Chemical forms and stability of selenium in soil

The chemical forms of selenium (Se) in soils derived from different parent rocks, and the stability of Se species in soils were investigated. Around 40% of the total Se occurred as organic Se on the average. Total selenite content was higher than that of total selenate. The content of organic Se decreased with heating of the soils at high temperatures, and most of it disappeared at heating temperatures above 500°C. However, the results of incubation experiments indicated that Se species were stable in soil, and that their chemical forms did not change appreciably with the variation of soil conditions. In soluble Se, organic Se accounted for about 50%, and in contrast to the total Se, the selenate content was higher than that of selenite.     

外源硒（Se(4＋)和Se(6＋））污染对土壤酶活性的影响

利用盆栽试验,研究了不同浓度、不同价态外源硒（Se4＋、Se6＋）对连续种植小白菜土壤中过氧化氢酶、脱氢酶、脲酶和碱性磷酸酶活性的影响,为土壤硒污染的生态风险评价和管理提供科学依据。结果表明：低浓度硒对土壤酶活性有不同程度的激活效应,而高浓度硒对4种土壤酶均产生抑制作用;外源硒对脲酶及脱氢酶活性的抑制作用大于碱性磷酸酶和过氧化氢酶。外源Se4＋及Se6＋浓度与土壤脲酶活性间都存在显著的负相关（P〈0.01）,且同浓度两个价态硒差异显著（P〈0.05）,说明脲酶可作为土壤硒污染程度的生态风险评价的生物指标;而过氧化氢酶、脱氢酶及碱性磷酸酶只能表征一定时间段内土壤硒污染的程度。土壤酶的ED5（0生态剂量）均随硒施入时间的延长而增大,以脲酶的ED50值最小,Se6＋的ED50小于Se4＋,生态毒性大于Se4＋。     

叶面喷施不同形态硒对草莓吸收和转运硒的影响

采用温室大棚试验,研究了叶面喷施硒酸钠和亚硒酸钠对提高草莓硒含量的影响以及硒在草莓植株中的转运和分布,旨在为富硒水果的开发和生产提供理论依据。本试验在草莓初花期进行叶面喷施亚硒酸钠和硒酸钠,当喷硒量为20、40、60 g Se·hm~(-2)时,亚硒酸钠处理40 d后草莓果实样品中硒含量分别为0.03、0.10、0.15 mg·kg~(-1)FW,分别为对照的3、20倍和30倍;硒酸钠处理40d后草莓果实硒含量分别为0.05、0.12、0.17 mg·kg~(-1) FW,是对照处理的5、24倍和34倍。但是叶面喷施的硒持续供给果实的能力有限,随着草莓果实的连续生长,后期采收的草莓中硒含量显著下降。叶面喷硒后,叶片吸收的亚硒酸钠约有30%转移到了根部,而硒酸钠则更多地累积于叶片中,转移到根部的量不到15%。草莓初花期叶面喷施硒可以显著提高前期草莓果实中硒的含量,随着采收期的延长,草莓中硒的含量显著下降,喷施亚硒酸钠的处理硒向根部转移的能力高于硒酸钠的处理。     

Germination-enhancing and zinc-sparing roles for selenium in broccoli

Large areas of China have soils low in both available selenium (Se) and zinc (Zn). In order to investigate whether Se supplied as either selenate or selenite can increase germination and growth compared with low-Se controls we used broccoli, an important vegetable with anticancer effects, especially when biofortified with Se. Broccoli was grown under both Zn adequacy and Zn deficiency to determine whether interactions between these minerals affect plant growth. Selenite and selenate at a wide range of doses increased the speed and extent of germination. Both inorganic Se forms increased early root and shoot growth at low concentrations, with selenite having a stronger effect than selenate. A sand culture trial showed a similar growth increase due to low-dose Se under Zn deficiency but not under Zn adequacy. Conversely, at high Se levels, the results provided evidence from biomass, water use, photosynthesis and gas exchange that broccoli growth was inhibited at high Se levels, with selenite being more toxic than selenate. In this broccoli trial, the two Se forms were equally effective in increasing leaf Se concentration, whereas in most plants selenite is largely converted to organic Se forms and stored in the roots. This study suggests that Se, supplied either as selenate or selenite, may improve germination and growth in broccoli, especially on Zn-deficient soils. Field trials conducted on soils which are very low in both plant-available Se and Zn are needed.     

