Selenium Supplementation in Horticultural Crops 3030704858, 9783030704858

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Alžbeta Hegedűsová · Ondrej Hegedűs Silvia Jakabová · Alena Andrejiová Miroslav Šlosár · Ivana Mezeyová  Marcel Golian

Selenium Supplementation in Horticultural Crops

Selenium Supplementation in Horticultural Crops

Alžbeta Hegedűsová • Ondrej Hegedűs •  Silvia Jakabová • Alena Andrejiová •  Miroslav Šlosár • Ivana Mezeyová •  Marcel Golian

Selenium Supplementation in Horticultural Crops

Alžbeta Hegedűsová Faculty of Horticulture and Landscape Engineering, Department of Vegetable Production Slovak University of Agriculture in Nitra Nitra, Slovakia

Ondrej Hegedűs Faculty of Education, Department of Chemistry J. Selye University Komárno, Slovakia

Silvia Jakabová Faculty of Biotechnology and Food Sciences, BioFood Centre Slovak University of Agriculture in Nitra Nitra, Slovakia

Alena Andrejiová Faculty of Horticulture and Landscape Engineering, Department of Vegetable Production Slovak University of Agriculture in Nitra Nitra, Slovakia

Miroslav Šlosár Faculty of Horticulture and Landscape Engineering, Department of Vegetable Production Slovak University of Agriculture in Nitra Nitra, Slovakia

Ivana Mezeyová Faculty of Horticulture and Landscape Engineering, Department of Vegetable Production Slovak University of Agriculture in Nitra Nitra, Slovakia

Marcel Golian Faculty of Horticulture and Landscape Engineering, Department of Vegetable Production Slovak University of Agriculture in Nitra Nitra, Slovakia

ISBN 978-3-030-70485-8    ISBN 978-3-030-70486-5 (eBook) https://doi.org/10.1007/978-3-030-70486-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Selenium is a naturally occurring essential microelement that plays an important role in the oxidative defense system of humans and animals. There are many countries in the world with low selenium content in the soil. Therefore, average daily human intake is limited through the food chain. Agricultural plants are an important source of selenium for humans. Selenium can accumulate in plants, and its binding in inorganic compounds is interesting in terms of increasing its bioavailability. Crop biofortification is a way to improve selenium status in humans. Various techniques for its supplementation in food have become popular for increasing its content in the food chain. The book provides an overview of the importance of selenium for humans and focuses on the possibilities of supplementing selenium in human nutrition with its income from plant sources. The book presents the results of selenium biofortification experiments with horticultural crops. The enrichment of various species of vegetables, herbs, and oyster fungi with selenium was realized by fertilizing the soil and foliage application of inorganic forms of selenium. The ability of selenium to accumulate in horticultural crops from different groups is presented. The aim was to produce edible plants with enhanced nutritional value. Nitra, Slovakia December 2020

Alžbeta Hegedűsová

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Acknowledgments

This publication was supported by the Operational Program Integrated Infrastructure within the project: Demand-driven research for the sustainable and innovative food, Drive4SIFood 313011V336, cofinanced by the European Regional Development Fund, was supported by the Grant KEGA no. 017/SPU-4/2019 Innovation of the content structure and e-Learning in the study programs of Food Safety and Control and Food and Technology in Gastronomy, and Grant KEGA no. 018SPU-4/2020 Development of Theoretical Knowledge and Practical Skills of Students for Teaching of Subject Vegetable Production.

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Contents

1 Introduction����������������������������������������������������������������������������������������������    1 2 The Role of Selenium in Human Nutrition��������������������������������������������    3 2.1 Selenium as an Essential Element����������������������������������������������������    3 2.1.1 Symptoms of Selenium Deficiency in Human Body��������������������������������������������������    3 2.1.2 Selenium Presence and Transport in Human Body��������������    6 2.1.3 Biochemical Function of Selenium in the Human Organism��������������������������������������������������������    8 2.2 Occurrence of Selenium in Nature and Its Potential Sources ����������   11 2.2.1 Selenium Occurrence in Soil������������������������������������������������   11 2.2.2 Selenium Occurrence in Water���������������������������������������������   21 2.2.3 Selenium Occurrence in Air��������������������������������������������������   22 2.3 Entering Selenium into the Food Chain��������������������������������������������   23 2.3.1 Selenium Uptake and Transport in the Plant������������������������   23 2.3.2 Incorporation of Selenium into Proteins ������������������������������   27 2.3.3 Distribution of Selenium in the Plant������������������������������������   30 2.3.4 Degradation of Selenium in the Plant ����������������������������������   31 2.3.5 Selenium Content in Food of Plant Origin ��������������������������   32 2.4 Options of Selenium Supplementation in Food Chain ��������������������   35 2.4.1 Experiences with Selenium Biofortification of Horticultural Crops Across the World������������������������������   37 3 Methods for the Determination of Selenium in Foodstuffs������������������   47 3.1 Methods for the Determination of Inorganic Selenium Compounds ����������������������������������������������������������������������   47 3.2 Methods for the Determination of Organic Selenium Compounds ����������������������������������������������������������������������   50

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4 Selenium Uptake by Selected Vegetable Species After Fortification of the Growing Substrate��������������������������������������������������   53 4.1 Garden Pea (Pisum sativum L. subsp. hortense (Neilr.) Asch. & Graebn.)������������������������������������������������������������������������������   53 4.1.1 Pot Experiments in Outdoor Conditions������������������������������   54 4.2 Cabbage (Brassica oleracea var. capitata L.)����������������������������������   58 4.2.1 Field Trials with Selenium Biofortification of Cabbage����������������������������������������������������������������������������   58 4.3 Selenium Accumulation in Various Vegetable Species After Soil Biofortification ����������������������������������������������������������������   59 5 Selenium Intake by Selected Vegetable Species After Foliar Application������������������������������������������������������������������������������������   63 5.1 Garden Pea (Pisum sativum L. subsp. hortense (Neilr.) Asch. & Graebn.)������������������������������������������������������������������������������   63 5.1.1 Characteristics of the Area����������������������������������������������������   63 5.1.2 Selenium Content������������������������������������������������������������������   65 5.1.3 Selected Qualitative Parameters After Foliar Treatment with Selenium������������������������������������������������������   69 5.2 Broccoli (Brassica oleracea L. var. italica)��������������������������������������   75 5.2.1 Selenium Content������������������������������������������������������������������   77 5.2.2 Selected Qualitative Parameters After Foliar Treatment with Selenium������������������������������������������������������   78 5.2.3 Effect of Foliar Application of Selenium on Quantitative Parameters ��������������������������������������������������   83 5.3 Basil (Ocimum spp.)��������������������������������������������������������������������������   84 5.3.1 Selenium Content������������������������������������������������������������������   85 5.3.2 Selected Qualitative Parameters After Foliar Treatment with Selenium������������������������������������������������������   89 5.3.3 Effect of Foliar Application of Selenium on Quantitative Parameters ��������������������������������������������������   99 5.4 Tomato (Lycopersicon esculentum Mill.) ����������������������������������������  103 5.4.1 Selenium Content������������������������������������������������������������������  104 5.4.2 Selected Qualitative Parameters After Foliar Treatment with Selenium������������������������������������������������������  106 5.4.3 Effect of Foliar Application of Selenium on Quantitative Parameters ��������������������������������������������������  110 5.5 Oyster Mushrooms (Pleurotus ostreatus) (Jacq.) P. Kumm��������������  114 5.5.1 Selenium Content������������������������������������������������������������������  114 5.5.2 Selected Qualitative Parameters After Foliar Treatment with Selenium������������������������������������������������������  118 5.6 Cabbage (Brassica oleracea var. capitata L.)����������������������������������  123 6 Changes in Selenium Content in Edibles During Processing��������������  125 References ��������������������������������������������������������������������������������������������������������  129 Index������������������������������������������������������������������������������������������������������������������  151

