Seleniferous Soils Research Articles

Interactions of selenium hyperaccumulators and nonaccumulators during cocultivation on seleniferous or nonseleniferous soil – the importance of having good neighbors

• This study investigated how selenium (Se) affects relationships between Se hyperaccumulator and nonaccumulator species, particularly how plants influence their neighbors' Se accumulation and growth. • Hyperaccumulators Astragalus bisulcatus and Stanleya pinnata and nonaccumulators Astragalus drummondii and Stanleya elata were cocultivated on seleniferous or nonseleniferous soil, or on gravel supplied with different selenate concentrations. The plants were analyzed for growth, Se accumulation and Se speciation. Also, root exudates were analyzed for Se concentration. • The hyperaccumulators showed 2.5-fold better growth on seleniferous than on nonseleniferous soil, and up to fourfold better growth with increasing Se supply; the nonaccumulators showed the opposite results. Both hyperaccumulators and nonaccumulators could affect growth (up to threefold) and Se accumulation (up to sixfold) of neighboring plants. Nonaccumulators S.elata and A.drummondii accumulated predominantly (88-95%) organic C-Se-C; the remainder was selenate. S.elata accumulated relatively more C-Se-C and less selenate when growing adjacent to S.pinnata. Both hyperaccumulators released selenocompounds from their roots. A.bisulcatus exudate contained predominantly C-Se-C compounds; no speciation data could be obtained for S.pinnata. • Thus, plants can affect Se accumulation in neighbors, and soil Se affects competition and facilitation between plants. This helps to explain why hyperaccumulators are found predominantly on seleniferous soils.

Influence of microbial associations on selenium localization and speciation in roots of Astragalus and Stanleya hyperaccumulators

Selenium (Se) hyperaccumulator plants can accumulate and tolerate Se up to 1% of their dry weight. Since little is known about below-ground processes of Se uptake and metabolism in hyperaccumulators, X-ray absorption spectromicroscopy was used to characterize the chemical composition and spatial distribution of Se in roots of Astragalus and Stanleya hyperaccumulators. Selenium was present throughout the roots, with the highest levels in the cortex. The main form of Se (48–95%) in both species collected from naturally seleniferous soil was an organic CSeC compound, likely methyl-selenocysteine. In addition, surprisingly high fractions (up to 35%) of elemental Se (Se0) were found, a form so far not reported in plants but commonly produced by Se-tolerant bacteria and fungi. Four fungi collected from hyperaccumulator roots were characterized with respect to their Se tolerance and ability to produce Se0, and then used to inoculate hyperaccumulators in a controlled greenhouse study. The roots of the greenhouse-grown Astragalus and Stanleya contained mainly CSeC; in most plants no Se0 was detected, with the exception of Astragalus nodules and roots of Astragalus inoculated with Alternaria astragali, an Se0-producing fungus. Apparently, Se0-producing endosymbionts including nitrogen-fixing bacteria and endophytic fungi or bacteria in the root can affect Se speciation in hyperaccumulator roots. Microbes that affect plant Se speciation may be applicable in phytoremediation and biofortification, especially if they are promiscuous and affect Se tolerance in crop species.

Exposure histories derived from selenium in otoliths of three cold-water fish species captured downstream from coal mining activity

Establishing exposure to contaminants within a given environment is often difficult for fish species with large home ranges. Chemical analyses of muscle or visceral tissue are useful indicators of recent exposure, but depuration, metabolic transformation, and tissue redistributions can alter temporal resolution. Otoliths are metabolically stable and thus provide complete chemical records within their calcified tissues that, when coupled to the annular structure, can provide temporal resolution for exposure to trace metals. Otoliths from bull trout, cutthroat trout, and mountain whitefish from an area rich in seleniferous soils and with active coal mining activity were analyzed for selenium to determine any history of exposure to elevated levels of selenium. Selenium concentrations in otolith primordia tended to be low, indicating that these fish emerged in low selenium areas. Later life stages showed peaks of high Se concentrations, suggesting that individuals moved into areas of increased selenium later in life. Individuals captured from the same area had a wide variety of selenium exposure profiles, indicating that these fish do not move en masse into and out of high-selenium areas. Year-to-year variability of selenium exposure patterns within an otolith suggests inconsistent utilization of high- and low-selenium areas by the individual. The inconsistent exposure profiles for these fish, in addition to their home range of tens of kilometres, indicate that soft tissue concentrations, while useful indicators of recent exposure, cannot be relied upon to provide a life history recording of exposure.

