Influence of soil mercury concentration and fraction on bioaccumulation process of inorganic mercury and methylmercury in rice (Oryza sativa L.)

Temporal mercury dynamics throughout the rice cultivation season in the Ebro Delta (NE Spain): An integrative approach

During the last few decades, inputs of mercury (Hg) to the environment from anthropogenic sources have increased. The Ebro Delta is an important area of rice production in the Iberian Peninsula. Given the industrial activity and its legacy pollution along the Ebro river, residues containing Hg have been transported throughout the Ebro Delta ecosystems. Rice paddies are regarded as propitious environments for Hg methylation and its subsequent incorporation to plants and rice paddies' food webs. We have analyzed how Hg dynamics change throughout the rice cultivation season in different compartments from the paddies' ecosystems: soil, water, rice plants and fauna. Furthermore, we assessed the effect of different agricultural practices (ecological vs. conventional) associated to various flooding patterns (wet vs. mild alternating wet and dry) to the Hg levels in rice fields. Finally, we have estimated the proportion of methylmercury (MeHg) to total mercury in a subset of samples, as MeHg is the most bioaccumulable toxic form for humans and wildlife. Overall, we observed varying degrees of mercury concentration over the rice cultivation season in the different compartments. We found that different agricultural practices and flooding patterns did not influence the THg levels observed in water, soil or plants. However, Hg concentrations in fauna samples seemed to be affected by hydroperiod and we also observed evidence of Hg biomagnification along the rice fields' aquatic food webs.

Soil organic matter degradation and methylmercury dynamics in Hg-contaminated soils: Relationships and driving factors

Biodegradation of soil organic matter (SOM), which involves greenhouse gas (GHG) emissions, plays an essential role in the global carbon cycle. Over the past few decades, this has become an important research focus, particularly in natural ecosystems. SOM biodegradation significantly affects contaminants in the environment, such as mercury (Hg) methylation, producing highly toxic methylmercury (MeHg). However, the potential link between GHG production from SOM turnover in contaminated soils and biogeochemical processes involving contaminants remains unclear. In this study, we investigated the dynamics of GHG, MeHg production, and the relationship between biogeochemical processes in soils from two typical Hg mining sites. The two contaminated soils have different pathways, explaining the significant variations in GHG and MeHg production. The divergence of the microbial communities in these two biogeochemical processes is essential. In addition to the microbial role, abiotic factors such as Hg species can significantly affect MeHg production. On the other hand, we found an inverse relationship between CH4 and MeHg, suggesting that carbon emission reduction policies and management could inadvertently increase the MeHg levels. This highlights the need for an eclectic approach to organic carbon sequestration and contaminant containment. These findings suggest that it is difficult to establish a general pattern to describe and explain the SOM degradation and MeHg production in contaminated soils within the specific scenarios. However, this study provides a case study and helpful insights for further understanding the links between environmental risks and carbon turnover in Hg mining areas.

Impacts of atmospheric deposition on the heavy metal mobilization and bioavailability in soils amended by lime

Atmospheric deposition is an important source of heavy metal in agricultural soils, but there is limited research on the mobility of these metals in soil and their impact on soil amendment. Here, we performed a dust incubation experiment in soils in the laboratory and a factorial transplant experiment at three field sites with a gradient of atmospheric deposition to examine the impacts of atmospherically deposited heavy metals (Cu, Cd, and Pb) on the mobility and bioavailability in soils with and without lime applications. Results showed that the atmospherically deposited heavy metals showed high mobility and were primarily presented in the soluble ionic fractions in the wet part and acid-exchangeable and reducible fractions in the dry part of atmospheric deposition. Atmospheric dust addition caused the 2p3/2 and 2p1/2 electrons of Cu atoms in uncontaminated soils to transition the 3d vacant states, resulting in similar copper absorption peaks as atmospheric particles by the observation of X-ray absorption near-edge spectroscopy (XANES). In the field, atmospheric deposition can only increase the mobile fractions in the surface soils, but not in the deeper layers. However, the deposition can increase the soluble and diffusive gradients in thin films (DGT)-measured bioavailable fractions in profile along with the soil depth. Lime applications cannot significantly reduce the mobile fractions of heavy metals in the surface soils exposed to atmospheric deposition, but significantly reduce the heavy metal concentrations in soil solutions and the DGT-measured bioavailable concentrations, particularly in the deeper layer (6-10 cm). The major implication is that atmospherically deposited heavy metals can significantly increase their bioavailable concentrations in the plough horizon of soil and constrain the effects of soil amendments on heavy metal immobilization, thereby increasing the risks of crop uptake.

