Endophytic bacteria promote biomass production and mercury-bioaccumulation of Bermuda grass and Indian goosegrass.

Plant growth-promoting endophytic bacteria can potentially improve the biomass production of Hg-accumulating grasses, resulting in improved Hg extraction from contaminated soils. This study aimed to analyze the effect of inoculation of Hg-resistant endophytic bacteria (i) Jeotgalicoccus huakuii (B1) and (ii) Bacillus amyloliquefaciens (B2), as single and consortium inoculant, on biomass production and Hg bioaccumulation of Bermuda grass (R1) and Indian goosegrass (R2) planted as monoculture and mixed cropping. The grass seeds were surface-sterilized before the inoculation. Both inoculated seeds with B1 and B2 (treatments) and uninoculated seeds were sown separately in sterilized sand. Grass seedlings of both treatments and control were replanted in the washed and sterilized sand medium, spiked with HgCl2 (100 mg kg-1). A subset of grass was harvested at 0, 4, 6, and 10 weeks after planting to measure biomass production and Hg bioaccumulation. The results showed that bacterial inoculation enhanced the grass biomass by 52.68% and Hg bioaccumulation by 47.76%. Mercury residue of Hg-spiked sand treated with the bacterial consortium was reduced by 80%. This suggests that endophytic bacteria can improve grass biomass production and enhance Hg bioaccumulation in grass biomass.

Mobilization, Methylation, and Demethylation of Mercury in a Paddy Soil Under Systematic Redox Changes.

Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation-reduction potential (Eh) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing Eh stepwise from -300 to +300 mV using a sophisticated biogeochemical microcosm. At the Eh range from -300 to -100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the Eh range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with Eh (-300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at Eh of +200 mV as compared to Eh of -300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at Eh of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.

Nanoactivated Carbon Reduces Mercury Mobility and Uptake by Oryza sativa L: Mechanistic Investigation Using Spectroscopic and Microscopic Techniques.

Mercury (Hg) contamination of paddy field poses a health risk to rice consumers, and its remediation is a subject of global scientific attention. In recent years focus has been given to in situ techniques which reduce the risk of Hg entering the food chain. Here, we investigate the use of nanoactivated carbon (NAC) as a soil amendment to minimize Hg uptake by rice plants. Application of 1-3% NAC to soil (by weight) reduced Hg concentration in the pore water (by 61-76%) and its bioaccumulation in the tissues of rice plants (by 15-63%), relative to the corresponding control. Specifically, NAC reduced the Hg concentration of polished rice by 47-63% compared to the control, to a level that was 29-49% lower than the food safety value (20 ng g-1) defined by the Chinese government. The NAC induced a change in Hg binding from organic matter to nano-HgS in the soil as a function of soil amendment. This Hg speciation transformation might be coupled to the reduction of sulfoxide to reduced sulfur species (S0) by NAC. The NAC amendment may be a practical and effective solution to mitigate the risk of Hg transferring from contaminated soil to rice grains at locations around the world.

Use of Mercury Isotopes to Quantify Mercury Exposure Sources in Inland Populations, China.

Mercury (Hg) isotopic compositions in hair and dietary sources from Wanshan (WS) Hg mining area, Guiyang (GY) urban area, and Changshun (CS) rural area were determined to identify the major Hg exposure sources of local residents. Rice and vegetables displayed low δ202Hg and small negative to zero Δ199Hg, and are isotopically distinguishable from fish which showed relatively higher δ202Hg and positive Δ199Hg. Distinct isotopic signatures were also observed for human hair from the three areas. Shifts of 2 to 3‰ in δ202Hg between hair and dietary sources confirmed mass dependent fractionation of Hg isotopes occurs during metabolic processes. Near zero Δ199Hg of hair from WS and CS suggested rice is the major exposure source. Positive Δ199Hg of hair from GY was likely caused by consumption of fish. A binary mixing model based on Δ199Hg showed that rice and fish consumption accounted for 59% and 41% of dietary Hg source for GY residents, respectively, whereas rice is the major source for WS and CS residents. The model output was validated by calculation of probable daily intake of Hg. Our study suggests that Hg isotopes can be a useful tracer for quantifying exposure sources and understanding metabolic processes of Hg in humans.

