Ecology of aerobic methanotrophs in controlling methane fluxes from wetlands

Methane (CH₄) is a critical greenhouse gas, with wetlands and flooded rice soils contributing >38% of the global emissions of CH₄. A key process that offsets CH₄ production is biological oxidation by methanotrophs that can consume as much as 90% of the CH₄ produced in aerobic soils. The objective was to review the literature on the biochemical pathways, ecology of methanotrophs, research methods, and environmental and management controls on CH₄ oxidation in wetlands. Methanotrophs have been extensively studied using phospholipid fatty acids (PLFA) but much less so using nucleic acid analysis or with the powerful stable-isotope probing technique of ¹³C-PLFA analysis. Of the environmental factors that affect CH₄ oxidation, temperature, water content and redox potential are the most important whereas a wide range of soil pH enables oxidation. Methane oxidation varies with season as functions of temperature and plant community shifts (changes with growth stage of C input chemistry and oxygenation from aquatic rhizospheres). Indirect evidence suggests there is greater CH₄ oxidation in pulsing than permanently flooded soils based on observations that static wetlands have greater emissions than pulsing wetlands. Basic research is needed on: the role of inorganic N species in controlling CH₄ oxidation pathways; phylogenetic analysis of methanotrophs; and environmental and edaphic effects on methanotroph growth and activity. Research is needed to develop wetland systems that optimize CH₄ oxidation relative to wetland type and environments and hydrology while maintaining and/or promoting other ecosystem services (e.g. C sequestration, pollution remediation, wildlife habitat, flood control).     

Response of CH<Subscript>4</Subscript> oxidation and methanotrophic diversity to NH<Subscript>4</Subscript><Superscript>+</Superscript> and CH<Subscript>4</Subscript> mixing ratios

Methane oxidising activity and community structure of 11, specifically targeted, methanotrophic species have been examined in an arable soil. Soils were sampled from three different field plots, receiving no fertilisation (C), compost (G) and mineral fertiliser (M), respectively. Incubation experiments were carried out with and without pre-incubation at elevated CH₄ mixing ratios (100 ml CH₄ l⁻¹) and with and without ammonium (100 mg N kg⁻¹) pre-incubation. Four months after fertilisation, plots C, G and M did not show significant differences in physicochemical properties and CH₄ oxidising activity. The total number of methanotrophs (determined as the sum the 11 specifically targeted methanotrophs) in the fresh soils was 17.0×10⁶, 13.7×10⁶ and 15.5×10⁶ cells g⁻¹ for treatment C, G and M, respectively. This corresponded to 0.11 to 0.32% of the total bacterial number. The CH₄ oxidising activity increased 10⁵-fold (20–26 mg CH₄ g⁻¹ h⁻¹), the total number of methanotrophs doubled (28–76×10⁶ cells g⁻¹) and the methanotrophic diversity markedly increased in treatments with a pre-incubation at elevated CH₄ concentrations. In all soils and treatments, type II methanotrophs (62–91%) outnumbered type I methanotrophs (9–38%). Methylocystis and Methylosinus species were always most abundant. After pre-incubation with ammonium, CH₄ oxidation was completely inhibited; however, no change in the methanotrophic community structure could be detected.     

Stimulation of methane oxidation by CH<Subscript>4</Subscript>-emitting rose chafer larvae in well-aerated grassland soil

