Soil Hg Concentrations Research Articles

Ecological drivers of methanotrophic communities in paddy soils around mercury mining areas

Methanotrophs play a crucial role in mitigating methane (CH4) emission by oxidizing produced CH4 in paddy soils; however, ecological drivers of methanotrophic community in the soils around heavy metal contaminated areas remain unclear. In this study, we evaluated the effects of heavy metal pollution and soil properties on the abundance, diversity and composition of methanotrophic community in paddy soils from two typical mercury (Hg) mining regions in southwest China. The results of random forest and structure equation models suggest that both heavy metal content and soil nutrients greatly influenced the attributes of methanotrophic community. In general, the abundance and diversity of methanotrophs were negatively related to soil Hg content, but showed positive correlation with soil organic carbon content. However, the other metals (cadmium (Cd), nickel (Ni), lead (Pb), arsenic (As), zinc (Zn)) had inconsistent associations with the microbial indexes of methanotrophic community in the soil. Elevated levels of heavy metal and nutrients in the soils shifted the community composition of methanotrophs. For example, Pb, As and Zn contents had negative associations with the relative abundance of Methylocaldum. In addition, changes in the relative abundance of ecological clusters within the co-occurrence network of methanotrophs were related to metal contents and soil properties. Together, our findings provide novel insights into understanding ecological drivers of methanotrophic community in paddy soils around Hg mining regions, with important implications for mitigating CH4 emissions in terrestrial ecosystems.

Mercury in Soil and Forage Plants from Artisanal and Small-Scale Gold Mining in the Bombana Area, Indonesia.

Mercury (Hg) contamination in soil and forage plants is toxic to ecosystems, and artisanal and small-scale gold mining (ASGM) is the main source of such pollution in the Bombana area of Indonesia. Hg contamination in soil and forage plants was investigated by particle-induced X-ray emission analysis of samples collected from three savannah areas (i.e., ASGM, commercial mining, and control areas) in the Bombana area. Hg contents of forage plants in the ASGM area (mean 9.90 ± 14 µg/g) exceeded those in the control area (2.70 ± 14 µg/g). Soil Hg contents (mean 390 ± 860 µg/g) were also higher than those in the control area (mean 7.40 ± 9.90 µg/g), with levels exceeding international regulatory limits. The Hg contents of 69% of soil and 78% of forage-plant samples exceeded critical toxicological limits. Thus, the Hg levels observed in this study indicate that contamination extending over large areas may cause major environmental problems.

Black pine (Pinus nigra) barks: A critical evaluation of some sampling and analysis parameters for mercury biomonitoring purposes

Abstract Tree barks are increasingly used as biomonitors of airborne pollutants. However, many authors stress the poor comparability of the results achieved in different studies. This drawback is mainly caused by a poor understanding of the critical sampling parameters to be considered. To minimize the biases that could be introduced during sampling, in this study the barks of Pinus nigra J.F. Arnold from thirteen sites were investigated in the abandoned Mt. Amiata mercury (Hg) mining district (Southern Tuscany, Italy) and surroundings. The influence of some sampling and analyzing parameters on Hg content was critically assessed. At each site, a total of eight bark samples were taken from a single tree at two heights (70 cm and 150 cm from soil) and at four different sides of the trunk, corresponding to the four cardinal directions; a composite soil sample was also collected. Mercury contents in barks range from 0.1 to 28.8 mg/kg, and are correlated with soil Hg contents (1–480 mg/kg), indicating that barks record both gaseous Hg concentrations in air, and wind-transported Hg-bearing particulate. For each tree, samples at 70 cm and 150 cm show Hg contents of the same order of magnitude, even if values for 150 cm are slightly less dispersed, possibly because barks at 70 cm are more influenced by random soil particles. There is no statistically significant dependence of Hg content on direction and tree age. Simulated rain events cause a negligible loss of Hg from barks. Results suggest that a convenient sampling practice for Pinus nigra is to collect a bark slice (typically 1–2 mm) within the outermost 1.5 cm layer.

