Dynamics of mercury and methylmercury in forest floor and runoff of a forested watershed in Central Europe

Forested watersheds are an important part of the terrestrialmercury and methylmercury cycle, and a link between theatmospheric and aquatic environment. This study was conducted todetermine the contribution of the forest floor to the pools andfluxes of total Hg (Hg_total) and methylmercury (MeHg) in aforested catchment, and to identify factors influencing themobility of both compounds. Throughfall deposition, litterfall,runoff and fluxes with forest floor percolate of Hg_total and MeHgwere sampled during one year in a coniferous catchment inGermany. Total deposition of Hg_total was 552 mg ha_–1 a_–1 withlitterfall contributing one third. Nearly 60% of the total inputof Hg_total reached the mineral soil with the forest floorpercolate, but less than half of this fraction was found in therunoff of the catchment. Total deposition of MeHg was 2.6 mg ha_–1a_–1, with litterfall as the dominating pathway. Only 19% of theMeHg deposition was discharged from the forest floor, but theflux of MeHg with runoff was nearly twice as high. Only fewcorrelations with other solution parameters were found. Fluxes ofboth compounds with forest floor percolates depended mainly onwater fluxes, which was not true for the runoff. The forest floorof the upland soil is an effective sink for MeHg, but not forHg_total. Differences in the mobility of both compounds in theforest floor disappeared at the catchment scale, probably becauseother processes (i.e. Hg_total immobilization and MeHg formation)dominated.     

Plant and soil microbial community responses to solid waste leachates diffusion on grassland

Litterfall from trees has been identified as an important pathway for deposition of mercury (Hg) and methylmercury (MeHg) in forested catchments, but very little is known about the role of ground vegetation in deposition and cycling of Hg compounds. This study was conducted to identify the origin of Hg compounds in the ground vegetation, and to estimate the role of its litterfall with respect to pools and fluxes of Hg in a coniferous forest in the German Fichtelgebirge mountains. Above and below ground biomass of the dominant ground vegetation (Vaccinium myrtillus, Deschampsia flexuosa and Calamagrostis villosa) were sampled at several plots successively during the growing season. The fluxes to the soil via litterfall of the ground vegetation were calculated using contents of Hg and MeHg in the annual fractions of aboveground biomass. With fluxes of 0.4 – 7.8 mg Hg_total ha^–1 a^–1 and 0.01 – 0.04 mg MeHg ha^–1 a^–1 (depending on the plant species) this pathway contributes only a few percent to the total deposition of both compounds in the catchment. To identify the uptake pathways of Hg compounds, the same plant species were grown in a pot experiment with addition of isotope labelled Hg compounds (²⁰²Hg²⁺, Me¹⁹⁸Hg) to a clean sand substrate. Only small proportions of ²⁰²Hg and Me¹⁹⁸Hg in the substrate were taken up by the plants, but in all cases the proportion translocated into aboveground biomass after uptake was greater in case of Me¹⁹⁸Hg. Thus, internal recycling in the plant-soil system is a source especially for MeHg in the ground vegetation. However, as compared to the input of Hg compounds by tree litterfall and storage in the forest floor, Hg_total and MeHg in ground vegetation are of minor importance. High volatilization of added Hg isotopes raises the question of a re-emission of Hg compounds by the transpiration flux of the ground vegetation.     

Impacts of Reservoir Creation on the Biogeochemical Cycling of Methyl Mercury and Total Mercury in Boreal Upland Forests