Effect of organic amendments on uptake of selenium and biochemical grain composition of wheat and rape grown on seleniferous soils in northwestern India

AbstractCrops grown in seleniferous soil may accumulate selenium (Se) to levels considered highly toxic for animal and human consumption. Furthermore, higher Se content in plant tissues leads to considerable deterioration in product quality. Application of organic amendments plays an important role in improving soil physical, chemical, and biological conditions and influencing nutrient availability. A field trial was conducted to evaluate the effect of organic amendments, namely poultry manure (PM), sugar cane press mud (SCPM), and farmyard manure (FYM), on Se uptake and grain quality of wheat and oilseed rape grown on a seleniferous soil in Punjab, India. Selenium accumulation by wheat and oilseed rape grains decreased significantly (75%–95%) with the application of PM and SCPM, while FYM application resulted in a significant decrease (23%) only in case of wheat grains. The amount of Se associated with seed proteins varied in proportion to its uptake under different treatments. Quality of wheat grains improved considerably with respect to total soluble sugars, reducing sugars, starch, lipids, and sulfur concentrations only after application of SCPM and PM. Treating a Se‐contaminated soil with organic amendments significantly increased the oil concentration and changed the proportion of various fatty acids in rape grains. It is concluded that applying organic amendments to Se‐contaminated soils can alleviate the deleterious effects of Se and restore the nutritional quality of grains.

Analytical Possibilities of Total Reflection X-ray Spectrometry (TXRF) for Trace Selenium Determination in Soils

Selenium content of soils is an important issue due to the narrow range between the nutritious requirement and toxic effects upon Se exposure. However, its determination is challenging due to low concentrations within complex matrices that hamper the analysis in most spectroscopic techniques. In this study, we explored the possibilities of several analytical approaches combined with total reflection X-ray (TXRF) spectrometry for soil Se determinations. The direct analysis of a solid suspension using 20 mg of fine ground material (<50 μm) has a relatively high Se limit of detection (LOD) of 1 mg/kg (worldwide Se average in soils = 0.4 mg/kg) and is therefore only suitable for seleniferous soils. Several fast and simple analytical strategies were developed to decrease matrix effects and improve the LOD for Se determination in soil digests. On one hand, the application of a liquid-liquid extraction procedure using ethyl ether and the introduction of a Cr absorbent in the instrument configuration were carried out to avoid the associated problems on TXRF analysis of soil extracts due to the high Fe concentrations (∼700 mg/L). On the other hand, a dispersive liquid-liquid microextraction procedure (DLLME) before the TXRF analysis of the soil digest was also developed. The effects of various experimental parameters such as sample volume, effect of major elements present in the soil matrix (Fe), and Se concentration in the sample were investigated. The LOD using this analytical methodology (0.05 mg/kg of Se) was comparable to or lower than those obtained in previous works using other popular spectrometric techniques such as GFAAS, ICPMS, and AFS. The calculated Se concentration for JSAC-0411 ([Se] = 1.32 ± 0.27 mg/kg) using the combination of DLLME and TXRF ([Se] = 1.40 ± 0.23 mg/kg) was in agreement with the certified value.

Hyperaccumulation of trace elements: from uptake and tolerance mechanisms to litter decomposition; selenium as an example

Hyperaccumulation of trace elements by plants is an attractive field of research with potential applications in phytoremediation of contaminated soils, biofortification of food with essential nutrients, and biofuel production. For these purposes, selenium is of special interest because areas with Se-deficient soils frequently are close to seleniferous soils. The use of Se-hyperaccumulating species for phytoremediation of soils with high Se burdens requires basic knowledge on the mechanisms of uptake, transport and tolerance of Se in plants. However, also a better understanding of the rhizosphere processes governing Se cycling is essential. Quinn et al. in this issue of Plant and Soil address, for the first time, the question of specialist decomposers presence and role in leaf litter from selenium hyperaccumulating plants in a seleniferous habitat. Their experimental results strongly support the involvement of selenium tolerant decomposers. This knowledge is of crucial importance for putting into practice integrated management systems for biofortification and phytoremediation technologies.

