Mercury cycling in the water column of a seasonally anoxic urban lake (Onondaga Lake,NY)

Onondaga Lake, New York, is a hypereutrophic, urban lake that was subjected to industrial discharges of mercury (Hg) between 1947 and 1988. Water samples were collected from April through November 1992 and analyzed for filtered and unfiltered total Hg, methylmercury (CH₃Hg), dimethylmercury, ionic Hg, and elemental Hg to characterize the biogeochemical cycling of Hg during water column stratification and hypolimnetic anoxia. In the spring and late fall when the water column was isothermal, total Hg and CH₃Hg concentrations were relatively constant throughout the water column, at approximately 3–7 ng/L and 0.3–1 ng/L, respectively. Through the summer and early fall, CH₃Hg concentrations systematically increased in the deeper waters, reaching peak concentration in August and September. In September 1992, CH₃Hg concentrations increased from 0.3 ng/L in the epilimnion to 10.6 ng/L in the hypolimnion, an increase of nearly 2 orders of magnitude. At the same time, total Hg increased from 6.6 ng/L in surface water to 21.7 ng/L at depth, a 3-fold increase. The spatial and temporal patterns observed for CH₃Hg agree well with manganese, suggesting that CH₃Hg and manganese are controlled by processes of the same or parallel cycles.     

The sulphur-mercury(II) system in natural waters

Sulphur is an essential element for aquatic biosystems, the life processes of which lead to the formation of low molecular weight S compounds in the water. The results of our calculations indicate a pronounced tendency for Hg(II) to form HgS (or HgOHSH) and Hg(SR)₂ complexes in the presence of H₂S and thiols. Likewise, McHg will form CH₃HgSH and CH₃HgSR complexes, but in this case the chloride complex will dominate at low concentrations of H₂S and thiols. In acidic low salinity water, CH₃HgCl is the dominant McHg species at the lowest concentration of sulphide/thiols (0.1 nM), whereas a hundredfold increase of the sulphide/thiol concentration, or an increase of the pH to neutral or slightly alkaline conditions, will result in a total dominance for CH₃HgSH and CH₃HgSR.     

Mercury Methylation Capacity and Removal of Hg Species from Aqueous Medium by Cyanobacteria

Most technologies used for decontamination presents good results for high concentrations, but limitations for lower ones. The desirable Hg concentration in the water is extremely low because of its toxicity. The aims of this study were to evaluate inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) toxicity in Nostoc paludosum, to assess the potential of this cyanobacteria strain to remove these Hg species from aqueous medium and also to investigate Hg methylation by the cyanobacteria. CH₃Hg⁺ determination was performed by gas chromatography-pyrolysis-atomic fluorescence spectrometry in cultures exposed to a concentration of 20 μg L^?1 for 30 days. Both Hg species were removed from the supernatant, ranging from 73 to 96% of Hg²⁺ and from 73 to 95% of CH₃Hg⁺. Ultrastructural Hg²⁺ effects in the cyanobacteria cells investigated by transmission electron microscopy revealed higher production of glycogen, cyanophycin, and intrathylacoidal spaces than the control group. When Hg was added to the culture in the form of CH₃Hg⁺, a decrease corresponding to approximately 60% of the initial concentration due to Hg volatilization was observed. The production of CH₃Hg⁺ by the cyanobacteria was detected in concentrations near the limit of detection (0.0025%) of the bioaccumulated THg. This is an advantage for biotechnological decontamination applications, as CH₃Hg⁺ is a very toxic specie and can be bioaccumulated and biomagnified. The results demonstrated that cyanobacteria cells are an efficient alternative to retain and/or remove Hg at low concentrations and they constitute a potential tool for a “final cleaning” of contaminated waste water.     

