Effects of straw amendment on selenium aging in soils: Mechanism and influential factors

Selenium spatial distribution and bioavailability of soil-plant systems in China: a comprehensive review

Selenium (Se) has a dual nature, with beneficial and harmful effects on plants, essential for both humans and animals, playing a crucial role in ecosystem regulation. Insufficient Se in specific terrestrial environments raises concerns due to its potential to cause diseases, while excess Se can lead to severe toxicity. Thus, maintaining an optimal Se level is essential for living organisms. This review focuses first on Se transformation, speciation, and geochemical properties in soil, and then provides a concise overview of Se distribution in Chinese soil and crops, with a focus on the relationship between soil Se levels and parent materials. Additionally, this paper explores Se bioavailability, considering parent materials and soil physicochemical properties, using partial least squares path modeling for analysis. This paper aimed to be a valuable resource for effectively managing Se-enriched soil resources, contributing to a better understanding of Se role in ecosystems.

Distribution and soil threshold of selenium in the cropland of southwest mountainous areas in China

To investigate the distribution characteristics of selenium (Se) in mountainous soil-crop systems and examine the threshold value of Se-rich soil, 275 soil samples and 153 associated crop samples (rice, maize, tea, nuts, konjac, mushrooms, buckwheat, and coffee) were collected in Ximeng County, a typical mountainous area in southwest China. The total Se, available Se, organic matter, pH, sampling point elevation, and crop Se content were analyzed to examine the distribution characteristics of soil Se and the ability of primary crops to enrich Se in Ximeng County. Random forest and multiple regression models were established to identify the factors influencing the available soil Se and the crop Se enrichment coefficient. Finally, the Se-rich soil threshold was examined based on the total Se, available Se, and Se content in primary crops (rice, maize, and tea). The results showed soil Se resource abundance in the study region, with high Se soil accounting for 64.72% of the entire area. The soil Se content displayed significant spatial autocorrelation. The average Se enrichment coefficient of the main cultivated crops included mushrooms > nuts > rice > coffee > tea > maize > buckwheat > konjac. The total Se content in the soil had the highest impact on the available Se content in the soil and the Se enrichment coefficient of crops. A Se-rich soil threshold of 0.3 mg·kg-1 was used for rice and maize, while that of tea was 0.4 mg·kg-1. This result provided a theoretical basis for developing and utilizing Se resources in mountainous soil in southwestern China and dividing the Se-rich soil threshold.

Straw with different fermentation degrees mediate Se/Cd bioavailability by governing the putative iron reducing bacteria

Straw returning is a crucial agronomic practice in fields due to its various benefits. However, effects and mechanisms of straw with different fermentation degrees on Se and Cd bioavailability have not been sufficiently investigated. In this study, straw with different fermentation degrees were applied to a Cd-contaminated seleniferous soil to investigate their effects on Se and Cd bioavailability. Results revealed that the effects of straw application on Se/Cd bioavailability in soil depended on the fermentation degrees of straw. Both original and slightly fermented straw had pronounced impacts on microbial iron reduction compared to fully fermented straw, and thus led to a significant increase in Se and Cd bioavailability. The linear discriminant analysis effect size (LEfSe) showed that norank_f_Symbiobacteraceae, Micromonospora, WCHB1-32, Ruminiclostrdium, and Cellulomonas were the major biomarkers at genus level in straw application soils, additional network analysis and random forest analysis suggested that Ruminiclostrdium and Cellulomonas might be implicated in microbial iron reduction. Furthermore, the microbial iron reduction had negative effects on mineral-associated Se with coefficient of -0.81 and positive effects on mineral-associated Cd with coefficient of 0.72, while Mn fractions exhibited positive effects on mineral-associated Se with a coefficient of 0.53 and negative effects on mineral-associated Cd. In conclusion, straw with different fermentation degrees governed Se and Cd mobility by regulating abundance of Ruminiclostrdium and Cellulomonas, subsequently affecting Fe and Mn fractions and consequently influencing Se and Cd bioavailability.

