Ppmv CH Research Articles

Activity of Type I Methanotrophs Dominates under High Methane Concentration: Methanotrophic Activity in Slurry Surface Crusts as Influenced by Methane, Oxygen, and Inorganic Nitrogen.

Livestock slurry is a major source of atmospheric methane (CH), but surface crusts harboring methane-oxidizing bacteria (MOB) could mediate against CH emissions. This study examined conditions for CH oxidation by in situ measurements of oxygen (O) and nitrous oxide (NO), as a proxy for inorganic N transformations, in intact crusts using microsensors. This was combined with laboratory incubations of crust material to investigate the effects of O, CH, and inorganic N on CH oxidation, using CH to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O depletion. With 10 ppmv CH there was no O limitation on CH oxidation at O concentrations as low as 2%, whereas CH oxidation at 10 ppmv CH was reduced at ≤5% O. As hypothesized, CH oxidation was in general inhibited by inorganic N, especially NO, and there was an interaction between N inhibition and O limitation at 10 ppmv CH, as indicated by consistently stronger inhibition of CH oxidation by NH and NO at 3% compared with 20% O. Recovery of C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high-concentration CH oxidation. Given the structural heterogeneity of crusts, CH oxidation activity likely varies spatially as constrained by the combined effects of CH, O, and inorganic N availability in microsites.

Inhibition of Methane Oxidation in a Slurry Surface Crust by Inorganic Nitrogen: An Incubation Study

Livestock slurry is an important source of methane (CH). However, depending on the dry matter content of the slurry, a floating crust may form where methane-oxidizing bacteria (MOB) and CH oxidation activity have been found, suggesting that surface crusts may reduce CH emissions from slurry. However, it is not known how MOB in this environment interact with inorganic nitrogen (N). We studied inhibitory effects of ammonium (NH), nitrate (NO), and nitrite (NO) on potential CH oxidation in a cattle slurry surface crust. At headspace concentrations of 100 and 10,000 ppmv, CH oxidation was assayed at salt concentrations up to 500 mM. First-order rate constants were used to evaluate the strength of inhibition. Nitrite was the most potent inhibitor, reducing methanotrophic activity by up to 70% at only 1 mM NO. Methane-oxidizing bacteria were least sensitive to NO, tolerating up to 30 mM NO at 100 ppmv CH and 50 mM NO at 10,000 ppmv CH without any decline in activity. The inhibition by NH increased progressively, and no range of tolerance was observed. Methane concentrations of 10,000 ppmv resulted in 50- to 100-fold higher specific CH uptake rates than 100 ppmv CH but did not change the inhibition patterns of N salts. In slurry surface crusts, MOB maintained activity at higher concentrations of NH and NO than reported for MOB in soils and sediments, possibly showing adaptation to high N concentrations in the slurry environment. Yet it appears that the effectiveness of surface crusts as CH sinks will depend on inorganic N concentrations.

The Formation of Sulfate and Elemental Sulfur Aerosols under Varying Laboratory Conditions: Implications for Early Earth

The presence of sulfur mass-independent fractionation (S-MIF) in sediments more than 2.45 × 10(9) years old is thought to be evidence for an early anoxic atmosphere. Photolysis of sulfur dioxide (SO(2)) by UV light with λ < 220 nm has been shown in models and some initial laboratory studies to create a S-MIF; however, sulfur must leave the atmosphere in at least two chemically different forms to preserve any S-MIF signature. Two commonly cited examples of chemically different sulfur species that could have exited the atmosphere are elemental sulfur (S(8)) and sulfuric acid (H(2)SO(4)) aerosols. Here, we use real-time aerosol mass spectrometry to directly detect the sulfur-containing aerosols formed when SO(2) either photolyzes at wavelengths from 115 to 400 nm, to simulate the UV solar spectrum, or interacts with high-energy electrons, to simulate lightning. We found that sulfur-containing aerosols form under all laboratory conditions. Further, the addition of a reducing gas, in our experiments hydrogen (H(2)) or methane (CH(4)), increased the formation of S(8). With UV photolysis, formation of S(8) aerosols is highly dependent on the initial SO(2) pressure; and S(8) is only formed at a 2% SO(2) mixing ratio and greater in the absence of a reductant, and at a 0.2% SO(2) mixing ratio and greater in the presence of 1000 ppmv CH(4). We also found that organosulfur compounds are formed from the photolysis of CH(4) and moderate amounts of SO(2). The implications for sulfur aerosols on early Earth are discussed. Key Words: S-MIF-Archean atmosphere-Early Earth-Sulfur aerosols.