The fine scale variability of dissolved methane in surface peat cores

Peat forming wetlands are globally important sources of the greenhouse gas CH₄. The variability of flux recordings from peatlands is however considerable and the distribution of CH₄ below the water table poorly described. Surface peat (0-500 mm below the water table) is responsible for the bulk of emissions and a localised region of intense CH₄ concentration may exist within this region but the structure of peat and presence of gas bubbles make the determination of in situ gas distributions problematic. We report on the in situ distribution and concentrations of CH₄, CO₂ and O₂ in surface bog peat cores using Quadrupole Mass Spectrometry and relate this to peat physical structure. Replicate cores collected in spring and autumn from both hollows and hummocks are used (n = 10). CH₄ recorded in almost every profile was localised in intense peaks reaching concentrations up to 350 μM at depths where O₂ was absent. Each CH₄ peak had a coincident CO₂ peak with a minimum mean ratio of ∼20:1 (CO₂:CH₄) and we found more CH₄ beneath hollows than hummocks. In statistical comparisons CH₄ concentration and distribution differed significantly between profiles for each depth. We demonstrate that variability found within a single core is at least as great as that between cores collected across the bog. The distribution of CH₄ was negatively correlated with bulk density and in some cases the location of roots matched those of intense CH₄ concentration where bubbles had formed and been trapped. Our comparisons suggest variability in gas distribution is caused by a heterogenous peat structure that controls the movement of gas bubbles and contains localised hotspots of gas production. The small and fine root systems of vascular plants on the peatland surface may cause high levels of methanogenic activity in their vicinity and also represent a physical barrier capable of trapping CH₄ bubbles.     

Detection of methane biogenesis in a shallow urban lake in summer

Purpose

Sediments are significant methane (CH₄) sources of the atmosphere. However, the mechanisms of CH₄ generation remain unclear in sediments of shallow urban lakes. The aims of this investigation were to study the characterization of environmental parameters, CH₄ generation, and methanogen populations in Wulongtan Lake, China, which is affected solely by nonpoint pollution.

Materials and methods

The concentrations of CH₄ in the atmosphere and of vertical sediment profiles and the methane flux at the air–water interface were monitored in the summer of 2012. Environmental parameters in the water column and in the vertical sediment profiles were assayed. The activities of cellulose, saccharase, polyphenol oxidase, and urease enzyme and 16S rRNA gene copy numbers of Archaea (ARC), Methanobacteriales (MBT), Methanococcales (MCC), Methanomicrobiales (MMB), Methanosarcinales (MSL), Methanosarcinaceae (MSC), and Methanosaetaceae (MST) were determined in the vertical sediment profiles. The abundance of methyl–coenzyme A reductase (ME) gene was also determined to evaluate the total activities of methanogens.

Results and discussion

High CH₄ concentrations were detected in the atmosphere above the lake, and the mean CH₄ flux at the air–water interface was 6.21 mM m^?2 h^?1. Dissolved oxygen decreased with an increase of water depth. Eh values and CH₄ contents increased, but total nitrogen, water content, and total organic carbon (TOC) decreased with an increase of sediment depth. Cellulose, saccharase, polyphenol oxidase, and urease activities were detected in all sedimentary layers. The copy number of 16S rRNA gene (wet weight) for Archaea reached the highest value in the surface sediment. Copy numbers of ME were higher at 12–33 cm than at 0–6 cm. In general, abundances of MMB, MBT, and MSL were higher than that of MCC in the same sedimentary layer. 16S rRNA gene copy numbers of MST decreased with increasing depth, while MSC was higher at 18–27 cm than that at other sections. These indicate that hydrogenotrophic, aceticlastic, and methylotrophic pathways coexisted in these sediments. Principal component analysis revealed that in the sediments, the level of CH₄ was closely related with several parameters including saccharase, urease, ME, and MBT, while TOC content was related to CEL, MST, ARC, water content, and Eh.

Conclusions

High CH₄ release potential was detected in this shallow urban lake and can be ascribed to the anaerobic aquatic environment, bacterial enzyme activities, and methanogens. The orders MMB, MBT, and MSL were dominant in sediments for CH₄ production. The presence of orders or families of methanogens might be determined by the types of available substrates in lake sediments.     