In this study, the impact of rose chafer (Cetonia aurata L.) larvae on net and gross methane (CH₄) fluxes in soil from an old permanent grassland site (Giessen, Germany) was investigated. Previous studies at this site suggested the existence of Scarabaeidae larvae-induced “CH₄-emitting hot spots” within the soil profile which may subsequently lead to increased CH₄ oxidation. The net (soil + larvae) and gross (soil and larvae separated) CH₄ fluxes were studied in a 3-month laboratory incubation. Addition of larvae changed the soil from a net sink (?330 ± 11 ng CH₄ kg^?1 h^?1) to a net source (637 ± 205 ng CH₄ kg^?1 h^?1). Supply of plant litter to the soil + larvae incubation jars tended to increase CH₄ emissions which was not significant due to large variability. After 11–13 weeks of incubation, the net soil CH₄ oxidation was significantly stimulated by 13–21% in the treatments containing larvae when these were taken out. Analysis of archaeal 16S rRNA genes revealed that the majority of the obtained clones were closely related to uncultured methanogens from guts of insects and other animals. Other sequences were relative to cultivated species of Methanobrevibacter, Methanoculleus, and Methanosarcina. Hence, Scarabaeidae larvae in soils (i) may represent an underestimated source of CH₄ emissions in aerobic upland soils, (ii) may stimulate gross CH₄ consumption in their direct soil environment, and, thus, (iii) contribute to the spatial heterogeneity often observed in the field with closed-chamber measurements. Long-term CH₄-flux balances may be wrongly assessed when “exceptional” net CH₄ flux rates (due to larvae hot spots) are excluded from data sets.     

Methane emissions from paddy fields in Bali Island,Indonesia

Abstract

Methane emission rates from plots with and without fertilizer and rice straw application, and growth of two rice varieties (an improved variety, IR74 or IR64, and a local variety, Krueng Aceh) in two Indonesian paddy fields (Inceptisol and Alfisol soils of volcanic ash origin) were measured every week throughout the growth period in the first and the second cropping seasons, 1994. The CH₄ emission rates from the fields were similar between the two varieties. The effect of chemical fertilizer on the increase of the emissions was observed only in the Tabanan paddy field for the plots treated with rice straw. Application of rice straw increased the CH₄ emission rates. The mean rates of CH₄ emission were 1.37-2.13 mg CH₄?C m^?2 h^?1 for the plots without rice straw and 2.14–3.62 mg CH₄?C m^?2 h^?1 for the plots with rice straw application in the Alfisol plots, and 2.32–3.32 mg CH₄ -C m-2 h-1 for the plots without rice straw and 4.18–6.35 mg CH₄?C m^?2 h^?1 for the plots with rice straw application in the Inceptisol plots, respectively. Total amounts of CH₄ emitted during the growth period were 3.9–6.8 and 2.6–3.3 g CH₄?C m^?2 for the Alfisol plots and 6.9–10.7 and 4.2–5.8 g CH₄?C m^?2 for the Inceptisol plots with and without rice straw application, respectively. These findings suggested that CH₄ emission from tropical paddy fields with soils of volcanic ash origin is low.     

Soil–methane sink increases with soil age in forefields of Alpine glaciers

In upland soils aerobic methane-oxidizing bacteria (MOB) catalyze methane (CH₄) oxidation, and thus regulate the sole terrestrial sink for atmospheric CH₄. While confirmed in mature upland soils, little is known about this important function in young mountainous soils in glacier forefields, which are progressively formed as a result of glacier recession. We assessed four attributes of the soil CH₄ sink, i.e., soil-atmosphere CH₄ flux (J_atm), CH₄ oxidation activity (k), MOB abundance and variation in community composition along the 6–120-yr soil chronosequence in two Alpine glacier forefields on siliceous and calcareous bedrock. At most sampling locations soil CH₄ profiles showed stable uptake of atmospheric CH₄, with J_atm in the range of −0.082 to −2.2 mg CH₄ m⁻² d⁻¹. Multiple-linear-regression analyses indicated that J_atm significantly increased with soil age, whereas k did not. Instead, water content and CH₄ profiles in the youngest soils often indicated dry, inactive top layers with k < 0.1 h⁻¹, and active deeper layers (0.2 h⁻¹ ≤ k ≤ 11 h⁻¹) with more favorable water content. With increasing soil age the zone of highest CH₄ oxidation activity gradually moved upwards and eventually focused in the 10–40-cm layer (0.2 h⁻¹ ≤ k ≤ 16 h⁻¹). Copy numbers of pmoA genes significantly increased with soil age at both sites, ranging from 2.4 × 10³ to 5.5 × 10⁵ copies (g soil w.w.)⁻¹, but also correlated with mineral nitrogen content. Terminal restriction-fragment-length-polymorphism and cluster analyses showed differences in MOB community composition apparently related to bedrock type rather than soil age. Yet, regardless of bedrock type, the soil CH₄ sink established within a few years of soil development, and J_atm increased to values comparable to mature soils within decades. Thus, young mountainous soils have the potential to consume substantial amounts of atmospheric CH₄, and should be incorporated into future estimates of global soil CH₄ uptake.     

