC=2.0 dSm^?1) conditions. Results of stress intensity showed that K⁺/Na⁺ ratio, biomass yield, and Na⁺ concentration were most affected by salt stress. There was no genetic relationship between Mg²⁺ and Ca²⁺ contents with salt tolerance. However, strong relationships were observed among K⁺/Na⁺ ratio, biomass yield, and stress tolerance index. Factor analysis revealed four factors, which explained 99.79% of the total variation among characters. Three‐dimensional plots based on the first three factor scores confirmed that the most salt‐tolerant cultivar was Roshan (an old Iranian cultivar), and Roshan×Alvand and Kharchia×Roshan and their reciprocal crosses were the best salt‐tolerant crosses.     

Response of lime seedling growth and chemical composition to salinity stress

Abstract

Responses of lime growth and chemical composition to salinity stress were evaluated on six‐month‐old lime seedlings raised under six salt levels with electrical conductivities (EC) ranging from 0.56–20 d‐Sm^‐1 together with Soba natural saline soil with an intrinsic salinity of 7.2 d‐Sm^‐1. Plantheight, number of leaves and branches, moisture content, and chlorophyll and nitrogen (N) contents were progressively reduced by high salt levels while leaf tip burn increased. Percentage total sugars (starch + soluble sugars) in leaf shoot and root tissues were higher in plants raised at higher (EC=16 and 20 d‐Sm^‐1) salinity level and Soba saline soil treatments than in control plants or plants raised at low to intermediate salinity levels. Accumulated carbohydrates in the tissues of the salt‐stressed lime plants were viewed as compatible solutes needed for osmoregulation.     

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.     

Evaluation of Continuous In Situ Monitoring of Soil Changes with Varying Flooding Regimes

Abstract

To support investigations of flood tolerance occurring at a field‐based research facility, changes in soil volumetric water content, temperature, redox potential, dissolved oxygen content, and pH over the course of flood events were monitored. Electronic sensors connected to dataloggers for continuous monitoring of these parameters were installed, and soil redox potential and pH were also monitored manually for comparison. Soil volumetric water content showed that soils became saturated quickly following inundation. Soil redox potentials revealed a reduction of the soil with inundation; however, stagnant water treatments did not result in lower redox potentials than flowing water treatments. Similarly, dissolved oxygen content was not lower in the stagnant water treatment. The automated and manual systems detected similar trends in redox potential response to flooding; however, redox potentials measured manually were generally higher and significantly different from those obtained with the automated system. Anomalous readings were detected with the automated measurement of soil pH, indicating further need for improvement of this system.     

Redox characterization of humins in sediments from the Yangtze Estuary to the East China Sea and their effects on microbial redox reactions

Purpose

Humins, as solid-phase redox mediators, are important for enhancing anaerobic biotransformation processes. Although humins are broadly distributed in estuarine and marine sediments, to date, their electron transfer capacity (ETC) from marine sediments has been poorly characterized. This study determined the potential of humins as redox mediators for microbial redox metabolic reactions in sediments along a transect from the Yangtze Estuary to the adjacent East China Sea and analyzed their electron-mediating characteristics.

Materials and methods

Comparative analyses of humins in sediments along this transect were conducted to investigate their ETC as redox mediators for microbial redox metabolic reactions, namely dissimilatory iron reduction (DIR) and dissimilatory nitrate reduction to ammonium (DNRA), using Shewanella oneidensis MR-1. Elemental composition, electrochemical, and Fourier transform infrared analyses were performed to better understand the functional redox groups in humins.

Results and discussion

All the humins functioned as solid-phase redox mediators and enhanced DIR and DNRA by Shewanella oneidensis, although with varying efficacy. The humins extracted from sampling site P1 (P1-HM), located close to the Yangtze Estuary, most effectively enhanced iron and nitrate reduction without nitrite accumulation. Electrochemical analysis confirmed the highest redox activity and ETC in P1-HM and a weakening trend of ETC with increasing distance from the estuary. Fourier transform infrared analysis suggested that quinone moieties might serve as key redox functional groups in humins.

