112Experimental Study on the Diseasepreventing Model of Culturing Chinese Sucker (Myxocyprinus asiatius) in River Crab (Eriocheir sinensis) Pond

Jiansheng YE Suzhen ZHAO Sheng YUAN Meijian LIU Jiamei WANG Jianhua DAI

AbstractIn recent years, production and processing of natural seleniumrich agricultural products has become a research hotspot. The main factor affecting the selenium content in plant is the content of effective selenium in soil. So it is important to understand the form and conversion relationship of selenium in soil and find ways to improve selenium effectiveness. By summarizing the researches on selenium at home and abroad, we analyze the classification of soil selenium form, the transformation among different selenium forms and the improving factors of selenium effectiveness. We understand the relationship between chemical form and extraction form of selenium, analyze the driving factors of conversion among different forms, and discuss the ways to improve selenium effectiveness of soil in high seleniumrich area. By combining characteristics of soil and crop in different seleniumrich regions, different selenium activation methods are used, which could provide more scientific evidence for further selenium research.

Key wordsSoil; Selenium; Form; Conversion factor

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Received: September 9, 2018Accepted: October 28, 2018

Supported by Guangxi Science Research and Technology Development Plan Program (Guikehe 41510400122); the National Natural Science Foundation of China（41761052）; Major Project of Guangxi Innovation Driven （Guike AA172020191, Guike AA172020192, Guike AA172020262）; Guangxi Youth Foundation Project (2016GXNSFBA380131); Science and Technology Development Fund Project of Guangxi Academy of Agricultural Sciences (Guinongke 2017JM01, Guinongke 2017JM03, Guinongke 2015JM23).

Ying XING (1986-), female, P. R. China, assistant professor, master, devoted to research about soil ecology and high value agriculture. *Corresponding author. Yongxian LIU (1981-), male, P. R. China, associate professor, master, devoted to research about efficient utilization of agricultural resources and sustainable development of ecoenvironment. Email: liuyx27@163.com.

Selenium is essential trace element for human and animal. Appropriate amount of selenium (40-400 μg/d) could enhance immunity, resist oxidation and defend peroxidative damage, but insufficient or excess selenium intake could affect human body and induce disease［1］. It is recognized as that selenium supplement by natural food is the most safe and effective［2］. Selenimum content in plant depends on the content of effective selenium in soil, and form distribution and valence change of selenium in soil directly affect its absorption and accumulation by plant［3-6］. Based on chemical form, selenium in soil is divided into selenate, selenite, elemental selenium, selenide and organic selenium［7］, and selenate and selenite containing Se4+ and Se6+ are easy to be absorbed and used by plant, which is effective form［8-10］. Based on extraction form, it is divided into water soluble state, exchangeable state, iron and manganese oxidation state, organic combination state and residue state［11-12］. Among them, water soluble state and exchangeable state are effective forms of selenium, and effective and invalid forms are not static but change in different environment. Ashworth et al.［13］ thought that many factors in soil commonly affected selenium effectiveness, such as pH, microorganism, oxidationreduction potential, element composition and organic matter. But the research only analyzed change relationship among different chemical forms. Su et al.［14］ thought that occurrence form of selenium (extraction form) was affected by physicalchemical properties of soil, and adsorption-desorption, oxidation-reduction, precipitation-dissolution reactions between organic matter, iron manganese oxide, clay minerals and selenium could be generated. But it only discussed change relationship of extraction form. There are many researches on selenium form in soil, but they mostly concentrate in exploring change and influence factors of chemical form, and the research on the relationship between different chemical forms and extraction forms is not thorough. But the content of each chemical form in extraction form is crucial to study selenium form in soil. Therefore, relevant researches at home and abroad are deeply analyzed. By exploring two kinds of forms, the research method suitable for selenium form in natural seleniumrich soil and improving ways of selenium effectiveness in soil are sought, which could provide the basis for further carrying out research.