Analysis of the Distribution of Some Potentially Harmful Elements (PHEs) in the Krugersdorp Game Reserve, Gauteng, South Africa

The Mintails Mogale Gold (MMG) and the Rand Uranium (RU) are two large-scale mining consortiums active in re-mining old tailings dams and dumps in Krugersdorp and are a source of mine discharge feed into the Krugersdorp Game Reserve (KGR). This has resulted in a noticeable accumulation of potentially harmful elements (PHEs) over a number of years. Efforts were implemented to interpret the concentration levels of PHEs in soils of the study areas of which a total of 36 georeferenced soil samples were collected (in triplicate) from the MMG, RU and KGR, including samples from farmlands and waterways adjacent to the mining sites. Samples were then analysed by both inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for 36 elements. From the 36 elements of this study, detailed evaluations of the occurrence of 12 selected elements were discussed. The geochemical landscape at the KGR is shown to be in flux. The major mediating influences on the behaviour of As, Co, Cu, Hg and Pb, as they enter the KGR largely in the form of acid mine drainage (AMD), are the geological substrate (mostly in carbonate form). Analysis of the soils showed high levels of contamination for As and Co in ppm. The mean maximum of As ranged from (5.00–170.30) with the highest level found in the Krugersdorp site. The mean maximum of Co ranged from (46.00–102.30) with the highest level found in MMG. All of these values were well above the recommended maximum acceptable concentration (MAC) values, i.e., As (15–20) and Co (20–50). The mean maximum values for Pb (12.40–92.30); Cu (18.50–115.30) and Hg (12.40–92.30) content in surface soils of all four segments studied falls well within the MAC range for agricultural soils i.e., Cu (60–150); Hg (0.5–5) and Pb (20–300).

A tale of three cities: Mercury in urban deciduous foliage and soils across land-uses in Poughkeepsie NY, Hartford CT, and Springfield MA USA.

Mercury is a global pollutant that harms human and wildlife health through chronic exposure. The role of urban forests in Hg biogeochemistry has been understudied in cities without historical mining or current coal combustion. This study aimed to quantify total Hg concentrations and pools in urban forests to determine whether adjacent land-use impacts Hg accumulation. Three cities in the northeastern United States were studied: Hartford, Connecticut; Poughkeepsie, New York; and Springfield, Massachusetts. We identified ~20 urban forests sites in a ~10km by ~10km grid for each city and sampled foliage and soil at each site. Foliage from Populus exhibited significantly lower Hg concentrations (15.6±2.1ngg-1) than mean foliar Hg concentrations (23.7±0.6ngg-1) but most deciduous genera had comparable concentrations. Average forest floor Hg concentrations (195±21ngg-1) and Hg pools (1.9±0.5mgm-2) were similar to previous, non-urban studies in the region. Average A horizon (182±19ngg-1) and B horizon (125±14ngg-1) Hg concentrations were double those of regional forest soils. Mineral soil Hg pools for the top 30cm (49±6mgm-2) averaged two to ten times higher than rural, montane forests in the region. Soil pH, LOI, and %clay were poorly correlated with mineral soil Hg concentrations. Instead, highest foliar and soil Hg concentrations and pools were in urban forests adjacent to high and medium intensity developed areas in Springfield and Hartford. To differentiate the impact of land-uses not captured by the National Land Cover Database (NLCD) system, we implemented new land-use categories. Industrial areas had highest foliar and soil Hg concentrations and pools of any land use. Our results show increasing land-use increases Hg accumulation in urban forests.