Insight into the availability and desorption kinetics of Se and Cd in naturally-rich soils using diffusive gradients in thin-films technique

Cadmium (Cd) contamination of selenium (Se)-rich soils may jeopardize the nutritional benefits of Se-biofortified crops. This study employed diffusive gradients in thin-films (DGT) technique and DIFS (DGT-induced fluxes in soils) model to understand the interdependency and driving factors of Se and Cd distribution and desorption kinetics across 50 soils from south China with naturally elevated levels. DGT-labile Se was the highest (up to 2.66 μg L-1) in non-carbonate/shale-derived soils, while Cd was maximal (5.53 μg L-1) in carbonate-based soils, reflecting soil background concentrations and soil characteristics. Over one-third of the soils showed labile Se:Cd molar ratio below 0.7, suggesting Cd phytotoxicity risks. The DIFS-derived response times (Tc) and desorption rate constants (k-1) suggested that Se was resupplied to the soil solution faster than Cd in soils with higher pH and SOM level, but Se resupply was still restricted due to the rapid depletion of its labile pool. As the first study of Se and Cd release kinetics in soils, our results reveal dependence on soil parent materials, with low labile Se:Cd soils presenting greater Cd hazards. By elucidating Se and Cd lability and interactions in soils, our findings help to inform management strategies to balance reduced Cd risk with adequate Se availability.

Effects of selenium on biogeochemical cycles of cadmium in rice from flooded paddy soil systems in the alluvial Indus Valley of Pakistan

This study delves into the pollution status, assesses the effects of Se on Cd biogeochemical pathways, and explores their interactions in nutrient-rich paddy soil-rice ecosystems through 500 soil-rice samples in Pakistan. The results showed that 99.6 % and 12.8 % of soil samples exceeded the World Health Organization (WHO) allowable Se and Cd levels (7 and 0.35 mg/kg). In comparison, 23 % and 6 % of the grain samples exceeded WHO's allowable Se and Cd levels (0.3 and 0.2 mg/kg), respectively. Geographically Weighted Regression (GWR) model results further revealed spatial nonstationarity, confirming diverse associations between dependent variables (Se and Cd in rice grain) and independent variables from paddy soil and plant tissues (root and shoot), such as Soil Organic Matter (SOM), pH, Se, and Cd concentrations. High Se:Cd molar ratios (>1) and a negative correlation (r = -0.16, p < 0.01) between the Cd translocation factor (Cd in rice grain/Cd in root) and Se in roots suggest that increased root Se levels inhibit the transfer of Cd from roots to grains. The inverse correlation between Se and Cd in paddy grains was further characterized as Se deficiency, no risk, high Cd risk, Se risk, Cd risk, and Se-Cd co-exposure risk. There was no apparent risk for human co-consumption in 42.6 % of grain samples with moderate Se and low Cd. The remaining categories indicate differing degrees of risk. In the study area, 31 % and 20 % of grain samples with low Se and Cd indicate Se deficiency and risk, respectively. High Se and low Cd levels in rice samples suggest a potential hazard for severe Se exposure due to frequent rice consumption. This study not only systematically evaluates the pollution status of paddy-soil systems in Pakistan but also provides a reference to thoroughly contemplate the development of a scientific approach for evaluating human risks and the potential dangers associated with paddy soils and rice, specifically in regions characterized by low Se and low Cd concentrations, as well as those with moderate Se and high Cd concentrations. SYNOPSIS: This study is significant for understanding the effects of Se on Cd geochemical cycles and their interactions in paddy soil systems in Pakistan.

Organic amendments for in situ immobilization of heavy metals in soil: A review

There is a growing need for soil remediation due to the increase in heavy metals (HMs) migrating into the soil environment, especially those from man-made sources dominated by industry and agriculture. In situ immobilization technology, because of its lower life cycle environmental footprint, can achieve "green and sustainable remediation" of soil heavy-metal pollution. Among the various in situ immobilization remediation agents, organic amendments (OAs) stand out as they can act as soil conditioners while acting as HMs immobilization agents, and therefore have excellent application prospects. In this paper, the types and remediation effects of OAs for HMs in situ immobilization in soil are summarized. OAs have an important effect on the soil environment and other active substances in soil while interacting with HMs in soil. Based on these factors, the principle and mechanism of HMs in situ immobilization in soil using OAs are summarized. Given the complex differential characteristics of soil itself, it is impossible to determine whether it can remain stable after heavy-metal remediation; therefore, there is still a gap in knowledge regarding the compatibility and long-term effectiveness of OAs with soil. In the future, it is necessary to develop a reasonable HMs contamination remediation program for in situ immobilization and long-term monitoring through interdisciplinary integration techniques. These findings are expected to provide a reference for the development of advanced OAs and their applications in engineering.