Methane production and consumption in a cultivated humisol

Summary Laboratory studies were conducted on a cultivated humisol containing populations of both methanotrophs and methanogens. The molar ratio CO₂ produced : O₂ consumed :CH₄ consumed was 0.27:1.0:1.0. Methane oxidation showed typical Michaelis-Menten kinetics with apparent K _m values for CH4 and O₂ of 66.2 M and 37.0 M, respectively. The low CO₂ yields and the effects of low dissolved oxygen indicated the presence of aerobic obligate methanotrophs. It is suggested that the methanotrophs in this soil are not entirely dependent on atmospheric CH₄ for growth and survival in situ.     

The effects of catena positions on greenhouse gas emissions along a seasonal wetland (dambo) transect in tropical Zimbabwe

Wetlands are major natural sources of greenhouse gases (GHGs). In central and southern Africa, one of the most extensive wetlands are dambos (seasonal wetlands) which occupy 20–25% of land area. However, there are very little data on GHG methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from dambos, and this study presents the first estimates from dambos in Zimbabwe. The objective was to evaluate the effects of catena positions; upland, dambo mid-slope and dambo bottom, on GHG emissions along an undisturbed dambo transect. Methane emissions were ?0.3, 29.5 and ?1.3 mg m^?2 hr^?1, N₂O emission were 40.1, 3.9 and 5.5 µg m² hr^?1, while CO₂ emissions were 2648.9, 896.2 and 590.1 mg m^?2 hr^?1 for upland, mid-slope and bottom catena, respectively. Our results showed that uplands were important sources of N₂O and CO₂, and a sink for CH₄, while the dambo mid-slope position was a major source of CH₄, but a weak source of CO₂ and N₂O. Dambo bottom catena was weak source GHGs. Overall, dambos were major sources of CH₄ and weak sources of N₂O and CO₂.We concluded that, depending on catenal position, dambos can be major or minor sources of GHGs.     

Methane oxidation in boreal forest soils: kinetics and sensitivity to pH and ammonium

In soil incubation experiments we examined if there are differences in the kinetic parameters of atmospheric methane (CH₄) oxidation in soils of upland forests and forested peatlands. All soils showed net uptake of atmospheric CH₄. One of the upland forests included also managed (clear-cut with or without previous liming or N-fertilization) study plots. The CH₄ oxidation in the forested peat soil had a higher K_m (510 μl l⁻¹) and V_max (6.2 nmol CH₄ cm⁻³ h⁻¹) than the upland forest soils (K_m from 5 to 18 μl l⁻¹ and V_max from 0.15 to 1.7 nmol CH₄ cm⁻³ h⁻¹). The forest managements did not affect the K_m-values. At atmospheric CH₄ concentration, the upland forest soils had a higher CH₄ oxidation activity than the forested peat soil; at high CH₄ concentrations the reverse was true. Most of the soils oxidised CH₄ in the studied pH range from 3 to 7.5. The pH optimum for CH₄ oxidation varied from 4 to 7.5. Some of the soils had a pH optimum for CH₄ oxidation that was above their natural pH. The CH₄ oxidation in the upland forest soils and in the peat soil did not differ in their sensitivities to (NH₄)₂SO₄ or K₂SO₄ (used as a non-ammonium salt control). Inhibition of CH₄ oxidation by (NH₄)₂SO₄ resulted mainly from a general salt effect (osmotic stress) though NH₄⁺ did have some additional inhibitory properties. Both salts were better inhibitors of CH₄ oxidation than respiration. The differences in the CH₄ oxidation kinetics in the forested peat soil and in the upland forest soils reveal that there are differences in the physiologies of the CH₄ oxidisers in these soils.     