Selenium Mobilization by Pseudomonas aeruginosa (SNT-SG1) Isolated from Seleniferous Soils from India

Selenium (Se) is a metalloid required at trace concentrations for normal metabolic activities of the cell. The bioavailable forms viz., selenate and selenite have been found in localized high concentrations in seleniferous environments. Studies are in progress on bacterial strains that were isolated from one such location in the North-West region of Punjab, India. A facultative anaerobe, identified as Pseudomonas aeruginosa by 16S rRNA gene homology, was isolated from the rhizosphere of crop plants from this region and was examined for selenium mobilization potential in the presence of selenium oxyanions. The isolate was observed to reduce 53 and 21% of sodium selenite and selenate to elemental selenium, respectively, and volatilize 4.7 and 5.1% within 72-hour duration. This is one of the few selenium tolerant aerobic bacteria isolated and reported from tropical seleniferous soils from India, and the first to show volatilization potential. These organisms are being considered for bioaugmenting Se-impacted soils for enhanced Se mobilization and removal.

Selenium in surface and irrigation water in the Kendrick irrigation district, Wyoming

High selenium (Se) concentrations have been found in surface waters in the Kendrick Reclamation Project, Wyoming. Precipitation and irrigation water moving over seleniferous soils are contributing causes, and drought may exacerbate this. This study surveyed Se concentrations and discharges in local surface streams, irrigation drains, and the delivery canal. Sites were sampled monthly and analyzed for Se and total suspended solids (TSS). A completely randomized design with two factors (soil parent material and location, inside or outside irrigation district) was used. Mean Se concentrations were 64 μg L(-1) inside the irrigation district on shale soils, 17 μg L(-1) inside the district off shale soils, 5 μg L(-1) outside the district on shale soils, and 3 μg L(-1) outside the district off shale soils. Correlations between discharge and Se concentrations were generally negative, while correlations between discharge and Se load were generally positive. There was little correlation between load and concentration, and little correlation between TSS and Se. A comparison of Se concentrations in streams and drains showed Se concentrations were significantly higher (p<0.001) in streams during the irrigation season, but not in the off-season (p=0.515). We conclude that higher discharges decrease Se concentration, but increase load. Conversion from flood to sprinkle irrigation may increase Se concentrations by reducing discharge, but decrease Se loads going into the N. Platte River, and will likely alter the timing and magnitude of flows. Both load and concentration should be considered when implementing Se regulations and standards.

Selenium accumulation by forage and grain crops and volatilization from seleniferous soils amended with different organic materials

Greenhouse and field experiments were conducted to study the direct and residual effect of applying different organic materials on Se accumulation by crops and volatilization from seleniferous soils of northwestern India. Among organic manures, press mud and poultry manures proved 4–5 times more effective in reducing Se accumulation by different crops than farmyard manure. Efficiency of organic manures increased with increase in application rates. Application of both press mud and poultry manures reduced Se accumulation by 44–97% in wheat ( Triticum aestivum) and rapeseed ( Brassica napus) shoots in the greenhouse; 85–92% in wheat straw, 45–74% in wheat grains, 45–74% in rapeseed straw and 76–92% in rapeseed grains under field conditions. Both the manures remained highly effective in reducing Se accumulation by the crops following wheat and rapeseed and the extent of reduction varied from 50% to 87% in maize ( Zea mays) and cowpeas ( Vigna sinensis) in the greenhouse and 40–89% in maize and rice ( Oryza sativa) crops under field situation. Rate of Se volatilization by wheat and rapeseed crops increased by 1.8–4.0 times; the greatest increase was observed with press mud followed by poultry manure, arhar ( Cajanus cajan) leaves and farmyard manure. After 134 d of incubation of 500 g soil amended with 2% of plant tissues, the maximum amount of Se was volatilized with cowpea leaves (385 ng) followed by wheat grains, leaves of maize, sugarcane ( Saccharum officcinarum), arhar, poplar ( Populus deltoides) and the control (91 ng). The results of this study convincingly prove the usefulness of applying press mud and poultry manure in enhancing volatilization and retarding the transfer of Se from soil to plant in seleniferous soils.