Methane oxidation in soil profiles of Dutch and Finnish coniferous forests with different soil texture and atmospheric nitrogen deposition

We studied methane oxidation capacity in soil profiles of Dutch and Finnish coniferous forests. The Finnish sites ( n = 9) had nitrogen depositions from 3 to 36 kg N ha −1 a −1. The deposition of N on the Dutch sites ( n = 13) was higher ranging from 50 to 92 kg N ha −1 a −1. The Dutch sites had also limed counterparts. Methane oxidation rates were determined by incubating soil samples in the laboratory at + 15°C with 10 μl CH 4 l −1 (10 ppmv CH 4). In general, CH 4 oxidation rates were highest in the uppermost mineral layers. The average CH 4 oxidation rate in the Finnish mineral soils was three times higher than that in the Dutch soils. The litter layers did not oxidize CH 4. In the Netherlands all organic horizons had a negligible capacity to oxidize CH 4. However, some Finnish organic horizons showed high CH 4 oxidation capacity. In the Netherlands, in contrast to Finland, there were some soil profiles lacking CH 4 oxidation. Higher contents of nitrate and ammonium, as well as greater production of nitrous oxide (N 2O) and lower production of carbon dioxide in the Dutch than in the Finnish forest soils reflected the high N deposition rate in the Netherlands. Not only the N deposition, but also the highly sorted soil texture (fine sand) with low amounts of both coarse and fine particles is an important reason for the low CH 4 oxidation in the Dutch soils. The proportions of fine and coarse particles, both well represented in moraine soils typical in northern Europe, correlated positively with the CH 4 oxidation. Fine particles provide a good surface for microbial growth. Coarse particles, on the other hand, enhance diffusion of CH 4 and oxygen into the soil. Methane oxidation in the Dutch mineral soils was slightly enhanced by liming.

Methyl fluoride, an inhibitor of methane oxidation and methane production

Abstract Methyl fluoride (CH 3 F) has been reported to inhibit effectively aerobic CH 4 oxidation, while not affecting methanogenesis. Currently it is applied in a variety of ecosystems, where oxidation and production of CH 4 take place simultaneously. We tested the effects of CH 3 F on both processes in a flooded soil, in wetland plants (rice, Oryza sativa , and cottontail, Typha latifolia ), and in a microbial mat. CH 4 emission from rice microcosms increased after treatment with up to 1.9% CH 3 F, but decreased to less than the initial rate after 3.3% CH 3 F had been added. In anoxic incubations of rice roots and Typha we observed in vitro an instantaneous methanogenesis that in rice was inhibited by CH 3 F. Cottontail-associated methanogenesis was not affected by CH 3 F. In anoxic slurries of ricefield soil CH 4 production was inhibited by CH 3 F. Even at concentrations as low as 1000 ppmv CH 3 F (≈ 40 μM) methanogenesis was reduced by about 75% as compared to the control without the inhibitor. Methanogenesis could be recovered partly when CH 3 F was flushed out with N 2 . In soil slurries with CH 3 F methanogenesis could be stimulated by addition of formate, but not by acetate. Acetate accumulated in soil slurries with CH 3 F to about the same amount as did CH 4 in the control experiment without inhibitor. Methanogenesis in the hypersaline microbial mat is probably driven by methylated amines; it was not affected by CH 3 F. Hence, measurements of aerobic CH 4 oxidation may be biased if acetoclastic methanogenesis plays a significant role, and if CH 4 production and oxidation zones are closely coupled. This is to be expected especially in freshwater sediments, wetlands and ricefields.

Methyl fluoride, an inhibitor of methane oxidation and methane production

Methyl fluoride (CH 3F) has been reported to inhibit effectively aerobic CH 4 oxidation, while not affecting methanogenesis. Currently it is applied in a variety of ecosystems, where oxidation and production of CH 4 take place simultaneously. We tested the effects of CH 3F on both processes in a flooded soil, in wetland plants (rice, Oryza sativa, and cottontail, Typha latifolia), and in a microbial mat. CH 4 emission from rice microcosms increased after treatment with up to 1.9% CH 3F, but decreased to less than the initial rate after 3.3% CH 3F had been added. In anoxic incubations of rice roots and Typha we observed in vitro an instantaneous methanogenesis that in rice was inhibited by CH 3F. Cottontail-associated methanogenesis was not affected by CH 3F. In anoxic slurries of ricefield soil CH 4 production was inhibited by CH 3F. Even at concentrations as low as 1000 ppmv CH 3F (≈ 40 μM) methanogenesis was reduced by about 75% as compared to the control without the inhibitor. Methanogenesis could be recovered partly when CH 3F was flushed out with N 2. In soil slurries with CH 3F methanogenesis could be stimulated by addition of formate, but not by acetate. Acetate accumulated in soil slurries with CH 3F to about the same amount as did CH 4 in the control experiment without inhibitor. Methanogenesis in the hypersaline microbial mat is probably driven by methylated amines; it was not affected by CH 3F. Hence, measurements of aerobic CH 4 oxidation may be biased if acetoclastic methanogenesis plays a significant role, and if CH 4 production and oxidation zones are closely coupled. This is to be expected especially in freshwater sediments, wetlands and ricefields.