Varying atmospheric methane concentrations affect soil methane oxidation rates and methanotroph populations in pasture,an adjacent pine forest,and a landfill

We describe experiments to better understand how CH₄ oxidation rates by different methanotroph communities respond to changing CH₄ concentrations. We used a novel system of automatically monitored chambers to investigate the response of CH₄ oxidation rates in a New Zealand pasture and adjacent pine forest soil exposed to varying atmospheric CH₄ concentrations.Type II methanotrophs that dominate CH₄ oxidation in the forest soil became progressively saturated as CH₄ concentrations rose from ambient (1.8 ppmv) to 570 ppmv, as shown by a decrease in uptake efficiency from 20% to 2% removal. By contrast, CH₄ oxidation in the pasture soil where Type I methanotrophs dominate increased in proportion to the increase in CH₄ inlet concentration, oxidising about 2% of the inlet CH₄ flux throughout. Modelling based on Michaelis-Menten kinetics revealed that low-affinity (Type I) methanotrophs were solely responsible for CH₄ oxidation in pasture soils, whereas high affinity (Type II) methanotrophs only contributed about 10% of the CH₄ oxidation in the forest soil. Increased aeration status using a soil–perlite (1:1) mixture doubled CH₄ oxidation rates at both ambient (1.8 ppmv) and 40 ppmv atmospheric CH₄. A similar volcanic soil previously exposed for 8 y to high CH₄ fluxes from a landfill had removal efficiencies consistently above 95% for atmospheric CH₄ concentrations up to 7500 ppmv when the CH₄ oxidation rate was7000 μg CH₄ kg⁻¹soil h⁻¹.     

Nitrous oxide and methane fluxes during the growing season from cultivated peat soils,peaty marl and gyttja clay under different cropping systems

Drainage of peatlands affects the fluxes of greenhouse gases (GHGs). Organic soils used for agriculture contribute a large proportion of anthropogenic GHG emissions, and on-farm mitigation options are important. This field study investigated whether choice of a cropping system can be used to mitigate emissions of N₂O and influence CH₄ fluxes from cultivated organic and carbon-rich soils during the growing season. Ten different sites in southern Sweden representing peat soils, peaty marl and gyttja clay, with a range of different soil properties, were used for on-site measurements of N₂O and CH₄ fluxes. The fluxes during the growing season from soils under two different crops grown in the same field and same environmental conditions were monitored. Crop intensities varied from grasslands to intensive potato cultivation. The results showed no difference in median seasonal N₂O emissions between the two crops compared. Median seasonal emissions ranged from 0 to 919?µg?N₂O?m^?2?h^?1, with peaks on individual sampling occasions of up to 3317?µg?N₂O?m^?2?h^?1. Nitrous oxide emissions differed widely between sites, indicating that soil properties are a regulating factor. However, pH was the only soil factor that correlated with N₂O emissions (negative exponential correlation). The type of crop grown on the soil did not influence CH₄ fluxes. Median seasonal CH₄ flux from the different sites ranged from uptake of 36?µg CH₄?m^?2?h^?1 to release of 4.5?µg?CH₄?m^?2?h^?1. From our results, it was concluded that farmers cannot mitigate N₂O emissions during the growing season or influence CH₄ fluxes by changing the cropping system in the field.     

Soil methane oxidation and methanotroph responses to afforestation of pastures with Pinus radiata stands