Effects of microorganisms on soil selenium and its uptake by pak choi in selenium-enriched lateritic red soil

Data on selenium (Se) transformation, specifically the mineralization or activation of Se bound by microorganisms in natural Se-enriched soil, is limited. Therefore, this study investigates the effects of microorganisms on Se availability of Se-enriched lateritic red soil and Se uptake by pak choi. Following the incubation of Stenotrophomonas maltophilia S1 and arbuscular mycorrhizal (AM) fungi agent, the available Se content of soils increased from 35 to 66.69-117.04 μg/kg, corresponding to an increase of 90.50-234.40%. The Se bioconcentration and translocation factors in pak choi increased after adding the AM fungi agent and strain S1. The soil acid phosphatase activity, and pak choi root length, surface area, and diameter also increased. Moreover, the soil acid phosphatase activity showed a significant positive correlation with soil available Se and phosphorus content (p < 0.01). Overall, the AM fungi agent and strain S1 increased Se bioavailability by enhancing soil acid phosphatase and promoting root activity, ultimately increasing pak choi's ability to absorb available Se.

Understanding soil selenium accumulation and bioavailability through size resolved and elemental characterization of soil extracts

Dietary deficiency of selenium is a global health threat related to low selenium concentrations in crops. Despite the chemical similarity of selenium to the two more abundantly studied elements sulfur and arsenic, the understanding of its accumulation in soils and availability for plants is limited. The lack of understanding of soil selenium cycling is largely due to the unavailability of methods to characterize selenium species in soils, especially the organic ones. Here we develop a size-resolved multi-elemental method using liquid chromatography and elemental mass spectrometry, which enables an advanced characterization of selenium, sulfur, and arsenic species in soil extracts. We apply the analytical approach to soils sampled along the Kohala rainfall gradient on Big Island (Hawaii), which cover a large range of organic carbon and (oxy)hydroxides contents. Similarly to sulfur but contrarily to arsenic, a large fraction of selenium is found associated with organic matter in these soils. However, while sulfur and arsenic are predominantly found as oxyanions in water extracts, selenium mainly exists as small hydrophilic organic compounds. Combining Kohala soil speciation data with concentrations in parent rock and plants further suggests that selenium association with organic matter limits its mobility in soils and availability for plants.

Selenium in Soil-Plant-Microbe: A Review

Selenium (Se) plays an important role in geochemistry and is an essential trace element for humans and animals. This review summarizes the transformation and accumulation of Se in the plant-soil-microbe system. As one of the important reservoirs of Se, soil is an important material basis of its entry into the food chain through plants. Soil with an appropriate amount of Se is beneficial for plant growth and plays a valuable role in a stress-resistant environment. Among the many migration and transformation pathways, the transformation of Se by microorganisms is particularly important and is the main form of Se transformation in the soil environment. In this review, the role and form transformation of Se in plants, soil, and microorganisms; the role of Se in plants; the form, input, and output of Se in soil; the absorption and transformation of Se by plants; and the role of microorganisms in Se transformation are presented. In addition to describing the migration and transformation laws of Se in the environment, this review expounds on the main directions and trends of Se research in the agricultural field as well as current gaps and difficulties in Se-related research. Overall, this reviews aims to provide necessary information and theoretical references for the development of Se-rich agriculture.