The FLooded Upland Dynamics Experiment (FLUDEX) at the Experimental Lakes Area (ELA) in northwest Ontario was designed to test the hypothesis that methylmercury (MeHg) production in reservoirs is related to the amount, and subsequent decomposition, of flooded organic matter. Three upland forest sites that varied in the amounts of organic carbon stored in vegetation and soils (Low C, 30,870 kg C ha⁻¹; Medium C, 34,930 kg C ha⁻¹; and High C, 45,860 kg C ha⁻¹) were flooded annually from May to September with low-organic carbon, low-MeHg water pumped from a nearby lake. Within five weeks of flooding, MeHg concentrations in the reservoir outflows exceeded those in reservoir inflows and remained elevated for the duration of the experiment, peaking at 1.60 ng L⁻¹ in the Medium C reservoir. We estimated the net production of MeHg in each reservoir by calculating annual changes in pools of MeHg stored in flooded soils, periphyton, zooplankton, and fish. Overall, there was an initial pulse of MeHg production (range = 120–1590 ng m⁻² day⁻¹) in all FLUDEX reservoirs that lasted for 2 years, after which time net demethylation (range = 360–1230 ng MeHg degraded m⁻² day⁻¹) began to reduce the pools of MeHg in the reservoirs, but not back to levels found prior to flooding. Rates of MeHg production were generally related to the total amount of organic carbon flooded to create the reservoirs. Large increases in MeHg stores in soils compared to those in water and biota indicate that flooded soils were the main sites of MeHg production. This study should assist hydroelectric utilities and government agencies in making informed decisions about selecting sites for future reservoir development to reduce MeHg contamination of the reservoir fisheries.     

Xylem sap as a pathway for total mercury and methylmercury transport from soils to tree canopy in the boreal forest

Conifer needles are an important link in the cycling of Total Mercury (THg) and Methylmercury (MeHg) in the boreal ecosystem due to the high THg and MeHg concentrations in litterfall. Translocation within the tree of Hg from soils to the crown canopy has been assumed to be a minor source of the Hg in litterfall. This paper, however, is the first to present direct observations of THg/MeHg transport from the soil via xylem sap. Xylem sap concentrations of THg and MeHg were measured in sap drained from different levels along the boles of freshly cut 100 year old Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). The trees came from a mixed stand growing on podzolized till soils at the Svartberget Forest Research Station in N. Sweden. Soil solution concentrations of THg and MeHg at different levels in the soil profile were measured for comparison.Concentrations of THg in xylem sap ranged from 10–15 ng L^-1 in both the Scots pine and Norway spruce. Concentrations of MeHg varied from 0.03 ng L^-1to 0.16 ng L^-1, with higher values in Scots pine than Norway spruce. If these concentrations are representative of the transport from soils to needles in xylem sap at this site, then only 3% of the MeHg in litterfall (0.12 mg ha^-1 yr^-1) and 11% of the THg (26 mg ha^-1 yr^-1) can originate via this pathway. The upward transport via xylem sap is larger relative to the open field inputs (84% of THg and 17% of MeHg). Comparison of soil solution and xylem sap THg/MeHg suggested some degree of THg exclusion during water uptake in Scots pine and Norway spruce, but MeHg exclusion only in Norway spruce.     

Effect of a modular extensive green roof on stormwater runoff and water quality

Runoff quantity and quality from a 248 m² extensive green roof and a control were compared in Connecticut using a paired watershed study. Weekly and individual rain storm samples of runoff and precipitation were analyzed for TKN, NO₃ + NO₂-N, NH₃-N, TP, PO₄-P, and total and dissolved Cu, Pb, Zn, Cd, Cr, and Hg. The green roof watershed retained 51.4% of precipitation during the study period based on area extrapolation. Overall, the green roof retained 34% more precipitation than predicted by the paired watershed calibration equation. TP and PO₄-P mean concentrations in green roof runoff were higher than in precipitation but lower than in runoff from the control. The green roof was a sink for NH₃-N, Zn, and Pb, but not for TP, PO₄-P, and total Cu. It also reduced the mass export of TN, TKN, NO₃ + NO₂-N, Hg, and dissolved Cu primarily through a reduction in stormwater runoff. Greater than 90% of the total Cu, Hg, and Zn concentrations in the green roof runoff were in the dissolved form. The growing media and slow release fertilizer were probable sources of P and Cu in green roof runoff. Overall, the green roof was effective in reducing stormwater runoff and overall pollutant loading for most water quality contaminants.     

