Se Availability Research Articles

Selenium, zinc and copper plasma levels in intrahepatic cholestasis of pregnancy, in normal pregnancies and in healthy individuals, in Chile

Background/Aims: Low blood Se levels have been previously shown in normal pregnancies (third trimester) and significantly lower levels in patients with intrahepatic cholestasis of pregnancy (ICP), in Finland and in Chile, suggesting that a low or marginal dietary availability of Se may contribute to the pathogenesis of this disease. The aim of this study was to investigate whether a temporal change in plasma concentration of Se, and seasonal fluctuations in plasma concentrations of Se, Zn and Cu, could coincide with changes in the prevalence of ICP. Methods: A cross-sectional cohort study was done including 21 ICP patients, 98 women in the third trimester of a normal pregnancy, 29 non-pregnant women, and also 13 individuals (seven non-pregnant women and six men) who had been studied 9 years before. Plasma Se, Zn and Cu were measured by atomic spectroscopy. Plasma Se levels in the present study were compared to the results obtained 5 to 7 years before, employing identical methodology in similar population samples. Results: Plasma Se concentrations in non-pregnant women were higher than in the previous study: 1.43±0.34 μmol/l vs 0.85±0.13; p<0.001. In comparison to non-pregnant women, normal pregnancies near term had lower plasma levels of Se: 1.08±0.25 μmol/l; p<0.01, and Zn: 17.90±3.61 μmol/l vs 19.71±3.21; p<0.05, but higher plasma levels of Cu: 34.35±7.12 μmol/l vs 20.62±3.34; p<0.01. In normal pregnancies, plasma Se concentration was significantly higher in summer (1.34±0.19 μmol/l) than in the other seasons, while Zn and Cu diminished. Similar to previous studies, ICP patients had significantly lower Se plasma levels than normal pregnancies: 0.94±0.12 μmol/l, p<0.05, and Cu levels were significantly higher: 50.80±7.02 μmol/l, p<0.01. Cu plasma levels correlated with the biochemical severity of the disease. Zn did not change in ICP. Conclusions: The present study shows that the decrease in the prevalence of ICP in Chile during the last decade coincides with an increase in plasma Se levels. Its lower incidence during summer coincides with a higher plasma Se concentration in summer than in other seasons, as observed in normal pregnancies.

Thioredoxin reductase

The mammalian thioredoxin reductases (TrxRs) are a family of selenium-containing pyridine nucleotide-disulphide oxidoreductases with mechanistic and sequence identity, including a conserved -Cys-Val-Asn-Val-Gly-Cys- redox catalytic site, to glutathione reductases. TrxRs catalyse the NADPH-dependent reduction of the redox protein thioredoxin (Trx), as well as of other endogenous and exogenous compounds. The broad substrate specificity of mammalian TrxRs is due to a second redox-active site, a C-terminal -Cys-SeCys- (where SeCys is selenocysteine), that is not found in glutathione reductase or Escherichia coli TrxR. There are currently two confirmed forms of mammalian TrxRs, TrxR1 and TrxR2, and it is possible that other forms will be identified. The availability of Se is a key factor determining TrxR activity both in cell culture and in vivo, and the mechanism(s) for the incorporation of Se into TrxRs, as well as the regulation of TrxR activity, have only recently begun to be investigated. The importance of Trx to many aspects of cell function make it likely that TrxRs also play a role in protection against oxidant injury, cell growth and transformation, and the recycling of ascorbate from its oxidized form. Since TrxRs are able to reduce a number of substrates other than Trx, it is likely that additional biological effects will be discovered for TrxR. Furthermore, inhibiting TrxR with drugs may lead to new treatments for human diseases such as cancer, AIDS and autoimmune diseases.

Tissue lipid peroxidative responses in Atlantic salmon (Salmo salar L.) parr fed high levels of dietary copper and cadmium