Phytoremediation of selenium‐contaminated soils: the efficiency of different cropping systems

AbstractField experiments were conducted at two locations in the seleniferous region of northwestern India from 2001 to 2006 to evaluate the efficiency of four cropping systems in removing Se from contaminated soil containing 2843–4345 μg Se per kg in the surface layer (0–15 cm). Rapeseed (Brassica napus) followed by arhar (Cajanus cajan), sunn hemp (Crotalaria juncea) or cotton (Gossypium arboretum) and wheat (Triticum aestivum) followed by rice (Oryza sativa) were the four cropping systems. The total biomass generated by Brassica‐based systems ranged from 16 to 21 t/ha when harvested at maturity. Corresponding values for a wheat–rice sequence were 22–26 t/ha. Among the different crops at both the experimental sites, the highest Se content was recorded in leaves (157–209 mg/kg), grains (64–201 mg/kg) and stems (42–93 mg/kg) of Brassica and the lowest in the shoots (10–27 mg/kg), grains (5–13 mg/kg) and straw (13–20 mg/kg) of the rice crop. Except for S and P, concentrations of other nutrients (Zn, Cu, Mn and Fe) were not significantly affected by variations in the Se content of plants. Significant correlation coefficients were observed between Se and S (r = 0.838, P ≤ 0.001), Se and P (r = 0.817, P ≤ 0.001) at the peak flowering stage (n = 16), and r = 0.743, P ≤ 0.001 and r = 0.498, P ≤ 0.05, respectively, at the maturity stage (n = 16). Total Se removal through harvested biomass of rapeseed‐based cropping sequences varied from 716 to 1374 g/ha/yr at peak flowering and 736–949 g/ha/yr at the maturity stage. Corresponding values for a wheat–rice system were 435–492 and 370–517 g/ha/yr, respectively. The amount of Se recycled through leaf senescence ranged from 255 to 500 g/ha/yr for Brassica‐based cropping systems. In the wheat–rice system, Se addition through irrigation varied from 170 to 243 g/ha/yr and was three to four times more than that added in Brassica‐based systems. On completion of the phytoremediation experiments at site I, Se removal through harvested biomass at maturity was 1.7–5.1% of total Se in the soil down to a depth of 120 cm and 4.8–13.2% at site II. Analysis showed that Se losses under different crop rotations were 18.5–24.5% at site I and 21–33% at site II of total soil Se. Thus, at both sites 16–20% of total Se lost from the soil was unexplained. Results show that Brassica‐based cropping systems lead to significant reductions in Se capital of contaminated soil over 2–3 years. Although a long‐term commitment is required, adoption of Brassica‐based systems as a regular agricultural practice must lead to sustainable management of seleniferous soils.

Selenium concentrations of common weeds and agricultural crops grown in the seleniferous soils of northwestern India