Acronyms

ABTS 2,2-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) AIDS Acquired immune deficiency syndrome ANOVA Analysis of variance AOA Antioxidant activity and Total antioxidant activity APS Reductase–adenosine phosphosulfate reductase APSe Adenosine phosphoselenate APSe Adenosine-5′-phosphoselenate AsA Ascorbic acid ATP Adenosine triphosphate AW Average weight BADH Betaine aldehyde dehydrogenase BBCH Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie (is used to identify the phenological development stages of plants) C Control variant cGSH-Px Cytozole glutathione peroxidase chla Chlorophyll a chlb Chlorophyll b CT Terms of cuts CVB3 Coxsackievirus B3 Cys Cysteine CZE Zone electrophoresis DM Dry matter DMBA 7,12-dimethylbenz[a]anthracene DMDSe Dimethyldiselenide DMSe Dimethylselenide DMSeP Dimethylselenopropionate DMSP Dimethylsulfoniopropionate DNA Deoxyribonucleic acid DPCSV Differential pulse cathode wiping voltammetry DPP Differential pulse polarography xi

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Acronyms

DPPH 2.2-diphenyl-1-picrylhydrazyl DW Dry weight EO Essential oils ESeMC γ-glutamyl selenomethylselenocysteine ETA-AAS Electrothermal atomization atomic absorption spectrometry EU European Union FAO Food and Agriculture Organization Fd Ferredoxin FM Fresh matter FRAP Ferric reducing antioxidant power GAE Gallic acid equivalent GC Gas chromatography GHB γ-1-glutaminyl-4-hydroxybenzene giGSH-Px Gastrointestinal glutathione peroxidase GLS Glucosinolates GPx Glutathione peroxidase G-SH Reduced glutathione GSH Glutathione GSH-Px Glutathione peroxidase GSL Glucosinolate GS-Se− Glutathione selenide GS-Se-SG Selenoglutathione intermediate GS-SeH Selenol GS-selenite Glutathione selenite GSSH Oxidized glutathione HG-AAS Hydride generation atomic absorption spectrometry HG-AFS Atomic fluorescence spectrometry HIV Human immunodeficiency virus HPLC High-performance liquid chromatography HSD Honestly significant difference Hyc Homocysteine I3C Indole-3-carbinol ICP Inductively coupled plasma ICP-AES Inductively coupled plasma atomic emission spectroscopy ICP-MS Inductively coupled plasma mass spectrometry LAD 27 Nitrogen fertilizer containing 27% nitrogen LOAEL Lowest observed adverse effect level LSD Least significant difference methylSeCys Se-methylselenocysteine MMT Methionine S-methyltransferase NAA Neutron activation analysis NADPH Nicotinamide adenine dinucleotide phosphate ND Not detected NMRI Nuclear magnetic resonance imaging NPK Nitrogen (N), phosphorus (P), and potassium (K) nutrition

Acronyms

OAS OPH PAGE PCL PGSH-Px PHP QE R RF2 RfDM RNA RNS ROS RP-HPLC S S-methylMet SAHyc SAM SD Se SeAp SeCys SeGSH SeHomoCys SELB SeMC SeMet SeMM SF SHST(1,2,3) Sl SRST1 SRST3 SUA TC TE TF TFSG TFSP TI TIRG Tmin TMSe+ TPC tRNA

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O-acetylserine O-phosphohomoserine Polyacrylamide gel Photo-chemiluminescence Phospholipid hydroperoxide glutathione peroxidase Photosynthetic pigments Quercetin equivalent Correlation coefficients Release factor Refractometric dry matter Ribonucleic acid Reactive nitrogen species Reactive oxygen species Reversed phase high performance liquid chromatography Substrate S-methylmethionine S-adenosyl-homocysteine S-adenosyl-methionine Standard deviation Selenium Selenium application Selenocysteine Selenoglutathione Selenohomocysteine Specific elongation factor Selenomethylselenocysteine Selenomethionine Se-methylmethionine Sulforaphane Carrier coding genes Solution High-affinity transporter gene Low-affinity transporter gene Slovak University of Agriculture in Nitra Total carotenoids Trolox equivalent Transfer factor Transfer factor from soil to pea seeds Transfer factor of whole pea biomass Transport index Transport index root-seed Minimal temperature Trimethylselenonium ion Total polyphenol content Transfer RNA

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tRNASec TS 10 UGA US UV VGC w/v WHO XRF

Acronyms

Selenocysteine transfer RNA The sum of daily mean temperatures above 10°C Stop codon United States Ultraviolet Vegetative growth characters Weight/volume World Health Organization X-ray fluorescence