Atlantic salmon (Salmo salar L.) parr were fed for one month on fish meal based diets supplemented with Cd (0, 0.7, or 204 mg Cd kg−1 DW) or Cu (0, 34, or 691 mg Cu kg−1 DW) to assess the effects of non-essential (Cd) and essential (Cu) dietary metals on lipid peroxidation and the oxidative defence system. Cadmium accumulated significantly in the liver, intestine, and kidney of 204 mg Cd kg−1exposed fish compared to controls. Copper accumulated significantly in the intestine, kidney, and liver of fish exposed to 691 mg Cu kg−1, and in the intestine of 34 mg Cu kg−1 exposed fish. Tissue Cu accumulation significantly increased intestinal and hepatic lipid peroxidation (as seen from thiobarbituric acid reactive substances, TBARS, levels) and subsequently decreased intestinal α-tocopherol levels and increased intestinal and hepatic selenium dependent glutathione peroxidase (SeGSH-Px) activity. Dietary Cd significantly reduced SeGSH-Px activity in the intestine and liver of 204 mg Cd kg−1 exposed fish compared to controls. No significant increase in tissue TBARS or reduction of α-tocopherol levels was observed in the intestine of fish exposed to dietary Cd, with exception of the highest exposure group (204 mg Cd kg−1). Dietary Cu caused depletion of tissue Se and glutathione levels, however the reduced availability of GSH and Se did not seem to explain the differences in SeGSH-Px activity. Dietary Cu had a direct effect on lipid peroxidation at a relatively low concentration (34 mg Cu kg−1). Cadmium indirectly affected tissue lipid peroxidation by damaging the oxidative defence system at the highest dietary concentration (204 mg Cd kg−1).

Selenium distribution in the local environment of selected villages of the Keshan Disease belt, Zhangjiakou District, Hebei Province, People's Republic of China

The distribution of Se in cultivated topsoils, grains, human hair and drinking water has been studied in 15 villages from a Keshan disease area of the People's Republic of China, villages being classified into 3 groups according to the Keshan disease incidence in the local population. In grain, hair and water the total Se follows expected trends; i.e. the highest concentrations are found in the villages where there is no incidence of Keshan disease. However, the soils from the high-incidence Keshan disease villages have the highest total Se content, an apparent contradiction, as Keshan disease is a response to a Se deficient environment. Soil analyses suggest that the organic content of the soils is a major factor in controlling the availability of Se and it is the high-incidence Keshan disease villages that have the most organic-rich soils. Although higher in total Se, the organic-rich soils have little bioavailable Se resulting in a Se deficient food chain. Soil pH is also seen to be a related factor in restricting the availability of Se and all the grain samples collected on soils with a pH <7.6 had a total Se content of less than 10% of the total soil Se. In an environment that can be classified as Se deficient small changes in the soil organic content and pH can have a critical affect on the Keshan disease status of a village.

Adsorption–desorption reactions of selenium in some soils of India

Adsorption/desorption behavior of selenium (Se) was investigated in laboratory studies for surface soil samples varying in physico-chemical characteristics and mineralogical composition. In general, greater amount of Se was sorbed by acidic soils than alkaline soils; the only exception was the alkaline seleniferous soil from Barwa in northwestern India. Adsorption data for Se fitted well to Freundlich isotherm and log K values were negatively correlated with pH ( r=−0.883***), electrical conductivity ( r=−0.823***), CaCO 3 ( r=−0.829**) and clay content (−0.639*) of the soils. Selenium adsorption data also conformed to Langmuir isotherm. Adsorption maxima was significantly correlated with organic carbon ( r=0.757**) and free iron ( r=0.773**) content of the soils. Constant related to bonding energy was significantly correlated with pH ( r=−0.851**) and organic matter content ( r=0.694*) of the soils. The distribution coefficient ( K d) showed that at the same value of surface coverage, the acidic soils had greater affinity for Se as compared to the alkaline soils. Amount of Se desorbed in the 0.25 M KCl and 0.1 M KH 2PO 4 was strongly influenced by pH of the soils. In acidic soils, greater amount of sorbed Se was desorbed in KH 2PO 4 than in KCl and the reverse was true for alkaline soils. As KCl desorbs predominantly selenate, the water-soluble form of Se in soil solution, the Se availability and absorption by plants should be more in alkaline soils as are found in northwestern India, rather than in soils with acidic reaction.

Low environmental selenium availability as an additional determinant for goiter in East Java, Indonesia?