Detoxification difference of cadmium between the application of selenate and selenite in native cadmium-contaminated soil

Cadmium (Cd) has strong mobility and could cause toxicity to plants, and selenium (Se) can effectively detoxify Cd stress. However, differences in the detoxification effects of different species and dosages of exogenous Se on Cd and its mechanism are still unclear. In this study, a pot experiment was conducted to determine the effects of different rates of selenite and selenate application on radish growth, the uptake and translocation of Cd, and the fractions of Cd transformation in native Cd-contaminated soil. Results indicated that the decrease in radish biomass in selenate treatment was significantly greater than that in selenite treatment at a high Se application rate (2.5 mg·kg^-1) (p < 0.05). In contrast to selenite treatments, the application of selenate significantly increased the translocation of Cd from radish roots to shoots (p < 0.05). Cadmium concentration and its bioaccumulation factor in radish decreased gradually with increasing selenite application rates, while these values decreased at low Se rate (1 mg·kg^-1) and increased at high Se rate for selenate treatment. Different Se application rates resulted in Cd fractions distributions to change in soil. Therefore, the application of selenite treatment had a greater detoxification effect on Cd in soil than that in selenate treatment, and the double toxic effect was observed between Se and Cd in high selenate treatment (2.5 mg·kg^-1). Combined with human health risk asseeement, the application of 2.5 mg·kg^-1 selenite could be a good approach for detoxification in native Cd-contaminated soil used in this study.

Effect of straw decomposition on organic carbon fractions and aggregate stability in salt marshes

Straw addition can increase the content of soil organic carbon (SOC), and affect the content of aggregates and organic carbon fractions. The changes in aggregates and organic carbon fractions in the natural salt marsh, 10-year and 15-year freshwater pumping areas in the Yellow River Estuary were studied by 120-day field in situ culture experiments with Phragmites australis straw addition. The results showed that straw addition mainly enhanced the soil aggregate stability in the 10-year freshwater pumping area, and the organic carbon content of small macro-aggregates increased significantly by 26.36% (P < 0.05). In particular, the content of coarse particulate organic carbon (cPOC) with small macro-aggregates in all areas increased significantly with the addition of straw (P < 0.05). For small macro-aggregates in the 10-year pumping area, the cPOC contents increased significantly by 21.73 g/kg (P < 0.05); and were significantly higher than the fine particulate organic carbon (fPOC) and mineral-associated organic carbon (mSOC) contents, as the fPOC contents in micro-aggregates increased by 85.92% (P < 0.05). Additionally, the cPOC contents of small macro-aggregates and fPOC contents of micro-aggregates increased by 34.59% and 43.24% in the 15-year pumping area. The contents of mSOC were the lowest in different aggregates across all areas. Thus, straw addition had a significant effect on the contents of cPOC and fPOC, while freshwater pumping in the YRE could affect the distribution of fPOC and mSOC with aggregates.

Selenium bioavailability in soil-wheat system and its dominant influential factors: A field study in Shaanxi province, China

Selenium (Se) content of crops depends on the local soil Se content and/or its bioavailability, and identifying the influence factors of soil Se bioavailability is a significant basis for adopting targeted agronomic measures to improve the Se nutritional status of humans. In this study, the main wheat-producing region in Shaanxi province with similar parent material and climate conditions was selected as the study area. The total Se contents of 602 soil samples and their corresponding wheat grains were determined, and the distribution characteristics of soil Se bioavailability and its dominant influential factors were investigated. Results showed that the total Se content ranged from 0.02 mg/kg to 1.67 mg/kg (average of 0.25 ± 0.25 mg/kg) in soil, which was lower than that content in China (0.29 mg/kg). The Se content of wheat grain was 0.001-1.50 mg/kg (average of 0.11 ± 0.19 mg/kg). The distribution trend of the Se content in wheat grains was different from that of the total soil Se, but it was consistent with the distribution of soil bioavailable Se content. The bioavailable Se accounted for 11.1% of the total soil Se. This could be attributed to relatively high soil Se bioavailability of the study area belonging to alkaline soil (with a pH of approximately 8). Both redundancy analysis and path analysis revealed that soil pH and organic matter were the dominant influential factors of soil Se bioavailability in Shaanxi wheat-producing area, and the soil Se bioavailability increased with these two parameters raising. On this basis, a prediction model was established to predict the Se content in wheat grain. The results show that the various agronomic measures could be used to produce Se-enriched wheat by regulating the soil pH and the organic matter content in Se biofortification practice.