Toward Financially Viable Phytoextraction and Production of Plant-Based Palladium Catalysts.

Although a promising technique, phytoextraction has yet to see significant commercialization. Major limitations include metal uptake rates and subsequent processing costs. However, it has been shown that liquid-culture-grown Arabidopsis can take up and store palladium as nanoparticles. The processed plant biomass has catalytic activity comparable to that of commercially available catalysts, creating a product of higher value than extracted bulk metal. We demonstrate that the minimum level of palladium in Arabidopsis dried tissues for catalytic activity comparable to commercially available 3% palladium-on-carbon catalysts was achieved from dried plant biomass containing between 12 and 18 g·kg-1 Pd. To advance this technology, species suitable for in-the-field application: mustard, miscanthus, and 16 willow species and cultivars, were tested. These species were able to grow, and take up, palladium from both synthetic and mine-sourced tailings. Although levels of palladium accumulation in field-suitable species are below that required for commercially available 3% palladium-on-carbon catalysts, this study both sets the target, and is a step toward, the development of field-suitable species that concentrate catalytically active levels of palladium. Life cycle assessment on the phytomining approaches described here indicates that the use of plants to accumulate palladium for industrial applications has the potential to decrease the overall environmental impacts associated with extracting palladium using present-day mining processes.

Effect of atmospheric mercury deposition on selenium accumulation in rice (Oryza sativa L.) at a mercury mining region in southwestern China.

Selenium (Se) is an important trace element for human nutrition and has an interactive effect on mercury (Hg) uptake by plants and Hg toxicity in animals. Rice (Oryza sativa L.) is the dominant source of dietary Se in China, however the effect of soil Hg contamination on the Se concentration in rice is unknown. We collected 29 whole rice plant samples and corresponding soils from an active artisanal mercury mining area and an abandoned commercial mercury mining area. The soil Se concentration was similar across the two mining areas and greater than the background concentration for China. However, the Se concentration in rice grain was dramatically different (artisanal area 51±3 ng g(-1); abandoned area 235±99 ng g(-1)). The total gaseous mercury (TGM) concentration in ambient air at the artisanal mining site was significantly greater than at the abandoned area (231 and 34 ng m(-3), respectively) and we found a negative correlation between TGM and the Se concentration in grain for the artisanal area. Principal component analysis indicated that the source of Se in rice was the atmosphere for the artisanal area (no contribution from soil), and both the atmosphere and soil for the abandoned area. We propose that TGM falls to soil and reacts with Se, inhibiting the translocation of Se to rice grain. Our data suggest that Se intake by the artisanal mining community is insufficient to meet Se dietary requirements, predisposing this community to greater risk from Hg poisoning.

Agromining: farming for metals in the future?

Phytomining technology employs hyperaccumulator plants to take up metal in harvestable plant biomass. Harvesting, drying and incineration of the biomass generates a high-grade bio-ore. We propose that "agromining" (a variant of phytomining) could provide local communities with an alternative type of agriculture on degraded lands; farming not for food crops, but for metals such as nickel (Ni). However, two decades after its inception and numerous successful experiments, commercial phytomining has not yet become a reality. To build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide "real-life" evidence for profitability.

Localization and speciation of mercury in brown rice with implications for pan-Asian public health.