Iodine deficiency, which is most visibly indicated by goiter, is highly prevalent in Indonesia. Since 1994, Indonesia has a decree that all salt used for human, livestock, and industry must be iodized. However, despite the increased distribution of iodized salt, pockets with significantly higher prevalence of goiter still remain. This situation may be consequence of selenium (Se) deficiency. This study aimed to assess the Se level in the environment of goiter prevalent areas. Five hundred eleven school children participated in this study. Goiter was measured using both ultrasound and palpation. Ninety-nine eggs were collected from free-living chicken in 11 villages, and the Se contents of egg yolk and egg white were determined by neutron activation analysis. In the villages studied, Se concentration in egg yolk ranged from 0.15 to 1.52 microg/g and in egg white from 0.18 to 2.97 microg/g. The prevalence of goiter measured by palpation ranged from 18.4% to 70% and by ultrasound from 0% to 100%. Because of the inconsistency of goiter rate measured by palpation and ultrasonography, the question remains whether low availability of Se in the environment might be an additional contributing factor for goiter.

Soil, grain and water chemistry in relation to human selenium-responsive diseases in Enshi District, China

Selenium deficiency (Keshan Disease) and toxicity diseases in humans occur within 20 km of each other in Enshi District in China and have been linked to environmental levels of Se. Low concentrations of Se are associated with Jurassic siltstones and sandstones, whereas high concentrations occur in areas underlain by Permian carbonaceous strata. Although these broad relationships between Se in the environment and the human population have been established previously, not all villages underlain by the carbonaceous strata suffer Se toxicity problems and the precise controls on Se distribution and availability have not been quantified. In the present study, soil, grain, drinking water and human hair samples are examined to determine the controls on Se availability in 3 Se environments in Enshi District. Five low-Se and Keshan Disease villages, 5 high-Se and no toxicity villages and 5 high-Se and toxicity villages were selected for the study. Results show that the majority of samples in the low-Se villages are deficient or marginal in Se, and that Se availability to plants is inhibited by adsorption onto organic matter and Fe oxyhydroxides in soil. Therefore, remediation strategies involving the application of Se fertiliser direct to the soil may not increase plant Se levels as expected. In the high-Se villages, localised lithological variations result in considerable ranges in Se concentrations in all sample types. Deficient and excessive levels of Se are recorded in samples from the same village. Selenium bioavailability in the high-Se toxicity villages is controlled by the total soil Se concentration and pH. A greater proportion of the Se is plant available in villages where the carbonaceous strata are interbedded with limestone. Villagers should be advised to avoid planting crops in these areas if possible.

Fractionation and distribution of selenium in soils

A modified selenium (Se) fractionation procedure was used to study Se distribution in three soils (two silt loams and one silty clay). This sequential procedure consisted of: i) 0.2 M potassium sulfate (K2SO4)‐soluble fraction, ii) 0.1 M potassium dihydrogen phosphate (KH2PO4)‐exchangeable fraction, iii) 0.5 M ammonium hydroxide (NH3H2O)‐soluble fraction, iv) 6 M hydrochloric acid (HCl)‐extractable fraction, and v) residual fraction digested with perchloric (HClO4) and sulfuric (H2SO4) acids. The fractionation procedure had high recovery rates (92.5 to 106%). The Se distribution in soil was controlled by soil properties, such as pH, oxide, clay, and calcium carbonate (CaCO3) contents. In the untreated soil samples, residual Se fraction was dominant. In the Se‐enriched soils, the silty clay had significantly more Se in the NH3H2O and residual fractions while in the two silt loams the largest were KH2PO4 and residual fractions. The Se availability in the two silt loams was higher than in the silty clay. The Se availability pattern in the untreated soils was: unavailable (HCl + residual fractions) >> potentially available (KH2PO4 + NH3H2O fractions) > available (K2SO4 fraction), while in the Se‐enriched soils it was potentially available > unavailable > available.

Effects of elevated dietary copper concentrations on growth, feed utilisation and nutritional status of Atlantic salmon ( Salmo salar L.) fry

The present experiment was conducted to study effects of elevated dietary Cu and establish upper limits of Cu in fish feed. Atlantic salmon fry were reared for 3 months on experimental diets containing either 5 (control), 35, 500, 700, 900, or 1750 mg Cu kg −1, provided as CuSO 4·5H 2O. Dietary Cr 2O 3 was included (1%) in all experimental diets for the last two weeks in order to assess apparent digestibility of both major food components and availability of minerals (Cu, Zn and Se). Growth was significantly ( P<0.01) reduced after 3 months at dietary Cu concentrations of ≥500 mg Cu kg −1 compared to the 35 mg Cu kg −1 group. Similarly, whole-body content of protein, glycogen and Se were significantly reduced at these dietary Cu concentrations compared to controls. Apparent digestibility (%) of the major food components and availability of Zn and Se did not differ among dietary treatments. However, apparent availability of dietary Cu was significantly reduced in fish fed experimental diets containing ≤900 mg kg −1 compared to controls. Whole-body minus intestine Cu concentrations were only significantly increased in fish fed dietary Cu concentrations ≥900 mg Cu kg −1 compared to controls, indicating a strong intestinal regulating capacity of dietary Cu uptake. Dietary Cu concentrations of 500 mg Cu kg −1 and above caused toxic responses in Atlantic salmon fry as concluded from the antagonistic interaction with Se, and reduced growth and whole-body energy stores (protein and glycogen).