Mercury accumulation in soil fractions of podzols from two contrasted geographical temperate areas: southwest Europe and southernmost America

The presence of Hg in soil fractions of different size in eight illuvial horizons of podzols from SW Europe and five from southernmost America was studied. Total Hg concentration (HgT) in the bulk soil was very variable, ranging from 11 to 293 ng g−1, attributed to a different intensity of the pedogenetic and weathering processes. The clay fraction showed the highest HgT average with 509 ng g−1 (range 93–2076 ng g−1), followed by the fine silt with a mean of 222 ng g−1 (range 38–836 ng g−1), while the lowest corresponded to the fine sand with an average of 50 ng g−1 (range 2–150 ng g−1). Mercury enrichment in the finer fractions was supported by the high values of the Hg accumulation factor (HgAF, Hg soil fraction / Hg bulk soil) found in the clay (range 1.4–11.5) and in the fine silt fractions (range 0.9–6.8). If the mass of each fraction with regard to bulk soil was considered, mean Hg content followed this sequence: fine silt (31.3 ng g−1), coarse sand (22.7 ng g−1), clay (18.2 ng g−1), coarse silt (15.2 ng g−1) and fine sand (12.5 ng g−1, n = 13). Silt and clay fractions from illuvial horizons of soils from SW Europe were richer in Hg than those fractions from southernmost America. Mercury levels in finer fractions (fine silt and clay), as well as some geochemical indices of Hg accumulation, were closely correlated to total organic C, organo-metal (Al, Fe) complexes and Al and Fe oxyhydroxides, which also exhibited an accumulation in those fractions, suggesting an active role in the Hg enrichment. The results confirmed that illuvial horizons of podzols, particularly those fractions with the lowest size, play a key environmental function preventing the mobilization of Hg in non-polluted soils towards other compartments of terrestrial ecosystems, such as surface waters and groundwater, in which Hg toxicity could be enhanced.

Observation and estimation of mercury exchange fluxes from soil under different crop cultivars and planting densities in North China Plain

The emission of mercury (Hg) from cropland soil greatly affects the global Hg cycle. Combinations of different crop cultivars and planting densities will result in different light transmittance under canopies, which directly affects the solar and heat radiation flux received by the soil surface below crops. In turn, this might lead to differences in the soil–air total gaseous mercury (TGM) exchange under different cropping patterns. However, soil–air TGM exchange fluxes in croplands under differing canopies have been poorly investigated. Here, a one-year observation of TGM exchange flux was conducted for cropland soils covering five different crop cultivars and three planting densities in North China Plain using the dynamic flux chamber method. The results showed that light transmittance under the canopies was the key control on soil–air TGM exchange fluxes. High light transmittance can enhance soil TGM emission rates and increase the magnitude of diurnal variations in soil–air TGM exchange fluxes. Furthermore, we found that there were piecewise–function relationships (Peak function–constant equation) between light transmittance under the different canopies and the numbers of days after crop sowing. The soil–air TGM exchange fluxes showed a parabolic response to changes in light transmittance under the different canopies. A second-order model was established for the response relationship between soil–air TGM exchange flux and soil Hg concentration, total solar radiation above the canopy, and numbers of days after sowing. The estimated annual average soil–air TGM exchange flux was 5.46 ± 21.69 ng m−2 h−1 at corn–wheat rotation cropland with 30 cm row spacing using this second-order model. Our results might a data reference and a promising foundation for future model development of soil–air TGM exchange in croplands under different crop cultivars and planting densities.

Mercury accumulation in soil from atmospheric deposition in temperate steppe of Inner Mongolia, China