The plants grown in seleniferous soils constitute a major source of toxic selenium levels in the food chain of animals and human beings. Greenhouse and field experiments were conducted to study selenium concentrations of weeds, forages and cereals grown on seleniferous soils located between 31.0417° to 31.2175° N and 76.1363° to 76.4147° E in northwestern India. Eleven winter season (November–April) weed plants were grown in the greenhouse in a soil treated with different levels of selenate-Se. Selenium concentrations of weed plants increased progressively with the levels of selenate-Se in soil. The highest Se concentration was recorded by Silene gallica (246 mg kg − 1 ) and the lowest by Avena ludoviciana (47 mg kg − 1 ) at 2.5 mg Se kg − 1 soil. A . ludoviciana and Spergula arvensis proved highly tolerant to the presence of 1.25 and 2.5 mg selenate-Se kg − 1 soil and the remaining weeds were sensitive to Se. Dry matter yield of Se-sensitive weed plants was 25 to 62% of the yield in the no-Se control at 1.25 mg selenate-Se kg − 1 and 6 to 40% at 2.5 mg selenate-Se kg − 1 soil. Other symptoms like change in leaf colour and size, burning of leaf tips and margins, and delayed flowering were also observed due to Se. Dry matter yield of Se-sensitive weed plants expressed as percentage of yield in the no-Se control at both the Se levels was inversely correlated with their Se content ( r = − 0.731, p < 0.01, N = 17). Among the weed plants grown in seleniferous soils under field situations, Mentha longifolia accumulated the highest Se (365 mg kg − 1 ) and Phalaris minor the lowest (34 mg kg − 1 ). Among agricultural crops grown on a naturally contaminated soil in the greenhouse, Se concentrations were the highest for oilseed crops (19–29 mg kg − 1 ), followed by legumes (6–13 mg kg − 1 ) and cereals (2–18 mg kg − 1 ). Helianthus annuus among the oilseed crops, A . ludoviciana among the winter season weeds, M . longifolia among the summer season (May–October) weeds and Cirsium arvense among the perennial weeds can be used for phytoremediation of seleniferous soils as these accumulate the highest amounts of Se.

Effect of Selenium-Rich Wheat on Rat Spermatogenesis/Wirkung von Selen-haltigem Weizen auf die Spermatogenese der Ratte

Wheat cultivated in seleniferous soils of some districts of Punjab/India contained on an average 12.5 ppm selenium. These grains when fed for 4 weeks to laboratory rats, were found to cause reduction of testicular weights and interruption of spermatogenesis. Seminiferous tubules were severely affected and contained spermatogenic cells at different stages of regression. The population of spermatids was first to be affected as they showed clumping leading to the formation of giant cells. Interstitial tissue in the testes of rats fed on seleniferous wheat grains appeared oedematous and widened than that of controls. The results, therefore, revealed that selenium toxicity, in addition to its known effects in female rats, has an adverse effect on reproduction in the male rats.

Selenium Concentrations of Common Weeds and Agricultural Crops Grown in The Seleniferous Soils of Northwestern India

The plants grown in seleniferous soils constitute a major source of toxic selenium levels in the food chain of animals and human beings. Greenhouse and field experiments were conducted to study selenium concentrations of weeds, forages and cereals grown on seleniferous soils located between 31.0417° to 31.2175° N and 76.1363° to 76.4147° E in northwestern India. Eleven winter season (November–April) weed plants were grown in the greenhouse in a soil treated with different levels of selenate-Se. Selenium concentrations of weed plants increased progressively with the levels of selenate-Se in soil. The highest Se concentration was recorded by Silene gallica (246 mgkg−1) and the lowest by Avena ludoviciana (47 mgkg−1) at 2.5 mg Sekg−1 soil. A. ludoviciana and Spergula arvensis proved highly tolerant to the presence of 1.25 and 2.5 mg selenate-Sekg−1 soil and the remaining weeds were sensitive to Se. Drymatter yield of Se-sensitive weed plants was 25 to 62% of the yield in the no-Se control at 1.25 mg selenate-Sekg−1 and 6 to 40% at 2.5 mg selenate-Sekg−1 soil. Other symptoms like change in leaf colour and size, burning of leaf tips and margins, and delayed flowering were also observed due to Se. Dry matter yield of Se-sensitive weed plants expressed as percentage of yield in the no-Se control at both the Se levels was inversely correlated with their Se content (r=−0.731, pb0.01, N=17). Among the weed plants grown in seleniferous soils under field situations, Mentha longifolia accumulated the highest Se (365 mgkg−1) and Phalaris minor the lowest (34 mgkg−1). Among agricultural crops grown on a naturally contaminated soil in the greenhouse, Se concentrations were the highest for oilseed crops (19–29 mgkg−1), followed by legumes (6–13 mgkg−1) and cereals (2–18 mgkg−1). Helianthus annuus among the oilseed crops, A. ludoviciana among the winter season weeds, M. longifolia among the summer season (May–October) weeds and Cirsium arvense among the perennial weeds can be used for phytoremediation of seleniferous soils as these accumulate the highest amounts of Se.