Plant availability of selenite and selenate as influenced by the competing ions phosphate and sulfate

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO4 (0.5 mM) and PO4 (2 μM) similar to those found in soil solution. Concentrations up to 5 mM SO4 and 200 μM PO4 allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more phytotoxic than selenate, especially for shoot growth. Selenate was less toxic, and tended to preferentially inhibit root growth. Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%). A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species. In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots. Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se.

Effect of soil amendments on fractionation of Selenium‐enriched soil

This study was to determine the effect of soil amendments on the fractionation of selenium (Se) using incubation experiments under simulated upland and flooded conditions. The treatments were as follows: 1) control [soil + sodium selenite (Na2SeO3) (1 mg Se kg‐1)]; 2) control + calcium carbonate (CaCO3) (5 g kg‐1); 3) control + alfalfa (40 g kg‐1); and 4) control + CaCO3 (5 g kg‐1) + alfalfa (40 g kg‐1). After a 90‐day incubation, soil was sampled and fractionated into five fractions: 1) potassium sulfate (K2SO4)‐soluble fraction (available to plants); 2) potassium dihydrogen phosphate (KH2PO4)‐exchangeable fraction (potentially available); 3) ammonium hydroxide (NH3H2O)‐soluble fraction (potentially available); 4) hydrochloric acid (HCl)‐extractable fraction (unavailable); and 5) residual fraction (unavailable). Compared with the control, CaCO3 increased the K2SO4 fraction at the expense of the NH3H2O fraction. Alfalfa increased both the K2SO4 and residual fractions but reduced the KH2PO4 and NH3H2O fractions. The CaCO3‐alfalfa treatment had a similar effect to the alfalfa treatment alone. The comparison between the upland and flooded conditions showed that the flooded condition generally increased the residual fraction and decreased the potentially‐available fractions. In general, CaCO3 was a better amendment because it not only increased the available fraction but also maintained the potentially available fractions at a high level. The application of Na2SeO3 and use of appropriate soil amendments can improve Se availability in soil.

Rate-limiting steps in selenium assimilation and volatilization by indian mustard

Se can be accumulated by plants and volatilized to dimethylselenide, providing an attractive technology for Se phytoremediation. To determine the rate-limiting steps in Se volatilization from selenate and selenite, time- and concentration-dependent kinetics of Se accumulation and volatilization were studied in Indian mustard (Brassica juncea). Time-dependent kinetic studies showed that selenate was taken up 2-fold faster than selenite. Selenate was rapidly translocated to the shoot, away from the root, the site of volatilization, whereas only approximately 10% of the selenite was translocated. For both selenate- and selenite-supplied plants, Se accumulation and volatilization increased linearly with external Se concentration up to 20 &mgr;M; volatilization rates were also linearly correlated with root Se concentrations. Se-volatilization rates were 2- to 3-fold higher from plants supplied with selenite compared with selenate. Se speciation by x-ray absorption spectroscopy revealed that selenite-supplied plants accumulated organic Se, most likely selenomethionine, whereas selenate-supplied plants accumulated selenate. Our data suggest that Se volatilization from selenate is limited by the rate of selenate reduction, as well as by the availability of Se in roots, as influenced by uptake and translocation. Se volatilization from selenite may be limited by selenite uptake and by the conversion of selenomethionine to dimethylselenide.

Changes in selenium speciation in wetland sediments induced by laboratory testing

Analysis of wet and laboratory‐dried samples of wetland sediment showed that the concentrations of soluble and adsorbed selenium (Se) increased greatly if samples were dried. This effect was more pronounced at higher temperatures. The release of organic Se through rapid decomposition of sediment organic material and Se oxidation to selenate appear to be the major processes resulting in the increase of the concentrations of the soluble and adsorbed Se in dried sediment. This study shows that sediment drying, especially at higher temperatures, could affect the evaluation of Se speciation and availability in wetland sediment.