Classification of Selenium Form in Soil

Classification of selenium chemical form in soil

Selenium form in soil is generally divided into two kinds: chemical valence and extraction form. According to chemical valence, it could be divided into selenide, elemental selenium, selenite, selenate, organic selenium and volatile selenium. After selenium converts into volatile selenium, it is very difficult to exist in soil and quickly enters into air. Under suitable environment, selenium with different valences converts mutually, and elemental selenium converts into inorganic selenium via hydrolysis, while selenide also could release some soluble selenium under weathering effect［15］. Therefore, any condition change may induce transformation of selenium valence, thereby affecting selenium effectiveness.

Classification of selenium extraction form in soil

The classification based on extraction form generally takes different chemical extractants as the standard. A lot of researches on form classification are conducted by scientists at home and abroad, and there are many classification methods, which is conducive to studying selenium effectiveness. For example, when studying the classification of heavy metals, Cutter［16］ found that it could be used for form classification of selenium, and fourpart classification method was generated. Selenium in soil could be divided into adsorption state, iron manganese oxide bound state, organic bound state and carbonate bound state. When studying selenium in soil of Swiss forest, Gustafssont et al.［17］ further divided organic bound selenium into NaOH solubleselenium, humic acidselenium, hydrophobic fulvic acidselenium and hydrophilic fulvic acidselenium. Martens et al.［18］ put forward that selenium in soil could be divided into water soluble state, adsorption state, organic selenium and insoluble selenium. Chinese scholars started to study selenium form in the 1980s-1990s, and the earliest selenium classification of China was conducted by Hou et al.［19］, which was divided into organic bound selenium and oxide bound selenium. Many scholars further refined it in subsequent research, and the representative was fivepart classification method. Based on a lot of researches, Qu et al.［11］ put forward that selenium in soil was divided into water soluble state, exchangeable state, carbonate bound state, organicssulphide bound state, iron manganese oxide bound state, elemental state and residue state. The method of Wu et al.［12］ was similar to that of Qu, but extraction method changed somewhat, and selenium in soil was divided into water soluble state, exchangeable state, acid soluble state, organic bound state and residue state. But above chemical extraction methods could affect the form of selenium itself in soil, and the result could not truely represent selenium form in soil. Therefore, some scholars started to study direct check of instrument. For example, Wen et al.［20］ used IR and X ray diffractometer to measure selenium form of soil in seleniumrich region of Enshi. But the method had certain limitation, and it was required that selenium content in soil must be more than 10 μg/g, while average selenium content of soil in the world was only 0.4 μg/g. Therefore, utilization range of the method was limited, and chemical extraction method was applied widely.

Conversion of selenium form in soil

Change of selenium valence or extraction form in soil is often caused by the adsorption, complexation and mineralization between it and mineral, organic matter, microorganism. Analyzed from physicalchemical angle of soil, selenium at different valences is under their respective energy states without external factors. When there is stimulation from external factors, interconversion could be generated. Therefore, suitable stimulation or condition is the basis of deciding selenium effectiveness in soil［21］. Before studying conversion among selenium forms, it should understand chemical valence of selenium in soil, which is the basis of studying interconversion. Content proportion of selenium with different valences in soil is shown as Fig. 1. Selenite accounts for 40% of total selenium amount in soil, which is main existence state. Selenite itself has very strong oxidation and water solubility, which is easy to be absorbed and used by plant. Meanwhile, it is easy to adsorbed and fixed by soil due to its own strong oxidation and water solubility. Selenate is the highest valence of selenium, accounting for 10% in soil. Selenate is easy to dissolve in water, which is easy to be absorbed and used by plant. Elemental selenium and selenide account for 25% of total selenium in soil, and organic selenium has the same proportion［15］. Elemental selenium is less in elemental selenium and selenide, which is difficult to dissolve in water and could not be directly used by plant. But it could convert into inorganic selenium under suitable condition, which could be absorbed and used by plant (Fig. 2). Selenide is a kind of form difficult to dissolve into water and be absorbed and used by plant, which mainly exists in semiarid soil. Under weathering effect, it could convert into soluble organic selenium or inorganic selenium, which could be absorbed and used by plant［22-25］. Organic selenium is mostly generated by plant decomposition process. Via biological decomposition, it converts into organic or inorganic selenium which is easy to be absorbed and used by plant, and it is main source of effective selenium. Meanwhile, organic selenium in soil also has volatilization loss, which is caused by the generation of alkylselenide by microbial decomposition［26］. Specific conversion condition and relationship among selenium with different valences are shown as Fig. 2. Suitable change of some factors in soil is conducive to transformation of selenium valence in soil, and it has an important role in studying selenium effectiveness in soil.

