Atmospheric CH4 Oxidation Research Articles

Endogenous methanogenesis stimulates oxidation of atmospheric CH(4) in alpine tundra soil.

Experiments were done to test the hypothesis that atmospheric CH(4) oxidizers in a well-drained alpine tundra soil are supported by CH(4) production from anaerobic microsites in the soil. Soil was subjected to 22 days of anaerobic conditions with elevated H(2) and CO(2) in order to stimulate methanogenesis. This treatment stimulated subsequent atmospheric CH(4) consumption, probably by increasing soil methanogenesis. After removal from anaerobic conditions, soils emitted CH(4) for up to 6 h, then oxidized atmospheric CH(4) at 111 (+/- 5.7) pmol (g dry weight)(-1) h(-1), which was more than 3 times the rate of control soils. Further supporting our hypothesis, additions of lumazine, a highly specific inhibitor of methanogenesis, prevented the stimulation of atmospheric CH(4) oxidation by the anaerobic treatment. The method used to create anaerobic conditions with elevated H(2) and CO(2) also elevated headspace CH(4) concentrations. However, elevated CH(4) concentrations under aerobic conditions did not stimulate CH(4) oxidation as much as preexposure to H(2) and CO(2) under anaerobic conditions. Anaerobic conditions created by N(2) flushing did not stimulate atmospheric CH4 oxidation, probably because N2 flushing inhibited methanogenesis by removing necessary precursors for methane production. We conclude that anaerobic conditions with elevated H(2) and CO(2) stimulate atmospheric CH(4) oxidation in this dry alpine tundra soil by increasing endogenous CH(4) production. This effect was prevented by inhibiting methanogenesis, indicating the importance of endogenous CH(4) production in a CH(4-) consuming soil.

Effect of moisture, texture and aggregate size of paddy soil on production and consumption of CH4

Laboratory experiments were conducted to find out under which conditions the soil from Italian rice fields could change from a source into a sink of atmospheric CH4. Moist (30% H2O=68% of the maximum water holding capacity (whc)) rice field soil oxidized CH4 with biphasic kinetics, exhibiting both a low (145ppmv CH4) and a high (20,200ppmv CH4) Km value and Vmax values of 16.8 and 839nmol gdw−1h−1, respectively. The activity with the low Km allowed the oxidation of atmospheric CH4. Uptake rates of high CH4 concentrations (16.5% v/v) and of O2 linearly decreased with aggregate size of soil between 2 and 10mm. Atmospheric CH4 (1.8 ppmv) was consumed in soil aggregates <6mm, but soil aggregates >6mm released CH4 into the atmosphere. Similarly, net uptake of atmospheric CH4 turned into net release of CH4 when the soil moisture was decreased below a water content of about 20% whc. The uptake rate of atmospheric CH4 increased threefold when the soil was amended with sterile quartz sand. Flooded microcosms with non-amended and quartz-amended soil emitted CH4 into the atmosphere. The CH4 emission rate increased when the flux was measured under an atmosphere of N2 instead of air, indicating that 30–99% of the produced CH4 was oxidized in the oxic soil surface layer. Removal of the flood water resulted in increase of CH4 emission rates until a water content of about 75–82% whc was reached, and subsequently in a rapid decrease. However, the soil microcosms never showed net uptake of atmospheric CH4. Our results show that the microorganisms consuming atmospheric CH4 were inactivated at an earlier stage of drainage than the microorganisms producing CH4, irrespective of the soil porosity which was adjusted by addition of quartz sand. Hence, it is unlikely that the Italian rice fields can act as a net sink for atmospheric CH4 even when drained.

Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils

The short-term (24 h) and medium-term (30 day) influence of N salts (NH4Cl, NaNO3 and NaNO2) and a non-N salt (NaCl) on first-order rate constants, k (h–1) and thresholds (CTh) for atmospheric CH4 oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g–1 dry weight (dw) soil. Additions of NH4Cl, NaNO3 and NaCl to 0.5 μmol g–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH4Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH4+-N (≤1 μmol g–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH4 oxidation by exogenous NH4+-N or NH4+-N released through cation exchange. The decrease in k was consistently less for NaNO3 than for NH4Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl– counter-anion. Thresholds for CH4 oxidation were less sensitive to salt addition than k for these three salts, as significant increases in CTh relative to controls were only observed at concentrations ≥1.0 μmol g–1 dw soil. Both soils were more sensitive to NaNO2 than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO2 g–1 dw soil that was clearly attributable to NO2–. Soils showed no recovery from NaCl, NH4+-N or NaNO3 addition with respect to atmospheric CH4 oxidation after 30 days. However, soils amended with NaNO2 to 1.0 μmol NaNO2 g–1 dw showed values of k that were not significantly different from controls. Recovery of CH4-oxidizing ability was due to complete oxidation of NO2–-N to NO3–-N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH4 oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH4 budget.

Molecular analyses of novel methanotrophic communities in forest soil that oxidize atmospheric methane.