Mercury speciation in selenium enriched wheat plants hydroponically exposed to mercury pollution

Mercury (Hg) pollution in agricultural soils and its potential pathway to the human food chain can pose a serious health concern. Understanding the pathway of Hg in plants and how the speciation may change upon interaction with other elements used for biofortification can be critical to assess the real implications for the final plant-based product. In that respect, selenium (Se) biofortification of crops grown in Se-poor soil regions is becoming a common practice to overcome Se deficient diets. Therefore, it is important to assess the interplay between these two elements since Se may form complexes with Hg reducing its bioavailability and toxicity. In this work, the speciation of Hg in wheat plants grown hydroponically under the presence of Hg (HgCl2) and biofortified with Se (selenite, selenate, or a 1:1 mixture of both) has been investigated by X-ray absorption spectroscopy at the Hg L3-edge. The main Hg species found in wheat grains was the highly toxic methylmercury. It was found that the Se-biofortification of wheat did not prevent, in general, the Hg translocation to grains. Only the 1:1 mixture treatment seemed to have an effect in reducing the levels of Hg and the presence of methylmercury in grains.

Mercury fractionation, bioavailability, and the major factors predicting its transfer and accumulation in soil-wheat systems

Soil mercury (Hg) and its bioaccumulation in food crops have attracted widespread concerns globally due to its harmful effects on biota. However, soil mercury fractionation, bioavailability, and the major factors predicting its transfer and accumulation in soil-wheat-systems have not been thoroughly explored. Twenty-one (21) soil samples collected throughout China with a wide spectrum of physico-chemical characteristics were contaminated with HgCl₂ and winter wheat (Triticum aestivum L.) was grown on the soils in a greenhouse pot-culture experiment for 180 days. A four-step sequential extraction was used segregating soil Hg into water-soluble (F1, 0.21 %), exchangeable (F2, 0.07 %), organically bound (F3, 16.40 %), and residual fractions (F4, 83.32 %). Step-wise multiple linear regression (SMLR) and path analysis (PA) were used to develop a prediction model and identify the major controlling factors of soil-wheat Hg transference. The SMLR results revealed that wheat Hg in leaves, husk, and grain was positively correlated with soil total and available Hg, and crystalline manganese (Cryst-Mn), while negatively correlated with soil pH, amorphous manganese (Amor-Mn) and crystalline aluminium (Cryst-Al). Bioconcentration factor (BCF) values were significantly higher in acidic soils (highest 0.05), with phytotoxic effects in some soils, as compared to alkaline soils (lowest 0.006). Furthermore, wheat grain Hg was significantly correlated with total (R² = 0.25), water-soluble (R² = 0.54) and NH₄Ac-extractable Hg (R² = 0.43) while also had a good correlation with soil pH (R² = -0.20). In conclusion, the soil total and available Hg (water-soluble + exchangeable fraction), pH, organic matter, and Amor-Mn are the most important soil variables that support Hg uptake in the wheat plants, which benefit managing Hg-enriched agricultural soils for safe wheat production.

Low-Temperature Thermal Desorption Effectively Mitigates Accumulation of Total Mercury and Methylmercury in Rice (Oryza sativa L.)

For maximally reserving soil fertility, two critical parameters (i.e. time and temperature) of low-temperature thermal desorption (LTTD) were initially optimized to remediate the mercury-contaminated soil from a mercury mining area. The effect of LTTD on soil properties was investigated, and the bioaccumulation of total mercury (THg) and methylmercury (MeHg) in rice (Oryza sativa L.) were researched via a pot experiment. Results indicated that the physicochemical properties and fertility of the soil after LTTD still meet the requirements of rice growth. Moreover, the concentrations of THg and MeHg in the remediated soil were decreased by 94.1% and 98.8%, respectively. Further, the bioavailability of Hg in soil was significantly reduced. More importantly, the concentrations of THg and MeHg in the seed of rice plant cultivated on the remediated soil were decreased by 57.6% and 80.2%, respectively. Overall, LTTD technology could efficiently remediate Hg-contaminated soil and be a promise remediation strategy.

Source Apportionment of Speciated Mercury in Chinese Rice Grain Using a High-Resolution Model

Rice grain consumption is a primary pathway of human mercury exposure. To trace the source of rice grain mercury in China, we developed a rice paddy mercury transport and transformation model with a grid resolution of 1 km × 1 km by using the unit cell mass conservation method. The simulated total mercury (THg) and methylmercury (MeHg) concentrations in Chinese rice grain ranged from 0.08 to 243.6 and 0.03 to 238.6 μg/kg, respectively, in 2017. Approximately, 81.3% of the national average rice grain THg concentration was due to atmospheric mercury deposition. However, soil heterogeneity, especially the variation in soil mercury, led to the wide rice grain THg distribution across grids. Approximately, 64.8% of the national average rice grain MeHg concentration was due to soil mercury. In situ methylation was the main pathway via which the rice grain MeHg concentration was increased. The coupled impact of high mercury input and methylation potential led to extremely high rice grain MeHg in partial grids among Guizhou province and junctions with surrounding provinces. The spatial variation in soil organic matter significantly impacted the methylation potential among grids, especially in Northeast China. Based on the high-resolution rice grain THg concentration, we identified 0.72% of grids as heavily polluted THg grids (rice grain THg > 20 μg/kg). These grids mainly corresponded to areas in which the human activities of nonferrous metal smelting, cement clinker production, and mercury and other metal mining were conducted. Thus, we recommended measures that are targeted at the control of heavy pollution of rice grain by THg according to the pollution sources. In addition, we observed a wide spatial variation range of MeHg to THg ratios not only in China but also in other regions of the world, which highlights the potential risk of rice intake.