Conclusions

Considering their high yield and ETC, P1-HM could be important redox mediators that influence microbial redox metabolic reactions in sedimentary ecosystems of the Yangtze Estuary. Thus, this study provides insights into the distribution of humins, their ECT, and their potential role as redox mediators for microbial redox reactions, which could improve understanding of elemental biogeochemical cycling processes involving microorganisms in the Yangtze Estuary and East China Sea sedimentary ecosystems and provide important reference information for researches on other estuary areas.

     

Effects of sodium chloride treatments on growth and ion accumulation of the halophyte Haloxylon recurvum

Abstract

Effects of increasing salt concentrations 0, 180, 360 mol im³ sodium chloride (NaCl), on growth, succulence, mineral composition, and glycinebetaine content in Haloxylon recurvum was investigated. Fresh and dry weight of plants increased with an increase in salinity. Succulence of shoots increased at low salinity and decreased at high salinity. Root and shoot Ca⁺, Mg⁺, and K⁺content decreased with increasing salinity while both Na⁺ and Cl^‐ content increased, reaching 4,900 and 5,300 mmol kg^‐1 dry weight, respectively. Glycinebetaine (mol m^‐3 tissue water) significantly increased in shoots at 360 mol m^‐3 NaCl, but did not differ significantly in roots treated with from 0 to 360 mol m^‐3 NaCl. Haloxylon recurvum is a highly salt tolerant stem succulent plant which accumulate a high quantity of salt, which makes it a good candidate to use for phytoremediation in highly saline areas of the sub‐tropics.     

Relationship between ammonium acetate‐acetic acid and morgan's solution for determining extractable P,K, Ca,and Mg in soils derived from calcareous glacial till

Abstract

Long filtration time, accumulation of salt residue on the atomic absorption or flame emission burner slot, and contamination of P and K in the sodium acetate are three problems encountered when using sodium acetate‐acetic acid or Morgan's solution (MS) for extracting P, K, Ca, and Mg from soil samples. Thus an ammonium acetate‐acetic acid (AA‐AA) solution was compared with MS for 144 soil samples which, although ranging in pH from 4.4 to 7.9, were all derived from calcareous glacial till. A highly significant linear correlation was obtained between the two methods for each of the four elements. Filtration time for the AA‐AA solution was 30–45 minutes shorter than that for MS. No salt residue accumulated on the atomic absorption burner slot when using AA‐AA. Because of these advantages, we have replaced MS with AA‐AA for samples analyzed in our laboratory.     

Iron and manganese transformation in louisiana salt and brackish marsh sediment

Abstract

The kinetics of the release of dissolved iron [Fe(II)], manganese [Mn(II)], and phosphate in salt and brackish marsh sediment and the exchange with the overlying water column were investigated. Sediment was incubated in laboratory microcosms and in sediment water columns in studying these exchanges. The rate constants of the dissolved Fe(II), Mn(II), and phosphate release in sediment suspensions were 2.02–2.28,0.08–0.117, and 4.18–5.38 μmol g^‐1 dry sediment d^‐1, respectively. In sediment‐water column studies, the rate constants (K) of dissolved Fe(II), Mn(II), and phosphate removal from the overlying water into the sediment were 0.755–0.989,0.0695–0.0949, and 0.315–0.448, day^‐1, respectively. The flux of dissolved Fe(II), Mn(II), and phosphates from the salt and brackish marsh sediment to the overlying water in the column studies were 2.56–4.93,1.05–1.689, and 208.6–428.9 mg m^‐2 d^‐1, respectively. The fluxes from salt marsh were slightly greater than those measured in brackish marsh, although these differences were not statistically significant.     