Evaluation of leaching and runoff losses of selenium from seleniferous soils through simulated rainfall

AbstractExperiments were conducted to study drainage and runoff losses of selenium (Se) from two seleniferous soils (from Simbly containing total Se 850 μg [kg soil]–1 and from Barwa containing 1310 μg [kg soil]–1) under simulated rainfall (250–260 mm in three rainstorms) conditions. Rainfall intensities ranged from 56 to 120 mm h–1 with uniformity coefficients ranging from 70.6% to 84.2%. Selenium lost through drainage (sum of drainage from initially saturated soil for 24 h and through dry and wet runs) was 0.15% and 0.11% of total Se content in the two soils. In soils having similar pH and organic‐C content, losses of Se through drainage as well as runoff were defined by total Se, water‐soluble Se, CaCO3 content, and texture of the soils. The amount of runoff water was almost two times in the soil with fine texture and less infiltration rate than in the other and that same trend was observed with respect to loss of sediment. The soil with higher CaCO3 content and water‐soluble Se lost more Se with moving water both through leaching and runoff, whereas the other soil with fine texture lost greater amount of Se with the sediment. Total Se lost through drainage as well as runoff was 0.29% of the native Se present in both the soils suggesting that significant amount of Se could be lost from seleniferous soils during irrigation and rainfall events.

Evaluation of different agroforestry tree species for their suitability in the phytoremediation of seleniferous soils

AbstractSeven agroforestry tree species were grown in a clay loam soil treated with different levels of selenate‐Se, viz. 0, 1.25, 2.5 and 5.0 mg/kg supplied through sodium selenate. After 1 year of growth, a progressive decrease in dry matter of leaves, stem and roots was observed with increasing levels of applied Se. However, a significant decrease in dry matter yield was observed only at or above 2.5 mg Se per kg soil and shisham (Dalbergia sissoo) proved to be highly sensitive to the presence of selenate‐Se in the soil. On average, the largest above‐ground and below‐ground biomass was accumulated by arjun (Terminalia arjuna) and the lowest by the acacia tree (Acacia tortillas). The selenium content of leaves, stem and roots of all the tree species increased significantly with increasing levels of applied Se, although a large variation within species was observed. In the stem portion of different trees, the highest concentration of Se was found in dek (Melia azedarach) (5.1 mg/kg) and the lowest in mulberry (Morus alba) (2.6 mg/kg). The efficiency of selenium removal (including leaves, stem and roots) was the highest in arjun followed by eucalyptus (Eucalyptus hybrid) – Clone 10, mulberry, jambolin (Syzygium cumini), dek, shisham and acacia. Effective removal of Se takes place through the stem portion of different trees where it constitutes 30–50% of total Se. Large variation in Se uptake by different tree species suggests that trees vary in their potential for phytoremediation of seleniferous soils. In one growing season, shisham aged 24 years, poplar (Populus deltoides)– Clone G 48 (10 years old) and eucalyptus – Clone 10 (10 years old) could remove 2385, 1845 and 1407 g Se per hectare respectively. Corresponding reductions in Se capital of the soil varied between 24 and 37, 19 and 29 and 14 and 32%, respectively, in the surface layer (0–15 cm) alone or 7–11, 6–9 and 4–7% for the whole soil profile (0–120 cm). Removal further increased to 4207 g Se per hectare under an agroforestry farming system of poplar–mentha/wheat with Se being reduced from 43 to 65% for the surface layer and from 13 to 20% for the whole profile.

Changes in biochemical components of wheat and rapeseed grown on selenium-contaminated soil

To study changes in some biochemical parameters in relation to selenium accumulation in wheat and rapeseed, plant samples at different growth stages of the crops were collected from the normal and selenium-contaminated region of Punjab. Plants grown in seleniferous soils accumulated 220–242 folds Se in wheat (Triticum aestivum L.var PBW 343) and 51–67 folds in rapeseed (Brassica napus L.var GSL-1) leaves; and 16–17 folds in grains of both the crops. Due to selenium accumulation, lipid and protein contents remained significantly low in leaves at both the stages of growth as well as grains of both the crops. Distinct differences were noticed in both the crops with respect to changes in total soluble sugars, reducing sugars and starch contents. In seleniferous soils, wheat leaves at stage I (55-day-growth) contained significantly higher amounts of total soluble sugars, reducing sugars and starch and thereafter these parameters recorded significantly low values in leaves at stage II (ear initiation) as well as in grains. On the other hand, a significant decrease in total soluble sugars and reducing sugars was recorded due to Se accumulation in rapeseed leaves at both growth stages. Quality of rapeseed grains harvested from seleniferous soil was not affected by Se accumulation except decrease in lipid content. Se accumulation did not influence the proportion of different lipid classes like phospholipids, glycolipids and sterols but reduced the proportion of unsaturated fatty acids.