Cultivation of paddy rice for human consumption is a dominant agricultural activity throughout Asia. High levels of mercury (Hg) in rice grain pose a potential threat to human health, although the extent of risk is dependent on the chemical speciation of Hg inside the grain. We have investigated the speciation and localization of Hg in three fractions of rice grain (hull, bran, and white rice) collected from a Hg-contaminated region in China. On a mass basis, the majority of inorganic mercury (IHg) in a rice grain is found in hull and bran. However, the majority of the more toxic species methyl mercury (MeHg) is found in edible white rice. Our data show that during grain processing, most of the IHg (∼78%) is eliminated, but the majority of the MeHg remains in the food product (∼80%). Synchrotron radiation microscopic X-ray fluorescence (SR-µXRF) mapping shows strong localization of Hg at the surface of brown rice grains, corresponding to the pericarp and aleurone layer. We infer that this Hg is predominantly IHg absorbed from the atmosphere. Based on X-ray absorption near-edge spectroscopy (XANES) data we propose that IHg in bran is primarily bound to cysteine, and is associated with phytochelatins. Consequently, IHg is largely immobile and restricted to the outer layers of rice grain. MeHg in bran is primarily bound to cysteine and is associated with proteins. However, this MeHg-cysteine association behaves like a mobile nutrient and is actively transported to the endosperm during seed ripening. Concentration of MeHg-cysteine in white rice has implications for public health. There is growing evidence for Hg contamination of rice throughout Asia due to point and diffuse sources of Hg pollution. The magnitude of the associated risk must be quantified through better understanding of the localization and speciation of mercury in rice. Our work makes an effort to contribute to this understanding.

Copper induces nitrification by ammonia-oxidizing bacteria and archaea in pastoral soils.

Copper (Cu) is the main co-factor in the functioning of the ammonia monooxygenase (AMO) enzyme, which is responsible for the first step of ammonia oxidation. We report a greenhouse-based pot experiment that examines the response of ammonia-oxidizing bacteria and archaea (AOB and AOA) to different bioavailable Cu concentrations in three pastoral soils (Recent, Pallic, and Pumice soils) planted with ryegrass (Lolium perenne L.). Five treatments were used: control (no urine and Cu), urine only at 300 mg N kg-1 soil (Cu0), urine + 1 mg Cu kg-1 soil (Cu1), urine + 10 mg Cu kg-1 soil (Cu10), and urine + 100 mg Cu kg-1 soil (Cu100). Pots were destructively sampled at Day 0, 1, 7, 15, and 25 after urine application. The AOB/AOA amoA gene abundance was analyzed by real-time quantitative polymerase chain reaction at Days 1 and 15. The AOB amoA gene abundance increased 10.0- and 22.6-fold in the Recent soil and 2.1- and 2.5-fold in the Pallic soil for the Cu10 compared with Cu0 on Days 1 and 15, respectively. In contrast, the Cu100 was associated with a reduction in AOB amoA gene abundance in the Recent and Pallic soils but not in the Pumice soil. This may be due to the influence of soil cation exchange capacity differences on the bioavailable Cu. Bioavailable Cu in the Recent and Pallic soils influenced nitrification and AOB amoA gene abundance, as evidenced by the strong positive correlation between bioavailable Cu, nitrification, and AOB amoA. However, bioavailable Cu did not influence the nitrification and AOA amoA gene abundance increase.

Nitrification rate in dairy cattle urine patches can be inhibited by changing soil bioavailable Cu concentration.

Ammonia oxidation to hydroxylamine is catalyzed by the ammonia monooxygenase enzyme and copper (Cu) is a key element for this process. We investigated the effect of soil bioavailable Cu changes induced through the application of Cu-complexing compounds on nitrification rate, ammonia-oxidizing bacteria (AOB) and archaea (AOA) amoA gene abundance, and mineral nitrogen (N) leaching in urine patches using the Manawatu Recent soil. Further, evaluated the combination of organic compound calcium lignosulphonate (LS) with a growth stimulant Gibberellic acid (GA). Treatments were applied in May 2021 as late-autumn treatments: control (no urine), urine-only at 600 kg N ha-1, urine + dicyandiamide (DCD), urine + co-poly-acrylic-maleic acid (PA-MA), urine + LS, urine + split-application of LS (2LS), and urine + combination of GA plus LS (GA + LS). In addition, another four treatments were applied in July 2021 as mid-winter treatments: control, urine-only at 600 kg N ha-1, urine + GA, and urine + GA + LS. Soil bioavailable Cu and mineral N leaching were examined during the experimental period. The AOB/AOA amoA genes were quantified using quantitative polymerase chain reaction. Changes in soil bioavailable Cu across treatments correlated with nitrification rate and AOB amoA abundance in late-autumn while the AOA amoA abundance did not change. The reduction in soil bioavailable Cu induced by the PA-MA and 2LS was linked to significant (P < 0.05) reduction in mineral N leaching of 16 and 30%, respectively, relative to the urine-only. The LS did not induce a significant effect on either bioavailable Cu or mineral N leaching relative to urine-only. The GA + LS reduced mineral N leaching by 10% relative to LS in late-autumn, however, there was no significant effect in mid-winter. This study demonstrated that reducing soil bioavailable Cu can be a potential strategy to reduce N leaching from urine patches.