Use of selenium fertilizers for production of Se‐enriched Kenaf (Hibiscus cannabinus): Effect on Se concentration and plant productivity

Due to selenium (Se) deficiency, Se fortification of food and feed is applied in many countries. Therefore, potential use of Se‐enriched kenaf was investigated based on its Se accumulation, its potential to transform accumulated Se to other Se species, and effect of Se accumulation on its growth. Kenaf was grown with different levels of two Se fertilizers (selenite and selenate) at concentrations ranging from 0 to 4 mg Se (kg soil)^–1. Total Se concentrations in the plants grown on selenate‐treated soil amounted to (1019 ± 136) mg Se (kg dry weight)^–1 and were much higher compared to plants grown on selenite‐treated soil. Identified Se species were selenite, selenate, Se‐methionine, and Se‐cystine. Biomass yield, net photosynthesis, and chlorophyll index of the plants decreased when plants were grown on soils treated with high doses of selenate.     

不同价态外源硒对冬小麦生长及生理代谢的影响

用盆栽试验研究了不同浓度外源硒（四价和六价）处理土壤对小麦生长和生理代谢的影响,旨在为富硒小麦的种植和开发提供理论依据。结果表明：外源硒抑制了苗期小麦的生长（株高和生物量）;而在生长后期,随外源硒浓度增加表现为先促进后抑制作用。Se（Ⅳ）和Se（Ⅵ）处理对小麦生物量影响的EC10分别从苗期的1.40mg·kg-1和0.28mg·kg-1增加到收获期的28.48mg·kg-1和4.56mg·kg-1,表明小麦对硒的耐受浓度随生长时间的延长逐渐增大。Se（Ⅵ）低浓度（≤1mg·kg-1）对小麦生长的促进作用及高浓度（≥2.5mg·kg-1）对小麦生长的抑制作用均大于Se（Ⅳ）。小麦籽粒产量随外源Se（Ⅳ）和Se（Ⅵ）浓度的增大先升高而后降低,分别在2.5mg·kg-1和1mg·kg-1浓度处理时达到最大。低浓度硒处理（Se（Ⅳ）≤5mg·kg-1和Se（Ⅵ）≤0.5mg·kg-1）能促进拔节期小麦叶片中GSH和POD等抗氧化酶活性,显著降低叶片过氧化物质MDA（Se≤2.5mg·kg-1）含量,高浓度硒处理则与之相反。     

Determination of selenium concentration of rice in china and effect of fertilization of selenite and selenate on selenium content of rice

A method of hydride generation atomic fluorescence spectrometry was applied to the determination of the selenium concentration of regular polished rice in China and selenium-enriched polished rice obtained by foliar application of selenium-enriched fertilizer in the forms of selenite and selenate. The average selenium content of regular rice was 0.025 +/- 0.011 microg g(-)(1). On the basis of a daily dietary rice intake of 300-500 g suggested by the China Nutrition Society, the total selenium intake from regular rice was calculated to be 7.5-12.5 microg per person per day for an adult. The selenium contents of rice were significantly increased to 0.471-0.640 microg g(-)(1) by foliar application of selenium-enriched fertilizer at rate of 20 g of Se ha(-)(1) in the forms of sodium selenite and sodium selenate. The selenium content of rice by application of a fertilizer of selenate was 35.9% higher than that by a fertilizer of selenite, which showed that Se-enriched fertilizer in selenate exhibited greater efficiency in increasing Se content in rice products. The Se-enriched rice products can increase daily Se intake on average by 100-200 microg of Se per day by the consumption of 400 g of rice products if the Se level of rice products is controlled at 0.3-0.5 microg of Se g(-)(1). Because rice is a staple food in China, selenium-enriched rice obtained by bioenrichment of selenium to increase the Se content of rice could be a good selenium source for the population in selenium-deficient regions.     