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.

Distribution, origin and speciation of soil selenium in the black soil region of Northeast China

Selenium (Se) is an essential trace element within human beings that hold with crucial biological functions. Investigating the complex origin of soil Se is of great importance to scientifically approach the land use of Se-rich land use, and the respective promotion of regional economic development. In this study, 160 soil samples from 10 profiles in farmland and woodland were collected in Hailun city, which is a typical black soil region in Northeast China, in order to characterize the distribution and speciation of Se in the black soil, and to identify the origin of soil Se. The total selenium content in the soil ranges from 0.045 to 0.444 μg g^-1, with an average selenium content in black soil (0.318 μg g^-1) of three times greater than that found in the yellow-brown soil (0.114 μg g^-1). The land-use type has a significant influence on the distribution of selenium in the black soil. Moreover, Se and heavy metals have a significant (positive or negative) correlation, in which TOC plays an important role. The black soil presents a consistent REE distribution pattern with underlying yellow-brown soil indicating black soil originates from yellow-brown soil. REE geostatistical analysis suggests that the soil Se partly originates from shale weathering and enriches in black soil. Moreover, elemental geochemical analysis and XRD results show that the paleoclimate change from humid and warm to dry and cold is favorable for organic matter accumulation, resulting in less leaching and enhanced adsorption of selenium into the black soil.

Immobilization of Cd, Pb and Zn through Organic Amendments in Wastewater Irrigated Soils

Due to the scarcity of water, raw sewage effluents are often used to irrigate arable suburban soils in developing countries, which causes soil contamination with toxic metals. Soil microorganisms involved in biochemical transformations are sensitive to heavy metals contamination. The study was designed to investigate the effect of organic amendments on the microbial activity of cadmium (Cd), lead (Pb) and zinc (Zn) fractions and their bioavailability in soils contaminated with wastewater irrigation. Three metal contaminated soils under wastewater irrigation were collected, ground, sieved and added to incubation jars. Two organic amendments: wheat straw and chickpea straw, were applied (1% w/w) to the soil before incubation for 84 days at 25 °C. The CO2-C evolution after 1, 2, 3, 5, 7, 10 and 14 days was measured and thereafter was also measured weekly. Soil samples collected at 0, 14, 28, 42, 56, 70 and 84 days after incubation were analyzed for microbial biomass carbon (MBC). Sequential extraction for metal fractionation of samples was carried out collected at 0, 28, 56 and 84 days. Three soils differed significantly in evolved MBC and ∑CO2-C. Chickpea straw addition significantly increased soil MBC as compared to the wheat straw. Organic amendments significantly increased ∑CO2-C evolution from the soils, which was higher from chickpea straw. The addition of crop residues did not affect total Pb, Cd and Zn contents in soils. The concentration of exchangeable, carbonate bound and residual fractions of Pb, Cd and Zn decreased (6–27%), while the organic matter bound fraction increased (4–75%) with straw addition. Overall, the organic amendments improved microbial activity and reduce the bioavailability of toxic metals in wastewater irrigated soils. Furthermore, organic amendments not only reduce economic losses as they are cheap to produce but also minimize human health risks from heavy metals by hindering their entry into the food chain.

Characteristics of selenium enrichment and assessment of selenium bioavailability using the diffusive gradients in thin-films technique in seleniferous soils in Enshi, Central China

Agricultural products from seleniferous areas commonly face problems associated with substantial variation in selenium (Se) concentration, which is mainly caused by the heterogeneity of Se bioavailability in soil. Many studies have assessed the bioavailability of Se and its influencing factors using soil samples treated with exogenous Se. Given the distinctly different characteristics of Se-spiked soils and naturally seleniferous soils, exploring Se bioavailability in naturally seleniferous soils is crucial to the stable production of Se-enriched agricultural products. In this study, we used the classical sequential extraction method to determine the Se fractionation and then applied the diffusive gradients in thin-films (DGT) technique to assess the Se bioavailability in naturally seleniferous soils. The results indicated that soluble and exchangeable Se fractions with high bioavailability accounted for only 0.7% and 5.1% of total Se, respectively. Both soluble and exchangeable Se concentrations were significantly positively correlated with soil pH (r = 0.329 and 0.262, respectively; P < 0.01). Se mainly exists in Fe-Mn oxide-bound, organic matter-bound, and residual Se fractions with low mobility (94.2% of total Se), among which organic matter-bound Se was the predominant fraction (49.5% of total Se). A significant positive correlation was found between total Se and soil organic matter (r = 0.539; P < 0.01). Multiple regression analysis revealed that the DGT-determined Se was mostly derived from soluble and exchangeable Se. The high correlation between the DGT-determined Se fraction and Se uptake by rice (r = 0.91; P < 0.01) confirmed that DGT can accurately assess Se bioavailability in naturally seleniferous soils in Enshi and other similar environmental settings.