Effect of salinity on metalloenzymes of oxygen metabolism in two leguminous plants

Abstract

Superoxide dismutase (SOD) pattern, catalase, Cyt c oxidase and fumarase activity were studied in leaves of Phaseolus vulgaris and Vigna unguiculata plants growth in two sodium chloride (NaCl) concentrations (35 mM and 100 mM). In bean plants growth with NaCl, leaf chloride (Cl^?) contents were higher than in control plants, and the same was found for sodium (Na⁺) and potassium (K⁺) contents, although to a lesser degree. In cowpea leaves, Na⁺ and Cl^? had a similar increase due to salt‐growth conditions. Under salinity, all changes in the antioxidant (SOD and catalase) enzymes levels were smaller in bean than in cowpea plants. In Phaseolus at 15 days growth, Cu, Zn‐SOD I showed an increase by the effect of salt treatment, but this induction did not occur at 30 days growth, and both Mn‐SOD and Cu, Zn‐SOD II did not show variations due to salt‐stress. In Vigna, Mn‐SOD was decreased by salinity but this was compensated by an increase in Cu, Zn‐SOD I activity in plants at 30 days growth, whereas in young leaves under saline conditions, both isozymes were also decreased. Likewise, there was a rise in cytochrome c oxidase and fumarase activity in leaves of NaCl‐treated plants compared to the control. The activity changes observed are discused in term of their possible relevance to plant sensitivity to saline conditions.     

Determining soil and water sodicity from electrode measurements

Abstract

A method is presented for estimating the sodium adsorption ratio (SAR) of the saturated soil‐paste extract from three electrode measurements made directly in the saturated soil‐paste. An analogous method is presented for the determination of the SAR of extracts and solutions solely from electrode measurements made in the extract or solution. Both methods are carried out without the use of pH and ionic strength buffers. The methods were tested on a widespread range of salt‐affected soils and their extracts; they are deemed suitable for field applications in the diagnosis, screening, and classification of sodic soils and waters.     

Influence of induced salinity and sodicity on manganese sorption in vertisols and aridisols

Abstract

Salinity and sodicity effects on manganese (Mn) sorption in a mixed sodium‐calcium (Na‐Ca) soil system were studied. Soil samples were taken at 0–30 cm depth from Vertisols (El‐Hosh and El‐Suleimi) and Aridisols (El‐Laota) at three sites in Gezira scheme (Sudan). No Mn was applied to these soils. Prior to analysis the soils were equilibrated with NaCl‐CaCL₂ mixed salt solutions to attain SAR values at different salt concentrations. The results indicated that saline soils sorbed less Mn and had higher equilibrium Mn concentrations. Sodic soils retained more Mn but had low equilibrium concentrations. Sodicity had a pronounced effect only on increasing Mn retention at higher SAR values. Salinity tended to alleviate sodicity effects on Mn retention, but soluble salts that increased soil pH decreased Mn concentration.     

Determination of exchangeable potassium in soil using ion-selective electrodes in soil suspensions

Summary Methods of determining exchangeable K⁺ of soil by mixing extracting solutions and analysing the soil suspension with ion‐selective electrodes were developed and evaluated on 30 samples of soils. From preliminary comparisons of the K⁺ extracted by BaCl₂ and NH₄OAc solutions and by batch and leaching treatments of soils, we established that suspensions of 5 g soil in 100 ml 0.5 m BaCl₂ and single batch treatments of 1 h should be used. The exchangeable K⁺ was determined with a K‐selective, valinomycin‐based PVC membrane electrode and electrochemical cells that did or did not include a liquid junction (the reference electrode being a double‐junction reference electrode assembly with a 10 m LiOAc salt bridge solution or a Cl‐selective electrode, respectively). The Ba‐exchangeable K⁺ values were sensibly the same whether a liquid junction was involved or not, a result that can be attributed to the beneficial effects of the salt bridge and the ionic strength of the extractant. Comparisons of these Ba‐exchangeable results with those obtained by various combinations of batch or leaching treatments, BaCl₂ or NH₄OAc extractants and filtrate analysis by the ion‐selective electrode method or atomic absorption spectrometry showed they were all highly correlated (r≥ 0.996). The selectivity of the K⁺‐selective electrode (k^pot_KNH4 = 0.004) significantly reduced the interference from indigenous soil NH₄⁺ in the BaCl₂ suspensions. Overall, the results show potentiometric measurements of K⁺ in soil suspensions can provide a simple, rapid, and reliable means of determining exchangeable K⁺ in soils.     