不同形态叶面硒肥对水稻吸收和转运硒的影响

通过田间试验,研究了不同形态叶面硒肥对水稻吸收和转运硒的影响。与空白对照相比,亚硒酸钠、硒酸钠、硒代蛋氨酸和化学纳米硒在水稻扬花期一次施用(15g/hm²)可使水稻籽粒、颍壳和秸秆的硒含量分别提高0.06～0.64、0.36～0.83和0.32～0.75 mg/kg。籽粒硒的回收率大小顺序为：硒代蛋氨酸(34.6%)>亚硒酸钠(15.7%)>硒酸钠(15.0%)>化学纳米硒(6.6%);硒在水稻各部位中的分配比例的高低顺序为：秸秆>籽粒>颍壳。此外,硒用量与水稻籽粒的硒含量呈极显著线性相关。按照黑龙江省富硒大米的地方标准(DB23T 790—2004),达到一等大米的硒含量指标(0.20～0.30 mg/kg),亚硒酸钠单施的施用量为6.01～10.62 g/hm²,腐植酸+亚硒酸钠复合肥的施用量为4.26～8.63 g/hm²。硒代蛋氨酸的富硒效率高于其他3种硒形态,腐植酸+亚硒酸钠复合肥的富硒效率高于亚硒酸钠单施。     

Retention and Uptake by Plants of Added Selenium in Peat Soils

In soil, adsorption of selenium (Se) onto mineral surfaces is accompanied by poorly known retention via organic matter. The effects of these components on the availability of Se were examined in two pot experiments. Spring wheat was grown with increasing amounts of selenate (SeO₄ ^2–) in one sand and three peat soils, and ryegrass with selenate and selenite (SeO₃ ^2–) in sphagnum peat manipulated by iron (Fe) hydroxide. Selenate persisted in soluble form, whereas selenite was fixed in the soil. In wheat, 5–50% of the selenate addition was recovered in the plant, the proportion increasing with increasing Se. In ryegrass, 30–40% of the added selenate but less than 2% of the selenite was found within the leaves. The Fe hydroxide enrichment enhanced the selenite uptake. Phosphate buffer desorbed a minor proportion of the added selenite, except in peat amply enriched with Fe hydroxide. The results suggest that the retention mechanism of selenite was changed due to the hydroxide amendment.     

Selenium speciation in soil and rice: influence of water management and Se fertilization

Rice (Oryza sativa) is the staple food for half of the world's population, but the selenium (Se) concentrations in rice grain are low in many rice-growing regions. This study investigated the effects of water management on the Se speciation dynamics in the soil solution and Se uptake and speciation in rice in a pot experiment. A control containing no Se or 0.5 mg kg(-1) of soil of selenite or selenate was added to the soil, and plants were grown under aerobic or flooded conditions. Flooding soil increased soluble Se concentration when no Se or selenite was added to the soil, but decreased it markedly when selenate was added. Selenate was the main species in the +selenate treatment, whereas selenite and selenomethionine selenium oxide were detected in the flooded soil solutions of the control and +selenite treatments. Grain Se concentration was 49% higher in the flooded than in the aerobic treatments without Se addition. In contrast, when selenate or selenite was added, the aerobically grown rice contained 25- and 2-fold, respectively, more Se in grain than the anaerobically grown rice. Analysis of Se in rice grain using enzymatic hydrolysis followed by HPLC-ICP-MS and in situ X-ray absorption near-edge structure (XANES) showed selenomethionine to be the predominant Se species. The study showed that selenate addition to aerobic soil was the most effective way to increase Se concentration in rice grain.     