The effect of straw-returning on antimony and arsenic volatilization from paddy soil and accumulation in rice grains

Pollution by antimony (Sb) and arsenic (As) in soil can pose a great threat to human health. Straw-returning is widely applied to paddy fields for improving and remediating soil. A pot experiment was conducted to investigate the effect of straw-returning on Sb and As transformation and translocation in a soil-rice system. In this study, Sb and As co-contaminated soil was thoroughly mixed with different proportions (0, 0.5, 1, and 2%) of straw and used for growing rice plants through the entire growing stage in a pot experiment and 4 weeks in a microcosm experiment. The straw application significantly increased Sb and As mobility. The concentrations of total Sb and As in soil-pore water increased after the application of straw in most growing stages. The Sb volatilization in the pot and microcosm experiments was also stimulated by straw application. With the high dose of straw application (2%), the concentration of Sb in brown grain was reduced by 72% compared with the control, but As concentrations increased by around 77%. These findings provide a new perspective in that straw-returning could affect the behavior of both Sb and As in soil and reduce the Sb accumulation in brown grain and some guidance in the use of straw-returning in Sb-contaminated paddy soil.

Enhancement of a constructed wetland water treatment system for selenium removal

Selenium (Se) is essential to most animals, whereas the gap between necessity and toxicity is narrow. Our previous work showed constructed wetlands were a promising solution to Se contamination in aquatic habitats. This study further examined effects of organic amendments and hydrologic regimes on Se removal by constructed wetlands. Our results suggest the removal efficiency exceeded 94% within 8 days for the systems with moderate and low organic carbon contents in the substrate, as a 98% removal of Se was obtained in three weeks for the system subjected to the 2-day wet/dry cycle. To mimic field wetlands, a litter layer was added to the cattail treatment system, which reduced waterborne Se much more rapidly than control, achieving a 77% removal of Se within 4 days. XAS results show all sediment Se was transformed to Se⁰ in the presence of litter, as SeMet (47%) dominated the Se adsorbed by the litter. The findings indicate the Se removal capacity of a constructed wetland would improve over time, especially via Se volatilization into the atmosphere and Se stabilization in the sediment with litter accumulating at the surface. Another mesocosm experiment showed the cattail floating system effectively removed Se, particularly selenate, by 99% in 48 h. To confirm that high performance, seven constructed wetland types were set up for comparison. The results show the cattail floating system was most effective in Se removal (93-100% at around 35 °C in summer and 51-100% at about 5 °C in winter). More research is needed to test the floating system under more field conditions and investigate the biomagnification and biotransformation of the removed Se along food chains. Seven constructed wetland types were set up for comparison.

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.

Impact of low-molecular weight organic acids on selenite immobilization by goethite: Understanding a competitive-synergistic coupling effect and speciation transformation