Effects of diet and chemical form of selenium on selenium metabolism in sheep.

The effects of diet composition and chemical form of Se on intestinal flow, absorption, and retention of Se were determined in sheep by the balance technique and by disappearance of Se from sites along the gastrointestinal tract with reference to dual-phase digesta markers. Six sheep with ruminal and duodenal cannulas were used in a crossover design with a split-plot arrangement of the Se isotope treatments. Sheep were fed a forage (alfalfa hay)-based (.37 mg Se/kg) or concentrate (barley)-based (.27 mg Se/kg) diet at 90% of ad libitum intake. Selenium stable isotopes (enriched [77Se]yeast, enriched [82Se]selenite) and fluid (Co-EDTA) and particulate (Cr-mordanted fiber) markers were administered simultaneously into the rumen four times daily for 7 d, and total collections of feces and urine were made every 24 h for these and the following 7 d. A larger proportion (51 to 61%) of the Se tracers flowing to the duodenum was associated with the particulate fraction, mainly as bacteria-associated Se, than with the fluid fraction. The [82Se]selenite was more available (P < .05) for absorption and retention than [77Se]yeast, indicating that inorganic chemical forms of Se are as available to the ruminant as organic forms of Se commonly found in feedstuffs. Selenium absorption and retention were greater (P < .05) in sheep receiving the concentrate-based diet than in sheep receiving the forage-based diet. Thus, the availability of Se from inorganic and organic sources in sheep seems to be influenced by diet composition.

Phase evolution in low-pressure Se vapor selenization of evaporated Cu/In bilayer precursors

Evolution of elemental binaries and single-phase copper indium diselenide, CuInSe2 (CIS) during Se vapor selenization of evaporated Cu/In bilayer metal precursors at pressures of 0.3–10 mbar and temperatures in the range of 260–400 °C has been investigated. At low pressures, the relative kinetics of selenization of Cu and In are changed resulting in the formation of single-phase CIS even at very low temperatures (260 °C). Optical, Auger, and x-ray photoelectron spectroscopy investigations are employed to characterize the chalcopyrite absorber layer. At higher pressures (≊7–10 mbar), simultaneous formation of the equilibrium binaries, CuSe and In2Se3 at low temperatures leads to the formation of CIS through a diffusion limited reaction of the binaries at higher temperatures. The availability of Se reacting species varies significantly in the pressure regime. At low reactor pressures and Se availability, the reaction CuSe+In(l)+Se→CIS, proceeds to completion even at low temperatures. The detailed study of the phase evolution is made by x-ray diffraction and scanning electron microscopy investigations and correlated with the Se availability in the reactor.

Estimation of selenium bioavailability from human, cow's, goat and sheep milk by an in vitro method.

The trace element selenium (Se) has been recognized to be essential for human health. The dependence of infants on milk as their principal food source, generally low in Se content, makes them more vulnerable to inadequate Se intake. The present study compared the Se availability as estimated by a simulated gastrointestinal digestion procedure, of human milk and some common ruminant milks, namely cow, goat and sheep milk. The Se availability of human milk (11.1%) was significantly higher compared to that of cow (6.8%), goat (6.2%) and sheep milk ( < 2%). Further study suggested that the Se availability may be related to the gastric digestibility of protein. The high Se availability of human milk might be attributed to the high gastric digestibility of human milk protein. It was found that removal of the milk fat fraction increases the Se availability.

Modeling Arsenic and Selenium Leaching from Acidic Fly Ash by Sorption on Iron (Hydr)oxide in the Fly Ash Matrix

The sorption of As and Se on iron (hydr)oxide in acidic fly ash is studied as a possible process controlling the leaching of these contaminants. Following controlled leaching experiments with an acidic fly ash, sorption of As and Se is investigated both on crystalline and amorphous iron (hydr)oxide, using partial chemical extractions and isotopic exchange experiments. Virtually all As and Se at the fly ash surface, which is potentially available for leaching, is shown to be associated with hydroxylamine extractable “amorphous” iron (hydr)oxides in the fly ash matrix. Isotopic exchange experiments suggest that at pH < 10 the oxyanions are partly coprecipitated with secondary-formed amorphous iron (hydr)oxide, a process reducing their availability. The leaching of As and Se from acidic fly ash is described by a simplified model of surface complexation on amorphous iron (hydr)oxide, taking the limited availability of iron-associated As and Se into account. This approach is shown to constitute a useful tool to model sorption control of contaminant leaching from waste materials.