Effect of selenium on the yield and quality of green tea leaves harvested in early spring

Foliar applications of a fertilizer of selenite or selenate were carried out to determine the influence of selenium on the yield and quality of green tea leaves harvested in early spring. Numbers of sprouts and the yield were significantly increased by the application of selenium. The sweetness and aroma of green tea leaves were also significantly enhanced, and bitterness was significantly decreased by the application of selenium. However, no significant differences were found in sweetness, bitterness, and aroma between tea leaves fertilized with selenite and selenate. Se concentration was significantly increased by selenium fertilization, and tea enriched by sodium selenate had a significantly higher selenium content than did tea enriched by sodium selenite. Total amino acid and vitamin C contents were significantly enhanced by the application of selenium. Tea polyphenol contents were significantly decreased by fertilization with selenium. The marked difference of tea polyphenols was also found between applications of selenite and selenate.     

Comparison of In Vitro PBET and Phosphoric Acid Extraction as an Approach to Estimate Selenite and Selenate Bioaccessibility from Soil

Several approaches have been used to estimate the bioaccessibility of trace metals from soils. Here, we applied phosphoric acid extraction and the in vitro test physiologically based extraction (PBET) to soils containing selenium (Se) and compared their performance in estimating the bioaccessibility of Se. For this purpose, we used two soil samples and two Certified Reference Material soil samples with a range of Se concentrations. The total Se contents were measured in the samples and in the extracts by hydride generation–atomic fluorescence spectroscopy. Moreover, we also measured selenite and selenate in the soil extracts (from phosphoric acid and from the PBET) using the coupled techniques liquid chromatography–UV photooxidation–atomic fluorescence spectroscopy and liquid chromatography–mass spectrometry with inductively coupled plasma. From the results obtained in the present study, the PBET showed that the selenium bioaccessible fraction was mainly attributed to the gastrointestinal step. When comparing the results from PBET with those of the phosphoric acid extraction, similar values of Se (IV) and Se (VI) were obtained for both extraction systems. An estimation of the bioaccessibility percentage of Se is also reported.     

紫甘薯对硒的吸收和累积特征

以紫甘薯为试验材料,采用盆栽试验的方法研究了基施硒酸钠[Se(VI)]和亚硒酸钠[Se(IV)]条件下,紫甘薯对外源硒的吸收累积规律,并比较了施用两种不同价态硒的紫甘薯富硒效果。结果表明:两种硒源均可显著提高紫甘薯各器官含硒量,且紫甘薯含硒量均随施硒量的增加而增大。当土壤施硒量为Se 8 mg/kg时,施用硒酸钠、亚硒酸钠收获期薯块的硒含量(干基)分别达到6.69、0.88 mg/kg。紫甘薯生育期40 d时各器官硒含量叶茎薯块,130 d时硒含量叶薯块茎。当硒酸钠施用量为Se 4 mg/kg时,紫甘薯薯块中的硒累积量最高达923.81μg/株,硒在紫甘薯块根中的分配率可达67%~70%,硒酸钠处理下,紫甘薯对硒的吸收利用率远远高于亚硒酸钠处理。综合紫甘薯含硒量和施硒量对生长的影响结果分析,施用硒酸钠和亚硒酸钠均能增加紫甘薯薯块的硒含量,紫甘薯对硒酸钠敏感性高于亚硒酸钠,生产过程中应充分考虑施用硒酸钠对作物造成的毒害。     

Selenium Speciation and Distribution Characteristics in the Rhizosphere Soil of Rice (Oryza sativa L.) Seedlings