The interactions between low-molecular weight organic acids (LMWOAs) and selenium (Se) on mineral/water interfaces affect the release, immobilization and bioavailability of Se in nature. Herein, the effects of three environmentally relevant LMWOAs (i.e., oxalic (Oxa), succinic (Suc) and citric (Cit) acids) on Se(IV) adsorption to goethite under oxic conditions were investigated using batch experiments, speciation fractionation, and ATR-FTIR and XPS analyses. The LMWOAs exhibited a competitive-synergistic coupling effect on Se(IV) adsorption to goethite, which inhibited the adsorption rate of Se(IV) by 14.1, 13.3 and 8.0 times. However, immobilization of Se(IV) was simultaneously enhanced by 39.1%, 34.6% and 14.1% in the following order Oxa > Suc > Cit. The results obtained by fractionation of the adsorbed Se(IV) revealed that the enhancement was due to surface binding as well as speciation transformation from ligand-exchangeable Se(IV) into residual fractions, which increased by approximately 18% in the presence of the LMWOAs. The dissolution of goethite significantly improved due to the LMWOAs and decreased to different degrees as the concentration of Se(IV) increased. The monodentate mononuclear complexes (58.2%) and Lewis base sites bonded Se (41.8%) were the predominant surface species of Se(IV) in goethite-Se(IV) system. The ATR-FTIR and high-resolution XPS analyses demonstrated that the formation of ≡FeO(SeO)O-CO surface complexes (22.8-27.0%) occurred in the presence of LMWOAs, which could be closely correlated with the interface-mediated reduction of Se(IV). In addition, the predominant mechanism for the formation of residual Se is LMWOA specific, in which ferric selenite-like precipitation was dominant for Suc (10.6%) and Cit (11.6%) and reduction was dominant for Oxa (17.5%). Overall, LMWOAs play an important role in Se(IV) immobilization and speciation transformation and may facilitate understanding the Se bioavailability in rhizosphere soils under oxic conditions.

Selenium translocation in the soil-rice system in the Enshi seleniferous area, Central China

Rice is an important source of selenium (Se) exposure; however, the transformation and translocation of Se in the soil-rice system remain poorly understood. Here, we investigated the speciation of Se in Se-rich soils from Enshi, Central China and assessed which Se species is bioavailable for rice grown in Enshi. Extremely high Se concentrations (0.85 to 11.46 mg/kg) were observed in the soils. The soil Se fractions, which include water-soluble Se (0.2 to 3.4%), ligand-exchangeable Se (4.5 to 15.0%), organically bound Se (57.8 to 80.0%) and residual Se (6.1 to 32.9%), are largely controlled by soil organic matter (SOM) levels. Decomposition of SOM promotes the transformation of organically bound Se to water-soluble Se and ligand-exchangeable Se, thereby increasing the bioavailability of Se. The bioaccumulation factors (BAFs) of Se decrease in the following order: roots (0.84 ± 0.30) > bran (0.33 ± 0.17) > leaves (0.18 ± 0.09) > polished rice (0.14 ± 0.07) > stems (0.12 ± 0.07) > husks (0.11 ± 0.07). Selenium levels in rice plants are affected by multiple soil Se fractions in the soil. Water-soluble, ligand-exchangeable and organically bound Se fractions are the major sources of Se in rice tissues.

Leaching behaviors and chemical fraction distribution of exogenous selenium in three agricultural soils through simulated rainfall

To clarify the leaching risk of selenium (Se) in agricultural soils, a laboratory column experiment was conducted to study the characteristics of leaching and chemical fractions of Se in three different soils treated with different levels of exogenous selenate under simulated local rainfall. Results demonstrated that the Se concentration in leachates of all tested soils decreased rapidly at the beginning of leaching and slowly decreased thereafter. After leaching, Se concentrations in leachates of all tested soils at 1, 3, and 6 mg/kg exogenous Se concentrations were 0.06-0.24, 0.25-0.84, and 0.60-1.65 mg/L, respectively, which exceeded the standard limit of the Chinese Environmental Quality Standards for Groundwater (<0.01 mg/L) (GB/T 14848-2017). The cumulative leached Se amount accounted for 51.27-86.22% of the total Se. Those results indicated the high risk of Se leaching in the tested soils. The Elovich model could better describe Se leaching processes in krasnozem, while the leaching processes of Se in black soil and loess soil accorded with the power function model. Se mainly existed in soluble fraction (61.33-81.05%) before leaching and residual fraction (48.91-68.04%) after leaching. The soluble and exchangeable Se fractions were the main contributors of Se in leachates. In addition, the parameters of the U_ts and I_R values could well describe the distribution of Se fractions in soil during leaching. In general, more attention should be placed on the assessment of Se leaching in soil.