Mercury (Hg) is a toxic and persistent pollutant and has long-term impacts on ecological systems and human health. Coal-fired power plants (CFPPs) are the main source of anthropogenic Hg emission, and the emitted atmospheric Hg is deposited to the surrounding environments which causes soil pollution. To assess the effects of atmospheric Hg from CFPPs in China on the temperate steppe, Hg contents in the topsoil and subsoil were analyzed for samples collected from 80 sites in central Inner Mongolia during 2012–2015. The average content of Hg in topsoil and subsoil were 14.9 ± 10.4 μg kg−1 and 8.9 ± 5.8 μg kg−1, respectively. The principal components analysis (PCA) indicated that the soil organic matter content and atmospheric deposition were the main factors determining soil Hg content in Inner Mongolia. We used the power plant impact factor (PPIF) to evaluate the impacts of the surrounding CFPPs. The PPIF results showed the most positive correlation with Hg content in topsoil at more than 400 km distances, indicating that the contribution of the long-range transport of Hg emitted from CFPPs is regional in scale. Considering the potential of Hg accumulation in soil, long-term and regional measurements of soil Hg and stricter emission-limit standards for power plants should be implemented to control soil Hg pollution in China.

IMPACT ASSESSMENT OF MERCURY PRODUCTION ON THE ENVIRONMENT, ON THE CASE OF A NON-OPERATING MINE IN VILLAGE OF OLENYOVO

The work is devoted to the study of the effect of non-working mercury mine (not operational since 1974) on the state of the environment, as exemplified by mine in the village of Olenyovo. Given the high toxicity, relative mobility and bioavailability, as well as the ability to bio-accumulate Hg compounds, the study of such effects is relevant. The work is planned in such a way that, on the basis of determination of the gross Hg content in the humus soil profile (H) of the adjacent areas and the natural water of the Rostoka stream, to estimate such an influx, to highlight the main tendencies of the processes of environmental pollution.It was found that the Hg content in soils in the vicinity of the mine is relatively low 0.24-0.46 mg×kg-1, with the lowest content observed in the background (control) area, with a gradual increase in concentration from the entrance to the mine in the direction of the road mercury ore. The trend of spreading pollution is clearly evident, although the mercury mine is no longer operational for about 45 years. The distribution of Hg content in the water of the Rostock stream has a distinct character (confirmed by mapping data), and the concentration of Mercury in the waters ranges from 0.066-0.093 μg×L-1. The highest Hg content in the water is observed in the stream at the mine entrance area (0.093 µg×L-1) and this is close to the fishery concentration limit. According to the environmental assessment (Hg content), the Rostoka stream water belongs to the second class of category 3 – good (as it is) and quite pure (in terms of purity).In general, the impact of a non-working mercury mine in the village of Olenyovo to the environmental status of the environment is acceptable in terms of soil and natural water contamination with Hg compounds, but it should be borne in mind that the mine is no longer operational for about 45 years. As a potential source of environmental pollution, mercury mines can be considered as dangerous objects of influence.

Pollution Characteristics and Source Analysis of Heavy Metals in Farmland Soils in the Taige Canal Valley

To determine the distribution characteristics of heavy metal pollution in farmland soils and related influencing factors in the Taige canal valley, and guarantee soil environmental quality and safety of agricultural products, 118 topsoil samples were collected from the Taige canal valley's farmland soils, and contents of chromium (Cr), mercury (Hg), arsenic (As), copper (Cu), zinc (Zn), nickel (Ni), lead (Pb), and cadmium (Cd) were measured. A single factor index and comprehensive index were used to assess soil heavy metal contamination with the soil background value of the Taihu Lake basin as the evaluation standard. The multivariate statistical analysis and the geostatistical analysis were combined to identify and apportion the pollution sources of soil heavy metals. The results showed that:The average concentrations of Cr, Hg, As, Cu, Zn, Ni, Pb, and Cd in soils were 63.25, 0.25, 7.83, 35.24, 77.25, 31.48, 38.37, and 0.16 mg·kg-1, respectively, all of which except for Cr and As were higher than the local soil background values. The content of each heavy metal in most soil samples were lower than the risk screening values for soil contamination of agricultural land. The comprehensive index showed that the degree of pollution of soil heavy metals were at a slightly polluted level in 87.29%, moderately polluted level in 5.93%, and severely polluted level in 6.78% of the sampling. Hg, Cu, Zn, Pb, and Cd in the watershed soil were affected by agricultural activities and atmospheric deposition. Cr and Ni were affected by the parent material and industrial production activities. As was mainly derived from the parent material.