Prediction of methyl mercury uptake by rice plants ( Oryza sativa L.) using the diffusive gradient in thin films technique.

Rice consumption is the primary pathway for methyl mercury (MeHg) exposure at inland mercury (Hg) mining areas of SW China. Mechanistic information on MeHg accumulation in rice is, however, limited. The process of MeHg exchange between paddy soil and rice plants predominantly occurs in pore water. The detection of bioavailable MeHg in pore water is therefore important to predict MeHg uptake by rice plants ( Oryza sativa L.). This study investigated MeHg dynamics and spatial MeHg trends in pore water during the rice growing season using the diffusive gradient in thin films (DGT) technique and tested the ability of DGT to predict MeHg uptake by rice. The MeHg uptake flux from soil to rice plants via roots was significantly correlated with the DGT-measured MeHg flux (R = 0.853, p < 0.01). Our study implies that DGT can predict the bioavailability of MeHg in rice paddy soil and that the DGT method can provide quantitative description of the rate of uptake of this bioavailable MeHg. The DGT technique is demonstrated as a useful indicator of the likely ecotoxicological risk that might be apparent where paddy rice is grown in MeHg contaminated soil.

Implications of mercury speciation in thiosulfate treated plants.

Mercury uptake was induced in two cultivars of Brassica juncea under field conditions using thiosulfate. Analysis was conducted to better understand the mechanism of uptake, speciation of mercury in plants, and redistribution of mercury in the soil. Plant mercury and sulfur concentrations were increased after thiosulfate treatment, and a linear correlation between mercury and sulfur was observed. Mercury may be absorbed and transported in plants as the Hg-thiosulfate complex. The majority of mercury in treated plant tissues (two cultivars) was bound to sulfur in a form similar to ß-HgS (66-94%). Remaining mercury was present in forms similar to Hg-cysteine (1-10%) and Hg-dicysteine (8-28%). The formation of ß-HgS may relate to the transport and assimilation of sulfate in plant tissues. Mercury-thiosulfate complex could decompose to mercuric and sulfate ions in the presence of free protons inside the plasma membrane, while sulfide ions would be produced by the assimilation of sulfate. The concomitant presence of mercuric ions and S(2-) would precipitate ß-HgS. The mercury concentration in the rhizosphere decreased in the treated relative to the nontreated soil. The iron/manganese oxide and organic-bound fractions of soil mercury were transformed to more bioavailable forms (soluble and exchangeable and specifically sorbed) and taken up by plants.

Assessment of environmental mercury discharge at a four-year-old artisanal gold mining area on Lombok Island, Indonesia.