A comparison of linear and exponential regression for estimating diffusive CH4 fluxes by closed-chambers in peatlands

The closed-chamber method is the most common approach to determine CH₄ fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non-linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge-covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-over-time curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH₄ concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty.     

Total mercury and methylmercury mass balance in an alkaline,hypereutrophic urban lake (Onondaga Lake,NY)

A total mercury (total Hg) and methylmercury (CH₃Hg) mass balance was developed for Onondaga Lake, NY, based on sampling of tributaries, sediments, water column, and biota in 1992. Thein situ flux of total Hg and CH₃Hg from sediments to the overlying water and the rate of net CH₃Hg production in the water column were determined experimentally. Fluxes from atmospheric deposition, groundwater, and volatilization were estimated from limited field data and the literature. Ultraclean sampling and analytical techniques developed specifically for Hg were used. Results indicate that tributaries contribute the majority of total Hg entering the lake (13.6 kg in 1992). Other sources of total Hg included groundwater flux (0.02 kg), atmospheric deposition (0.44 kg), and flux from sediments (0.056 kg). Net sedimentation (11.1 kg), outflow (2.8 kg), and volatilization (0.016 kg) were sinks for total Hg. The two major sources of CH3_Hg were tributaries (0.26 kg) and net CH₃Hg production in the water column (0.60 kg). Flux from sediments accounted for only 0.017 kg CH₃Hg. Net sedimentation (0.47 kg), outflow (0.24 kg), and net uptake, by fish (0.20 kg) were sinks for CH₃Hg. Gross sedimentation of CH₃Hg exceeded net sedimentation by 90%, suggesting that release of CH₃Hg from settling particles is a significant process.     

Methanotrophic bacteria in the rhizosphere of rice microcosms and their effect on porewater methane concentration and methane emission

CH₄ emission from irrigated rice field is one of the major sources in the global budget of atmoshperic CH₄. Rates of CH₄ emission depend on both CH₄ production in anoxic parts of the soil and on CH₄ oxidation at oxic-anoxic interfaces. In the present study we used planted and unplanted rice microcosms and characterized them by numbers of CH₄-oxidizing bacteria (MOB), porewater CH₄ and O₂ concentrations and CH₄ fluxes. Plant roots had a stimulating effect on both the number of total soil bacteria and CH₄-oxidizing bacteria as determined by fluorescein isothiocyanate fluorescent staining and the most probable number technique, respectively. In the rhizosphere and on the root surface CH₄-oxidizing bacteria were enriched during the growth period of tice, while their numbers remained constant in unplanted soils. In the presence of rice plants, the porewater CH₄ concentration was significantly lower, with 0.1–0.4mM CH₄, than in unplanted microcosms, with 0.5–0.7mM CH₄. O₂ was detected at depths of up to 16 mm in planted microcosms, whereas it had disappeared at a depth of 2 mm in the unplanted experiments. CH₄ oxidation was determined as the difference between the CH₄ emission rates under oxic (air) and anoxic (N₂) headspace, and by inhibition experiments with C₂H₂. Flux measurements showed varying oxic emission rates of between 2.5 and 29.0 mmol CH₄m^-2 day^-1. An average of 34% of the anoxically emitted CH₄ was oxidized in the planted microcosms, which was surprisingly constant. The rice rhizosphere appeared to be an important oxic-anoxic interface, significantly reducing CH₄ emission.     