Trace Metal Accumulation in Rice Variety Kainat Irrigated with Canal Water

Due to the rapid increase in industrial and urban areas, environmental pollution is increasing worldwide, causing unwanted changes in the air, water, and soil at biological, physical, and chemical levels, ultimately causing negative effects for living things. This work was performed in Jhang, Punjab, Pakistan, and examined and measured heavy metal levels in various plant parts of the rice (Oryza sativa) variety Kainat (roots, shoots, and grains) with results been set in relation to the soil around the root area. The samples were taken from five different sites. The mean level of trace metals (mg/kg) in grains was soil-dependent and varied from cadmium (Cd) (2.49–5.52), zinc (Zn) (5.8–10.78), copper (Cu) (4.82–7.85), cobalt (Co) (1.48–6.52), iron (Fe) (8.68–14.73), manganese (Mn) (6.87–13.93), and nickel (Ni) (2.3–8.34). Excluding Cd, the absorption of all metals under inspection was recorded within permissible limits, as recommended by the FAO and WHO. The pollution load index for Cd was highest at all sites. The enrichment coefficient of Co, Cd, and Cu were greater. The bioaccumulation factor at all studied sites was present, in order: Cu ˃ Zn ˃ Fe ˃ Mn ˃ Co ˃ Ni ˃ Cd. The translocation factor was present at five different sites: Mn ˃ Fe ˃ Cu ˃ Zn ˃ Co ˃ Cd ˃ Ni. The health risk index of all inspected metals was lower than 1 and was within safe limits. The higher pollution of Cd suggested maintenance of rice crop is recommended, decreasing health risks in humans.

Mercury accumulation response of rice plant (Oryza sativa L.) to elevated atmospheric mercury and carbon dioxide

New observations and updated models now suggest terrestrial ecosystems are net sink of atmospheric mercury (Hg), and the critical constrained process to identify the strengths of terrestrial sink is whether the large amount of Hg stored in vegetation originates from the soil as well as from the atmosphere. In this study, field open top chambers (OTCs) experiments reveal that rice plant can assimilate gaseous elemental mercury (GEM, Hg0) from the atmosphere through stomata, and Hg concentrations in rice leaves, upper and bottom stalks and grains increased with Hg0 levels in air, showing significantly quadratic linear relationships. Coupling field stable isotope soil amendment experiments, atmospheric source of Hg in rice plant is quantified with more than 90% of Hg accumulation in rice aboveground biomass from air and approximately 80% of rice root Hg from soil. Furthermore, elevated atmospheric carbon dioxide (CO2) exposure led to lower Hg concentration in rice tissues through reduction stomatal conductance of rice leaf, and subsequently impact the capacity of Hg storage in rice aboveground parts from the atmosphere. The findings from experiments provide a foundation for future quantification of atmospheric sink of crops in local and larger scales and comprehensive evaluation atmosphere - terrestrial processes and exposure risks in the global Hg cycling.

Physiological effects of mercury-contaminated Oxisols on common bean and oat

The heavy metal mercury (Hg) is one of the most complex and toxic pollutants. When present in soils, it may impair plant growth, but the intensity of damage depends on the physical-chemical properties of the soil such as pH, clay, and organic matter content, which in turn affects Hg sorption and bioavailability. Understanding Hg potential damage to staple food crops is of paramount relevance. Here, we evaluated the physiological effects of Hg in Phaseolus vulgaris (common bean) and Avena sativa (oat) cultivated in two Oxisols with contrasting properties: Rhodic Acrudox (RA) and Typic Hapludox (TH). We performed four independent experiments (one per species/soil combination) that lasted 30 days each. Treatments were composed by HgCl₂ concentrations in soils (0 to 80 mg kg^-1 Hg). At the end of the experiment, we determined the impact of Hg on photosynthesis, nutritional status, and oxidative stress. Cultivation in TH contaminated with Hg resulted in oxidative stress in common bean and decreased photosynthesis/P accumulation in oat. No deleterious effects on physiological variables were detected in both species when cultivated in the RA soil. In general, we conclude that the lower Hg sorption in the TH soil resulted in toxicity-like responses, while acclimation-like responses were observed in plants cultivated in RA, reinforcing soil physical-chemical properties as key features driving Hg toxicity in Oxisols. Graphical abstract.