Indonesian artisanal and small-scale gold mining activities (ASGM) have been described for the islands of Borneo (Kalimantan) and Sulawesi. But the increased gold price over recent years has seen operations extend to the islands of Lombok and Sumbawa. For the current research, an environmental assessment was conducted across three new ASGM locations. Gold is recovered by miners through a two-stage process of whole-ore amalgamation and cyanidation. Waste (tailings) is discharged to land or sea with no concern for contaminants in the tailings. The gold grade of ore is up to 5000 mg kg(-1). The mean gold grade of the amalgamation tailings is 7 mg kg(-1), dropping to 1.2 mg kg(-1) for the cyanidation tailings. The mean mercury concentration of the amalgamation tailings is about 3000 mg kg(-1) and greater than 1600 mg kg(-1) for the cyanidation tailings. Samples of paddy rice grain collected adjacent to cyanidation tailings ponds showed methyl mercury concentrations greater than 100 ng g(-1). This is five times above the Chinese permissible level for total mercury in food crops. The mean total mercury concentration in hair of Lombok ASGM workers was greater than that in a non-exposed population; however there was no difference in methyl mercury concentration. This indicates the primary pathway of mercury exposure is inhalation of volatile mercury in the atmosphere. Future exposure may come from ingestion of methyl mercury contaminated rice where discharge of cyanide tailings to paddies continues. To protect the environment and to enhance the sustainability of ASGM, appropriate tailings management must be implemented. The gold grade of the tailings indicates that the residual value might be recoverable with appropriate technology. Ongoing research is investigating systems such as phytoextraction that might assist ASGM operators in Lombok and Sumbawa to improve their environmental performance.

Gold phytomining. A review of the relevance of this technology to mineral extraction in the 21st century.

The precious metal gold can be found at high concentration in tailings dumps and waste rock piles at many mining locations around the world. Conventional technology is generally unable to economically recover this residual gold, and, as a result, the potential resource is wasted, presenting environmental risk to the wider ecosystem through particulate and dissolved metal leaching and erosion. For the past 14 years, the idea of gold phytomining to recover this gold resource has been researched by various scientific groups worldwide. A number of plant species have been tested under laboratory, greenhouse, and field conditions to determine their potential for use in the phytoextraction of gold. This paper presents a review of reported gold phytomining trials developed in the laboratory, the greenhouse under soil and hydroponic conditions, as well as in the field, between 1998 and 2011. A summary economic assessment for gold phytomining in Mexico is also presented. Mexico is an example of a developing country with a long history of gold mining that has a large resource of sites that might be suitable to gold phytomining. The technology remains limited by certain environmental and plant physiology factors. However, the increase in the market price for gold during the first decade of the 21st century and into 2011, and advances recorded for the gold concentration and biomass yield of a range of plant species, suggest that gold phytomining might be an economically viable technology.

Nitrate Leaching Mitigation Options in Two Dairy Pastoral Soils and Climatic Conditions in New Zealand.

This lysimeter study investigated the effect of late-autumn application of dicyandiamide (DCD), co-poly acrylic-maleic acid (PA-MA), calcium lignosulphonate (LS), a split-application of calcium lignosulphonate (2LS), and a combination of gibberellic acid (GA) and LS (GA + LS) to reduce N leaching losses during May 2020 to December 2020 in lysimeter field sites in Manawatu (Orthic Pumice soil) and Canterbury (Pallic Orthic Brown soil), New Zealand. In a second application, urine-only, GA only and GA + LS treatments were applied during July 2020 in mid-winter on both sites. Results showed that late-autumn application of DCD, 2LS and GA + LS reduced mineral N leaching by 8%, 16%, and 35% in the Manawatu site and by 34%, 11%, and 35% in the Canterbury site, respectively when compared to urine-only. There was no significant increase in cumulative herbage N uptake and yield between urine-treated lysimeters in both sites. Mid-winter application of GA and GA + LS reduced mineral N leaching by 23% and 20%, respectively in the Manawatu site relative to urine-only treated lysimeters, but no significant reduction was observed in the Canterbury site. Our results demonstrated the potential application of these treatments in different soils under different climate and management conditions.

Forage crops and cadmium: How changing farming systems might impact cadmium accumulation in animals.