Afforestation of pastures in New Zealand reduces methane (CH₄) production from soil, while also stimulating oxidation of atmospheric CH₄ by soil methanotrophs. However, neither the mechanisms by which soil CH₄ oxidation is enhanced by afforestation, nor how long after forest planting tree-dependent responses in CH₄ oxidation become detectable are fully known. Here, we investigated the effects of different-aged stands (5-20 y) of the exotic pine (Pinus radiata (D. Don)) on CH₄ oxidation and methanotrophic community structure in soils, compared with adjacent, long-established pastures. Two of the pastures were on volcanic soils and two were on non-volcanic soils. Although the CH₄ fluxes in soils from these young stands were not significantly different from those in the associated pastures, the rate of oxidation of added ¹³CH₄ was higher in the pine soils. Both fluxes and ¹³CH₄ oxidation rates were higher in the volcanic than the non-volcanic soils. Combined phospholipid fatty acid (PLFA) and stable isotope probe (SIP) analyses suggested that type II methanotrophs (PLFA C18:1ω7) were most active in all soils followed by uncultivable bacteria (C17:0ai). Molecular analysis of the methanotrophic community structure using pmoA (particulate methane monooxygenase) genes suggested that a particular type II methanotroph (TRF 35) was dominant in all soils, but more so in the pine than in pasture soils. A type I methanotroph (TRF 245) was more prevalent in the pasture than in associated pine soils, whereas TRF 128 (a type II methanotroph) was slightly more dominant in soils under pine. Cloning and sequencing data suggest TRFs 35 and 128, which differ from one another, belong to distant relatives of Methylocapsa sp; TRF 245 is related to Methylococcus capsulatus. Land-use change resulted in changes in soil bulk density, porosity, moisture contents and in methanotrophic community structure. Methane oxidation rates were most closely related to soil moisture, as well as to the methanotrophic community structure, and nitrate-N, extractable C and total C concentrations. Stepwise multiple regression also suggested a weak effect (P = 0.06) of stand age on CH₄ oxidation rate. By contrast, the responses of the methanotrophic community structure to this land-use change were more readily detected by the specific molecular analyses, and indicated a predominance of type II methanotrophs in pine soils.     

Methane uptake in soils from Pinus radiata plantations, a reverting shrubland and adjacent pastures: Effects of land-use change, and soil texture, water and mineral nitrogen

Afforestation and reforestation of pastures are key land-use changes in New Zealand that help sequester carbon (C) to offset its carbon dioxide (CO₂) emissions under the Kyoto Protocol. However, relatively little attention has been given so far to associated changes in trace gas fluxes. Here, we measure methane (CH₄) fluxes and CO₂ production, as well as microbial C, nitrogen (N) and mineral-N, in intact, gradually dried (ca. 2 months at 20 °C) cores of a volcanic soil and a heavier textured, non-volcanic soil collected within plantations of Pinus radiata D. Don (pine) and adjacent permanent pastures. CH₄ fluxes and CO₂ production were also measured in cores of another volcanic soil under reverting shrubland (mainly Kunzea var. ericoides (A. Rich) J. Thompson) and an adjacent pasture. CH₄ uptake in the pine and shrubland cores of the volcanic soils at field capacity averaged about 35 and 14 μg CH₄-C m⁻² h⁻¹, respectively, and was significantly higher than in the pasture cores (about 21 and 6 μg CH₄-C m⁻² h⁻¹, respectively). In the non-volcanic soil, however, CH₄-C uptake was similar in most cores of the pine and pasture soils, averaging about 7-9 μg m⁻² h⁻¹, except in very wet samples. In contrast, rates of CO₂ production and microbial C and N concentrations were significantly lower under pine than under pasture. In the air-dry cores, microbial C and N had declined in the volcanic soil, but not in the non-volcanic soil; ammonium-N, and especially nitrate-N, had increased significantly in all samples. CH₄ uptake was, with few exceptions, not significantly influenced by initial concentrations of ammonium-N or nitrate-N, nor by their changes on air-drying. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of only the pine and pasture soils showed that different methanotrophic communities were probably active in soils under the different vegetations. The C18 PLFAs (type II methanotrophs) predominated under pine and C16 PLFAs (type I methanotrophs) predominated under pasture. Overall, vegetation, soil texture, and water-filled pore space influenced CH₄-C uptake more than did soil mineral-N concentrations.     