The role of selenium in protecting plants against prairie dog herbivory: implications for the evolution of selenium hyperaccumulation

Some plants can hyperaccumulate the element selenium (Se) up to 10,000 mg Se kg(-1) dry weight. Hyperaccumulation has been hypothesized to defend against herbivory. In laboratory studies high Se levels protect plants from invertebrate herbivores and pathogens. However, field studies and mammalian herbivore studies that link Se accumulation to herbivory protection are lacking. In this study a combination of field surveys and manipulative field studies were carried out to determine whether plant Se accumulation in the field deters herbivory by black-tailed prairie dogs (Cynomys ludovicianus). The Se hyperaccumulator Astragalus bisulcatus (two-grooved milkvetch) occurs naturally on seleniferous soils in the Western USA, often on prairie dog colonies. Field surveys have shown that this Se hyperaccumulator is relatively abundant on some prairie dog colonies and suffers less herbivory than other forb species. This protection was likely owing to Se accumulation, as judged from subsequent manipulative field experiments. When given a choice between pairs of plants of the Se hyperaccumulator Stanleya pinnata (prince's plume) that were pretreated with or without Se, prairie dogs preferred to feed on the plants with low Se; the same results were obtained for the non-hyperaccumulator Brassica juncea (Indian mustard). Plants containing as little as 38 mg Se kg(-1) DW were protected from herbivory. Taken together these results shed light on the functional significance of Se hyperaccumulation and the possible selection pressures driving its evolution. They also have implications for the use of plants in Se phytoremediation, or as Se-fortified crops.

Distribution and bioavailability of selenium fractions in some seleniferous soils of Punjab, India

Soil-solid phase associations of Se in seleniferous soils of Punjab were investigated by following sequential extraction procedures involving multiple extractions with 0.2 M K2SO4 (2 times), 0.1 M Na2SeO3 (4 times), 0.05 M NH4OH (4 times), 6 M HCl (2 times) and 9 M HNO3 (2 times) vis-a-vis single extractions with 0.25 M KCl, 0.1 M KH2PO4, 4 M HCl and concentrated HCl. Soil samples were equilibrated with 75Se (as Na2SSeO3) @ 9.25 kBq g - 1 soil by incubating at field capacity moisture regime and subjecting to alternate wetting and drying cycles. Following multiple extraction procedure, out of total 75Se added, 8.8 - 26.1% was present in readily available form (0.2 M K2SO4 extractable); 27.6 - 49.0% as isotopically exchangeable (0.1M Na2SeO3 extractable) and 5.3 - 12.0% as organic Se (0.05 M NH4OH extractable). Selenium extractable in K2SO4 was significantly correlated with free iron (r = - 0.774, p < 0.05) and CaCO3 (r = 0.670, p <0.10) content of the soils. Negative relationship was observed between Se uptake by maize (Zea mays L.) and ammonium hydroxide extractable (r = - 0.752, p <0.05) as well as residual Se (r = - 0.726, p <0.05) in soils. Highly positive coefficients of correlation between isotopically exchangeable Se and Se content (r = 0.851, p <0.01) as well as its uptake by maize (r = 0.841, p <0.01) indicated that the isotopically exchangeable form of Se may be considered as an index of bioavailable Se in seleniferous soils of Punjab. None of the fractions defined by following single extraction procedure was correlated with either the soil characteristics or Se uptake by maize plants. Multiple extraction procedure could, thus, better explain the distribution of Se in different fractions and uptake by plants.