Red-backed salamander (Plethodon cinereus) as a bioindicator of mercury in terrestrial forests of the northeastern United States

Despite concerns over the widespread deposition of mercury (Hg) in remote forested areas of the northeastern United States (U.S.), little information is available on the bioaccumulation of Hg in this region's terrestrial fauna. There is a strong need to develop baseline data on appropriate bioindicator species for this area, and here we report Hg concentrations in one of the most widely distributed vertebrates in forested areas, the red-backed salamander (Plethodon cinereus). To inform the use of this species as a bioindicator of Hg accumulation, we assessed ratios of bioavailable methylmercury (MeHg) to total Hg, and techniques for non-destructive sampling along an elevational gradient of increasing forest floor Hg concentrations. Total Hg in body samples was 70% MeHg, and Hg concentrations in tail-clips, which can be collected non-lethally, were positively correlated with body concentrations. Mercury concentrations in salamanders increased 2.4-fold along an increasing elevational gradient of Hg in soils. We conclude that Hg concentrations in P. cinereus can act as a biomonitoring tool for broad areas of remote terrestrial forests, and may help identify regions and landscape characteristics of particular concern for Hg bioaccumulation.     

Biogeochemistry of trimethyllead and lead in a forested ecosystem in Germany

Lead compounds, especially ionic organolead compounds (OLC), are highly toxic and mobile pollutants strongly affecting many ecosystems. Soil pools and fluxes with precipitation, litterfall and runoff of trimethyllead (TML), one of the dominant ionic OLC in the environment, and Pb_total were investigated in a forested ecosystem in NE-Bavaria, Germany. In addition, ad/desorption of TML to soils was studied in batch experiments and its degradation in soils was investigated using long term incubations. Total soil storage in the catchment was 11.56 mg Pb ha^–1 for TML and 222 kg Pb ha^–1 for Pb_total. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30% of the area. Most Pb_total (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. The annual total deposition from the atmosphere, estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha^–1 year^–1 for TML and 52 g Pb ha^–1 year^–1 for Pb_total. The contribution of litterfall was 1.5 and 32%, respectively. The concentrations of TML and Pb_total in wet precipitation were: fog > throughfall > bulk precipitation. The annual fluxes with runoff from the catchment was 0.5 mg Pb ha^–1 year^–1 for TML and 2.8 g Pb ha^–1 year^–1 for Pb_total. TML degraded rapidly in the forest floor (Oa horizon) with a half-life (t _1/2) of 33.5 days. The degradation of TML in Fen (t _1/2 = 421 days) and in the mineral soil (Bw-C horizon, t _1/2 = 612 days) was much slower. Emission of tetramethyllead from wetland soils was not observed during the 1 year incubation. The adsorption affinity of TML to different soils was Fen > Oa > A Bw-C. The ratio of total soil storages to the present annual input were 3.6 years for TML. TML and Pb_total are still deposited in remote areas even after the use of tetraalkyllead as additives has been terminated for years. The rates of deposition are, however, much lower than in the past. Forest soils act as a sink for deposited TML and Pbtotal. TML is accumulated mostly in wetland soils and seems to be stable under anoxic conditions for a long time. In upland soils, TML decomposes rapidly. Only small amounts of TML are transferred from soils into runoff.     

中亚热带常绿阔叶林凋落物分解过程中汞的动态变化及迁移机理

森林凋落物对于汞在林地土壤的生物地球化学循环中起到重要作用,为研究森林凋落物分解过程中汞的迁移转化特征,以重庆四面山风景名胜区典型林分(常绿阔叶林)作为研究对象。于2014年3月—2015年3月连续监测典型林分凋落物中各形态汞浓度和有机质变化量,同时监测周围土壤中汞浓度变化。结果表明:四面山典型林分凋落物分解过程中汞浓度整体上升,总汞浓度(初始浓度:78 ng/g)的增幅最高达53%,甲基汞浓度(初始浓度:0.32 ng/g)最高增幅达138%;在春季和夏季,水溶态和酸溶态两种活性态汞含量分别增加了851%和96%,在分解前期和末期,凋落物汞的中惰性汞比例最高,占比达75%。土壤腐殖质层中总汞和甲基汞浓度比较稳定。凋落物中活性态汞通过雨水淋洗进入土壤与有机质络合并发生甲基化/去甲基化过程,通过地表径流、地下径流进入水体。凋落物中C含量减少了22%,N含量增加了15%,总汞浓度与C/N比呈负相关,与N含量呈正相关。凋落物中微生物C、N含量整体增加,与汞浓度峰值同步,且夏季含量显著高于冬季含量(P0.05),说明微生物与凋落物固定汞和汞的甲基化过程密切相关。     