Determination of (Bio)-available mercury in soils: A review

Despite the mercury (Hg) control measures adopted by the international community, Hg still poses a significant risk to ecosystem and human health. This is primarily due to the ability of atmospheric Hg to travel intercontinentally and contaminating terrestrial and aquatic environments far from its natural and anthropogenic point sources. The issue of Hg pollution is further complicated by its unique physicochemical characteristics, most noticeably its multiple chemical forms that vary in their toxicity and environmental mobility. This meant that most of the risk evaluation protocols developed for other metal(loid)s are not suitable for Hg. Soil is a major reservoir of Hg and a key player in its global cycle. To fully assess the risks of soil Hg it is essential to estimate its bioavailability and/or availability which are closely linked to its toxicity. However, the accurate determination of the (bio)-available pools of Hg in soils is problematic, because the terms 'bioavailable' and 'available' are ill-defined. In particular, the term 'bioavailable pool', representing the fraction of Hg that is accessible to living organisms, has been consistently misused by interchanging with other intrinsically different terms e.g. mobile, labile, reactive and soluble pools. A wide array of physical, chemical, biological and isotopic exchange methods were developed to estimate the (bio)-available pools of Hg in soil in an attempt to offer a plausible assessment of its risks. Unfortunately, many of these methods do not mirror the (bio)-available pools of soil Hg and suffer from technical drawbacks. In this review, we discuss advantages and disadvantages of methods that are currently applied to quantify the (bio)-availability of Hg in soils. We recommended the most feasible methods and give suggestions how to improve the determination of (bio)-available Hg in soils.

Methylmercury and inorganic mercury in Chinese commercial rice: Implications for overestimated human exposure and health risk

China is the largest rice producer and consumer in the world, and mercury (Hg) levels, particularly methylmercury (MeHg), in rice and health exposure risks are public concerns. Total Hg (THg) and MeHg levels in 767 (domestic = 709 and abroad = 58) Chinese commercial rice were investigated to evaluate Hg pollution level, dietary exposures and risks of IHg and MeHg. The mean rice THg and MeHg levels were 3.97 ± 2.33 μg/kg and 1.37 ± 1.18 μg/kg, respectively. The highest daily intake of MeHg and IHg were obtained in younger groups, accounted for 6% of the reference dose-0.1 μg/kg bw/day for MeHg, 0.3% of the provisional tolerance week intake-0.571 μg/kg bw/day for IHg. Residents in Central China and Southern China meet the highest rice Hg exposure, which were more than 7 times of those in Northwest China. Lower concentrations than earlier studies were observed along the implementations of strict policies since 2007. This may indicate that a declining temporal trend of Hg in Chinese grown rice and associated exposures could be obtained with the implementations of strict policies. Though there exist Hg pollution in commercial rice, Hg levels in Chinese commercial rice is generally safe compared with Hg polluted sites. Populations dwelling in China have relatively a quite low and safe MeHg and IHg exposure via the intake of commercial rice. Strict policies contributed to the decrease in THg and MeHg levels in Chinese-grown rice. More attention should be paid to younger groups.

The bioavailability and contribution of the newly deposited heavy metals (copper and lead) from atmosphere to rice (Oryza sativa L.)

Despite the global importance of atmospheric heavy metal input into agricultural soils, research has primarily focused on the amount of the depositions with limited attention given to the risk of the newly deposited heavy metals. To understand the remobilization of the newly deposited copper (Cu) and lead (Pb) from the atmosphere and explore the metals' mobility and bioavailability to rice (Oryza sativa L.), a soil transplant experiment was conducted in three areas along a gradient of atmospheric depositions. Approximately 61% of the Cu and 76% of the Pb depositions tended to be present in potentially mobile fractions. The soil retention of newly deposited Cu and Pb presented as higher mobile fractions than these in the original soil. The newly deposited Cu and Pb in soils only accounted for 0.34-8.7% and 0.07-0.29% of the total soil Cu and Pb pools, but they contributed 30-84% and 6-41% in rice tissues, respectively. A major implication of these findings is that once the heavy metal is deposited, it may be reactivated in soils and transported to aerial parts or foliar uptake into plant tissues, emphasizing the important role of the newly deposited Cu and Pb in contributing to the edible parts of crops.

Comparative study of the effects of different chelating ligands on the absorption and transport of mercury in maize (Zea mays L.)

Mercury (Hg) pollution seriously threatens food safety and has attracted global attention. Phytoextraction, due to its low cost, applicability, and environmental friendliness, is considered a new technology for clean-up of heavy metal contamination in the environment. However, the low bioavailability of Hg in polluted areas greatly limits the applicability of phytoextraction. Here, we compared the effects of six common chelating ligands on the absorption and transport of Hg in maize (Zea mays L.), which has a high biomass and short growth cycle. The results showed that the root length and biomass of maize seedlings of the groups treated with the six chelating ligands (EDTA, iodide, ammonium, thiosulfate, thiocyanate, and thiocarbamide) did not change compared with those of the non-treated groups. Co-exposure to Hg and each chelating ligand markedly alleviated the inhibitory effect induced by Hg. Iodide treatment resulted in the lowest root Hg content and highest translocation factor (TF) value, while ammonium treatment gave rise to the highest shoot Hg concentration and lowest TF. Compared with other chelating ligands, thiosulfate exhibited the maximum alleviation of Hg toxicity and achieved the highest concentration of Hg in the roots and aerial parts. Moreover, the TF and Hg accumulation in the thiosulfate and Hg co-exposed group were much higher than those in the group exposed to Hg alone. This finding suggests that, among these common chelating ligands, thiosulfate compounds have great potential for Hg phytoextraction, while the others can immobilize Hg in polluted areas.