A five-step sequential extraction procedure was used for the fractionation of selenium (Se) in rhizosphere and nonrhizosphere soils with rice (Oryza sativa L.) seedlings. Results showed that in rhizosphere soils without the addition of Se, the soluble Se (Sol-Se), exchangeable Se and Se bound to carbonates (Exc-Se), Se bound to organic-sulfide matter and elemental Se (OM-Se), and total Se contents were significantly greater than those in nonrhizosphere soils, whereas the residual Se (Res-Se) was less than that in the nonrhizosphere soils. After the addition of Se, the Sol-Se and OM-Se contents in the rhizosphere soils were still evidently greater than those in nonrhizosphere soils, but the Exc-Se was significantly less in rhizosphere soils than in nonrhizosphere soils. Our overall results suggest that the Se bioavailability in rice rhizosphere soils is greater than that in nonrhizosphere soils. Selenium bioavailability in the rhizosphere soil is not correlated with Se accumulation in rice seedlings.     

Foliar application of selenite and selenate to potato (Solanum tuberosum): effect of a ligand agent on selenium content of tubers

The effect of a foliar spray of selenium on potatoes was investigated for 2 years. Amounts of 0, 50, and 150 g of Se ha(-)(1) were applied both as sodium selenate and as sodium selenite in water, either pure or with the addition of 0.15% of soluble leonardite as a source of humic acids (pH 7). Tuber selenium concentration increased with the application levels, both with sodium selenate and with sodium selenite, when only aqueous solutions were used. When humic acids were added, the tuber selenium level rose more markedly after the application of sodium selenate as compared to the case of the aqueous solutions; however, in the case of sodium selenite, the level showed a large increase only after the application of 50 g of Se ha(-)(1). Kinetics showed that humic acids raised the selenate availability, but no differences were found in the distribution of selenium in the tuber fractions. Foliar application of selenium with humic acids was proven to be a good way to increase the selenium content of potatoes, but the assimilation process of selenium was simpler with selenate than with selenite.     

Effect of phosphorus status of the soil on selenium availability

Due to its importance for human and animal health, low bioavailability of selenium (Se) is of concern in large parts of the world. Among the factors determining Se availability is competition for binding sites by other anions. In order to evaluate the effect of different soil P status on Se availability from fertilizer, adsorption studies were conducted with soils ranging from low to very high available P as measured in ammonium lactate (P‐AL) and addition of Se as either selenate or selenite. Generally selenite, and to some extent also selenate, adsorption decreased with increasing P‐AL status of the soil. However, in a silt loam, the increase in P‐AL from 140 (high) to 210 mg P kg^–1 (very high) did not result in a corresponding decrease in Se adsorption. Phosphorus saturation, on the other hand, was found to be lower in the sample that was very high in available P, suggesting that both the total amount of P on binding sites and the amount of plant‐available P influence Se availability. Selenate addition caused an increase in P availability, especially when added together with phosphate to a silt loam with very high P‐AL status.     

Effects of Organic Matter-Rich Amendments on Selenium Mobility in Soils

Soil organic matter (SOM) plays an important role in the Se dynamics in soil. The potential effects of vermicompost and digestate as important sources of SOM on selenium (Se) mobility were assessed in this study. Three soils differing in their physicochemical parameters, fluvisol, chernozem, and luvisol, were chosen, and three types of vermicomposts based on various bio-waste materials as digestate (vermicompost 1), kitchen waste with woodchips (vermicompost 2), and garden bio-waste (vermicompost 3) were used due to their high organic matter content. Additionally, digestate samples alone were applied. To evaluate the potential effect of vermicompost application on sorption characteristics of soils, batch sorption experiments were performed. The results showed a predominant effect on Se species in the soils, where selenite sorbed more intensively compared to selenate, regardless of the soil and ameliorative material applied. In the control, the soil sorption ability of selenite tended to decrease in the order:fluvisol > luvisol > chernozem. However, these differences were not significant. Moreover, the effects of the ameliorative materials depended on both soil and amendment used. In fluvisol, all the amendment applications resulted in a decrease in distribution coefficient (K_d values) of Se, whereas in chernozem, this effect was observed only for the digestate-based vermicompost 1. Increasing K_d levels were reported in luvisol treated with digestate; the application of garden bio-waste-based vermicompost 3 tended to decrease the K_d values. Further studies are required on long-term effects of these amendments on Se mobility in soils and the role of individual organic matter fractions in this context.     