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Fig. 1Distribution of different forms of selenium in soil

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Fig. 2Interconversion among selenium with different valence［15］

Occurrence form and chemical valence of selenium in soil mutually include. Selenite, selenate or organic selenium dissolve into water, which becomes watersoluble selenium, while elemental selenium could convert into organic bound selenium or exchangeable selenium and even water soluble selenium under microorganism, which could be absorbed and used by plant. Water soluble selenium or exchangeable selenium could be reduced into elemental selenium or convert into residue selenium by the fixation of soil colloid, and the conversion among different valences is accompanied by form change. The classification from extraction form is conducive to understanding transfer and transformation of selenium in soil, and linking with plant absorption and use. Via the analysis and research on literatures at home and abroad, it is thought that pH, soil moisture content, oxidationreduction condition, organic matter content, microorganism and other ions affect selenium form or valence change in some soil. Therefore, it could be thought that they are main factors of affecting selenium form, thereby deciding absorption and accumulation of selenium by plant.

Improving Way of Selenium Effectiveness in Soil

Improving process of selenium effectiveness in soil could be thought as conversion process of selenium form. Therefore, the method conducive to improving water soluble selenium and exchangeable selenium could be understood as the way of improving selenium effectiveness in soil. There are a lot of researches on form and influence factor of selenium in soil, and research results show that main ways of improving selenium effectiveness in soil are pH, soil moisture content, oxidationreduction potential, organicinorganic adsorption, microbial action and interaction of other ions.

pH

pH is one of main factors affecting selenium form, and its change directly affects selenium form. For example, selenite mainly exists in the soil of pH 3-7, and selenite in such soil is easy to be absorbed and used by plant and fixed by soil colloid. Therefore, pH is conducive to improving selenium effectiveness in soil. When pH rises, selenite adsorbed by soil is released［27］. When pH is 8-12, selenate is main existence form, and it is also easy to be fixed by soil and could not be absorbed［28］. Therefore, suitable measures could be taken in acid seleniumrich region of South China to improve pH of soil and increase effective selenium content in soil.

Soil moisture and redox potential

Both soil moisture and redox potential significantly affect selenium form in soil, but variation of any condition could induce pH change. Therefore, it needs combining soil moisture and redox potential with pH for exploration. It is because that change of soil moisture often could induce variations of pH and redox potential. Under submergence condition, selenate content quickly decreases, while selenite content quickly rises. Reducibility of soil also has similar change. In the soil with strong reducibility, Se4+ is reduced into elemental selenium or adsorbed by soil, thereby affecting its effectiveness［29］. In arid and semiarid region, selenite can be oxidized to selenate when pH≥8.5［30］. It could be thought that change of soil moisture indirectly affects redox reaction of selenate and selenite in soil with different pH, thereby affecting change of selenium form in soil［31］. Soil environment itself is complex, especially pH and redox potential could not be singly considered, and selenium form is commonly affected by them. Fig. 3 is form change of selenium under different pH and Eh. When pH is 7.00 and Eh is 500 mV, Se6+ is main existence form of selenium in soil. According to Fig. 3, Eh of soil could decide selenium effectiveness by affecting valence change of selenium. When Eh is high, selenite is main valence of selenium in soil. In the soil of pH 3-7, the improving of redox potential could increase selenite effectiveness, which is easy to be absorbed and used by plant. With the reducing of redox potential, selenium valence changes, and selenite, selenide and elemental selenium are main existence valences, thereby causing that selenium effectiveness declines. The influence of pH and Eh on soil selenium is as below. On the one hand, common change of pH and Eh causes the difference of adsorption ability by soil. When pH is lower and it is in reduction condition, selenite is main existence form. When pH is higher and it is in oxidation condition, selenium exists in selenate form. On the other hand, pH and Eh change could induce changes in surface charge of soil colloids, causing adsorption change of selenium by soil colloids. In soil with strong acidity, there are more positive charges on soil colloids surface, thereby causing stronger adsorption ability on selenium and affecting selenium effectiveness. Generally speaking, adsorption ability of anion by soil declines with pH rise［32］. But under actual situation, form change of selenium in soil is more complex than theoretical expectation. Therefore, to improve selenium effectiveness by the means, it needs combining external environmental factors for comprehensive analysis.