Influence of ferrous-metal production on mercury contamination and fractionation in farmland soil around five typical iron and steel enterprises of Tangshan, China

Iron and steel production is one of the main anthropogenic sources of mercury (Hg) emission and release. Oxidized and particulate Hg discharged from iron and steel enterprises deposit into the surrounding soil, which accumulate and introduce environmental risks. Therefore, it is important to assess Hg pollution in the soil surrounding iron and steel enterprises. In this study, the Hg pollution, Hg distribution from steel plants and Hg fractionation in farmland soil around five typical steel plants were analysed in Tangshan of China. The Hg pollution indexes (Pi) of more than 90% soil samples were greater than 3 by the single factor pollution index method, which showed that most soil samples around the five steel plants were strongly contaminated by Hg. The Hg contents in soil increased first and then decreased, and the maximum content presented at 250–300 m away from the boundary of the steel plants. The order of Hg fraction proportion in the soil samples was extractable (35%–43%) > volatile (24%–36%) > residual (10%–26%) > reducible (0–15%) > oxidizable (0–12%). The distribution of Hg fraction in farmland soil had no regular trend with the distance from the steel plants. The volatile Hg and extractable Hg were dominant in farmland soil, and their combined proportion was greater than 60%. These two fractions of Hg are at risk of re-volatilization into the atmosphere or potential absorption by plants.

Occurrence and environmental impact of industrial agglomeration on regional soil heavy metalloid accumulation: A case study of the Zhengzhou Economic and Technological Development Zone (ZETZ), China

Abstract To clarify whether industrial agglomeration affects soil heavy metalloid (HM) accumulation and pollution and whether industrial agglomeration results in less soil HM pollution than in areas without industrial agglomeration, an investigation of the HM concentration and pollution in soils in an industrial agglomeration area, the national Zhengzhou Economic and Technological Development Zone (ZETZ), was conducted. Determinations of the Hg, Pb, Cr, As, Zn, Cd, Ni, Co, Cu and Mn concentrations in soils were performed using inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectrometry and atomic fluorescence spectrometry (ICP-MS, ICP-AES). The contamination factor (CF) of different elements, and the comprehensive pollution load index (PLI) of multiple HMs were calculated. Furthermore, to quantify the relationships between industrial output and soil HM accumulation, the concept of the toxic equivalent mass of soil HM accumulation per capital output (TEM) is proposed. The key results are as follows: (1) The average concentrations for all the studied HMs were higher than the local background concentration (BC) values. The distribution of the soil HM concentrations showed significant spatial heterogeneity, and the regions with the highest soil HM concentrations were generally affected by local industrial activities. (2) The pollution load index (PLI) values indicate that the extent of the human impacts on the soil environment in the study area correspond to the slight pollution level as a whole. (3) The PLI values, the total accumulated mass of soil HMs over time per million industrial capital output (PMt), and the TEM values for the ZETZ, Kaifeng Carbon Factory (KCF), and Wanyang Lead Smelter (WLS) were subjected to comparative analyses, with the KCF and WLS showing markedly higher levels than the ZETZ. Accordingly, although slight HM accumulation occurred in the soil of the ZETZ, the industry-related accumulation of soil HMs in the ZETZ is significantly less than that in the KCF and WLS; thus, industrial agglomeration may be effective for reducing the intensity of HM accumulation.