Chapter 1

Introduction

The unfavorable state of health of the Slovak population, and the inappropriate nutritional composition, led to the adoption of documents that should contribute to the overall improvement of the state of health of the population. These documents are the National Health Promotion Program, the Principles of the State Health Policy of the Slovak Republic, the Concept of the State Health Policy, and the Action Plan for the Environment and Health of the Population of the Slovak Republic and the Nutrition Recovery Program. These documents are part of a strategy aimed at preventing cardiovascular diseases, which are the main cause of mortality in Slovakia. The international project “Healthy nutrition for a healthy heart” also belongs to the system of preventive measures. The Slovak Republic is the fourth country in which the project is being implemented—after Canada, Australia, and Hungary. One of the main conditions for ensuring proper nutrition of the population is a stabilized food market and meeting the requirement to maintain the food security of the population of the Slovak Republic by producing domestic agricultural primary production and processing it by our food industry, in the form of new health-­ promoting products. Primary agricultural production can be a permanent source of contaminants in food, but also of nutritionally necessary compounds and elements. Targeted intervention in primary agricultural production can result in a reduction of the risk of contamination and also significantly improves quality by ensuring the production of health-promoting products. All interventions in biosystems will ultimately affect the health of the human population. Vegetable production has a special position in primary agricultural production. For a large number of species, types, and varieties, it requires different conditions of cultivation, treatment, harvesting, and storage. Vegetable production is an important crop sector. It is engaged in the cultivation of fields, fast vegetables, vegetable seeds, and seedlings. It ensures the production of a very important raw material of the food industry and an irreplaceable component of human nutrition, which is fresh vegetables. Vegetables are a source of vital

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 A. Hegedűsová et al., Selenium Supplementation in Horticultural Crops, https://doi.org/10.1007/978-3-030-70486-5_1

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vitamins and other valuable substances that affect human health. Vegetables are also an important commodity for the Slovak national economy. It is not gratifying that the annual consumption of vegetables per capita in the Slovak Republic is 104.7 kg (Meravá 2018). The recommended total consumption of 127.9  kg per capita per year is not being achieved. The presented work is focused on the possibilities of improving the quality of raw food materials based on vegetables. The possibilities for quality improvement are demonstrated by the example of an essential element, selenium, which in recent years has got into the attention of the wider professional public. Central Europe has been identified as a region with suboptimal concentrations of selenium in agricultural soils that reflects in low selenium levels in the agricultural products coming from the area (Sager 2006). In the past, great efforts have been focused only on increasing crop yields, but enhancing the concentrations of mineral micronutrients including selenium has become an urgent task. Biofortification of trace elements can be achieved by their application within the agronomic process such as soil or foliar fertilization or crop breeding (El-Ramady et  al. 2014). Agronomic biofortification with selenium (Se) could be a powerful tool to remedy Se deficiency as it may increase the Se concentration in the plant sources used in food production (Poblaciones et al. 2014). Use of common fertilizers with Se for crop production is considered as an effective way to produce selenium-rich food and feed. As the cultivation of vegetables is one of the most intensive agricultural production areas with high demands on nutrition and fertilization, but also with high demands on the health of the grown products, its high cultivation quality is the basis of the production of quality, nutritional, and safe canning products.

Chapter 2

The Role of Selenium in Human Nutrition

2.1  Selenium as an Essential Element The essentiality of selenium was proven in 1957, when selenium was found in the so-called factor 3, which prevents liver necrosis in rats. In 1976, many experiments clearly proved that selenium is necessity for humans; however, until the 1940s, only negative effects had been presented. Recently, interest in the role and importance of selenium in human nutrition has increased considerably, especially as more and more research results have pointed to the essentiality of this element for human health. Only its toxic effect on the body was known in the past, but recent studies have shown that selenium deficiency is related to an increase in cardiovascular diseases and tumors. Diseases associated with selenium deficiency occur in many countries around the world, and it is estimated that selenium deficiency causes more economic losses than its toxicity (Merian and Clarkson 1991; Strmisková 1992; Benstoem et al. 2015).

2.1.1  Symptoms of Selenium Deficiency in Human Body Keshan disease is a potentially fatal cardiomyopathy—irregular pulse, enlargement and hardening of the heart, and mitochondrial crystallization. This has been found mainly in girls and women in some areas of China where selenium levels are low in food resources (Chen 1987). Keshan disease is likely to be of dual origin, caused by both nutritional deficiency of the essential trace element Se and the enteroviral infection, which merits further investigation of the relationship between nutrition and viral infections. Coxsackieviruses B3, CVB3/0, that affect heart muscle converted to a virulent form when injected into Se-deficient mice. Recent studies have shown that mild forms of the influenza virus, influenza A/Bangkok/1/79, also

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 A. Hegedűsová et al., Selenium Supplementation in Horticultural Crops, https://doi.org/10.1007/978-3-030-70486-5_2

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showed an increase in virulence when the virus was administered to Se-deficient mice. The epidemic neuropathy in Cuba was caused by a combination of nutritional origin and viruses (Ordúnez-Garcia et al. 1996). Some diseases of civilization are associated with decreased selenium status. Maintaining optimal concentrations and activity of selenoproteins seems to be important for the prevention of so-called civilization diseases (Holben and Smith 1999; Whanger 2000). Low levels of selenium in the blood are associated with the occurrence of cardiovascular diseases. Selenium deficiency can cause the accumulation of fatty acid peroxides in the heart and lead to the formation of substances that increase the formation of a blood clot (thrombus) (Holben and Smith 1999). Factors such as stress or infection lead to the formation of hydrogen peroxide, which is not adequately metabolized in tissues with inactive glutathione peroxidase. Hydrogen peroxide can irreparably damage the heart muscle (Yusuf et al. 2002). Sufficient selenium status and maintenance of cellular and phospholipid hydroperoxide peroxidase activity appears to be an effective aid in the protection of vascular endothelium from oxidative damage by low-density lipoproteins (Salonen et al. 1982) and from lipid peroxide damage (Oster and Prellwitz 1990). Yusuf et al. (2002) reported that there is a reversible cardiomyopathy caused by selenium deficiency observed in patients with malnutrition (e.g., with the occurrence of Crohn’s disease and cystic fibrosis). Various civilization diseases, including cancer, have been associated with low selenium status (Kieffer 1987; Tinggi 2008). The relationship between selenium intake and cancer risk was described in the 1970s. Epidemiological studies have shown that the incidence of cancer is higher in areas where the soil is poor in selenium, which translates into low selenium status in the population and contributes to cancer development (Li et al. 2004). Selenium may have an auxiliary chemopreventive effect when consumed in amounts higher than those recommended in the diet. Administration of multiple selenium compounds in amounts exceeding nutritional needs inhibited tumor growth in some animals (Holben and Smith 1999). There are several hypotheses to explain the procarcinogenic effect of low selenium levels and the chemopreventive effect of high selenium levels (including changes in protection against free radicals, carcinogenic metabolism, endocrine function, and immune system function). In addition, selenium can inhibit the synthesis of specific enzymes in cancer cells and can develop the apoptosis, resulting in slower tumor growth (Fleet 1997). At a high selenium intake, the tumor cells survive a shorter time, subject to apoptosis, and the tumor may shrink, depending on the stage of the cancer. The sooner the disease is diagnosed, the better effect is obtained (Hrušovský 2000). There is evidence that selenium can inhibit carcinogenesis through multiple mechanisms, but the relationship between selenium dose and response through tumor growth is not linear. The antitumor effect of selenium in animal models was observed at levels higher than those required to maintain selenoproteins. It appears that the primary mechanism by which selenium inhibits carcinogenesis is associated