Critical influence of reactant pressure on the evolution of single phase CuInSe2 during selenization at low temperatures

Single phase chalcopyrite CuInSe2 (CIS) phase formation at low temperatures (≊260 °C) is demonstrated by Se vapor selenization of evaporated metal precursors at a pressure of ≊0.3 mbar. The low pressure changes the relative kinetics of selenization of Cu and In due to the modification of the availability of Se reacting species. Consequently, the thermodynamically favorable reaction CuSe+In(1)+Se→CIS proceeds to completion at lower temperatures. This is confirmed by the absence of In2Se3 at intermediate pressures (0.5–1.0 mbar). At higher pressures (≊7–10 mbar) and Se flux, simultaneous formation of the equilibrium binaries, CuSe and In2Se3 at low temperatures, leads to the formation of CIS through a diffusion limited reaction of the binaries at higher (≥400 °C) temperatures.

Effects of supplementation of fertilizers on human selenium status in Finland.

The Se concentration of foods can be increased by supplementing fertilizers with soluble Se compounds. In Finland the availability of soil Se for plants is poor owing to the relatively low Se concentration, low pH and high iron content of the soil. Since 1984 multimineral fertilizers have been supplemented with Se (16 mg kg-1 to fertilizers for grain production and 6 mg kg-1 to those for fodder production) in the form of sodium selenate. Within two years a three-fold increase of mean Se intake was observed. The supplementation affected the Se content of all major food groups with the exception of fish. The concomitant human serum Se concentration increased by 70%. In 1990 the amount of Se that was supplemented was reduced to 6 mg kg-1 for all fertilizers. This reduced the mean Se intake by 30% and the serum Se concentration decreased by 25% from the highest levels observed in 1989. Plants take up part of the supplemented selenate and transform it into organic Se compounds, mainly selenomethionine. This affects human nutrition by increasing the Se content of foods of both animal and vegetable origin. According to data obtained in Finland, supplementation of fertilizers with Se is a safe and effective means of increasing the Se intake of both animals and humans that is feasible in countries with relatively uniform geochemical conditions. This kind of intervention requires careful monitoring of the effects on both animal and human nutrition and the environment.

Redox transformations and plant uptake of selenium resulting from root-soil interactions

Batch studies were conducted with Mn oxides (birnessite-hausmannite mixture, BHM) and samples of four soil series from the Mid-Atlantic region of the USA to determine effects of reducing organic acids, similar to those found in the rhizosphere, on the SeO3/SeO4 distribution. Jackland (Typic Hapludalf), Myersville (Ultic Hapludalf), Christiana (Aeric Paleaquult), and Evesboro (Typic Quartizipsamment) A and B horizon soil samples with and without prior Mn oxide reduction were incubated aerobically for 10 d with 0.1 mmol kg-1 SeO3 and 0 or 25 mmol kg-1 of ascorbic acid, gallic acid, oxalic acid, or citric acid. Selenite was also added to BHM (10 mmol kg-1) with 0 or 0.1 mmol kg-1 ascorbic acid. The availability of Se for plant uptake as a result of root-soil interactions was examined using growth chamber studies with barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seedlings grown in 150-mL cone-shaped containers to maximize root-soil surface interactions and to create ‘rhizosphere’ soil throughout the root zone. In the BHM system ascorbic acid increased oxidation of SeO3 to SeO4 to 33% of added SeO3. In the presence of ascorbic and gallic acids and Mn oxides, oxidation of SeO3 to SeO4 occurred in the B horizons of all the soils and in the A horizons of Jackland and Myersville soils. Removal of Mn oxides decreased the oxidation in some samples. Wheat and barley plants were able to accumulate up to 20 μmol Se kg-1 from the Jackland soil when soluble Se was not measurable. The root-soil interactions in the Jackland soil with barley and wheat provided the plant with Se from insoluble sources. The results also indicate that Mn oxides coming in contact with reducing root exudates have a greater ability to oxidize SeO3 to SeO4. Thus, rhizosphere processes play an important role in the availability of Se for plant uptake.