Dactylis glomerata L. cultivation on mercury contaminated soil and its physiological response to granular sulphur aided phytostabilization

Most mercury (Hg) deposition in the environment results from anthropogenic inputs, Chlor-Alkali Plants (CAPs) particularly had a significant Hg impact on the environment at a regional scale. Exposure to mercury compounds resulting in various toxic effects for living organisms. The aim of this study was to investigate the capacity of granular sulphur (S) soil amendment and cultivation of Dactylis glomerata to decrease gaseous mercury emission to the atmosphere and mercury mobility in soils affected by CAP activity in the past. The effect of this approach on D. glomerata physiological status was also assessed (Hg concentration in biomass, chlorophyll a fluorescence, pigment contents and oxidative stress). Stabilization of mercury in soil and reduction of root and shoot concentration did not influence biomass production. Despite similar yields, photosynthetic efficiency was higher for plants grown in sulphur amended soil compared to unamended soil, particularly observed in phenomenological energy fluxes. Relative chlorophyll content was 30% lower for amended soil plants, however based on chlorophyll fluorescence data those were in high portion ineffective. Oxidative stress products and catalase activity did not differ significantly between experimental treatments. Sulphur amendment was a key factor for reduction of Hg mobility in soil (reduced by about 30%) while plant cover was significant for the reduction of Hg atmospheric emission (emissions were 2-times higher in sulphur amended soil without plant cover). Due to the very high concentration of Hg in soil (798.2 ± 7.3 mg kg−1), growth inhibition was consistent regardless of treatment, demonstrated in the overload Reactive Oxygen Species scavenging mechanism and similar biomass yields. This leads to the conclusion that Hg may have greater impact on Calvin-Benson cycle associated enzymes than on the light-dependent photosynthesis phase. Despite these limitations this approach may still decrease environmental risks by reducing Hg emission to the atmosphere and reducing groundwater contamination.

Climate and Vegetation As Primary Drivers for Global Mercury Storage in Surface Soil.

Soil is the largest Hg reservoir globally. Data of Hg concentration in surface soil are fundamental to understanding environmental Hg cycling. However, present knowledge on the quantity and global distribution of Hg in soil remains deficient. Using stable Hg isotopic analyses and geospatial data, the concentration and global spatial distribution of Hg in surface soil of 0-20 cm depth have been developed. It is estimated that 1088 ± 379 Gg of Hg is stored in surface soil globally. Thirty-two percent of the surface Hg storage resides in tropical/subtropical forest regions, 23% in temperate/boreal forest regions, 28% in grassland and steppe and shrubland, 7% in tundra, and 10% in desert and xeric shrubland. Evidence from Hg isotopic signatures points to atmospheric Hg0 dry deposition through vegetation uptake as the primary source of Hg in surface soil. Given the influence of changing climate on vegetative development, global climate change can act as an important forcing factor for shaping spatial distribution of Hg in surface soil. This active forcing cycle significantly dilutes the impacts caused by Hg release from anthropogenic sources, and needs to be considered in assessing the effectiveness of reducing Hg use and emissions as specified in Minamata Convention on Mercury.

Degassing and Cycling of Mercury at Nisyros Volcano (Greece)

Nisyros Island (Greece) is an active volcano hosting a high-enthalpy geothermal system. During June 2013, an extensive survey on Hg concentrations in different matrices (fumarolic fluids, atmosphere, soils, and plants) was carried out at the Lakki Plain, an intracaldera area affected by widespread soil and fumarolic degassing. Concentrations of gaseous elemental mercury (GEM), together with H2S and CO2, were simultaneously measured in both the fumarolic emissions and the atmosphere around them. At the same time, 130 samples of top soils and 31 samples of plants (Cistus creticus and salvifolius and Erica arborea and manipuliflora) were collected for Hg analysis. Mercury concentrations in fumarolic gases ranged from 10,500 to 46,300 ng/m3, while Hg concentrations in the air ranged from high background values in the Lakki Plain caldera (10-36 ng/m3) up to 7100 ng/m3 in the fumarolic areas. Outside the caldera, the concentrations were relatively low (2-5 ng/m3). The positive correlation with both CO2 and H2S in air highlighted the importance of hydrothermal gases as carrier for GEM. On the other hand, soil Hg concentrations (0.023-13.7 μg/g) showed no significant correlations with CO2 and H2S in the soil gases, whereas it showed a positive correlation with total S content and an inverse one with the soil pH, evidencing the complexity of the processes involving Hg carried by hydrothermal gases while passing through the soil. Total Hg concentrations in plant leaves (0.010-0.112 μg/g) had no direct correlation with soil Hg, with Cistus leaves containing higher values of Hg with respect to Erica. Even though GEM concentrations in the air within the caldera are sometimes orders of magnitude above the global background, they should not be considered dangerous to human health. Values exceeding the WHO guideline value of 1000 ng/m3 are very rare (<0.1%) and only found very close to the main fumarolic vents, where the access to tourists is prohibited.