2.1 Selenium as an Essential Element

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with alterations in carcinogen metabolism or the production of toxic selenium metabolites that are different from known selenoproteins. The function of selenium in thioredoxin reductase is one of the many possible ways that selenium acts a chemopreventive agent (Holben and Smith 1999). Ginter (1995) listed possible mechanisms of the chemopreventive effect of selenium: • • • • • • • •

Stimulation of the immune system Decrease of the oxidative effect of carcinogens Affects the metabolism of carcinogens Protection against damage by oxygen radicals Modification of proteins in the body Protection of metabolic enzymes Presumed production of selenium metabolites (dimethylselenide) Direct connection with DNA

Research dealing with the relationship between dietary intake of selenium and the risk of prostate cancer has shown that selenium supplementation can reduce the risk of prostate cancer by up to three times. Research on the effect of selenium supplementation on reducing the risk of skin cancer has shown an approximately 50% reduction in the incidence of other cancers, including lung, colon, and prostate cancer. Other experimental studies have compared the relationship between selenium in diet and the cancer risk in Europe and the United States (Johnson 2001). Recently, more researchers have noted the value of Se as an antimetastasis agent (including antimigration, anti-invasion, and antiangiogenesis) (Chen et al. 2013). Patients suffering from AIDS have been found to have low levels of selenium in the heart muscle. Pathological examination of the heart of AIDS patients revealed abnormalities in all patients. The abnormalities showed a high similarity to the symptoms of Keshan disease, probably related to selenium deficiency. In HIV-­ positive patients, selenium supplementation in nutrition significantly increases survival chances (Yusuf et al. 2002). Further research, both basic and applied, will be needed to identify the possible role of malnutrition in contributing to the output of new viral diseases (Beck et al. 2003). Janek and Muntág (1992) reported the following selenium functions: • • • • • • •

Synergistic with vitamin E acts as antioxidant Inhibits the development of cardiovascular diseases Protects erythrocytes Maintains tissue elasticity Suppresses headache and other changes during menopause Helps in the treatment and prevention of dandruff in the hair Protects against some cancers

Selenium intervenes in metabolic processes and is particularly important as an antitoxic agent. It is approximately a thousand times more efficient than vitamin E, which as a fat-soluble vitamin is much more slowly absorbed. It suppresses the

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formation of blood clots, increases the effectiveness of the immune system, enhances resistance to viral and bacterial infections, and decreases the negative effects of heavy metals and other toxic substances in the body (Blattná 1998). The results of recent years have shown the importance of selenium, GSH-Px, and selenoprotein for reproductive processes—the maturation of both sexes, sperm maturation, and placental transfer of selenium from mother to fetus (Bedwall and Bahuguna 1994). Selenium is involved in spermiogenesis; spermatocytes are high in selenium. Its deficiency may lead to male infertility (Vezina et al. 1996). Selenium acts anticarcinogenically through selenium proteins where it is incorporated as seleno-amino acids. The protective effect of selenium is closely related to the intake of some vitamins with antioxidative effects (Ginter 1995). There is an inverse correlation between the amount of selenium in the body and cancer mortality. The incidence of certain cancers in areas with low selenium content in soil and food is demonstrably higher. High doses of sodium selenite reduced the incidence of tumors by 15–30% (Maďarič et al. 1993). Selenium and vitamin E act synergistically; thus, their effects ensuring healthy heart function and antibody production are enhanced when interacting together in comparison with each agent acting alone. Selenium in the etiology of cardiovascular diseases acts as part of glutathione peroxidase and thereby protects the endothelium of the arteries from damage by lipid peroxides. In the deficiency of selenium, lipoperoxides accumulate in the heart. They damage cell membranes and lead to disturbation of the calcium transport leading to its accumulation in the cell (Oster and Prellwitz 1990).

2.1.2  Selenium Presence and Transport in Human Body The amount of selenium in the human body depends on its intake from food and the physiological state of the organism. Selenium in organic form is taken in and absorbed in different ways to inorganic, and therefore, the bioavailability is also clearly different. Most of the organic compounds containing selenium are absorbed almost completely (85–95%), while inorganic selenium is absorbed with a very different intensity in humans (40–70%), depending on the concrete form, whether selenite or selenate. Organic selenium is stored in tissues (Mosnáčková et al. 2003). In the body of an adult male, 5–10 mg of selenium is commonly found, and in natural areas poor in selenium, only 5–6 mg (Chovancová and Krajňáková 2004). Velíšek (2002) reported that the adult body contains about 10–15 mg of selenium. The state of selenium in the organism is determined primarily by its intake from food and secondarily by physiological condition of the organism. After the oral administration of selenites, selenates, or selenomethionine, selenium is well absorbed from the duodenum (Maďarič et al. 1993). The absorption of selenium in the gastrointestinal tract is relatively high and is dependent on the form and amount of selenium present. Selenomethionine has a resorption of 95–97%, while selenite is absorbed at 44–76%. Selenomethionine is