Methane turnover and temperature response of methane-oxidizing bacteria in permafrost-affected soils of northeast Siberia

The abundance, activity, and temperature response of aerobic methane-oxidizing bacteria were studied in permafrost-affected tundra soils of northeast Siberia. The soils were characterized by both a high accumulation of organic matter at the surface and high methane concentrations in the water-saturated soils. The methane oxidation rates of up to 835 nmol CH₄ h⁻¹ g⁻¹ in the surface soils were similar to the highest values reported so far for natural wetland soils worldwide. The temperature response of methane oxidation was measured during short incubations and revealed maximum rates between 22 °C and 28 °C. The active methanotrophic community was characterized by its phospholipid fatty acid (PLFA) concentrations and with stable isotope probing (SIP). Concentrations of 16:1ω8 and 18:1ω8 PLFAs, specific to methanotrophic bacteria, correlated significantly with the potential methane oxidation rates. In all soils, distinct 16:1 PLFAs were dominant, indicating a predominance of type I methanotrophs. However, long-term incubation of soil samples at 0 °C and 22 °C demonstrated a shift in the composition of the active community with rising temperatures. At 0 °C, only the concentrations of 16:1 PLFAs increased and those of 18:1 PLFAs decreased, whereas the opposite was true at 22 °C. Similarly, SIP with ¹³CH₄ showed a temperature-dependent pattern. When the soils were incubated at 0 °C, most of the incorporated label (83%) was found in 16:1 PLFAs and only 2% in 18:1 PLFAs. In soils incubated at 22 °C, almost equal amounts of ¹³C label were incorporated into 16:1 PLFAs and 18:1 PLFAs (33% and 36%, respectively). We concluded that the highly active methane-oxidizing community in cold permafrost-affected soils was dominated by type I methanotrophs under in situ conditions. However, rising temperatures, as predicted for the future, seem to increase the importance of type II methanotrophs, which may affect methane cycling in northern wetlands.     

The differential effects of ammonium and nitrate on methanotrophs in rice field soil

It has been known that nitrogenous fertilizers can either stimulate or inhibit methane oxidation in soils. The mechanism, however, remains unclear. Here we conducted laboratory incubation experiments to evaluate the effects of ammonium versus nitrate amendment on CH₄ oxidation in a rice field soil. The results showed that both N forms stimulated CH₄ oxidation. But nitrate stimulated CH₄ oxidation to a greater extent than ammonium per unit N base. The 16S rRNA genes and the pmoA genes were analyzed to determine the dynamics of total bacterial and methanotrophic populations, respectively. The methanotrophic community consisted of type I and type II methanotrophs and was dominated by type I group after two weeks of incubation. Nitrate promoted both types of methanotrophs, but ammonium promoted only type I. DNA-based stable isotope probing confirmed that ammonium stimulated the incorporation of ¹³CH₄ into type I methanotrophs but not type II, while nitrate caused almost homogenous distribution of ¹³CH₄ in type I and type II methanotrophs. Our study suggests that nitrate can promote CH₄ oxidation more significantly than ammonium and is probably a better N source for both types of methanotrophs in rice field soil. More investigations, e.g. using ¹⁵N labeling, are necessary to elucidate this possibility.     

Mercury and Methylmercury in Upland and Wetland Acid Forest Soils of a Watershed in NE-Bavaria,Germany

Mercury (Hg) and methylmercury (CH₃Hg⁺) are global pollutants, but little information is available on their distribution and mobility in soils and catchments of Central Europe. The objective of this study was to investigate the pools and mobility of Hg and CH₃Hg⁺ in different forest soils. Upland and wetland forest soils, soil solutions and runoff were sampled. In upland soils the highest contents of total-Hg were found in the Oh layer of the forest floor (>400 ng g^-1) and the storage of non geogenic total-Hg (calculated for 60 cm depth) was about 120 mg/m². The storage of total-Hg was one order of magnitude lower in wetland soils as compared to the upland soils. By far the largest proportion of total-Hg in soils was bound in immobile fractions. The depth gradients of CH₃Hg⁺ did not correspond to those of total-Hg and the highest contents of CH₃Hg⁺ in upland soils were observed in the litter layer of the forest floor and in the Bsv horizon. The CH₃Hg⁺ content of the wetland soils was generally much higher in comparison with upland soils. CH₃Hg⁺ in solution was found in the forest floor percolates of upland soils and in wetland soils, but not in soil solutions from mineral soil horizons. Gaseous losses of Hg as well as methylation of Hg are likely in wetland soils. The latter might be highly relevant for CH₃Hg⁺ levels in runoff.     