汉江上游水体氮素污染特征

2011年对汉江上游干流进行了6次采样,分析了水体氮素的污染特征。结果表明,汉江上游水体NH₃-N,NO₃-N和NO₂-N的年平均值分别为:1.037,1.751和0.044 mg/L。氮素时间分布曲线表现为:NH₃-N浓度的周年变化趋势为双峰型,呈现:丰水期 >枯水期 >平水期的规律。NO₃-N,NO₂-N浓度的周年变化趋势为单峰型曲线,NO₃-N呈现:丰水期 >枯水期 >平水期的规律,NO₂-N丰水期最高,枯水期和平水期相差不大。汉江上游水体氮素空间分布表现为:NH₃-N,NO₃-N浓度自源头向下游呈现出"低值-升高-降低"的趋势。NH₃-N污染主要来自农村生活污水和畜禽养殖排放物,NO₃-N污染主要来自水土流失。     

Field sampling and analytical intercomparison for mercury and methylmercury determination in natural water

In fall 1993, two intercomparisons were conducted with laboratories performing mercury (Hg) and methylmercury (CH₃Hg) analysis in water. The first, conducted at the La Grande complex in northern Québec, involved two provincial universities and evaluated their respective sampling and analytical protocols. Some samples were spiked with a known amount of CH₃HgCl and HgCl₂ to evaluate the accuracy of the results. Samples preserved with HCl at pH<2 were analyzed up to 45 days after sampling without any change in the total Hg concentration. Filtration at the analytical lab of samples that had been previously frozen resulted in almost complete loss of Hg and CH₃Hg. It is recommended that filtration, if required, should be performed at the sampling point. The second intercomparison examined only analytical performances and included a total of seven laboratories which analyzed two samples from the Experimental Lake Area, in Northwestern Ontario. The standard deviation was 24 and 14% in total Hg for the first five labs and 31 and 21% in CH₃Hg for the first four labs. The average spike recovery in total Hg measured by three labs was 101±9%, but the two labs that performed spike recoveries on CH₃Hg obtained variable results in one case and a constant negative bias in the other case. Standard addition on all samples is recommended to assure the accuracy of CH₃Hg analysis.     

Bench Scale Evaluation of a Chemo-biochemical Process for Desulphurization of Gaseous Streams Containing Hydrogen Sulphide

Gases and suspended particulate matter are sampled and measured in situ at different sites within the city of Madrid to determinethe air pollution in the city. Lead concentrations in air are also determined. The information needed to assess levels of contaminants is obtained combining several analytical techniques,which provide real time concentration data of particles andgases. Particulate matter (PM), SO₂, CO, O₃, NO_x,NO₂, CH₄, total hydrocarbons and Pb were measured. Concentrations of pollutants are averaged over periods of hours,days, months and years. Variations of contaminant concentrationsin the last years are presented. Spatial and temporal distribution follows that of the traffic, the main source of pollution in Madrid City. Some specific measurements were carriedout to understand the influence of traffic emissions at themeasured site. Further, all measured concentrations never exceeded the European Union Directives.     

Faunal effects on the distribution of gamma emitting radionuclides in four forested soils

Brown trouts, Salmo trutta, were exposed via aquarial water to ²⁰³Hg²⁺ or to CH₃ ⁻²⁰³Hg⁺ alone or together with one of the following S-containing substances: sodium dimethyldithiocarbamate (SMC), sodium diethyldithiocarbamate (SEC), potassium ethylxanthate (PEX), sodium isopropylxanthate (SIX), sodium diethyldithiophospbate (SEP), sodium diisopropyldithiophosphate (SIP) or sodium pyridinethione (SPyr). The distribution of the ²⁰³Hg²⁺ and the CH₃−²⁰³Hg⁺ in the fishes were then studied by whole-body autoradiography and gamma spectrometry. The results showed that the examined complexing substances can induce increased uptake of both ²⁰³Hg²⁺ and CH₃−²⁰³Hg⁺ by the fishes: for ²⁰³Hg²⁺ most of the complexing substances induced a similar increase in various tissues; for CH₃−²⁰³Hg⁺ marked variations were seen for different substances. Determinations of chloroform/water partition coefficients showed that the examined substances are able to form lipophilic complexes both with Hg²⁺ and CH₃−Hg⁺. A facilitated penetration of the lipophilic complexes over the membranes of the gills and other tissues may underly the increments of the tissue-levels of the Hg. It is possible that increased uptake of Hg²⁺ and CH₃−Hg⁺ induced by complexing substances of this type may have toxicological implications for fishes and for other aquatic organisms.     