2.1 Selenium as an Essential Element

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resorbed by a mechanism of active transport in the duodenum that uses active sites for methionine. In contrast, selenite and selenocystine are resorbed passively (in the direction of the concentration gradient) (Velíšek et  al. 1999; Velíšek 2002) and remain unchanged (Maďarič et  al. 1993). The absorption of selenomethionine is inhibited by methionine, high dietary fiber, arsenic, cadmium, and mercury. The positive effect of a higher concentration of selenium in the blood can also be supported by sufficient intake of vitamins, especially vitamins A, C, and E (Maďarič et  al. 1993). Hertlová et  al. (1994) reported that organically bound selenium is absorbed very well and rapidly after hydrolytic release from proteins. Elemental selenium is poorly absorbed. From a comparison of the absorption of inorganic forms of selenium (selenate and selenite), Blattná (1998) found better absorption of selenites compared to selenates. Brtková (1996) presented that the enemies of selenium are generally confectionery and alcohol, which reduce blood levels of selenium. In addition to these factors, some diseases of the gastrointestinal tract, especially gastric acid secretion, may reduce absorption in the body. On the contrary, stress increases the absorption. Selenium absorption appears not to be regulated and most studies confirm that it is high (over 80%). Selenomethionine is absorbed in the same way as methionine, but less is known about the absorption of selenocysteine. The absorption of inorganic selenium is very efficient and is not affected by the selenium state (Miko 1994a). The absorbed selenium is partially taken up by the blood cells in the blood and is partially transported to the tissues by blood plasma containing a specific selenoprotein (Velíšek 2002). Blood contains about 20% more selenium than serum, but with a higher intake of selenomethionine, a significant proportion of selenium is imported nonspecifically into hemoglobin (Kvíčala 1999). Selenium is transported from the digestive system by lipoproteins, especially low- and very low-density lipoproteins (Miko 1994b). The serum selenium content correlates with essential acids, especially eicosapentaenoic acid, which has been shown to protect the heart (Blattná 1998). In blood, selenites bind easily to erythrocytes (50–70% in 1 min). Selenites are reduced by thiols to hydrogen selenide (H2Se), immediately released, bound by plasma proteins, and distributed to tissues where they are imported into newly synthesized selenoproteins. Most of the selenium binds to selenoprotein P and to glutathione peroxidase (GSH-Px), and about 9% binds to albumin (Maďarič et al. 1993). Opinions on selenium concentration in the human body vary. Some authors report 3–6  mg, others 2–12  mg. Selenium is not present in all parts of the body evenly (Melicherčík and Melicherčíková 1997). Determination of the selenium concentration in the whole blood, plasma, serum, erythrocytes, and thrombocytes is used for the estimation of the selenium level in the body. The level of selenium in the blood should be in the range of 1.0–1.9μmol dm−3. In plasma, the level of selenium changes immediately because its absorption is not controlled homeostatically (Fox 1992). According to Chovancová and Krajňáková (2004), about half of the total selenium in the human body is accumulated in the liver and is also found in the kidneys and thyroid gland. The concentration in the blood should be about 0.1 mg dm−3, and

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2  The Role of Selenium in Human Nutrition

60% of this amount is present in red blood cells and 40% in blood serum. Blood transports selenium into all tissues of the body. Velíšek (2002) reported selenium concentration in blood in the range of 40–350μg dm−3.The highest selenium concentrations are found in the kidneys (0.2–1.5 mg kg−1), in the liver (0.24–0.4 mg kg−1), in hair (0.6–6 mg kg−1), and in bones (1–9 mg kg−1); muscles have the lowest content (0.07–0.1 mg kg−1) (Velíšek 2002). Melicherčík and Melicherčíková (1997) reported selenium in skeletal muscle of 40% and in the liver 30%, while Blattná (1998) reported up to 50% of all selenium in the liver, and high concentrations were also found in adrenal glands and spleen. According to Daniels et  al. (1997), selenium generally accumulates in the brain, hypophysis, thyroid, heart muscle, and ovaries. Unlike the liver, from which selenium is easily released, it remains in the brain for a long time. Most of the selenium in tissues is in proteins. More proteins have been identified to which selenium is bound. Proteins containing bound selenium are called selenoproteins. Most of selenium in the body is found in selenium-containing amino acids, especially selenocystine and selenomethionine, where it replaces sulfur. Selenomethionine in tissues is derived only from dietary sources and cannot be synthesized in the body; this form does not have a regulatory function and is considered rather as a storage form of selenium. Selenocysteine, which is incorporated into proteins by a specific mechanism, is responsible for the biological activity of selenium, and there is no evidence that it replaces cysteine (which contains sulfur instead of selenium) in animal systems (Miko 1994a). Organic selenium is stored in tissues, and inorganic is excreted in the urine when the organism is sufficiently saturated (Blattná 1998). In the case of excess selenium intake, selenium is methylated to the trimethylselenonium cation (CH3)3Se+, which can be excreted in the urine. If it is in the form of dimethylselenide (CH3)2Se, it is mainly eliminated by exhalation (Ďuračková 1998). At normal intake, 60% selenium is excreted in the urine, 35% in the feces, 1% in the lungs, 1% in exhalation, and 1% in the form of sweat (Maďarič et al. 1993).

2.1.3  B  iochemical Function of Selenium in the Human Organism Organic compounds of selenium play a key role in biological processes—seleno-­ amino acids, peptides containing selenium, selenoderivatives of nucleic acids, and other compounds. In selenoproteins, selenium is present in the form of one amino acid containing selenium—selenocysteine, which, as one of the amino acids, can be directly incorporated into proteins and into the genetic code of organisms (Arthur et al. 1993; Clark et al. 1996). Thousands of different oxidation processes take place in living nature. In these reactions, oxygen radicals containing unpaired electrons are formed, which causes extra chemical reactivity of the radicals. Free radicals are a normal part of

2.1 Selenium as an Essential Element

9

metabolism and their high reactivity is utilized, for example, to fight pathogenic microorganisms. On the other hand, they are harmful if their production exceeds certain limits. Free radicals react with nucleic acids, proteins, and fatty acids that are part of phospholipids in cell membranes. This can interfere with the transfer of genetic information, protein synthesis. This is manifested, for example, by the faster proliferation of the affected cells compared with other cells of the tissue. Sometimes, the growth of such a genus of cells (cell clone) becomes completely uncontrolled; cells only grow according to how many nutrients and building materials they receive. This is the case of the proliferation of cancerous cells. Selenium is characterized by its strong antioxidant properties. The antioxidant effect of selenium is that the body ensures sufficient production of selenoproteins that capture or neutralize free oxygen radicals. This prevents oxidative stress, inflammation, and short-term tissue and organism damage (Hrušovský 2000). In the absence of selenium in the diet, the organism is unable to generate protective enzymes, as a result of which oxygen radicals and their harmful products multiply and interfere with cell structures (Ginter 1995). Antioxidants most likely prevent some types of cancer and heart disease as well as premature aging (Salonen et al. 1982; Wilet et al. 1983; Froslie et al. 1985; Oster and Prellwitz 1990). In the human organism, organic forms of selenium can be utilized more efficiently than inorganic forms, particularly because selenomethionine can replace methionine in various proteins, thus selenoproteins are formed. Selenium is essential for the production of the thyroid hormone responsible for healthy skin, hair, and eye preservation. Together with vitamin E, it helps in preventing cancer and cardiovascular diseases. The human organism most readily accepts selenium bound in organic compounds (Baghour et al. 2002). In plants, selenium is uptaken from the growing substrate, where it is present mostly in the form of inorganic compounds, and is gradually incorporated by the plants through selenocysteine into its own proteins (Maďarič and Kadrabová 1997). This means that a substantial part of the organically bound Se is present in amino acids in proteins. Reduced glutathione (G-SH) and cysteine play an important role in the resorption and metabolism of selenium compounds. The daily intake of selenium from food consumption is very different across the world. In Slovakia, it is 38μg  Se  day−1 (Rayman 2008). The World Health Organization has established a recommended daily intake of selenium of 50–200μg. Optimal intake is considered to be 1μg per day per 1 kg of body weight, 55μg average for adult women, and 70μg per day for adult men (Maďarič and Kadrabová 1998; Velíšek et al. 1999). Higher intake is recommended for nursing mothers and elderly men (Ježek et al. 2012). Lee et al. (1996) described biological compounds containing selenium as follows: • Iodothyronine 5-deiodinase controls thyroid metabolism, and its deficiency causes a slower rate of growth. • Selenoprotein P: in this form, approximately 60–80% of plasma selenium is present. It has a binding capacity and an antioxidizing ability, and it is a chemically inactive form of selenium.