Production advantages, environmental benefits and increasing parasite resistance are changing the composition of New Zealand pastures. Traditional ryegrass/clover pasture mixes are being replaced by forage herb crops such as lucerne, chicory and plantain that accumulate a higher concentration of contaminants such as cadmium (Cd). To explore the relationship between Cd in forage crops and the Cd concentration accumulated by animals, four-month-old lambs at four farms across the central North Island of New Zealand were grazed on different forage crops (ryegrass, chicory, lucerne and plantain) between weaning and slaughter. Soil and pasture samples, and sequential liver biopsies, were collected and analysed for total Cd. There were significant differences in Cd concentration between the forage crops (chicory > plantain > lucerne > ryegrass) and this ordering was repeated for Cd in liver. There was no exceedance of maximum limits (ML) for Cd in offal set by the EU and NZ/Australia food safety standards authorities for animals of this study, although the highest concentration of Cd in chicory (0.85 mg/kg DW) was considerably lower than has been recorded elsewhere in New Zealand (4.5 mg/kg DW). Provisional Soil Management Values (SMVs) were developed to explore compliance of liver with EU food standards as a function of grazing chicory. For a soil pH of 5, exceedance might occur at a soil cadmium concentration of 0.34 mg/kg. This concentration falls within Tier 0 of the New Zealand Tiered Fertiliser Management System which seeks to ensure soil Cd remains within acceptable limits over the next 100 years and beyond. Increased Cd uptake by fodder crops and its management in these Tier 0 pastoral soils is therefore an emerging issue for pastoral agriculture. The risk of ML exceedance for animals grazing forage crops such as chicory on low Cd soils should be further considered to ensure uninterrupted access to export markets.

Effects of biological nitrification inhibitor in regulating NH3 volatilization and fertilizer nitrogen recovery efficiency in soils under rice cropping.

Biological nitrification inhibitors are exudates from plant roots that can inhibit nitrification, and have advantages over traditional synthetic nitrification inhibitors. However, our understanding of the effects of biological nitrification inhibitors on nitrogen (N) loss and fertilizer N recovery efficiency in staple food crops is limited. In this study, acidic and calcareous soils were selected, and rice growth pot experiments were conducted to investigate the effects of the biological nitrification inhibitor, methyl 3-(4-hydroxyphenyl) propionate (MHPP) and/or a urease inhibitor (N-[n-butyl], thiophosphoric triamide [NBPT]) on NH3 volatilization, N leaching, fertilizer N recovery efficiency under a 20% reduction of the conventional N application rate. Our results show that rice yield and fertilizer N recovery efficiency were more sensitive to reduced N application in the calcareous soil than in the acidic soil. MHPP stimulated NH3 volatilization by 13.2% in acidic soil and 9.06% in calcareous soil but these results were not significant. In the calcareous soil, fertilizer N recovery efficiency significantly increased by 19.3% and 44.4% in the MHPP and NBPT+MHPP groups, respectively, relative to the reduced N treatment, and the rice yield increased by 16.7% in the NBPT+MHPP treatment (P < 0.05). However, such effects were not significant in the acidic soil. MHPP exerted a significant effect on soil ammonia oxidizers, and the response of abundance and community structure of ammonia-oxidizing archaea, ammonia-oxidizing bacteria, and total bacteria to MHPP depended on the soil type. MHPP+NBPT reduced NH3 volatilization, N leaching, and maintaining rice yield for a 20% reduction in conventional N fertilizer application rate. This could represent a viable strategy for more sustainable rice production, despite the inevitable increase in cost for famers.

Effect of soil cadmium on root organic acid secretion by forage crops.

The two forage species used in New Zealand pastoral agricultural systems, chicory (Cichorium intybus) and plantain (Plantago lanceolata) show differential ability to absorb and translocate cadmium (Cd) from roots to shoots. Chicory can accumulate Cd from even low Cd soils to levels that might exceed regulatory guidelines for Cd in fodder crops and food. Chicory and plantain were grown in soil-filled rhizocolumns under increasing Cd levels (0 (Control), 0.4, 0.8 and 1.6 mg Cd/kg soil) for 60 days and showed variable secretion of oxalic, fumaric, malic and acetic acids as a function of Cd treatment. Plant roots secrete such Low Molecular Weight Organic Acids into the rhizosphere soil, which can influence Cd uptake. Chicory showed significantly (P < 0.05) lower secretion of fumaric acid, and higher secretion of acetic acid than plantain at all Cd treatments. We propose that the significant secretion differences between the two species can explain the significantly (P < 0.05) higher shoot Cd concentration in chicory for all Cd treatments. Understanding the mechanism for increased uptake in chicory may lead to breeding or genetic modification which yield low Cd uptake cultivars needed to mitigate the risk of Cd accumulation in pastoral agricultural food chains from this increasingly important fodder crop.