Forest and other upland soils are important sinks for atmospheric CH(4), consuming 20 to 60 Tg of CH(4) per year. Consumption of atmospheric CH(4) by soil is a microbiological process. However, little is known about the methanotrophic bacterial community in forest soils. We measured vertical profiles of atmospheric CH(4) oxidation rates in a German forest soil and characterized the methanotrophic populations by PCR and denaturing gradient gel electrophoresis (DGGE) with primer sets targeting the pmoA gene, coding for the alpha subunit of the particulate methane monooxygenase, and the small-subunit rRNA gene (SSU rDNA) of all life. The forest soil was a sink for atmospheric CH(4) in situ and in vitro at all times. In winter, atmospheric CH(4) was oxidized in a well-defined subsurface soil layer (6 to 14 cm deep), whereas in summer, the complete soil core was active (0 cm to 26 cm deep). The content of total extractable DNA was about 10-fold higher in summer than in winter. It decreased with soil depth (0 to 28 cm deep) from about 40 to 1 microg DNA per g (dry weight) of soil. The PCR product concentration of SSU rDNA of all life was constant both in winter and in summer. However, the PCR product concentration of pmoA changed with depth and season. pmoA was detected only in soil layers with active CH(4) oxidation, i.e., 6 to 16 cm deep in winter and throughout the soil core in summer. The same methanotrophic populations were present in winter and summer. Layers with high CH(4) consumption rates also exhibited more bands of pmoA in DGGE, indicating that high CH(4) oxidation activity was positively correlated with the number of methanotrophic populations present. The pmoA sequences derived from excised DGGE bands were only distantly related to those of known methanotrophs, indicating the existence of unknown methanotrophs involved in atmospheric CH(4) consumption.

Effect of nitrogen fertilization on atmospheric methane oxidation in boreal forest soils

Field plots of aspen and black spruce in the Alaskan boreal forest were fertilized repeatedly with nitrogen during the 1993 summer growing season, and weekly determinations of the influence of fertilization on atmospheric CH4 oxidation were made with static chambers. Repeated fertilization with (NH4)2SO4 solution or nutrient media used to culture methanotrophic or nitrifying bacteria gave a total addition of 140 or 580 kg N ha−1. Time-integrated CH4 oxidation was not significantly different in fertilized soils versus watered controls because CH4 oxidation was localized in a subsurface soil zone that was probably not penetrated by surface-applied aqueous phase fertilizer. Insensitivity of CH4 oxidation by these soils to a high rate of N fertilization and the low current rate of atmospheric N deposition suggest that future increases in atmospheric N deposition will not alter the sink strength of high latitude boreal forest soils in the atmospheric CH4 budget.

Short-term effects of nitrogen on methane oxidation in soils

The short-term effects of N addition on CH4 oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH4, the arable soil oxidised CH4 at a rate of 0.013 μg CH4 kg–1 h–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH4)2SO4 caused an immediate reduction in the rate of atmospheric CH4 oxidation approximately in inverse proportion to the amount of NH4 + added. The addition of KNO3 caused no change in the rate of CH4 oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH4 oxidation had decreased from 0.22 μg CH4 kg–1 h–1 to 0.13 μg CH4 kg–1 h–1 in soil treated with the equivalent of 192 kg N ha–1. A 15N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH4 oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg–1 day–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg–1 day–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH4 oxidation at this site must be methanotroph mediated.

Low-concentration kinetics of atmospheric CH4 oxidation in soil and mechanism of NH4+ inhibition

NH4+ inhibition kinetics for CH4 oxidation were examined at near-atmospheric CH4 concentrations in three upland forest soils. Whether NH4+-independent salt effects could be neutralized by adding nonammoniacal salts to control samples in lieu of deionized water was also investigated. Because the levels of exchangeable endogenous NH4+ were very low in the three soils, desorption of endogenous NH4+ was not a significant factor in this study. The Km(app) values for water-treated controls were 9.8, 22, and 57 nM for temperate pine, temperate hardwood, and birch taiga soils, respectively. At CH4 concentrations of </=15 &mgr;l liter-1, oxidation followed first-order kinetics in the fine-textured taiga soil, whereas the coarse-textured temperate soils exhibited Michaelis-Menten kinetics. Compared to water controls, the Km(app) values in the temperate soils increased in the presence of NH4+ salts, whereas the Vmax(app) values decreased substantially, indicating that there was a mixture of competitive and noncompetitive inhibition mechanisms for whole NH4+ salts. Compared to the corresponding K+ salt controls, the Km(app) values for NH4+ salts increased substantially, whereas the Vmax(app) values remained virtually unchanged, indicating that NH4+ acted by competitive inhibition. Nonammoniacal salts caused inhibition to increase with increasing CH4 concentrations in all three soils. In the birch taiga soil, this trend occurred with both NH4+ and K+ salts, and the slope of the increase was not affected by the addition of NH4+. Hence, the increase in inhibition resulted from an NH4+-independent mechanism. These results show that NH4+ inhibition of atmospheric CH4 oxidation resulted from enzymatic substrate competition and that additional inhibition that was not competitive resulted from a general salt effect that was independent of NH4+.

Wetting stimulates atmospheric CH4 oxidation by alpine soil

Studies were done to assess the effects of soil moisture manipulations on CH4 oxidation in soils from a dry alpine tundra site. When water was added to these soils there was a stimulation of CH4 oxidation. This stimulation of CH4 oxidation took time to develop. One to three days after water additions no stimulation was observed. Nine days after rewetting, CH4 oxidation was greatly stimulated. This time delay suggests that methanotrophs grew in response to water additions, or that they take a long time to recover from metabolically inactive resting stages.

Tropospheric chemistry: A global perspective

A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.