Newly deposited atmospheric mercury in a simulated rice ecosystem in an active mercury mining region: High loading, accumulation, and availability

Mercury (Hg) mining activities are an important anthropogenic source of atmospheric Hg. The Xunyang Hg mine located in Shaanxi Province is the largest active Hg producing centre in China. To understand the biogeochemical processes of atmospheric Hg through Hg mining activities, six groups of experimental pots were carefully designed to investigate the effect of Hg mining activities on Hg contamination from atmospheric deposition in the local surface soils. Based on the variations of Hg in the soil from the experimental pots, the deposition flux and loading of Hg in the Xunyang Hg mining district were investigated. The results showed that the average concentration of total gaseous mercury (TGM) as high as 193 ± 122 ng m^-3 was observed in the ambient air, which was orders of magnitude higher than that in remote areas. The average deposition flux and annual loading of atmospheric Hg were 72 mg m^-2 y^-1 and 10 t y^-1, respectively. The dominant atmospheric Hg deposition is within a distance range of 6.0-12 km from the Hg retorting facility, accounting for approximately 85% of the total Hg loading. After 14 months of exposure, total mercury (THg) concentrations in the soil from the experimental pots increased 0.35-9.5 times, and the highest concentrations of methylmercury (MeHg) (3.7 ± 2.9 μg kg^-1) in soil were observed in February. Concentrations as high as 643 μg kg^-1 THg and 13 μg kg^-1 MeHg in rice were observed in the second experimental year. Elevated concentrations of both THg and MeHg in rice indicated that the newly deposited atmospheric Hg was bioavailable, readily methylated, and taken up by rice, suggesting that the ongoing Hg mining activities cause serious Hg contamination in the soil-rice ecosystem and posed a threat to local residents in the Xunyang Hg mining area.

Thiosulphate-induced phytoextraction of mercury in Brassica juncea: Spectroscopic investigations to define a mechanism for Hg uptake

Thiosulphate is extensively used to enhance mercury (Hg) phytoextraction due to its efficient in prompting plant Hg uptake. However, the mechanism by which thiosulphate promotes Hg uptake is poorly understood. We determined the concentrations of Hg and potassium (K), and their spatial distribution, in the tissues of Brassica juncea grown in Hg-contaminated soils treated by thiosulphate and compared this to a non-treated soil (control). The spatial distribution of Hg and K was characterized using micro-X ray fluorescence spectroscopy. The subcellular localization and speciation of Hg in the root of plant treated by thiosulphate were elucidated using Transmission electron microscope coupled energy-dispersive X-ray (TEM-EDX) spectroscopy. Thiosulphate increased significantly the Hg concentration in the roots (mainly in the epidermis and xylem) and shoots (mainly in the vascular bundles), while Hg was accumulated in the root (mainly in the epidermis) of the control plant. Thiosulphate promoted the movement of Hg from the epidermis to the xylem of roots, with subsequent loading into the stem via vascular bundles. Thiosulphate decreased the K concentration in plant tissues, relative to the control plant, and we propose this is due to leakage of electrolyte from roots via increased plasma membrane permeability as a consequence of physiological damage caused by the added thiosulphate. Mercury was distributed mainly at the extracellular space in the roots and was shown by TEM-EDX to be predominately amorphous nano-clusters of HgS. We conclude that thiosulphate-promoted Hg accumulation in the plant may happen through increased plasma membrane permeability, a changed pathway of Hg movement within plants, and extracellular co-transportation of Hg-S complexes in the roots. Our results may underpin the ongoing development of phytomanagement as an environmental strategy for Hg contaminated soils around the world.

Exposure risk of local residents to copper near the largest flash copper smelter in China

Copper (Cu) smelting released large amounts of Cu and contaminated the environment. However, few studies have investigated the Cu exposure risks for people located near Cu smelters. In this study, atmospheric bulk deposition, food from local families, drinking water and biological samples (hair and urine) were collected in three villages near the largest flash Cu smelter in China. The objective of the current study was to investigate how non-ferrous metals smelting affect the human health. Total atmospheric Cu depositions (56-767μgm^-2yr^-1) were one or two orders of magnitude greater than that of unpolluted rural areas. The Cu concentrations in locally grown vegetables and dietary chronic daily intake (CDI) of local residents showed a consistently decreasing trend with atmospheric Cu depositions. Dietary intake of vegetables and rice were the two major pathways of total CDI, which accounted for >93% totally. The Cu exposure showed higher potential non-carcinogenic risk to human health of local residents, especially children living around the Cu smelter through food consumptions. Health impact monitoring data revealed that mean Cu concentrations in hair and urine samples were ranged from 5.13 to 28.85mgkg^-1 and 19.90 to 54.61μgL^-1 in the three villages, respectively. Significant correlation between hair Cu concentrations and the CDI of Cu indicated food ingestion had adverse effects on the health of the local residents. The result suggested that nonferrous metal smelter should be away from residential area and locally produced crops became unsuitable for consumption. Therefore, effective measures on Cu pollution management and control in the surrounding area should be formulated and implemented.