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• Selenoprotein W has an antioxidant function (Whanger 2000). • Selenium-binding proteins probably have antitumor effect. • Sperm capsule selenoprotein: lack of it results in the production of abnormal sperm. • Cytozole glutathione peroxidase (cGSH-Px) is an antioxidant and a selenium store. For its optimal activity, about 100μg Se dm−3 in serum is required. It contains four selenium atoms in its molecule. • Plasma glutathione peroxidase (p1GSH-Px) is an extracellular secretory protein. It is found in breast milk. • Gastrointestinal glutathione peroxidase (giGSH-Px) is present in the liver and colon. • Phospholipid hydroperoxide glutathione peroxidase (PGSH-Px) is present in cell membranes and has antioxidant and control function. The body has several enzymatic systems (such as glutathione peroxidase) at its disposal for the disposal of free radicals. These enzymes work very efficiently, but in the overproduction of radicals, it is not enough to prevent the chain multiplication of the radicals. The formation of oxygen radicals is multiplied by factors such as sunlight, cigarette smoke, and foreign substances. The most important role of selenium is that it is part of the glutathione peroxidase enzyme. Under the influence of certain factors, free radicals are formed. These free radicals can react with unsaturated fatty acids, leading to the formation of hydrogen peroxide (H2O2) or lipoperoxides (ROOH) (Fig. 2.1). Glutathione peroxidase breaks down lipoperoxides into water and alcohol. In order to break down

Fig. 2.1  Glutathione peroxidase system

2.2 Occurrence of Selenium in Nature and Its Potential Sources

11

lipoperoxides, this enzyme needs glutathione (GSH), which provides hydrogen while oxidizing itself (GSSH). Vitamin E prevents peroxide formation, and therefore, its function overlaps with that of glutathione peroxidase (Miko 1994a, b). The enzyme glutathione peroxidase (GSH-Px) is based on four identical subunits having one selenium atom in the form of selenocysteine. GSH-Px is found in the cytosol of cells and in the mitochondrial matrix. The an oxidation system balance is homeostatically maintained by the activity of the corresponding enzymes, and free radical overproduction can be effectively reduced by the supply of exogenous and micronutrient antioxidants—selenium and vitamins E and C (Maďarič et al. 1993).

2.2  O  ccurrence of Selenium in Nature and Its Potential Sources 2.2.1  Selenium Occurrence in Soil Selenium in nature accompanies sulfur and is a minor component of copper, silver, lead, and mercury sulfides. Froslie (1993) states that selenium enters the soil mainly by natural manners—by weathering of sulfide (ore) minerals and by anthropogenic activity. In soils, it easily oxidizes and migrates until it is attached to volcanic barriers, where it binds to mineral and organic colloids. So far, geochemical research has shown that the behavior of selenium in soils is very complicated and depends on the soil properties, in particular soil response, mother rock, anthropogenic activities, secondary Fe oxide content, humus quality, and quantity (Heikens 1992). In soils, selenium occurs most often in the range of 0.1–2  mg  kg−1, an average of about 0.41 mg kg−1. Compared to rocks, the content of soils is higher. Soils formed from substrates rich in Se (andesites, basalt, clay rocks, saline soils, slate) usually have higher selenium concentrations, which range from 30 to 300 mg kg−1. Much of the selenium is also present in soils where the sulfur is in elemental form or in organically bound or adsorbed in the Fe and Mn clays. In light soils, it is found in low contents (Polański and Smulikowski 1978). In different territories of the world, there are areas with higher or lower concentrations of selenium in the soil. Soils with a total selenium content less than 0.6 mg kg−1 are considered to be deficient (Gupta and Gupta 2000). Mean selenium concentrations occur in soils in Canada (except Ontario Province) and in most parts of the United States, particularly in states on the Atlantic and Pacific coasts. In Europe, selenium content in arable soils is usually low and varies within a narrow range. Some European countries (Finland, Switzerland, Czech Republic, Slovakia) have very low concentrations of selenium in the soil. In Austrian soils, soil monitoring revealed that the most common median values of total Se content in different soil types ranged between 0.11 and 0.41 mg kg−1, depending on geological features (Sager 2006; Abrams et al. 1990). Selenium is a chalcophilic element and is therefore associated with sulfide ore sediments containing it in a ratio of 1:6000 to the sulfur content. Selenium content in soils mainly reflects maternal rock weathering,

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although the state of the atmosphere, and more recently anthropogenic inputs, may affect their composition (Alloway 1995). Sager (2006) states that in Central Europe, land use has a greater impact on the content of Se in it than the impact of geological factors. In particular, fertilization techniques and transfer to crops have an impact. Elution to deeper soil horizons is low. In Austria, it was found that soils in maize areas have significantly less Se (0.20  mg  kg−1) than permanent grassland (0.29 mg kg−1). In Finland, the average Se content in the topsoil was 0.21 mg kg−1. Higher values were measured in organogenic soils. In Norway, levels of Se in soils ranged from 0.2 to 1.4 mg kg−1 (Eurola et al. 2003). In central Spain, the Se content in the soil surface horizon was found to be adequate (in the range of 0.17–0.39 mg kg−1) or high (in the range of 0.50–4.38 mg kg−1) and Se appeared in forms highly accessible by plants, or potentially available (Rodriguez et al. 2005). In addition to natural resources (mainly metal-sulfur minerals), selenium compounds are widespread throughout the environment and come from the combustion of fossil fuels, the glass and electrical industries, and agriculture (Bajčan et al. 2001). To study the behavior of selenium in the soil–plant system, it is necessary to know the properties of this trace element in terms of the following characteristics (Kabata-Pendias 2001): –– –– –– –– –– ––

