Karaj S. Dhillon

Quality of underground water and its contribution towards selenium enrichment of the soil-plant system for a seleniferous region of northwest India

Quality of underground water drawn from 90 tubewells located in the seleniferous region of northwestern India was evaluated on the basis of Se content, electrical conductivity and residual sodium carbonate (RSC). All the water samples were found to be suitable for irrigation on the basis of total dissolved salts. With respect to RSC ratings, 86% of the samples were marginally fit and 12% of the samples were unfit for irrigation purposes. Selenium content of tubewell waters at or near the toxic sites ranged between 0.25 and 69.5 mg l21 with an average value of 4.7 mg l21. The maximum contamination level (MCL) of 10 mg Se l21 for drinking purposes was exceeded by 11.1% of the tubewell waters and the maximum permissible level (MPL) of 20 mg Se l21 for irrigation purposes was exceeded by 4.4% of the waters. In water samples having .1 mg Se l21, a significant positive relationship (r ¼ 0.287p) of Se with pH was observed and the r values increased to 0.919 as the level of Se in the water increased. Underground water pumped from shallow tubewells (24–36 m depth) contained 2–3 times more Se than that from deep tubewells (76 m depth). Selenium enrichment in the soil obtained by subtracting Se removed by crops from Se additions through irrigation water, was found to be highly positive in the case of rice and sunflower. Selenium balances turned out to be negative for mustard followed by Egyptian clover, pearl millet and wheat. In rice-based cropping sequences, Se balances were positive, suggesting that cultivation of rice may be discouraged if the aim is to reduce accumulation in shallow groundwater. More area should be brought under mustard, as its cultivation results in depletion of Se from the soils. Even irrigation with water containing Se at MPL should result in accumulation of Se in the soil under rice, sunflower, sugarcane, maize or oat.

Selenium adsorption in soils as influenced by different anions

Studies on selenium adsorption were conducted on seleniferous and non-seleniferous soils of north-west India. Soils were equilibrated with graded levels of Se ranging from 1 to 100 μg ml -1 tagged with 75 Se in the presence of sulphate, nitrate and phosphate ions, generally being applied to soils as inorganic fertilizers. The adsorption of Se on different soils, both in the presence and absence of competing anions, increased with increase in the level of Se added. Adsorption of Se conformed to Langmuir equation. In the absence of any competing anions, adsorption maxima of Se for different soils ranged from 270 to 461 μg g -1 . The corresponding values decreased appreciably in the presence of competing anions; per cent decrease ranged from 3 to 21 at 10 μg SO 4 -S ml -1 , from 8 to 40 at 60 μg NO 3 -N ml -1 and 32 to 56 at 15 μg H 2 PO 4 -P ml -1 . The bonding energy of Se in different soils decreased by 33 to 66 per cent in the presence of only phosphate ions. The changes in bonding energy were inconsistent in the case of nitrate and sulphate ions. At equal concentration of added P and Se, the amount of P adsorbed was 2 to 3 times the amount of Se adsorbed. With increasing concentration of Se, greater amounts of S were released in the equilibrium solution. The distribution coefficients (K d ) decreased significantly in the presence of different anions; the effect was conspicuous in the case of phosphate ions.

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 134d of incubation of 500g soil amended with 2% of plant tissues, the maximum amount of Se was volatilized with cowpea leaves (385ng) followed by wheat grains, leaves of maize, sugarcane (Saccharum officcinarum), arhar, poplar (Populus deltoides) and the control (91ng). 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.

Selenium accumulation by sequentially grown wheat and rice as influenced by gypsum application in a seleniferous soil

A field experiment was conducted for 2 years on an alkaline calcareous seleniferous soil to study the effect of different levels of gypsum (0.2 – 3.2 t ha−1) applied to wheat only in the first year on Se accumulation by wheat (Triticum aestivum L.) – rice (Oryza sativa L.) cropping sequence. With gypsum application, grain yield of both rice and wheat crops increased by 0.4 – 0.5 t ha−1; the increase in straw yield was 0.4 – 1.1 t ha−1. Significant reduction in Se accumulation by wheat was observed with gypsum application up to 0.8 t ha−1 and its residual effect was evident on the following crops for 2 years. Reduction in Se accumulation varied from 53 to 64% in wheat grain, 46 to 49% in wheat straw, 35 to 63% in rice grain and 36 to 51% in rice straw with an application of gypsum at 0.8 t ha−1. A corresponding increase in S concentration was observed. In the gypsum-treated plots, the ratio of S:Se increased by 6 – 8 times in wheat and 3 – 6 times in rice. Reduction in Se accumulation by crop plants through gypsum application may help in lowering the risk of Se over-exposure of animals and humans that depend on diet materials grown on high selenium soils.