Effect of summer throughfall exclusion, summer drought, and winter snow cover on methane fluxes in a temperate forest soil

Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH₄ uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH₄ uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm⁻³ may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH₄ consumption, with maximum rate at 0.23 g H₂O g⁻¹ soil. On average, about 74% of atmospheric CH₄ was consumed in the top 4-5 cm of the mineral soil. By contrast, little or no CH₄ consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH₄ uptake by about 700-1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH₄ uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH₄ oxidation and that summer droughts may increase the soil CH₄ sink of temperate forest soils.     

Generation,sink, and emission of greenhouse gases by urban soils at different stages of the floodplain development in Moscow

Purpose

The purpose of this research was to study the generation, sink, and emission of greenhouse gases by soils on technogenic parent materials, created at different stages of the Moskva River floodplain development (1—construction and 2—landscaping of residential areas).

Materials and methods

Field surveys revealed the spatial trends of concentration and emission of the greenhouse gases in following groups of soils: Retisols (RT-ab-ct) and Fluvisols (FL-hu, FL-hi.gl) before land engineering preparation for the construction, Urbic Technosols Transportic (TC-ub-ar.tn and TC-ub-hu.tn) at stage 1 and Urbic Technosols Folic (TC-ub-fo) at stage 2. CO₂ and CH₄ concentration in soils and their emission were determined using subsurface soil air equilibration tubes and the closed chamber method, respectively. Bacterial methane generation rate (MGR) and methane oxidation rate (MOR) were measured by kinetic methods.

Results and discussion

In natural soils MOR is caused only by intra-aggregate methanogenesis. The imbalance of methane generation and oxidation was observed in FL-hi.gl. It caused CH₄ accumulation in the profile (7.5 ppm) and its emission to the atmosphere (0.11 mg CH₄ m^?2 h^?1). RT-ab-ct acted as the sink of atmospheric methane. CO₂ emission was 265.1?±?24.0 and 151.9?±?37.2 mg CO₂ m^?2 h^?1 from RT-ab-ct and FL-hi.gl, respectively. In Technosols CH₄ concentration was predominantly low (median was 2.7, 2.9, and 3.0 ppm, in TC-ub-ar.tn, TC-ub-hu.tn, and TC-ub-fo, respectively), but due to the occurrence of peat sediments under technogenic material, it increased to 1–2%. Methane emission was not observed due to functioning of biogeochemical barriers with high MOR. In TC-ub-ar.tn and TC-ub-hu.tn, the barriers were formed at 60-cm depth. In TC-ub-fo, the system of barriers was formed in Folic and Technic horizons (at 10- and 60-cm depth). CO₂ emission was 2 times lower from TC-ub-ar.tn and TC-ub-hu.tn and 1.5 times higher from TC-ub-fo than from natural soils.

Conclusions

Greenhouse gas generation, sink, and emission by natural soils and Technosols in floodplain were estimated. CO₂ and CH₄ content in Technosols varied depending on the properties of parent materials. Technosols at stage 1 did not emit CH₄ due to formation of biogeochemical barriers—soil layers of high CH₄ utilization rates. Urbic Technosols (Folic) at stage 2 performed as a source of significant CO₂ emission.

     

Soils,surficial geology,and geomicrobiology of saline-sodic wetlands,North Platte River Valley,Nebraska, USA