Nitrification potentials in early successional black locust and in mixed hardwood forest stands in the southern Appalachians,USA

Soil nitrogen mineralisation and nitrification potentials, and soil solution chemistry were measured in black locust (Robinia pseudo-acacia L.), in pine-mixed hardwood stands on an early successional watershed (WS6), and in an older growth oak-hickory forest located on an adjacent, mixed hardwood watershed (WS14) at Coweeta Hydrologic laboratory, in the southern Appalachian mountains, U.S.A. Nitrification potentials were higher in black locust and pine-mixed hardwood early successional stands than in the oak-hickory forest of the older growth watershed. Ammonification rates were the main factor controlling nitrification in the early successional stands. There was no evidence of inhibition of nitrification in soils from the older growth oak-hickory forest site.Within the early successional watershed, black locust sites had net mineralisation and nitrification rates at least twice as high as those in the pine mixed-hardwood stands. Concentrations of exchangeable nitrate in the soil of black locust stands were higher than in pine-mixed hardwoods at 0–15 cm in March and they were also higher at 0–15, 16–30 and 31–45 cm depth in the black locust dominated sites in July. Soil solution nitrate concentrations were higher under black locust than under pine-mixed hardwoods. Areas dominated by the nitrogen fixing black locust had greater nitrogen mineralisation and nitrification rates, resulting in higher potential for leaching losses of nitrate from the soil column in the early successional watershed.     

Lead dynamics in the forest floor and mineral soil in south-central Ontario

Recent studies have suggested that the residence time of Pb in the forest floor may not be as long as previously thought, and there is concern that the large pulse of atmospheric Pb deposited in the 1960s and early 1970s may move rapidly through mineral soils and eventually contaminate groundwater. In order to assess Pb mobility at a woodland (JMOEC) in south-central Ontario, a stable Pb isotope tracer ²⁰⁷Pb (8 mg m^–2) was added to the forest floor in white pine (Pinus strobus) and sugar maple (Acer saccharum) stands, respectively, and monitored over a 2-year period. Excess ²⁰⁷Pb was rapidly lost from the forest floor. Applying first-order rate coefficients (k) of 0.57 (maple) and 0.32 (pine) obtained from the tracer study, and estimates of Pb deposition in the region, current predicted Pb concentrations in the forest floor are 1.5–3.1 and 2.1–5.8 mg kg^–1 in the maple and pine plots, respectively. These values compare favorably with measured concentrations (corrected for mineral soil contamination) of 3.1–4.3 mg kg^–1 in the maple stand and 2.6–3.6 mg kg^–1 in the pine stand. The response time (1/^k) of Pb in the forest floor at the sugar maple and white pine plots was estimated to be 1.8 and 3.1 years, respectively. The rapid loss of Pb from the forest floor at the JMOEC is much greater than previously reported, and is probably due to the rapid rate of litter turnover that is characteristic of forests with mull-type forest floors. In a survey of 23 forested sites that border the Precambrian Shield in south-central Ontario, Pb concentrations in the forest floor increased exponentially with decreasing soil pH. Lead concentrations in the forest floor at the most acidic survey sites, which exhibited mor-type forest floors, were approximately 10 times higher (80 mg kg^–1) than at the JMOEC, and pollution Pb burdens were up to 25 times greater. Despite the rapid loss of Pb from the forest floor at the JMOEC, the highest pollution Pb concentrations were found in the upper (0–1 cm) mineral soil horizon. Lead concentrations in the upper 30 cm of mineral soil were strongly correlated with organic matter content, indicating that pollution Pb does not move as a pulse down the soil profile, but instead is linked with organic matter distribution, indicating groundwater contamination is unlikely.     

Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from <0.1 ng gdw^-1 sediment in the ENR to 5 ng gdw^-1 in WCA3 sediments; and MeHg constituted <0.2% of total Hg (Hg_T) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimated using tracer-level injections of²⁰³ Hg(II) into intact sediment cores, ranged from 0 to 0.12 d^-1, or about 1 to 10 ng g^-1 d^-1when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controls concentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota, suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments.Sulfate concentrations in surficial pore waters (up to 400 µm), microbial sulfate-reduction rates (up to 800 nm cc^-1 d^-1) and resultant pore water sulfide concentrations (up to 300 µm) at the eutrophic northern sites were all high relative to most freshwater systems. All declined to the south, and sulfate concentrations in WCA2B and in central WCA3 resembled those in oligotrophic lakes (50–100 µm). MeHg concentration and production were inversely related to sulfate reduction rate and pore water sulfide. Control of MeHg production in the northern Everglades appears to mimic that in an estuary, where sulfate concentrations are high and where sulfide produced by microbial sulfate reduction inhibits MeHg production.     

Soil-productivity relationships and organic matter turnover in dry tropical forests of the Florida Keys

Soils and aboveground production in five types of upland forest in the Florida Keys were examined. Throughout the habitat gradient represented by these forest types, the soils were predominantly shallow and organic, forming in place directly on the limestone bedrock. However, the well-drained soils in the most productive broadleaved forests were deep enough to qualify as Histosols (Folists). Soils decreased in electrical conductivity and increased in nutrient content with increasing aboveground production. At 3–12 Mg ha^–1 yr^–1, production was within the range reported for dry tropical forests. Measured rates of decomposition were moderate or fast, and estimates of the organic C turnover of several soils based on their bomb radiocarbon signature were 100 years or less. In the face of these rapid turnover rates, we attribute the development of organic soils to the absence of mineral residues from weathering of the underlying limestone bedrock. Fast turnover of organic matter, and rapid and efficient cycling of nutrients are necessary to sustain the high production rates obtained on these shallow organic soils.     

Mercury dynamics in Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River

The accumulation of total mercury (Hg_T) andmethylmercury (MeHg) was evaluated in sediments ofTivoli South Bay, a freshwater tidal mudflat wetlandin the Hudson River National Estuarine ResearchReserve system. Hg_T concentrations in sedimentcores were measured to evaluate the spatialvariability of Hg_T deposition, and to establisha chronology of Hg_T accumulation. Cores takenfrom the northern, middle, and southern sections ofthe bay had similar distribution patterns andconcentrations of Hg_T, suggesting a common sourceof Hg_T throughout the bay. Sedimentconcentrations ranged from 190 to 1040 ng Hg g^–1,2 to 10 times greater than concentrations expected insediments from non-anthropogenic sources. Hg_Tdeposition rates were similar in different regions ofthe bay, and increased from 200 ng Hg cm^–2yr^–1in the 1930s to a maximum of 300 ngHg cm^–2 yr^–1 in the 1960s. Deposition rateshave steadily declined since the 1970s and arecurrently at 80 ng Hg cm^–2 yr^–1. Transportof Hg_T by tidal waters from the Hudson River islikely the main source of Hg_T in the bay.Distribution patterns and absolute concentrations ofMeHg in sediment cores were similar throughout thebay, with concentrations ranging from 0.43 to 2.95ng g^–1. Maxima in MeHg concentration profilesoccurred just below the sediment-water interface andat a depth of 30 cm. The maximum at 30 cm wascoincident with maximum Hg_T concentrations. MeHgconcentrations in suspended particulate matter (SPM)from the Hudson River suggest that MeHg in the baycould be derived from riverine SPM rather than formedin situ.     