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Fig. 3Eh-pH relationship of selenium under 25 ℃ and 1 atm

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Organicinorganic adsorption

Organic matter in soil has twoside influence on selenium. On the one hand, organic selenium combining with organic matter is released under mineralization to improve selenium effectiveness. On the other hand, it has certain adsorption ability on functional groups themselves of the organic matter, and could adsorb inorganic selenium to form organicselenium complex and reduce selenium effectiveness. How the two functions happen and which action is dominant have close relationship with composition of organic matter in soil. It is thought that when fulvic acid is more than humic acid in organic matter, selenium effectiveness in soil is high. When humic acid is more than fulvic acid, selenium effectiveness declines［33］. In many researches, the influence of internal components of organic matter is often neglected, which also causes that the result of research between organic matter and selenium is inconsistent. It is clear that internal component of organic matter is crucial to selenium form in soil. Therefore, it should more concern its composition when selecting organic fertilizer to improve selenium effectiveness in soil.

Microorganism

Soil microorganism has an important role in selenium transformation. Besides decomposition and synthesis of organic acids, microorganism could form metabolites of organic selenium via methylation: methyl selenide［34］. Such organic matter containing selenium is easy to volatile than inorganic selenide, and has larger toxicity on higher plant. Selenate could convert into selenite nitrate reductase A or Z (narGHIJ or narZUWV) or elemental selenium by dissimilate sulfur reduction pathway［35］. Herbel et al.［36］ found that microorganism could reduce highvalence selenium and form FeSe precipitation. Dhanjal et al.［37］ found that Bacillus cereus could reduce selenite into red elemental selenium nanoparticles, while some microorganisms could oxide elemental selenium into highvalence selenium to obtain energy. It becomes an important research direction to develop seleniumrich microorganism by selenium transformation of microorganism.

Synergy or competition of other ions

Due to complex environment and various ions in soil, there may be synergy or competition among ions under specific environment. The effect could change selenium form and improve selenium effectiveness in soil, in which the interaction between phosphate, sulfate and selenate or selenite is the most significant. It is thought that phosphate and selenite have competition in plant adsorption［38］. When phosphate content is higher than selenate in soil, plant mainly absorbs phosphate, which affects selenium absorption. Actually, the relationship between selenium and phosphor in soil is very complex, which is related to comprehensive factor of soil itself. The research shows that soil pH does not affect selenite absorption improved by high phosphor condition［39］. It is also found that simultaneous utilization of lime and phosphate fertilizer in volcanic ash soil could affect selenium absorption by Trifolium repens［40］. It is clear that their relationship could change under different soil conditions. On the other hand, sulfur and selenium are cognate elements, and have similar physicalchemical properties. Sulfur content has significant impact on absorption and accumulation of selenium by plant［41］. It is thought that selenite and sulfate compete because that the same transporter is needed when absorbed by plant, and high sulfate content could affect selenium absorption［42］. In many researches, it is thought that sulfate and selenite have synergistic effect, and sulfate utilization promotes selenium absorption by lettuce［43］, wheat and spring rape［42］, barely and rice［44］.

Prospect

At present, most of researches concentrate in studying chemical form of selenium, and do not simultaneously study chemical and extraction forms. Selenium in several valences may exist in each extraction form, while selenium in the same valence may exist in different extraction forms. The research on their relationship is conducive to understanding form change and transfer rule of selenium, and studying selenium activation and utilization in natural seleniumrich soil. Significant improving methods of selenium effectiveness in soil are changing pH and organic matter, but they are not suitable for all seleniumrich regions. Although microbial activation has certain effect, it also becomes the key and difficulty due to complex soil environment. The research on soil moisture and element interaction mostly stays in theoretical stage, and simulation test and pot experiment are dominant. Therefore, it also becomes future research emphasis to improve selenium effectiveness in soil by two kinds of methods.

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Editor: Ping SONGProofreader: Xinxiu ZHU

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