Impact of acidification on the methylmercury cycle of remote seepage lakes

Concentrations of monomethylmercury [CH₃Hg] were measured in the water and seston of five nearly pristine Wisconsin lakes, which span a range of pH from about 4.6 to 7.2. Previous studies had established a clear inverse relationship between [CH₃Hg] in fish and the pH of lakes in this region. Here, we examined the pH dependency of [CH₃Hg] in lake water and explored the partitioning of CH₃Hg between water, seston, and fish as a function of pH. Results indicate that [CH₃Hg] in lake water tends to increase as pH decreases, but that seasonal and spatial variability of [CH₃Hg] in individual lakes confounds a simple analysis of the relationship. The partitioning of CH₃Hg was related only weakly, if at all, to pH. Average partitioning coefficients (log k_d=log (C_p/C_w)) were higher for yearling yellow perch (6.0 to 6.5) than for seston (5.5 to 6.0) but did not vary significantly between lakes. This suggests that acidification has a stronger effect on the supply of CH₃Hg to the ecosystem than on specific rates of uptake by the biota.     

Principal reactions of airborne mercury,ozone and some other microcomponent in pure water or very dilute sulfuric acid

Numerous bubblings of pure H₂O or very dilute H₂SO₄-solutions with ambient air have been undertaken. All of them have shown the formation of HgIIa and HgIIb in the liquid phase (nomenclature according to Brosset, 1987). With increasing air volume such systems clearly approach a steady state. Earlier it has been shown that Hg° in water is oxidized by O₃ primarily to Hg²⁺, (Brosset, l.c., Iverfeldt and Lindqvist, 1986, Munthe, Thesis 1991). However, the attainment of a steady state indicates that a reaction involving a removal of Hg from the solution must exist as well. It has now been anticipated that a substance ‘X’, present in the ambient air, should be responsible, at least partly, for this removal. Starting from this assumption a model has been developed, which seems to be supported by all experimental tests hitherto performed by us. Further it has been tried to establish the chemical nature of ‘X’. So far, some experiments show that ‘X’ may be (CH₃)₂S. Finally it has been shown that HgIIb observed in precipitation is not identical with the corresponding compound formed by bubbling of ambient air at the ground level through it.     

Seasonal variability in the Mercury speciation of Onondaga Lake (New York)

Dissolved and particulate Hg speciation was determined on four occasions in the Spring to Fall interval of 1989, at three depths of the water column of Onondaga Lake, New York; an urban system in which the sediments and fish flesh are contaminated with H_g. Species determined included total Hg (Hg_t), reactive (‘ionic’) Hg (Hg_i), monomethylmercury (CH₃HgX), elemental Hg (Hg°) and dimethylmercury (CH₃)₂Hg). Onondaga Lake was found to contain very high levels of Hg_t (2 to 25 ng L^?1 Hg), Hg_j (0.5 to 10 ng L^?1 Hg), and CH₃HgX (0.3 to 7 ng L^?1 Hg), which generally increased with depth in the lake. These concentrations represent a significant level of contamination, based upon comparisons with other polluted and pristine sites. Elemental Hg levels were typically about 0.05 ng L^?1 and (CH₃)₂Hg was near the limits of detection (?0.001 ng) L^?1 in most samples. The greatest CH₃HgX concentrations in the hypolimnion, as well as the largest gradients of both CH₃HgX and (Hg_t), were observed upon the first onset of stratification, in early summer. These concentrations did not become more pronounced, however, as stratification and H₂S levels in the hypolimnion increased throughout the summer. The very low concentrations of (CH₃)₂Hg in these MeHg and sulfide-rich waters calls into question the belief that CH₃HgX and H₂S will react to yield volatile dimethyl-mercury, which can then escape to the atmosphere by diffusion. Mercury speciation was highly dynamic, indicating active cycling within the lake, and an apparent sensitivity to changes in attendant Iimnological conditions that track the stratification cycle.     