The natural content of the element in soils The threshold content of the element in soils The phytotoxic equivalents of the effect of the element on plants The sensitivity of the plants to the trace element Ratios between individual trace elements Soil characteristics (soil pH, redox potential, soil texture, organic carbon content, etc.) –– Trace element inputs and outputs Kabata-Pendias (2001) reported the average selenium contents and the total range of selenium levels present in topsoil worldwide. The values are represented in Table 2.1. 2.2.1.1  Selenium Content in Soils of the Slovak Republic By the Directive of the Ministry of Agriculture of the Slovak Republic no. 531/1994-540, a limit value for selenium in soils was established by the value 0.8  mg  Se  kg−1. Since May 2004, new limit values of hazardous substances in Table 2.1  Mean total contents and range of total trace element contents in topsoil in mg Se kg−1 of dry soil worldwide (Kabata-Pendias 2001—selected) Se content Mean Content range

Podzols 0.25 0.005–1.32

Cambisols 0.34 0.02–1.9

Rendzina 0.38 0.1–1.4

Orthic Luvisols and Chernozems 0.33 0.1–1.2

Organosols 0.37 0.1–1.5

2.2 Occurrence of Selenium in Nature and Its Potential Sources

13

agricultural land have been in force in the Slovak Republic, as stipulated by Act No. 220/2004 Coll. The limit values of the content of risk elements in agricultural land are assessed on the basis of soil degradation by the aqua regia (Table 2.2). In practice, the classification of soils into seleniferous soils containing more than 1.0 mg Se kg−1 and nonseleniferous containing from 0.1 to 1.0 mg Se kg−1 is used (Terry and Zayed 2000). A more detailed classification of soils (Table 2.3) according to selenium content worldwide has been reported by Wells (1967). According to surveys of selenium content in soils in Slovakia, individual authors present the following data: Grones et al. (1999) reported a mean soil selenium content of 0.25 mg Se kg−1. Based on the results of soil monitoring in Slovakia, Linkeš et  al. (1997) reported mean content of total selenium in soils in Slovakia to be 0.34 mg Se kg−1 in soil profile from 0.0 to 0.1 m and 0.28 mg Se kg−1 in soil profile from 0.35 to 0.45 m (Table 2.4). The highest selenium content above the limit value, 0.8 mg kg−1, was found in the clay soils of the Eastern Slovak Lowland, mainly in the north of Pavlovce upon Uh and in the floodplain of Bodrog and Latorica. Above-mean selenium content, from 0.34 to 0.7 mg Se kg−1, was found on the Ondava floodplain (from Vranov to the mouth of Latorica), in the Bukovské vrchy Mts. and in the entire Laborecká vrchovina Mts., Pieniny Mts., Popradská kotlina basin, in the northern part of Strážovské vrchy Mts., Myjavská pahorkatina upland, which is probably associated with the abundance of moldy shales in these areas. Above-mean selenium content is also found in the southeastern part of the Danube Lowland, dominated by heavy soils and floodplains of the lower part of the rivers Váh, Nitra, and Hron (Linkeš et al. 1997). Kadrabová et  al. (1996) reported that in two localities (around Modra, Senec, Galanta and Michalovce, Trebišov, and Vranov), the concentration of selenium in Table 2.2  Limit values for Se content in soils with different soil texture (Act No. 220/2004 Coll.)

Soil texture Sandy, loamy—sandy Sandy—loamy, loam

Limit value for Se content mg kg−1 0.25 0.40

Table 2.3  Soil classification according to selenium content (Wells 1967) Classification Se content mg kg−1

Very low 1.5

Se content mg kg−1  dry soil 0.04–1.34 – 0.02–2.05

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2  The Role of Selenium in Human Nutrition

grain is three to four times higher than the average in Slovakia, which is closely related to the above-average content of selenium in the soil. The below-average selenium content, from 0.2 to 0.33  mg  Se  kg−1, is in the soils of other parts of Slovakia. It consists of the predominant part of the Danube Lowland (the upper part of the Žitný ostrov and the uplands), the whole area of the Považské podolie, Orava, Kysuce, volcanic mountains, northern Spiš, and South Slovak basins (Linkeš et al. 1997). The Veterinary and Food Institute and the Institute of Hygiene and Epidemiology in Dolný Kubín analyzed soils at 58 localities in Orava. According to their results, selenium content in this region is very low, only 0.2 mg Se kg−1on average. Areas with low selenium levels have also been identified in the Záhorie lowland (areas of flooded sands), in the area between Hurbanovo and Kravany upon Danube (sandy chernozems and wind sands), and in Cerová vrchovina upland (prevalence of soils on neogene sands). Linkeš et al. (1997) reported a mean selenium content in the range of 0.04–0.19 mg kg−1. The content of total selenium in the soil was compared with the limit value valid until 2004 Act no. 220/2004 Coll., which sets three limit values depending on the soil type and requires a current assessment of selenium status in the soils of the Slovak Republic. Several authors (Čurlík and Šefčík 1999; Linkeš et al. 1997 and others) agreed that the soils in Slovakia are poor for selenium. Till 2004, the Se content up to 0.8  mg  kg−1 was considered to be below the limit in Slovakia according to the Decision of the Ministry of Agriculture of the Slovak Republic No.531/1994-540. In 2004, there was a legislative change and the new Act No. 200/2004 Coll. sets a limit values for Se in dependence with the soil species in the range of 0.25–0.60 mg kg−1 of dry matter. Soil selenium content is very different even at relatively small distances. Analysis of the soils in the Nitra region showed large variability between the samples collected throughout the region. The mean total selenium content in soil substrates is 1.2  mg  kg−1, which is higher than the limit set for clay soils (0.60  mg  kg−1). Its content varies within the region between 0.70 and 2.1 mg Se kg−1. Compared to the mean Se value in slovak soils (0.25 mg kg−1), it presents from 2.0 to 7.5 times higher values of this content (Hegedűsová et al. 2005). The average content of selenium in the soils and wheat of the Nitra region is given in Table 2.5 and Fig. 2.2. Since most of arable soils in Slovakia have low selenium content, this results in low levels of selenium in food and consequently in human organisms. In food of plant origin, the selenium content depends on its level in the soil or the fertilizers used and how its form is used by the plant. The selenium content of food of animal origin is determined by the selenium content and form in nutrition of the animals. Some animals are feed intentionally by selenium-fortified feed. Based on its insufficiency in nutrition, selenium has been ranked among the main food ingredients whose intake requires monitoring, according to the Nutrition Recovery Program of the Slovak Republic (Mosnáčková et al. 2003). The marginal selenium deficiency probably leads to a decrease of the body’s antioxidant protection and indirectly allows a faster development of atherosclerosis

2.2 Occurrence of Selenium in Nature and Its Potential Sources

15

Table 2.5  Average selenium content in soils and wheat of the Nitra region (Hegedűsová et al. 2005) District Komárno Nové Zámky Nitra Levice Topoľčany Šaľa Zlaté Moravce