Evaluation of different extractants for the estimation of bioavailable selenium in seleniferous soils of Northwest India

Efficacy of various extractants—AB-DTPA, 0.5 m Na2CO3, 0.25 m KCl, 0.1 m KH2PO4, hot water, and isotopically exchangeable selenium (Se)—was studied for estimating bioavailable Se in 15 naturally occurring alkaline seleniferous soils in north-western India. Total Se concentration in these soils varied from 0.6 to 3.1 µg/g. Amount of Se extracted as isotopically exchangeable was the greatest, and that extracted with 0.5 m Na2CO3 was the smallest. When grown in the seleniferous soils, raya (Brassica juncea) accumulated (μg Se/g dry matter) 1.5–86.6, wheat (Triticum aestivum L.) 0.7–58.3, maize (Zea mays L.) 1.7–8.6, and rice (Oryza sativa L.) 1.5–4.6. Raya, wheat, and maize plants absorbed Se more than the maximum permissible level for animal consumption (5 μg Se/g dry matter) in 9, 8, and 4 experimental soils, respectively. Selenium concentration of maize was significantly correlated with the amount of Se extracted by 0.25 m KCl (r = 0.646,P &lt; 0.01), 0.1 m KH2PO4 (r = 0.498,P &lt; 0.10), and with total Se concentration (r = 0.628,P &lt; 0.05) of the soils; Se concentration in rice was correlated with AB-DTPA extractable Se (r = 0.443,P &lt; 0.10). Highly significant relationships between hot water soluble Se and concentration of Se in raya (r = 0.705,P &lt; 0.01), wheat (r = 0.696,P &lt; 0.01), maize (r = 0.698,P &lt; 0.01), and rice (r = 0.559,P &lt; 0.05) suggest that it can reliably quantify bioavailable Se in seleniferous soils of north-western India. Hot water soluble Se was positively correlated with electrical conductivity (r = 0.514,P &lt; 0.05), total Se concentration (r = 0.710,P &lt; 0.01), and KCl-extractable Se (r = 0.712,P &lt; 0.01) of the soils.

Speciation and weathering of selenium in upper cretaceous chalk and shale from South Dakota and Wyoming, USA

In geologic materials, petroleum, and the environment, selenium occurs in various oxidation states (VI, IV, 0, -II), mineralized forms, and organo-Se complexes. Each of these forms is characterized by specific chemical and biochemical properties that control the element’s solubility, toxicity, and environmental behavior. The organic rich chalks and shales of the Upper Cretaceous Niobrara Formation and the Pierre Shale in South Dakota and Wyoming are bentoniferous stratigraphic intervals characterized by anomalously high concentrations of naturally occurring Se. Numerous environmental problems have been associated with Se derived from these geological units, including the development of seleniferous soils and vegetation that are toxic to livestock and the contamination of drinking water supplies by Se mobilized in groundwater. This study describes a sequential extraction protocol followed by speciation treatments and quantitative analysis by Hydride Generation–Atomic Absorption Spectroscopy. This protocol was utilized to investigate the geochemical forms and the oxidation states in which Se occurs in these geologic units. Organic Se and di-selenide minerals are the predominant forms of Se present in the chalks, shales, and bentonites, but distinctive variations in these forms were observed between different sample types. Chalks contain significantly greater proportions of Se in the form of di-selenide minerals (including Se associated with pyrite) than the shales where base-soluble, humic, organo-Se complexes are more prevalent. A comparison between unweathered samples collected from lithologic drill cores and weathered samples collected from outcrop suggest that the humic, organic-Se compounds in shale are formed during oxidative weathering and that Se oxidized by weathering is more likely to be retained by shale than by chalk. Selenium enrichment in bentonites is inferred to result from secondary processes including the adsorption of Se mobilized by groundwater from surrounding organic rich sediments to clay mineral and iron hydroxide surfaces, as well as microbial reduction of Se within the bentonitic intervals. Distinct differences are inferred for the biogeochemical pathways that affected sedimentary Se sequestration during periods of chalk accumulation compared to shale deposition in the Cretaceous seaway. Mineralogy of sediment and the nature of the organic matter associated with each of these rock types have important implications for the environmental chemistry and release of Se to the environment during weathering.