Subchronic Toxicity of 4-Vinylcyclohexene in Rats and Mice by Inhalation Exposure

This study was conducted to evaluate the subchronic toxicity of 4-vinylcyclohexene (VCH). Male and female Sprague–Dawley rats and B6C3F1mice were exposed by inhalation to VCH 6 hr/day, 5 days/week for 13 weeks. Rats were exposed to 0, 250, 1000, or 1500 ppm, and mice were exposed to 0, 50, 250, or 1000 ppm. In addition, another group of rats and mice was exposed to 1000 ppm butadiene so that a comparison could be made between the two compounds. Exposure to 1000 ppm VCH resulted in deaths of all male mice and 5/10 female mice on Test Days 11 or 12. Three additional female mice exposed to 1000 ppm VCH died prior to study completion. The most notable compound-related clinical sign was lethargy observed in the 1500 ppm VCH-exposed rats and 1000 ppm VCH-exposed mice. Male rats exposed to 1500 ppm VCH had significantly lower body weights compared to controls, and male and female rats in the 1500 ppm group had significantly lower body weight gains. None of the VCH-exposed animals or butadiene-exposed rats showed any compound-related hematological effects. However, mice exposed to 1000 ppm butadiene exhibited mild macrocytic anemia. Clinical chemistry evaluation and urinalysis showed no compound-related effects in rats exposed to either VCH or butadiene. Male and female rats exposed to 1000 or 1500 ppm VCH or 1000 ppm butadiene had increased absolute and/or relative liver weights, and male rats in these same exposure groups had increased relative kidney weights. Microscopically, increased accumulation of hyaline droplets was observed in the kidneys of male rats from all VCH exposure groups. Although compound-related, the droplets were not accompanied by cytotoxicity. In mice, the most notable adverse histopathological effect was ovarian atrophy in females exposed to 1000 ppm VCH or 1000 ppm butadiene. The atrophy was slightly more severe in the VCH-exposed females than in the butadiene-exposed females. There were no other compound-related pathological effects in male or female mice exposed to VCH. Additionally, butadiene-exposed male mice had decreased testicular weights, accompanied by slight testicular degeneration and atrophy. For VCH exposure, the no-observed-adverse-effect-level is 1000 ppm for rats based on lethargy and lowered body weights and 250 ppm for mice based on mortality and ovarian atrophy.

Subchronic Toxicity of 4-Vinylcyclohexene in Rats and Mice by Inhalation Exposure

This study was conducted to evaluate the subchronic toxicity of 4-vinylcyclohexene (VCH). Male and female Sprague–Dawley rats and B6C3F1 mice were exposed by inhalation to VCH 6 hr/ day, 5 days/week for 13 weeks. Rats were exposed to 0, 250, 1000, or 1500 ppm, and mice were exposed to 0, 50, 250, or 1000 ppm. In addition, another group of rats and mice was exposed to 1000 ppm butadiene so that a comparison could be made between the two compounds. Exposure to 1000 ppm VCH resulted in deaths of all male mice and 5/10 female mice on Test Days 11 or 12. Three additional female mice exposed to 1000 ppm VCH died prior to study completion. The most notable compound-related clinical sign was lethargy observed in the 1500 ppm VCH-exposed rats and 1000 ppm VCH-exposed mice. Male rats exposed to 1500 ppm VCH had significantly lower body weights compared to controls, and male and female rats in the 1500 ppm group had signifi cantly lower body weight gains. None of the VCH-exposed animals or butadiene-exposed rats showed any compound-related hemato logical effects. However, mice exposed to 1000 ppm butadiene exhibited mild macrocytic anemia. Clinical chemistry evaluation and urinalysis showed no compound-related effects in rats exposed to either VCH or butadiene. Male and female rats exposed to 1000 or 1500 ppm VCH or 1000 ppm butadiene had increased absolute and/or relative liver weights, and male rats in these same exposure groups had increased relative kidney weights. Microscopically, in creased accumulation of hyaline droplets was observed in the kid neys of male rats from all VCH exposure groups. Although compound–related, the droplets were not accompanied by cytotoxicity. In mice, the most notable adverse histopathological effect was ovarian atrophy in females exposed to 1000 ppm VCH or 1000 ppm butadiene. The atrophy was slightly more severe in the VCH exposed females than in the butadiene–exposed females. There were no other compound–related pathological effects in male or female mice exposed to VCH. Additionally, butadiene–exposed male mice had decreased testicular weights, accompanied by slight testicular degeneration and atrophy. For VCH exposure, the no–observed-adverse–effect–level is 1000 ppm for rats based on leth argy and lowered body weights and 250 ppm for mice based on mortality and ovarian atrophy.