Methane emissions from stems of Fraxinus mandshurica var. japonica trees in a floodplain forest

We measured methane (CH₄) emissions from the stem surfaces of mature Fraxinus mandshurica var. japonica trees in a floodplain forest. Flux measurements were conducted almost monthly from May to October 2005, and positive CH₄ fluxes were detected throughout the study period, including the leafless season. The mean CH₄ flux was 176 and 97 μg CH₄ m⁻² h⁻¹ at the lower (15 cm above the ground) and upper (70 cm above the ground) stem positions, respectively. The CH₄ concentration was lower in soil gas than in ambient air to a depth of at least 40 cm. One possible source of CH₄ emitted from the stems might be the dissolved CH₄ in groundwater; maximum concentrations were 10,000 times higher than atmospheric CH₄ concentrations. Our results suggest that CH₄ transport from the submerged soil layer to the atmosphere may occur through internal air spaces in tree bodies.     

More evidence that anaerobic oxidation of methane is prevalent in soils: Is it time to upgrade our biogeochemical models?

Estimating future fluxes of CH₄ between land and atmosphere requires well-conceived process-based biogeochemical models. Current models do not represent the anaerobic oxidation of methane (AOM) in land surface soils, in spite of increasing evidence that this process is widespread. Our objective was to determine whether AOM, or potential AOM, commonly occurs in 20 hydromorphic soils spanning a wide range of chemical properties. Bulk soil samples were collected under shallow water near the shoreline of 15 recently drained fish ponds in southern Bohemia (Czech Republic), as well as from below the water table at 3 peatland locations in northeast Scotland and 2 acid sulfate soils on the southern coast of Finland. Each soil slurry was incubated under both oxic and anoxic conditions, with or without the addition of alternative electron acceptors (SO₄²⁻ and NO₃⁻) or H₂PO₄⁻. Here, “oxic” and “anoxic” conditions refer to anoxic soil respectively incubated in a headspace containing air or argon. Using the isotope dilution method, we determined the gross production and oxidation rates of CH₄ after 2 days incubation under oxic headspace conditions, and after 2, 21 and 60 days incubation under anoxic conditions. Large differences in net CH₄ fluxes were observed between soil types and between incubation conditions. AOM was detected in each of the 20 bulk soil samples, which spanned >6 pH units and 2 orders of magnitude in organic C content. Significant positive relationships were found between AOM and gross CH₄ production rates under anoxic conditions, resulting in AOM rates that were sometimes higher than CH₄ oxidation rates under oxic headspace conditions. There was no relationship between net and gross CH₄ production rates, such that 2 soil types could display similar low net rates, yet conceal very large differences in gross rates. The effects of alternative electron acceptors on AOM were idiosyncratic and resulted in no net trend. We did find, however, a negative effect of SO₄²⁻ and H₂PO₄⁻ on gross CH₄ production rates under anoxic and oxic conditions respectively. Under oxic headspace conditions, CH₄ oxidation was related to soil organic C content. Taken collectively, our results suggest that AOM, or potential AOM, is prevalent over a wide range of soil types, that AOM may contribute substantially to CH₄ oxidation in soils, and that AOM in soils should be integrated to current process-based CH₄ cycling models.     