Saline-sodic wetlands along a 200-km stretch of the North Platte River Valley in western Nebraska, USA lie within an important agricultural region, but their processes, salt mineralogy, and geomicrobiology have not previously been investigated. Putative anthropogenic salinization has long been a concern, yet early historical accounts of widespread surface salts in the area have never been applied as comparative standards. Surface salts in the area originate from soil capillarity and surface evaporation. Thenardite (Na₂SO₄) and/or mirabilite (NaSO₄·10 H₂O) dominate, depending on ambient conditions. Bloedite (Na₂Mg[SO₄]₂·4[H₂O]), halite (NaCl), burkeite (Na₆CO₃[SO₄]₂), and calcite (CaCO₃) are minor constituents. Historical accounts indicate that salts accumulated naturally long before Euramerican settlement, apparently as a result of rock–water interaction in nearby volcaniclastic sediments of the Brule Formation.Ephemeral to permanent water-holding basins in the wetlands contain Na⁺-rich waters that vary widely in electrical conductivity (as high as 159 mS/cm) and in ionic composition, but local spring waters are extremely dilute. Basin floors exhibit a unique type of microrelief, which appears to form by the filling of microlows with water and the dispersal of soil material therein by Na⁺, followed by dewatering and collapse of the soil with drying. Illite dominates basin surface soils, but smectite dominates at depth; high soil pH, available K⁺, and frequent wetting–drying cycles in the wetlands suggest that in-situ illitization may have occurred. Soil crusts and vesicular surface horizons are common as are upward increases in electrical conductivity. The activity of sulfate-reducing microbes forms prominent near-surface horizons of sulfate reduction in saturated soils, which retract or disappear entirely during dry episodes.Saline-sodic wetland soils in the study area change on daily to seasonal scales. Cycles of surface salt development, microbial activity, and microrelief genesis are all controlled by regular wetting–drying cycles and the interaction of ponded surface waters and shallow groundwaters. Relatively unique aspects of microbial ecology and surface processes make the soils important as “geomicrobial reactors” wherein important parts of hydrological and geochemical cycles occur.     

Methane uptake in salt-affected soils shows low sensitivity to salt addition

Methane oxidation in aerated soils is a significant sink for atmospheric methane (CH₄). Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about CH₄ turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of CH₄ consumption in saline soils, CH₄ fluxes were measured in intact cores of the slightly (ECe = 3.2 mS cm⁻¹), moderately (ECe = 7.1 mS cm⁻¹) and extremely (ECe = 50.7 mS cm⁻¹ and 112.6 mS cm⁻¹) saline soils from the Yellow River Delta, China. CH₄ uptake of cores from the slightly saline soil ranged from 14 to 24 μg CH₄-C m⁻² h⁻¹, comparable to those in the non-saline forest soils with similar texture. CH₄ uptake of cores from the moderately saline soil was only about 6% of that in the slightly saline soil. CH₄ uptake was too low to be measurable in the extremely saline soil. Compared with the non-saline soil, CH₄ uptake in the saline soils was much less sensitive to salt, suggesting the higher salt-tolerance of CH₄ oxidizers in the saline soil. The result also indicated an underestimate in CH₄ uptake for the naturally-occurring saline soils by adding salt to non-saline soils. These results should be useful to study the global CH₄ budget and to explore the physiological and ecological characteristics of methanotrophic bacteria in the salt-affected soils.     

Short-term effect of urea on CH4 flux under the oil palm (Elaeis guineensis) on tropical peatland in Sarawak,Malaysia

Abstract

Methane flux was measured monthly from August 2002 to July 2003 at an oil palm plantation on tropical peatland in Sarawak, Malaysia, using a closed chamber technique. Urea was applied twice, once in November 2002 and once in May 2003. The monthly CH₄ flux ranged from ?32.78 to 4.17 µg C m^?2 h^?1. Urea applications increased CH₄ emissions in the month of application and emissions remained slightly higher a month later before the effect disappeared in the third month after application (i.e. back to CH₄ uptake). This effect was the result of increased soil NH⁺ ₄ content that was not immediately absorbed by the oil palm following urea application, which reduced the oxidation of CH₄, resulting in its enhanced emission. By using the Cate–Nelson linear-plateau model, the critical soil NH⁺ ₄ content causing CH₄ emissions in the oil palm ecosystem was 42.75 mg kg^?1 soil. However, the inhibitory effect of NH⁺ ₄ on the oxidation of CH₄ was mitigated by low rainfall and the pyrophosphate solubility index (PSI), where the former might increase oxidation of CH₄ and the latter was a reflection of the low soluble substrate for methane production. Thus, the splitting and timing of urea applications are important not only to optimize oil palm yield, but also to reduce soil NH⁺ ₄ content to minimize CH₄ emissions and, therefore, its potential negative impact on the environment.