A field experiment on the influence of the postulated global climatic change on coastal marshland soils

A climatic scenario postulates an increase of atmospheric CO₂, 55 cm water‐level rise combined with a 30 cm higher tidal amplitude for the North Sea. Further, more frequent and stronger west storms, a 2.5°C mean‐annual‐temperature increase together with 15% more rainfall during winter time are to be expected until 2050 due to recent human impact. The possible consequences of the postulated changes in temperature and flooding dynamics were studied for soil formation as well as for soil‐ecological attributes and processes of two sites at Hedwigenkoog foreshore in N Germany: a Hypersali‐tidalic Fluvisol (protothionic, calcaric) (1.5 m asl with >500 annual floods) and a Gleyi‐sali‐tidalic Fluvisol (calcaric) under a salt meadow (2.2 m asl with 70 annual floods). Temperature, water, redox, nutrient and gas dynamics were measured at different soil depths for a period of 2 y. The topsoil of the Gleyi‐sali‐tidalic Fluvisol was permanently heated over an area of 14 m² to simulate a temperature rise of 1.5–2.5°C. The results are discussed in relation to two realistic scenarios. If the ground level of the Gleyi‐sali‐tidalic Fluvisol is heaved up (due to sedimentation) as intensively as the sea level rises, it may be assumed that the soil unit including vegetation cover in principle will not noticeably change. Then, the temperature increase will intensify the activity of soil organisms, bioturbation, and formation of a crumb structure, however, there will be no further accumulation of soil humus. If there was no sedimentation during the next 50 y, the salt meadow itself together with the upper part of the Gleyi‐sali‐tidalic Fluvisol would be destroyed by erosion, and a Hypersali‐tidalic Fluvisol would be formed similar to the studied one.     

Effects of equilibrating salt solutions on phosphate sorption by soils

Abstract

Several equilibrating salt solutions have been used in the studies of P sorption by soils and sediments. This study was conducted to evaluate the effects of 10 salt solutions on estimation of P sorption by soils. Results obtained showed that, when the equilibrating solution was made to contain 0.01M with respect to CaCl₂, Ca(NO₃)₂, CaSO₄, MgCl₂, KCl, LiCl, Nacl, or KHCO₃, the amount of P sorbed by soil always exceeded the amount sorbed from the soil‐water system. In comparison with the amount of P sorbed from water, 0.01M NaHCO₃ reduced P sorption by soils. Use of THAM buffer (0.05M pH 7.0) to control the pH increased P sorption by some soils and decreased P sorption by others, relative to that sorbed from the soil‐water system. The results indicated that inclusion of salts in the equilibrating solution for P‐sorption studies should be avoided, especially in studies related to water quality.     

New method of phosphorus determination with improved pretreatment of occluded iron phosphate fraction in soils

Abstract

In the sequential fractionation of phosphorus (P), a modified approach in the oxidation process of the occluded fraction of P (in iron oxides in soils) using redox titration in nitric acid (HNO₃) medium is presented. Also a new spectrophotometric method to determine that P fraction as the phosphomolybdate complex using a mixture of isobutyl acetate and methyl isobutyl ketone to extract the formed complex into the organic phase. Radioactive tracer (³²P) and sequential extraction is used to state the yield of extraction of the phosphomolybdate complex. Accuracy, precision, detection limit, and the linearity of the present method are 1.04%, 0.0322 μg mL^‐1, 0.0038 ug mL^‐1, and 0–15 ug mL^‐1, respectively. The results correlate significantly with the results of the standard method of Petersen and Corey (1966), and the procedure saves time (50 min) and chemicals.     