Se content in soil mg kg−1 0.817 0.739 1.701 0.719 1.339 2.096 0.941

Se content in wheat μg kg−1 38.3 54.3 20.3 53.1 ND 203.0 ND

Soil type F, T, CH T, CH T, CH CH, F, H H T, CH H

Soil reaction pH 7.3–8.4 6.5–8.4 7.3–7.8 5.5–7.8 6.0–6.5 7.3–8.4 5.5–6.5

F fluvisol, CH chernozem, T blackberry, H brown earth, ND not detected

Se in soil

Se in wheat

900 800 700

%

600 500 400 300 200 100 0

Fig. 2.2  Comparison of selenium content in soil and wheat of Nitra region with its mean content in Slovakia (Hegedűsová et al. 2005)

and many other diseases associated with oxidative damage. According to the latest data, the level of selenium in the blood of the Slovak population is low compared to other European countries. An overdose of selenium leads to gastrointestinal disorders and cachexia. It occurs in animals in areas with high Se content in soil (Ireland, some areas of the United States).

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2.2.1.2  Selenium Forms in Soil Trace elements are present in soil in various concentrations and in various forms. Their accumulation in soil creates a potential source of their further release into the biological cycle and thus into the food chain. Merian and Clarkson (1991) distinguished the most important forms of trace elements in the soil: soluble, exchangeable, organically bound, in occluded forms of Mn and Fe oxides, in forms of defined compounds (carbonates, phosphates, and sulfides), and forms bound in the silicate structure (the so-called residual fraction). For an essential element such as selenium, its total soil content and potentially releasable content in the soil are monitored. Subsequently, the representation and type of mobile and acceptable forms are experimentally investigated. Total selenium content represents all forms of selenium in soil. It provides basic information on its natural soil content, along with air pollution content. From the viewpoint of the bioavailability of selenium for plants as well as the soil hygiene status, the content of potentially releasable selenium, which includes extractable forms of selenium, is crucial. Potentially releasable element contents are, relatively, a more sensitive indicator for assessing the soil hygiene than total contents. There are several ways of extracting potentially releasable selenium. HNO3 with a concentration of 2 mol dm−3 (Čurlík and Šefčík 1999) was used to obtain soil extract in the investigation of geochemical properties of soils in Slovakia. Total Soil Selenium All inorganic and organic compounds, together with elemental selenium, represent total soil selenium. The soils in the world are characterized by considerable variability of forms and amounts of selenium. The content varies in the soil from trace amounts up to a maximum of 82 mg kg−1. These analyses indicate that the normal level of Se in selenium-rich soils is in the range of 1–6 mg kg−1. In general, Se-rich soils do not occur in wet areas. The content of Se in common soils of England is on average 0.6 mg kg−1. Published Se values for other countries are 0.41 mg kg−1 for soils in India, 0.6 mg kg−1 for New Zealand, and 0.7 mg kg−1 for soils in Japan. In some regions, selenium occurs in the soil at far higher concentrations. These soils are found in the dry to semidry regions of China, Mexico, Colombia, the western regions of the United States, and Canada (Adriano 1986; Habeck 2003). The content of selenium in the soil within a territory can be very variable. In China, for example, there are areas with extremely high concentrations of selenium in soil and there are also areas with selenium deficiency (Polański and Smulikowski 1978). Climatic conditions contribute significantly to the content of selenium in the soil. Selenium can be expected in increased amounts only in dry and semidry areas, in soils originating from the Cretaceous shales. In areas with a humid climate and in irrigated areas, most selenium is washed out of the soil. In some cases, the lack of selenium in the soil may be due to the nutritional requirements of the animals,

2.2 Occurrence of Selenium in Nature and Its Potential Sources

17

especially in volcanic soils. These low levels of selenium in the surface layers can be further reduced by intensive irrigation. Potentially Releasable Soil Selenium Knowledge of the total content of the element is sufficient to estimate the environmental status of the soil, whereas in order to determine the availability of selenium for plants, it is necessary to determine those forms that are exchangeable and water soluble (Borowska and Koper 2002). The total amount of selenium in the soil, as well as the content of other trace elements, is not a reliable value for the amount of the element available to plants. In soils with vegetation which contain toxic amounts of selenium, water-soluble selenium values in the range of tenths of mg kg−1 can be expected. Selenium has been identified as a soluble form of selenium in soil samples from the western United States, Wyoming, Ireland, and Canada. The soluble Se content varied between 1% and 7% of the total selenium content (0.197–0.744 mg kg−1) in soil from certain areas of Canada and between 0.33% and 2.90% of the total selenium content (20.4–850.0 mg kg−1) in Ireland’s soils. Watersoluble selenium level in soils with low total content can be hardly considered as the amount available for plants, but it is a good criterion for monitoring the behavior of selenium after its application to soil (Adriano 1986). In a large survey in China, it was found that the content of water-soluble Se in soils with a low Se content is 2μg kg−1 of soil (which represents 1–2% of total soil Se), whereas soils with a higher Se content have approximately 18μg kg−1 of soil (4–6% of total Se). However, only part of this Se is bioavailable to plants. It is generally reported that only 45% of Se in soil solution can be used by plants (0.5–1.0% of total soil Se in low-content soils) (Fordyce et al. 2000). 2.2.1.3  Soil Properties Affecting Selenium Mobility Silanpää and Jansson (1992) reported the soil factors that affect the bioavailability of selenium: pH, redox potential, iron oxide levels, soil texture, organic carbon content, and cation exchange capacity. The authors assessed these soil properties in terms of the extractability of selenium from the soil: Soil pH: It strongly influences selenium uptake by plants. Minimum selenium extractability was found to be around pH = 6. Increasing the acidity of the soil environment is associated with a gradual increase in selenium extractability. In the direction of increasing alkalinity of the soil environment, the extractability of selenium increases even more intensively. According to several authors, correction of soil pH can increase the bioavailability of selenium for plants. In acidic and neutral soils, a ferrous selenite complex is formed which is poorly soluble, making this form of selenium practically unavailable to plants (Merian and Clarkson 1991). Soil liming experiments (Adriano 1986) revealed that increas-

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ing the pH from 6.0 to 6.9 also increased selenium intake in the alfalfa. Selenium is poorly mobile at pH 4.2–6.6, based on the mobility properties of the trace element depending on soil reaction. In general, as the pH of the soil increases, negative charges on the soil sorption complex increase, and therefore, the selenium anions are more easily available in alkaline soils. Redox potential: The soil pH values may have an effect on redox changes of various selenium forms while each form is characterized by certain availability. At pH values 400 Medium 200–400 Low