Analysis of the Microbial Community Structure by Monitoring an Hg Methylation Gene (hgcA) in Paddy Soils along an Hg Gradient

Knowledge of the diversity of mercury (Hg)-methylating microbes in the environment is limited due to a lack of available molecular biomarkers. Here, we developed novel degenerate PCR primers for a key Hg-methylating gene (hgcA) and amplified successfully the targeted genes from 48 paddy soil samples along an Hg concentration gradient in the Wanshan Hg mining area of China. A significant positive correlation was observed between hgcA gene abundance and methylmercury (MeHg) concentrations, suggesting that microbes containing the genes contribute to Hg methylation in the sampled soils. Canonical correspondence analysis (CCA) showed that the hgcA gene diversity in microbial community structures from paddy soils was high and was influenced by the contents of total Hg, SO₄²⁻, NH₄⁺, and organic matter. Phylogenetic analysis showed that hgcA microbes in the sampled soils likely were related to Deltaproteobacteria, Firmicutes, Chloroflexi, Euryarchaeota, and two unclassified groups. This is a novel report of hgcA diversity in paddy habitats, and results here suggest a link between Hg-methylating microbes and MeHg contamination in situ, which would be useful for monitoring and mediating MeHg synthesis in soils.     

Vertical distribution of biological and geochemical phosphorus subcycles in two southern Appalachian forest soils

We measured Al, Fe, and P fractions by horizon in two southern Appalachian forest soil profiles, and compared solution PO₄ ^–1 removal in chloroform-sterilized and non-sterilized soils, to determine whether biological and geochemical P subcycles were vertically stratified in these soils. Because organic matter can inhibit Al and Fe oxide crystallization, we hypothesized that concentrations of non-crystalline (oxalate-extractable) Al (Al₀) and Fe (Fe₀), and concomitantly P sorption, would be greatest in near-surface mineral (A) horizons of these soils.Al₀ and Fe₀ reached maximum concentrations in forest floor and near-surface mineral horizons, declined significantly with depth in the mineral soil, and were highly correlated with P sorption capacity. Small pools of readily acid-soluble (AF-extractable) and readily-desorbable P suggested that PO₄ ^3– was tightly bound to Al and Fe hydroxide surfaces. P sorption in CHCl₃-sterilized mineral soils did not differ significantly from P sorption in non-sterilized soils, but CHCl₃ sterilization reduced P sorption 40–80% in the forest floor. CHCl₃ labile (microbial) P also reached maximum concentrations in forest floor and near-surface mineral horizons, comprising 31–35% of forest floor organic P. Combined with previous estimates of plant root distributions, data suggest that biological and geochemical P subcycles are not distinctly vertically stratified in these soils. Plant roots, soil microorganisms, and P sorbing minerals all reach maximum relative concentrations in near-surface mineral horizons, where they are likely to compete strongly for PO₄ ^3– available in solution.     

Patterns of Bacterial Diversity Along a Long-Term Mercury-Contaminated Gradient in the Paddy Soils

Mercury (Hg) pollution is usually regarded as an environmental stress in reducing microbial diversity and altering bacterial community structure. However, these results were based on relatively short-term studies, which might obscure the real response of microbial species to Hg contamination. Here, we analysed the bacterial abundance and community composition in paddy soils that have been potentially contaminated by Hg for more than 600 years. Expectedly, the soil Hg pollution significantly influenced the bacterial community structure. However, the bacterial abundance was significantly correlated with the soil organic matter content rather than the total Hg (THg) concentration. The bacterial alpha diversity increased at relatively low levels of THg and methylmercury (MeHg) and subsequently approached a plateau above 4.86 mg kg^?1 THg or 18.62 ng g^?1 MeHg, respectively. Contrasting with the general prediction of decreasing diversity along Hg stress, our results seem to be consistent with the intermediate disturbance hypotheses with the peak biological diversity under intermediate disturbance or stress. This result was partly supported by the inconsistent response of bacterial species to Hg stress. For instance, the relative abundance of Nitrospirae decreased, while that of Gemmatimonadetes increased significantly along the increasing soil THg and MeHg concentrations. In addition, the content of SO₄ ^2?, THg, MeHg and soil depth were the four main factors influencing bacterial community structures based on the canonical correspondence analysis (CCA). Overall, our findings provide novel insight into the distribution patterns of bacterial community along the long-term Hg-contaminated gradient in paddy soils.     