Responses of the grass Paspalum distichum L. to Hg stress: A proteomic study

Paspalum distichum L. was tested to evaluate its ability to phytoremediate mercury (Hg) contaminated soil over a 60-d period by analysis of the total Hg concentrations in roots and leaves. Hg concentration in Hg-contamination soil decreased by 70.0 μg g−1 after 60 day of grass cultivation and Hg was readily taken up by the roots (4.51 ± 1.90 μg g−1) rather than the leaves (0.35 ± 0.02 μg g−1). In addition, a comparative proteomic study was performed to unravel the protein expression involved in the Hg stress response in P. distichum L. A total of 49 proteins were classified as differentially proteins in the roots by the ‘top three’ proteomic analysis, of which 32 were up-regulated and 17 down-regulated in response to Hg stress. These changed proteins were classified by gene ontology analysis into five complex molecular functions involving photosynthesis and energy metabolism (31%), oxidative stress (14%), protein folding (16%), sulfur compound metabolism (10%), metal binding, and ion transport (29%). Moreover, the protein expression patterns were consistent with the metabolism pathway results. Overall, the results contribute to our understanding of the molecular mechanisms of the Hg response in P. distichum and we propose a theoretical basis for the phytoremediation of Hg-contaminated soils.

An experimental study of the impacts of solar radiation and temperature on mercury emission from different natural soils across China.

Mercury (Hg) emission from natural soil is one of the most important contributors to global Hg cycles. Research on Hg emission from soil to air has been carried out in China. Currently, most of the research focuses on contaminated sites in China, while research in other regions is rare. To provide more accurate information on Hg emissions from soil to air in China and obtain additional laboratory data to verify the role of solar radiation and temperature in this process, we sampled and measured Hg emission fluxes from various natural soils (range, 48-240 ng/g) across mainland China under different solar radiation (0-900 W·m-2) and temperature (15-45 °C) conditions in a laboratory. We found that in different places in China, Hg emissions from natural soils occurred more easily when the soil Hg concentration, temperature, and solar radiation were high, but the impacts were different among the regions due to different soil types. Hg emissions from natural soils (0.071-24 ng·m2·h-1) were typically lower than those from contaminated sites, suggesting that additional measurements in natural soils are desirable. The results of this study could provide more accurate information on Hg emission from natural soil to air and help establish a nationwide natural soil Hg emission inventory in China.

Bioavailability and sequential extraction of mercury in soils and organisms of a mangrove contaminated by a chlor-alkali plant

Botafogo river estuary poses the highest Hg concentration reported for mangrove soils in Brazil. Such high contamination took place owing to the improper waste disposal for 24 years from a chlor-alkali plant nearby the estuary. Here we determined Hg concentrations in soils, mangrove plants (Rhizophora mangle and Laguncularia racemosa), and an aquatic organism (the oyster species Crassostrea rhizophorae) to assess Hg bioavailability. Besides, a sequential extraction procedure was used to separate soil Hg into five fractions: water-soluble; ‘human stomach acid’ soluble; organically bound; elemental Hg; mercuric sulfide. Results showed that environmentally available concentration of Hg in the mangrove soils were up to 150-fold higher than a pristine mangrove under the same geological context used as a reference. Additionally, Hg concentration in soils was also above sediment quality guidelines and Hg adverse effects towards sensitive estuarine organisms are likely. Mercury concentrations in oysters were the highest reported in Brazil, but within safety standards according to the country food security agency. It seems that Hg stocks in the studied soils are governed by organic matter and redox conditions, but changing on environmental conditions and land use can alter this balance and convert these mangrove areas from sink to source of Hg to the environment.