Selenium distribution and speciation in plant parts of wheat (Triticum aestivum) and Indian mustard (Brassica juncea) from a seleniferous area of Punjab, India

The concentration, distribution, and speciation of selenium in different parts of wheat and Indian mustard, grown in a seleniferous area in Punjab, were investigated using synchrotron based (XAS) and classical acid digestion and extraction methods. The analyses revealed a high Se enrichment in all investigated plant parts, with Se levels in the range of 133-931 mg/kg (dry weight, dw). Such high Se enrichment is mainly due to the considerable amounts of easily available Se detected in the soil, which are renewed on a yearly basis to some extent via irrigation. Speciation analysis in soil and plants indicated selenate and organic Se as major Se species taken up by plants, with a minor presence of selenite. The analyses also revealed that the highest Se enrichment occurs in the upper plant parts, in agreement with the high uptake rate and mobility of selenate within plants. In both wheat and mustard, highest Se enrichments were found in leaves (387 mg/kg·dw in wheat and 931 mg/kg·dw in mustard). Organic species (dimethylselenide and methylselenocysteine) were found in different parts of both plants, indicating that an active detoxification response to the high Se uptake is taking place through methylation and/or volatilization. The high proportion of selenate in wheat and mustard leaves (47% and 70%, respectively) is the result of the inability of the plant metabolism to completely transform selenate to non-toxic organic forms, if oversupplied. Methylselenocysteine, a common Se species in accumulating plants, was detected in wheat, suggesting that, in the presence of high Se concentration, this plant develops similar response mechanisms to accumulator plants.

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 degrees to 31.2175 degrees N and 76.1363 degrees to 76.4147 degrees 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. Dry matter yield of Se-sensitive weed plants was 25 to 62% of the yield in the no-Se control at 1.25mg selenate-Sekg(-1) and 6 to 40% at 2.5mg 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, p<0.01, 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.

Accumulation of selenium in sugarcane (Sachharum officinarum Linn.) in seleniferous areas of Punjab, India

A survey was conducted during 1986–88 to assess the level and pattern of accumulation of selenium in sugarcane plants in seleniferous areas of Punjab (India). Total and water-extractable (available) selenium ranged from 0.55 to 2.58 (mean 1.43 ± 0.67) mg kg−1 and from 0.02 to 0.05 (mean 0.033 ± 0.007) mg kg−1, respectively, in seleniferous areas. Corresponding values from non-seleniferous areas were 0.23–0.55 (mean 0.36 ± 0.08) mg kg−1 and 0.015–0.025 (mean 0.020 ± 0.003) mg kg−1, respectively. Sugarcane tops from seleniferous areas accumulated high levels of selenium ranging from 7.9 to 67.5 mg kg−1. These selenium levels were 6–14 times higher than those from non-seleniferous areas. During the early stages of growth (June), the selenium content was highest but decreased during the months of July and August and then did not change up to maturity. In the seleniferous areas sugarcane tops and canes at maturity contained 5.7–9.5 and 1.8–2.1 mg Se kg−1, respectively. However, the tops and canes of plants growing near the permanent boundary (bundh) contained 9.5–18.8 and 2.1–2.4 mg Se kg−1, respectively.In a field experiment on sugarcane, application of gypsum up to 1 ton ha−1 resulted in a significant reduction of selenium content in sugarcane tops as well as in the cane. Selenium content in sugarcane tops at maturity was reduced from 15.16 to 5.08 mg kg−1 by applying gypsum of 1 ton ha−1.

Development and mapping of seleniferous soils in northwestern India.

Periodic surveys were undertaken to identify and characterize Se-contaminated soils in northwestern India. Total Se content varied from 0.023 to 4.91mgkg(-1) in 0-15cm surface soil and 0.64-515.0mgkg(-1) in samples of vegetation. Selenium-contaminated land occupying an area of 865ha was classified into different categories based on total Se content of soils as moderately contaminated (0.5-2.0mg Sekg(-1)) and highly contaminated (>2.0mg Sekg(-1)). The normal soils contained <0.5mg Sekg(-1). The soil map was prepared using village level cadastral maps. Se-contaminated soils were silty loam to silty clay loam in texture and tested pH 7.9-8.8, electrical conductivity 0.3-0.7dSm(-1), calcium carbonate 0.1-4.1% and organic carbon 0.4-1.0%. Selenium was present throughout the soil profile up to 2m depth; 0-15cm surface soil layer contained 1.5 to 6.0 times more Se than in subsurface layers. Selenium content in rock samples collected from lower and upper Shiwalik sub-Himalayan ranges varied from 1864 to 2754 and 11 to 847μgkg(-1), respectively. The sediments transported through seasonal rivulets linking the Shiwalik ranges to affected sites contained 0.57-2.89mg Sekg(-1). The underground water containing 2.5-69.5μg SeL(-1) used for irrigating transplanted rice grown in Se-contaminated area resulted in a net Se addition in soil up to 881gha(-1)y(-1); possibly further aggravating the Se-toxicity problem. Presence of substantial amount of Se in rock samples and sediments of seasonal rivulets suggests that Se-rich materials are being transported from Shiwalik hills and deposited in regions where seasonal rivulets end up.

The influence of zinc and sulphur deficiency on oil-filling in peanut (Arachis hypogaea L.) kernels

In the developing peanut (Arachis hypogaea L.) kernels, the period between 15 and 35 days after podding (DAP) was identified as the active period of oil-filling. The period of active oil-filling was associated with a decrease in the starch, soluble sugars and proteins so as to make available the energy and carbon skeleton for the synthesis of oil. The oil content in the mature kernels decreased by 11, 12 and 25 per cent with Zn, S and Zn+S deficiency, respectively. In addition, proteins and starch content decreased significantly while that of soluble sugars increased slightly. The activity of malate dehydrogenase and glucose-6-phosphate dehydrogenase also decreased due to Zn as well as S deficiency. The deficiency treatments resulted in a decrease in phospholipids, free fatty acids and triacylglycerols in mature kernels. Further the proportion of 16∶0 and 18∶2 decreased while that of 18∶1 increased in developing kernels.

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.