Effect of urban sewage treatment on total and methyl mercury concentrations in effluents

The purpose of this study was to investigate the effect of sewage treatment on total mercury (THg) and methylmercury (MeHg) concentrations in domestic effluents and the contribution of urban sewage treatment facilities to THg and MeHg in rivers. We determined the concentrations of THg and MeHg in unfiltered samples of untreated and treated domestic sewage from the three treatment facilities and receiving river water within the City of Winnipeg. The concentrations of THg in the Red and Assiniboine rivers ranged from 3–31 ng/L. THg was related positively to suspended sediment concentrations in the rivers. The concentrations of MeHg in these rivers were usually 0.2–0.3 ng/L. THg concentrations in raw sewage varied widely, from 2–150 ng/L. Treatment removed an average of 88% of this mercury. MeHg concentrations in raw sewage were 0.5–4.3 ng/L, however, after treatment at two treatment facilities, MeHg was greatly reduced, usually to 0.1–0.4 ng/L. Most treated sewage, therefore, had MeHg concentrations that were similar to levels in the receiving rivers and the effect of discharged effluent was usually a change of about 2% or less on concentrations in the rivers. However, one of the facilities (the West End plant) was discharging higher concentrations of MeHg, up to 2 ng/L, causing calculated increases of up to 11% in the concentration of MeHg in the Assiniboine River.     

Seasonal influences on partitioning and transport of total and methylmercury in rivers from contrasting watersheds

Seven Wisconsin rivers with contrasting, relativelyhomogeneous watershed composition were selected toassess the factors controlling mercury transport.Together, these watersheds allow comparisons ofwetland, forest, urban and agricultural land-uses.Each site was sampled nine times between September1993 and September 1994 to establish seasonalsignatures and transport processes of total mercury(Hg_T) and methylmercury (MeHg). Our resultsclearly show that land use and land cover stronglyinfluence mercury transport processes. Under base-flowconditions, unfiltered MeHg yield varies by a factorof sixteen (12–195 mg km^-2 d^-1), andincreases with the fraction of wetland area in thewatershed. Elevated mercury yields during high floware particle-phase associated in agricultural sites,but filtered-phase associated in wetland sites.Methylmercury represented less than 5% of totalmercury mobilized during the spring thaw across allwatersheds. Autumn MeHg yield was generally 11–15%of Hg_T in wetland influenced watersheds, thougha maximum of 51% was observed. In some cases, singlehigh-flow events may dominate the annual export ofmercury from a watershed. For example, one high-flowevent on the agricultural Rattlesnake Creek had thelargest Hg_T and MeHg yield in the study (107 and2.32 mg km^-2 d^-1, respectively). The mass ofmercury transported downstream by this single eventwas an order of magnitude larger than the eight other(non-event) sampling dates combined. These resultsunderscore the importance of watershed characteristicsand seasonal events on the fate of mercury in freshwater rivers.     

Total mercury and methylmercury dynamics in upland–peatland watersheds during snowmelt

Wetlands, and peatlands in particular, are important sources of methylmercury (MeHg) to susceptible downstream ecosystems and organisms, but very little work has addressed MeHg production and export from peatland-dominated watersheds during the spring snowmelt. Through intensive sampling, hydrograph separation, and mass balance, this study investigated the total mercury (THg) and MeHg fluxes from two upland–peatland watersheds in Minnesota, USA during the 2005 spring snowmelt and proportionally attributed these fluxes to either peatland runoff or upland runoff. Between 26% and 39% of the annual THg flux and 22–23% of the annual MeHg flux occurred during the 12-days snowmelt study period, demonstrating the importance of large hydrological inputs to the annual mercury flux from these watersheds. Upland and peatland runoff were both important sources of THg in watershed export. In contrast to other research, our data show that peatland pore waters were the principal source of MeHg to watershed export during snowmelt. Thus, despite cold and mostly frozen surface conditions during the snowmelt period, peatland pore waters continued to be an important source of MeHg to downstream ecosystems.     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.