Bioaccessibility-corrected risk assessment of urban dietary methylmercury exposure via fish and rice consumption in China

The role of seafood consumption for dietary methylmercury (MeHg) exposure is well established. Recent studies also reveal that rice consumption can be an important pathway for dietary MeHg exposure in some Hg-contaminated areas. However, little is known about the relative importance of rice versus finfish in MeHg exposure for urban residents in uncontaminated areas. Especially, the lack of data on MeHg bioaccessibility in rice hinders accurately assessing MeHg exposure via rice consumption, and its importance compared to fish. By correcting commonly used risk models with quantified MeHg bioaccessibility, we provide the first bioaccessibility-corrected comparison on MeHg risk in rice and fish for consumers in non-contaminated urban areas of China, on both city- and province-scales. Market-available fish and rice samples were cooked and quantified for MeHg bioaccessibility. Methylmercury bioaccessibility in rice (40.5±9.4%) was significantly (p<0.05) lower than in fish (61.4±14.2%). This difference does not result from selenium content but may result from differences in protein or fiber content. Bioaccessibility-corrected hazard quotients (HQs) were calculated to evaluate consumption hazard of MeHg for consumers in Nanjing city, and Monte Carlo Simulations were employed to evaluate uncertainty and variability. Results indicate that MeHg HQs were 0.14 (P50) and 0.54 (P90). Rice consumption comprised 27.2% of the overall dietary exposure to MeHg in Nanjing, while fish comprised 72.8%. Employing our bioaccessibility data combined with literature parameters, calculated relative contribution to MeHg exposure from rice (versus fish) was high in western provinces of China, including Sichuan (95.6%) and Guizhou (81.5%), and low to moderate in eastern and southern provinces (Guangdong: 6.6%, Jiangsu: 17.7%, Shanghai: 15.1%, Guangxi: 20.6%, Jiangxi: 22.8% and Hunan: 25.9%). This bioaccessibility-corrected comparison of rice versus fish indicates that rice consumption can substantively contribute to dietary MeHg exposure risk for urban populations in Asia, and should be regularly included in dietary MeHg exposure assessment.

Mercury in rice (Oryza sativa L.) and rice-paddy soils under long-term fertilizer and organic amendment

High levels of mercury (Hg), especially methylmercury (MeHg), in rice is of concern due to its potential of entering food chain and the high toxicity to human. The level and form of Hg in rice could be influenced by fertilizers and other soil amendments. Studies were conducted to evaluate the effect of 24 years application of chemical fertilizers and organic amendments on total Hg (THg) and MeHg and their translocation in soil, plants, and rice grain. All treatments led to significantly higher concentrations of MeHg in grain than those from the untreated control. Of nine treatments tested, chemical fertilizers combining with returning rice straw (NPK1+S) led to highest MeHg concentration in grain and soil; while the nitrogen and potassium (NK) treatment led to significantly higher THg in grain. Concentrations of soil MeHg were significantly correlated with THg in soil (r = 0.59^***) and MeHg in grain (r = 0.48^***). Calcium superphosphate negatively affected plant bioavailability of soil Hg. MeHg concentration in rice was heavily influenced by soil Hg levels. Phosphorus fertilizer was a main source contributing to soil THg, while returning rice straw to the field contributed significantly to MeHg in soil and rice grain. As a result, caution should be exercised in soil treatment or when utilizing Hg-contaminated soils to produce rice for human consumption. Strategic management of rice straw and phosphorus fertilizer could be effective strategies of lowering soil Hg, which would ultimately lower MeHg in rice and the risk of Hg entering food chain.

Combined Amendments of Nano-hydroxyapatite Immobilized Cadmium in Contaminated Soil-Potato (Solanum tuberosum L.) System

The toxicity of cadmium (Cd) has posed major public health concern in crops grown in the Cd-contaminated soils. The effects of five amendments, nano-hydroxyapatite (n-HA) and it combined with lime, zeolite, bone mill and fly ash on Cd immobilization in soils and uptake in potatoes, were investigated in a contaminated soil by pot experiments. The result showed that the applications of combined amendments significantly decreased the bioavailable Cd concentrations extracted by TCLP, DTPA-TEA and MgCl₂ in the contaminated soils, and changed the soluble and exchangeable and specifically sorbed fractions to oxide-bound and organic-bound fractions. Compared to the control group, the concentrations of Cd in the potato tubers grown in n-HA, n-HA + Fly ash, n-HA + Lime, n-HA + Bone mill and n-HA + Zeolite soil were reduced 17.4%, 20.7%, 15.2%, 32.6% and 39.1%, respectively. Nano-hydroxyapatite combined amendments was more effective in reducing bioavailable Cd concentrations and Cd accumulations in potatoes, especially for n-HA + Z.

