Mechanisms underlying mercury detoxification in soil-plant systems after selenium application: a review.

Feasible countermeasures to mitigate mercury (Hg) accumulation and its deleterious effects on crops are urgently needed worldwide. Selenium (Se) fertilizer application is a cost-effective strategy to reduce Hg concentrations, promote agro-environmental sustainability and food safety, and decrease the public health risk posed by Hg-contaminated soils and its accumulation in food crops. This holistic review focuses on the processes and detoxification mechanisms of Hg in whole soil-plant systems after Se application. The reduction of Hg bioavailability in soil, the formation of inert HgSe or/and HgSe-containing proteinaceous complexes in the rhizosphere and/or roots, and the reduction of plant root uptake and translocation of Hg in plant after Se application are systemically discussed. In addition, the positive responses in plant physiological and biochemical processes to Se application under Hg stress are presented to show the possible mechanisms for protecting the plant. However, application of high levels Se showed synergistic toxic effect with Hg and inhibited plant growth. The effectiveness of Se application methods, rates, and species on Hg detoxification is compared. This review provides a good approach for plant production in Hg-contaminated areas to meet food security demands and reduce the public health risk.

Assessing the potential availability of selenium in the soil-plant system with manure application using diffusive gradients in thin-films technique (DGT) and DOM-Se fractions extracted by selective extractions.

Knowledge of the Se fractionation and the role of dissolved organic matter (DOM) in soil is the key to understanding Se mobility and its bioavailability in the soil-plant system. In this study, single extractions using phosphate-buffer (PBS), sequential extraction procedures (SEP), and diffusive gradients in thin-films (DGT) were used to measure Se bioavailability in soil supplemented with selenite and organic amendment (cow and chicken manures). Selenium fraction was isolated into DOM-Se fractions, such as hydrophilic acid-bound Se (HY-Se), fulvic acid-bound Se (FA-Se), humic acid-bound Se (HA-Se), and hydrophobic organic neutral-bound Se (HON-Se), by a rapid batch technique using XAD-8 resin (AMBERLITE XAD™, USA). Simultaneous application of either cow or chicken manure with selenite could result in the decrease of Se availability in the soil. Isolating Se available fraction into DOM-Se fractions showed that low-molecular-weight DOM-Se as an available fraction and even HY-Se as a less available fraction (OM-Se) were likely the major sources for Brassica juncea (L.) Czern. et Coss uptake in soil. Moreover, knowledge of the DOM-Se composition, especially the low-molecular-weight DOM-Se fractions, is important for assessing the bioavailability of Se in soil, the results of which are more accurate than the chemical extraction method. The high value of Pearson correlation coefficients between CDGT-Se and Se concentrations in shoots, tubers and roots of Brassica juncea (L.) Czern. et Coss in cow and chicken manures treatment were 0.95 and 0.99, 0.96 and 0,96, and 0.89 and 0.97 (p < 0,05), respectively, indicating that DGT-Se can reflect the Se uptake ability by plants and can be used to predict the bioavailability of Se when manure and selenite are simultaneously applied.

DOM derivations determine the distribution and bioavailability of DOM-Se in selenate applied soil and mechanisms.

Straw amendment and plant root exudates modify the quality and quantities of soil dissolved organic matter (DOM) and then manipulate the fractions of soil selenium (Se) and its bioavailability. Two typical soils with distinct pH were selected to investigate the effect of different contributors on DOM-Se in soil. The mechanisms relying on the variation in DOM characteristics (quality, quantity and composition) were explored by UV-Vis, ATR-FTIR and 3D-EEM. Straw amendment significantly (p < 0.05) suppressed the selenate bioavailability. The reduction in wheat Se content was greater in krasnozems than in Lou soil, as more HA fraction appeared in krasnozems. The root exudates of wheat mainly elevated the low molecular hydrophilic compounds (Hy) in soil, which contributed to the SOL-Hy-Se fractions and thus grain Se in soils (p < 0.01). However, straw amendment promoted DOM transforming from small molecules (Hy and FA) to aromatic large molecules (HA), when accompanied with the reduction and retention of Se associated with these molecules. As a result, selenium bioavailability and toxicity reduced with DOM amendment and DOM-Se transformation.

Effects of selenium combined with zinc amendment on zinc fractions and bioavailability in calcareous soil.

Selenium (Se) and zinc (Zn) are two important trace elements for human being and animals. The interaction between Se and Zn on the bioavailability of Zn in soil is still unclear. Therefore, pot experiments exposed to different dosages of zinc sulfate (ZnSO4) (0, 20, and 50 mg/kg soil) and sodium selenite (Na2SeO3) (0, 0.5, 1.0, and 2.5 mg/kg soil) were conducted to investigate the effects of selenite application on Zn bioavailability in calcareous soil and its related mechanisms. The total Zn content of different tissues (roots and shoots) of pak choi (Brassica chinensis L.) and the changes in Zn fraction distribution in soil before planting and after harvest were determined, and the mobility factor (MF) and distribution index (DI) of Zn in soils were calculated. In addition, the Pearson correlation and path analysis were conducted to clarify the relationships between Zn fractions in soil and the Zn uptake of pak choi. Results showed that Se amendment elevated soil Zn bioavailability at appropriate levels of Se and Zn. When 1.0 and 2.5 mg/kg of Se and 20 mg/kg of Zn were applied in soil, the proportion of exchangeable Zn (Ex-Zn) and Zn weakly bound to organic matter (Wbo-Zn) to the total content of Zn was significantly increased by 28.14%-82.52% compared with that of the corresponding single Zn treatment. Therefore, the Zn concentration in the shoots of pak choi was significantly increased by 27.2%-31.1%. High Zn (50 mg/kg) and Se co-amended treatments showed no significantly beneficial effect on the bioavailability of Zn. In addition, the potential available Zn content in soil (weakly bound to organic matter and carbonate bound Zn) and MF and DI values were all positively correlated with the Zn concentrations in pak choi, indicating that these indexes can be used to predict the bioavailability of Zn in soil. This study can provide a good reference for Se and Zn biofortification of plants in calcareous soil.

Selenium distribution in the Chinese environment and its relationship with human health: A review.

This paper reviewed the Se in the environment (including total Se in soil, water, plants, and food), the daily Se intake and Se content in human hair were also examined to elucidate Se distribution in the environment and its effects on human health in China. Approximately 51% of China is Se deficiency in soil, compared with 72% in the survey conducted in 1989. Low Se concentrations in soil, water, plants, human diet and thus human hair were found in most areas of China. The only significant difference was observed between Se-rich and Se-excessive areas for Se contents in water, staple cereal, vegetables, fruits, and animal-based food, no remarkable contrast was found among other areas (p>0.05). This study also demonstrated that 39-61% of Chinese residents have lower daily Se intakes according to WHO/FAO recommended value (26-34µg/day). Further studies should focus on thoroughly understanding the concentration, speciation, and distribution of Se in the environment and food chain to successfully utilize Se resources, remediate Se deficiency, and assess the Se states and eco-effects on human health.

Role of organic acids on the bioavailability of selenium in soil: A review.

Organic Acids (OAs) are important components in the rhizosphere soil and influence Se bioavailability in soil. OAs have a bidirectional contrasting effect on Se bioavailability. Understanding the interaction of OAs with Se is essential to assessing Se bioavailability in soil and clarifying the role of OAs in controlling the behavior and fate of Se in soil. This review examines the mechanisms for the (im)mobilization of Se by OAs and discusses the practical implications of these mechanisms in relation to sequestration and bioavailability of Se in soil.