Mercury distribution in the foliage and soil profiles of a subtropical forest: Process for mercury retention in soils

Forest canopy can exert great influence on the global mercury (Hg) accumulations and soil Hg pools. However, there exists a debate that forests are net sinks or sources of atmospheric Hg. The current study aimed to study whether forest soils in the southwestern China are net sinks for atmospheric Hg and the soil residence time of Hg in the subtropical forest soils. Foliage samples of fern and pine in sixty-six plots and six soil profiles were sampled at the Tieshanping Forest Park (TFP) in southwestern China. To study the process of Hg budget, the flux of Hg input and the output from the forest soils were estimated and the mean soil Hg residence time (MRT) were calculated using a simple two-box model. The Hg concentrations in foliage (98 ng g−1) was relatively higher than remote areas because the elevated atmospheric Hg concentrations. Mercury concentrations in the understory foliage were positively correlated with altitude, which was resulted from the altitudinal distribution of increasing soil Hg concentrations. The annual Hg deposition flux via litterfall was estimated at 42 μg m−2 and the total Hg input to the forests was approximately 108 μg m−2. The annual Hg retention in forest soil was about at 76 μg m−2. The MRT was longer in surface horizon (0–10 cm, 134 ± 14 yr) than that in the mineral horizon (20–80 cm, 595 ± 34 yr). Subtropical forests in southwestern China act as net sinks for atmospheric Hg and 69% of the total Hg deposition was sequestrated in the forest soils. Short residence time of Hg in surface soil enhanced the deposited Hg output from the forest, which may be the reason for the comparable soil Hg concentration with some other remote forests with lower Hg depositions.

Determining the Reliability of Measuring Mercury Cycling Gene Abundance with Correlations with Mercury and Methylmercury Concentrations.

Methylmercury (MeHg) is a bioaccumulative toxic contaminant in many ecosystems, but factors governing its production are poorly understood. Recent work has shown that the anaerobic microbial conversion of mercury (Hg) to MeHg requires the Hg-methylation genes hgcAB and that these genes can be used as biomarkers in PCR-based estimators of Hg-methylator abundance. In an effort to determine reliable methods for assessing hgcA abundance and diversity and linking them to MeHg concentrations, multiple approaches were compared including metagenomic shotgun sequencing, 16S rRNA gene pyrosequencing and cloning/sequencing hgcAB gene products. Hg-methylator abundance was also determined by quantitative hgcA qPCR amplification and metaproteomics for comparison to the above measurements. Samples from eight sites were examined covering a range of total Hg (HgT; 0.03-14 mg kg-1 dry wt. soil) and MeHg (0.05-27 μg kg-1 dry wt. soil) concentrations. In the metagenome and amplicon sequencing of hgcAB diversity, the Deltaproteobacteria were the dominant Hg-methylators while Firmicutes and methanogenic Archaea were typically ∼50% less abundant. This was consistent with metaproteomics estimates where the Deltaproteobacteria were steadily higher. The 16S rRNA gene pyrosequencing did not have sufficient resolution to identify hgcAB+ species. Metagenomic and hgcAB results were similar for Hg-methylator diversity and clade-specific qPCR-based approaches for hgcA are only appropriate when comparing the abundance of a particular clade across various samples. Weak correlations between Hg-methylating bacteria and soil Hg concentrations were observed for similar environmental samples, but overall total Hg and MeHg concentrations poorly correlated with Hg-cycling genes.