Mercury species in formerly contaminated soils and released soil gases

Total mercury (T-Hg), elemental mercury (Hg⁰), methylmercury (MeHg⁺), phenylmercury (PhHg⁺), and gaseous elemental mercury (GEM) species were determined in soils formerly contaminated by different processes from two sites in the Czech Republic. Analytical methods involved atomic absorption spectrometry (AAS) using a single-purpose Advanced Mercury Analyser AMA-254 and radiochemical neutron activation analysis (RNAA) for T-Hg determination, a thermal desorption method was used for Hg⁰ determination, gas chromatography coupled with atomic fluorescence spectrometry (GC-AFS) was employed for assay of MeHg⁺ and PhHg⁺, while GEM measurement was carried out using a portable Zeeman-AAS device Lumex RA-915⁺. The first sampling site was in the surroundings of a former PhHgCl-based fungicide processing plant next to Příbram (central Bohemia). Although the use of Hg-based fungicides as seed mordant have been banned, and their production stopped at the end of 1980's, highly elevated Hg contents in soil are still observed in the vicinity of the former plant, reaching T-Hg values >13mgkg^-1. The second sampling site was an abandoned mining area named Jedová hora Hill near Hořovice (central Bohemia), where cinnabar (HgS) was occasionally mined as by-product of Fe ores hematite and siderite. Mining activities have been stopped here in 1857. Very high contents of T-Hg are still found at this site, up to 144mgkg^-1. In most cases we found a statistically significant correlation between T-Hg and Hg⁰ values regardless of the pollution source. On the contrary, insignificant correlation was observed neither between T-Hg and GEM values, nor between GEM and Hg⁰. Concentrations of the investigated organomercury species were above a limit of detection (LOD) only in the most contaminated samples, where their levels were about two to three orders of magnitude lower compared to those of T-Hg.

Mercury content in agricultural soils (Vojvodina Province, Serbia)

The Vojvodina Province in northern Serbia is well known for its intensive field crops production. Over 90 % of total arable land, which represents more than 1500.000 ha, is used for field or vegetable crop production. A grid superimposed on Vojvodina land by means of a GIS tool (GIS ArcView 10) has divided land into 4 × 4 km units, each representing an area of 1600 ha. Total number of 1370 bulked soil samples were taken (0-30 cm depth) from agricultural land and analysed for total mercury content THg. The samples were analysed using Direct Mercury Analyzer DMA 80 Milestone. Quality control was carried out with IRMM BCR reference materials 143R. The aim of this study was to determine the total content of Hg in agricultural soils and its spatial distributions in different parts of Vojvodina Province. The obtained results were within interval 0.008-0.974 mg kg^-1. The average concentration of Hg was 0.068, with median 0.048 mg kg^-1. The correlation was determined between Hg concentration and organic matter content in the soil. Content of Hg coincides with main geomorphological units of Vojvodina Province. Average values of Hg concentrations for soils formatted on different geomorphological units were 0.031 for sandy area with dune fields, 0.048 for alluvial terraces, 0.055 for upper Pleistocene terraces, 0.058 for loess plateaus, 0.083 for mountains and 0.092 mg kg^-1 for alluvial plains. Hg spatial distribution confirmed that most of Vojvodina Province area has geochemical origin of Hg. Higher concentration of Hg on alluvial plains indicated that the origin of Hg near rivers could be from anthropogenic source. The main rivers in Vojvodina have been dammed more than a century ago. Thus, higher concentrations of Hg in their alluvial plains out of narrow dammed zone around the rivers must be related to natural and anthropogenic sources located in their huge catchments. Higher content of Hg in mountain region can be explained by high clay content in these soils. Additional hotspots of Hg concentration of top soils are related to geographical locations of major towns. The obtained results also indicated that the measured levels of Hg in the soil are not limiting factors for production of safe food in Vojvodina.

Accumulation and translocation of methylmercury and inorganic mercury in Oryza sativa: An enriched isotope tracer study

Methylmercury (MeHg) accumulation in rice is an emerging human health issue, but uptake pathways and translocation into the grain remain poorly understood. We grew Oryza sativa plants in pots of wetland soil amended with an enriched mercury isotope (94.3% ²⁰⁰Hg) tracer, alongside unvegetated control pots, and assessed both ambient and tracer MeHg and inorganic Hg (IHg) concentrations in soil and plant tissues at three growth stages. Based on similar ratios of ambient:tracer MeHg concentrations in soil and plant tissues, we provide the first direct evidence that MeHg is first synthesized in saturated soil and subsequently translocated to rice grains. There is no evidence of in planta methylation of IHg, but significant losses of MeHg from plant tissues between flowering and maturity indicates likely in planta demethylation. In this greenhouse experiment, lower percent of tracer MeHg in vegetated soils at late growth stages suggests that rice plants reduce the net MeHg accumulation capacity of soils, although the mechanism remains unclear. For IHg, roots accumulated Hg from the soil, straw from the soil and the atmosphere, and grain almost entirely from the atmosphere. Management strategies that aim to reduce MeHg accumulation in rice should focus on mercury methylation in paddy soils, but IHg reductions will depend on regional controls of atmospheric Hg.