Significant mercury efflux from a Karst region in Southwest China - Results from mass balance studies in two catchments.

Karst regions have long been recognised as landscapes of ecological vulnerability, however the mass balance and fate of mercury (Hg) in karst regions have not been well documented. This study focused on the largest contiguous karst area in China and investigated Hg mass balance in two catchments, one with high geological Hg (Huilong) and the other representative of regional background Hg (Chenqi). The mass balance of Hg was calculated separately for the two catchments by considering Hg in throughfall, open field precipitation, total suspended particulate matter (TSP), litterfall, fertilizer, crop harvesting, air-surface Hg0 exchange, surface runoff and underground runoff. Results show that litterfall Hg deposition is the largest loading (from atmosphere) of Hg in both catchments, accounting for 61.5% and 38.5% of the total Hg input at Huilong and Chenqi, respectively. Air-surface Hg0 exchange is the largest efflux, accounting for 71.7% and 44.6% of the total Hg output from Huilong and Chenqi, respectively. Because both catchments are subject to farm and forest land use, cultivation plays an important role in shaping Hg fate. Mercury loading through fertilizer was ranked as the second largest input (28.5%) in Chenqi catchment and Hg efflux through crop harvest was ranked as the second largest output pathway in both Huilong (27.0%) and Chenqi (52.9%). The net Hg fluxes from the catchments are estimated to be 1498 ± 1504 µg m-2 yr-1 and 4.8 ± 98.2 µg m-2 yr-1. The significantly greater magnitude of net Hg source in Huilong is attributed to higher air-surface Hg0 exchange. The output/input ratio of Hg in this study was much greater than has been reported for other forest or agricultural ecosystems and indicates that the karst region of Southwest China is a significant source of atmospheric Hg. The results of this study should be considered in the development of pollution control policies which seek to conserve fragile karst ecosystems characterised by high geological background of Hg.

Fluorine and white clover: Assessing fluorine's impact on Rhizobium leguminosarum.

The soil fluorine (F) concentration in New Zealand agricultural soils has increased with time as a direct result of the widespread application of phosphate fertilizer to land. Elevated soil F concentrations may potentially harm soil microorganisms, which are important for nutrient cycling and soil formation. Rhizobium leguminosarum is a N2 -fixing soil bacterium that is a fundamental component in New Zealand legume-based pastoral farming. Any impact of F on Rhizobium leguminosarum would have an adverse effect on New Zealand pasture production. In this study, F toxicity to Rhizobium leguminosarum was examined as a first step to develop F guideline values for New Zealand agricultural soils. Bottle-based experiments were conducted to examine the effect of the F- ion on Rhizobium-white clover (Trifolium repens L.) symbiosis by observing nodule morphology and growth. Results indicate that the F- concentration that causes 10% inhibition of Rhizobium respiration (IC10 ) for F- toxicity to Rhizobium leguminosarum was >100 mg F-  L-1 . Significant morphological changes occurred when Rhizobium was exposed to F concentrations of 500 and 1000 mg L-1 . Both light and transmission electron micrographs confirmed that the Rhizobium leguminosarum-white clover interaction was not influenced by F- concentrations >100 mg L-1 . The toxic F- concentration for Rhizobium leguminosarum determined in this study is orders of magnitude higher than the F- concentration in New Zealand agriculture soils under "normal conditions." There appears to be no indication of imminent risk of soil F to Rhizobium leguminosarum.