National-scale estimation of methane emission from paddy fields in Japan: Database construction and upscaling using a process-based biogeochemistry model

We have estimated methane (CH₄) emission from total rice (Oryza sativa L.) paddies in Japan by means of a process-based biogeochemistry model, DeNitrification-DeComposition(DNDC)-Rice, combined with a geographic information system (GIS) database of climate, soil and farming practices. In the GIS database, 2 million ha of rice paddies were divided into 17,408 units according to 136 climate areas, 16 soil types, four classes of drainage rate and two classes of groundwater level, to simulate CH₄ flux from each of the units applying the DNDC-Rice model. As a result, the national-scale CH₄ emission in 1990 was estimated to be 216 Gg carbon (C), 13% lower than a previous inventory estimated by the Tier 2 method. By our Tier 3 approach, a relatively higher CH₄ flux was estimated from eastern regions than from western regions of Japan, presumably due to the differences in climate and water management. Sensitivity analysis and uncertainty assessment indicated that it is important to account for the heterogeneity in soil properties such as field water capacity, iron (Fe) concentration and drainage rate, in order to reduce the uncertainty in regional estimates.     

Diffusion probe for gas sampling in undisturbed soil

Soil‐atmosphere fluxes of trace gases such as methane (CH₄) and nitrous oxide (N₂O) are determined by complex interactions between biological activity and soil conditions. Soil gas concentration profiles may, in combination with other information about soil conditions, help us to understand emission controls. This paper describes a simple and robust diffusion probe for soil gas sampling as part of flux monitoring programmes. It can be deployed with minimum disturbance of in‐situ conditions, and also at sites with a high or fluctuating water table. Separate probes are used for each sampling depth, in this study ranging from 5 to 100 cm. The probe has a 10‐ml diffusion cell with a 3‐mm diameter opening covered by a 0.5‐mm silicone membrane. At sampling the diffusion cell is flushed with 10 ml N₂ containing 50 µl l^?1 ethylene (C₂H₄) as a tracer; tracer recovery is used to calculate sample concentrations. Ethylene is immediately removed by flushing with unamended N₂. Equations are presented to correct for dead volumes of connecting tubing and valves. Laboratory tests evaluated recovery of CH₄, N₂O and carbon dioxide (CO₂), removal of C₂H₄ and equilibration of CH₄, N₂O and CO₂ in air and water. Field tests on peat soils used for grazing showed soil gas concentrations of CH₄ and N₂O as influenced by topography, site conditions and season. The applicability of the diffusion probe for trace gas monitoring is discussed.     

Presence of Eriophorum scheuchzeri enhances substrate availability and methane emission in an Arctic wetland

Here we present results from a field experiment in an Arctic wetland situated in Zackenberg, NE Greenland. During one growing season we investigated how dominance of the sedge Eriophorum scheuchzeri affected the below-ground concentrations of low molecular weight carbon compounds (LMWOC) and the fluxes of CO₂ and CH₄ in comparison to dominance of other sedges (Carex stans and Dupontia psilosantha). Three groups of LMWOC were analysed using liquid chromatography-ionspray tandem mass spectrometry, i.e., organic acids (OAs), amino acids (AAs) and simple carbohydrates (CHs). To identify the effect of plant composition the experiments were carried out in a continuous fen area with very little between species variation in environmental conditions, e.g., water-table and active layer thickness and soil temperature. The pool of labile LMWOC compounds in this Arctic fen was dominated by OAs, constituting between 75 and 83% of the total pore water pool of OAs, CHs and AAs. The dominant OA was acetic acid, an easily available substrate for methanogens, which constituted ≥85% of the OA pool. We estimated that the concentration of acetic acid found in pore water would support 2–2.5 h of CH₄ flux and an additional continuous input of acetic acid through root exudation that would support 1.3–1.5 h of CH₄ flux. Thus, the results clearly points to the importance of a continuous input for acetoclastic methanogenesis to be sustainable. Additionally, Eriophorum had a very strong effect on parts of the carbon cycle in the Arctic fen. The mean seasonal CH₄ flux was twice as high in Eriophorum dominated plots, most likely due to a 1.7 times higher concentration of OAs in these plots. Further, the ecosystem respiration was 1.3 times higher in Eriophorum dominated plots. In conclusion, the results offer further support to the importance of certain vascular plant species for the carbon cycle of wetland ecosystems.