Mercury Fractionation in Superficial Sediment and Paddy Soil Samples from Tianjin, Northern China

Sediment and soil samples from the Beitang River (BR) and the Haihe River (HR) in Tianjin were analyzed to investigate the extent of mercury contamination. The results show that total mercury (THg) contents in the BR and HR sediments were 2241 ± 1024 and 653 ± 450 ng g(-1), and THg in rice paddy soils were 907 ± 345 and 328 ± 286 ng g(-1), respectively. Industrial and domestic sewage were regarded as the main sources of mercury in the two river basins. Sediment-bound mercury in the BR and the HR were found to be predominantly associated with the organic-bound fraction (55 %) and residual fraction and (54 %), while soil-bound mercury was mainly in organic-bound fraction in paddy soils (61 % and 57 %, respectively). The availability of this element (soluble and exchangeable and specifically sorbed fraction) seemed restricted, but significantly higher in the paddy soils than in sediments. Higher soluble and exchangeable, specifically sorbed fraction and organic-bound fraction may promote the higher toxic methylmercury and bioavailable fraction formation in the soils during the rice cultivation.

Advantages and limitations of chemical extraction tests to predict mercury soil-plant transfer in soil risk evaluations

In this study, we compared the size of the mobile Hg pool in soil to those obtained by extractions using 2 M HNO3, 5 M HNO3, and 2 M HCl. This was done to evaluate their suitability to be used as proxies in view of Hg uptake by ryegrass. Total levels of Hg in soil ranged from 0.66 to 70 mg kg(-1) (median 17 mg kg(-1)), and concentrations of Hg extracted increased in the order: mobile Hg < 2 M HNO3 < 5 M HNO3 < 2 M HCl. The percentage of Hg extracted relative to total Hg in soil varied from 0.13 to 0.79 % (for the mobile pool) to 4.8-82 % (for 2 M HCl). Levels of Hg in ryegrass ranged from 0.060 to 36 mg kg(-1) (median 0.65 mg kg(-1), in roots) and from 0.040 to 5.4 mg kg(-1) (median 0.34 mg kg(-1), in shoots). Although results from the 2 M HNO3 extraction appeared to the most comparable to the actual total Hg levels measured in plants, the 2 M HCl extraction better expressed the variation in plant pools. In general, soil tests explained between 66 and 86 % of the variability of Hg contents in ryegrass shoots. Results indicated that all methods tested here can be used to estimate the plant total Hg pool at contaminated areas and can be used in first tier soil risk evaluations. This study also indicates that a relevant part of Hg in plants is from deposition of soil particles and that splashing of soil can be more significant for plant contamination than actual uptake processes. Graphical Abstract Illustration of potential mercury soil-plant transfer routes.

Mercury in terrestrial forested systems with highly elevated mercury deposition in southwestern China: The risk to insects and potential release from wildfires

Forests are considered a pool of mercury in the global mercury cycle. However, few studies have investigated the distribution of mercury in the forested systems in China. Tieshanping forest catchment in southwest China was impacted by mercury emissions from industrial activities and coal combustions. Our work studied mercury content in atmosphere, soil, vegetation and insect with a view to estimating the potential for mercury release during forest fires. Results of the present study showed that total gaseous mercury (TGM) was highly elevated and the annual mean concentration was 3.51 ± 1.39 ng m(-2). Of the vegetation tissues, the mercury concentration follows the order of leaf/needle > root > bark > branch > bole wood for each species. Total ecosystem mercury pool was 103.5 mg m(-2) and about 99.4% of the mercury resides in soil layers (0-40 cm). The remaining 0.6% (0.50 mg m(-2)) of mercury was stored in biomass. The large mercury stocks in the forest ecosystem pose a serious threat for large pluses to the atmospheric mercury during potential wildfires and additional ecological stress to forest insect: dung beetles, cicada and longicorn, with mercury concentration of 1983 ± 446, 49 ± 38 and 7 ± 5 ng g(-1), respectively. Hence, the results obtained in the present study has implications for global estimates of mercury storage in forests, risks to forest insect and potential release to the atmosphere during wildfires.

Effects of sulfate and selenite on mercury methylation in a mercury-contaminated rice paddy soil under anoxic conditions

Biogeochemical cycling of sulfur and selenium (Se) could play an important role in methylmercury (MeHg) dynamics in soil, while their potential effects on MeHg production in rice paddy soil are less understood. The main objective of this study was to explore the effects of sulfate and selenite on net MeHg production in contaminated rice paddy soil, characterized with massive MeHg production and thus MeHg accumulation in rice. A series of microcosm incubation experiments were conducted using a contaminated paddy soil amended with sulfate and/or selenite, in which sulfate-reducing bacteria were mainly responsible for MeHg production. Our results demonstrated that sulfate addition reduced solid and dissolved MeHg levels in soils by ≤18 and ≤25 %, respectively. Compared to sulfate, selenite was more effective in inhibiting net MeHg production, and the inhibitory effect depended largely on amended selenite doses. Moreover, sulfate input played a dual role in affecting Hg-Se interactions in soil, which could be explained by the dynamics of sulfate under anoxic conditions. Therefore, the effects of sulfate and selenium input should be carefully considered when assessing risk of Hg in anoxic environments (e.g., rice paddy field and wetland).