High-precision direct measurements of ^13CH_4/^12CH_4 and ^12CH_3D/^12CH_4 ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer

Measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane (CH(4)) sources provide important information about the global CH(4) budget as well as about CH(4) production and consumption processes occurring within the various sources. As an alternative to the conventional mass spectrometer (MS) technique, which requires conversion of CH(4) to CO(2) and H(2), we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Δδ(13)C = 0.44‰ and ΔδD = 5.1‰. In the present system the minimum mixing ratios required are 50 parts in 10(6) by volume (ppmv) CH(4) (sample size 2 µmol CH(4)) for direct δ(13)C measurements and 2000 ppmv (sample size 80 µmol CH(4)) for direct δD measurements. These mixing-ratio limits are adequate for most CH(4) source characterization studies without requiring sample preconcentration.

Kinetics of methane oxidation in oxic soils

We investigated the kinetics of CH 4 oxidation in three different soils (cutivated cambisol, forest cambisol, meadow cambisol) under oxic conditions. CH 4 oxidation was apparently due to aerobic microorganisms which were inhibited by autoclaving, and followed a Michaelis-Menten kinetic with a half saturation constant (Km) of about 20–45 ppmv and a maximum oxidation rate (Vmax) of about 0.6–3.6 nmol CH 4 h −1g −1 dry weight soil. The determined Km values were much smaller than those known from previous experiments with soil or methanotrophic bacteria. Extrapolation of the linear part of the Michaelis-Menten kinetic (range of first- order kinetics) to zero CH 4 oxidation usually resulted in threshold mixing ratios (Tha) of about 0.2–0.3 ppmv CH 4, i.e. lower than the atmospheric mixing ratio. CH 4 was oxidized even below these threshold mixing ratios, but at a rate that was much lower than expected from the first-order rate constant of CH 4 oxidation. These results show that oxidation of atmospheric CH 4 in soil is not a simple first-order reaction emphasizing careful interpretation of kinetic data with respect to field conditions.

Kinetics of CH 4 oxidation in oxic soils exposed to ambient air or high CH 4 mixing ratios

The kinetic parameters of CH 4 oxidation ( K m, V max, apparent threshold = Th a) were measured using different oxic soils (cultivated cambisol, forest luvisol, meadow cambisol, paddy soil) both in a fresh state and after 3 weeks preincubation under high CH 4 mixing ratios (20%). The preincubation resulted in an increase of the most probable number of methanotrophic bacteria. In fresh soils, CH 4 oxidation followed Michaelis-Menten kinetics with a low K m (30–51 nM CH 4), low V max (0.7–3.6 nmol CH 4 h −1g −1dw soil), and low Th a (0.2–2.7 ppmv CH 4). In preincubated soils, CH 4 oxidation exhibited biphasic kinetics in which two different CH 4 saturation curves were apparently superimposed on each other. Eadie-Hofstee plots of the data showed two activities with different kinetic parameters: a high-affinity activity with low K m (13–470 nM CH 4), low V max (2.1–150.0 nmol CH 4 h −1g −1dw) and low Th a (0.3–4.1 ppmv CH 4) being similar to the kinetic parameters in fresh soils; and a low-affinity activity with high K m (1740–27 900 nM CH 4), high V max (270–3 690 nmol CH 4 h −1g −1dw) and high Th a (11–45 ppmv CH 4) being similar to the kinetic parameters known from methanotrophic bacteria. The low-affinity activity was also observed in a soil over a deep natural gas source which was permanently exposed to high CH 4 mizing ratios (>5% CH 4). Bacteria culturable as methanotrophs are probably responsible for the low-affinity activity which is typical for the soils exposed to high CH 4 mixing ratios. However, the bacteria responsible for the high-affinity activity are still unknown. This activity is typical for the soils exposed to only ambient CH 4 mixing ratios. Both high- and low-affinity activities were inhibited by autoclaving and by acetylene.