Selenium Accumulation and Biochemical Composition of Brassica Grains Grown in Selenate- or Selenite-Treated Alkaline Sandy Loam Soil

In a greenhouse experiment, Brassica plants were grown in an alkaline sandy loam soil treated with different levels of selenate selenium (Se) or selenite Se ranging from 0 to 4 mg Se kg^?1. Plants grown in Se-treated soil were stressed at an early stage of pod setting and produced fewer pods per plant. Selenium accumulation increased by 2- to 35-fold in shoots, 3- to 19-fold in roots, and 2- to 57-fold in grains. Selenium accumulation in grains resulted in significant increases in contents of reducing sugars, starch, glucosinolate, and free and sulfur-containing amino acids and a decrease in lipid content. Selenium accumulation significantly increased the proportions of different lipid classes such as glycolipids, sterols, and free fatty acids whereas triacylglyceride content showed the reverse trend. Oil extracted from Brassica grains grown in the seleniferous region contained Se within safe limits and thus is safe for human consumption.     

Influence of submergence and subsequent drainage on the partitioning and lability of added selenium fertilizers in a sulphur‐containing Fluvisol

We used an isotope dilution method to examine the time‐dependent changes in the partitioning and lability of selenium (Se) in a Sri Lankan rice soil after adding fertilizer with selenite (Se(IV)) and selenate (Se(VI)) (1 mg kg^?1) and incubation under anaerobic (submerged) (30 days) and subsequent aerobic (drained) conditions (7 days) in controlled reaction vessels mimicking rice paddy water management practices. The K_d (the ratio of sorbed ion to that in solution) values for Se(IV) were significantly (P≤ 0.001) larger than those for Se(VI) in all treatments and at all sampling times. The K_d values for Se(IV) and Se(VI) decreased significantly (P≤ 0.001) with time during the anaerobic and subsequent aerobic phases. Applied Se(IV) fertilizer was rapidly removed into non‐labile pools during the anaerobic phase (day 0 = 60% labile and day 14 = no labile Se), with no significant increase in the labile pool following short‐term aeration. The results suggest that the rapid decrease in Se(IV) lability may be caused by the strong non‐reversible (at least for 7 days) sorption of Se (IV). In contrast, applied Se(VI) fertilizer was 90% labile at 0 day and decreased during the anaerobic phase to 30% after 30 days. There was no significant change in the lability of Se(VI) following the short‐term aerobic phase following anaerobic conditions. These results indicate that Se(IV) would not be an effective pre‐planting fertilizer for rice production. Selenate is likely to be more effective, but losses to non‐labile forms during the submerged phase of rice production may mean that efficiency of pre‐planting Se(VI) fertilizer is also compromised.     

植物对硒的吸收转运和形态转化机制

阐述了植物对不同形态硒的吸收、转运和形态转化机制。植物主要吸收水溶性硒,包括部分有机硒、硒酸盐和亚硒酸盐。多数研究表明植物对硒酸盐的主动吸收是通过高亲和力的硫酸盐转运子完成,最近的研究表明磷酸盐可以调节亚硒酸盐的吸收,磷酸盐转运子在亚硒酸盐的主动吸收过程中有重要作用;植物吸收的硒酸盐很快从根部转移到地上部,在叶片中被还原成亚硒酸盐,进而转化为有机硒化物进入其他组织;而亚硒酸盐及其代谢产物主要积累在根部,极少转移到地上部。进入植物体中的硒转化为含硒氨基酸和硒蛋白参与植物的代谢。