The use of dialysis cells for investigating pore water composition in wetland substrata,with particular reference to dissolved iron and sulphide

Abstract

Details and some evaluation are given of the use of a dialysis method for sampling pore water from depth in wetland substrata (peats), with particular reference to measurement of concentrations of ion species with stabilities dependent on redox potential. The method is based upon the burial and subsequent retrieval (after equilibration) of cells made from dialysis membrane filled with deionized water. Preliminary results of field investigations of concentrations of dissolved iron and sulphide in the pore water of base‐rich mires, as sampled by this method, are given. Results suggest that the method could have very considerable application and that its potential, and possible problems, deserve further examination.     

Ion content and ion excretion of Spartina anglica in relation to salinity and redox potential of salt marsh soil

Concentrations of Na, K, Ca, Mg, Fe, Mn, and Zn were determined in polluted estuarine (Western Scheldt) and non-polluted (Eastern Scheldt) salt marsh soil, in the shoot tissue of plants of Spartina anglica and in the excretion of the salt glands of Spartina anglica. Excretion of ions by the salt glands of Spartina anglica was analysed with increasing salinity (0, 300, and 500 mM NaCl) and with increasing values of the redox potential of the salt marsh soil (from ?300 mV to +600 mV). Salt glands of Spartina anglica, growing in containers filled with salt marsh soil in the greenhouse excreted Na-ions at a rate of 1.0–1.8 mmol Na⁺ g^?1 dry wt of the shoot tissue over a period of 18 days implying that about every 6–18 days the amount of Na⁺ present in the shoot tissue is being removed by the salt glands. The excretion rate of K-ions was 0.02–0.14 mmol g^?1 dry wt/18 days. Zinc ions are excreted by the salt glands to such a rate (0.03–0.11 μmol g^?1 dry wt/18 days) that every 36–900 days the amount of zinc present in the leaves is removed. There was no relation between excretion of Zn ions by the plants and the concentration of zinc in the soil. The excretion of Fe and Mn is reported and was found to be related to increasing values of the redox potential.     

Intra‐specific variation for salt tolerance in a potential oil‐seed crop,brown mustard (Brassica juncea (l.) Czern. and Coss.)

Thirty eight accessions of brown mustard (Brassica juncea (L.) Czern. and Coss.) were screened after two weeks growth in solution culture containing 120 mol m‐³ NaCl. Considerable variation for salt tolerance was observed in this set of germplasm, since some accessions showed relatively vigorous growth in saline medium.

In order to determine the consistency of degree of salt tolerance at different growth stages of crop life cycle two salt tolerant accessions, P‐15 and KS‐51 and two salt sensitive 85362 and 85605 were tested at the adult stage in 0(control), 100 and 200 mol m^‐3 NaCl. Both the tolerant accessions produced significantly greater fresh and dry biomass and had considerably higher seed yield than those of the salt sensitive accessions. Analysis of different ions in the leaves showed that salt tolerant accessions contained greater amounts of Na⁺, K⁺ and Ca²⁺ than the salt sensitive accessions, although they did not differ significantly for leaf Cl^‐. Only one salt tolerant accession P‐15 had greater leaf K/Na ratio and K⁺ versus Na⁺ selectivity compared with the tolerant KS‐51 and the two salt sensitive accessions.

From this study it was established that there is a considerable variation for salt tolerance in B.juncea which can be exploited by selection and breeding for improvement of its salt tolerance. Since the degree of salt tolerance in B.juncea does not change at different growth stages of the crop life cycle, selection for salt tolerance at the initial growth stages could provide individuals that would be tolerant at all other growth stages. Accumulation of Na⁺, K⁺ and Ca²⁺ in the leaves are important components of salt tolerance in B.juncea.