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.     

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.     

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.     

Soil–atmosphere greenhouse gas exchange in a cool,temperate Eucalyptus delegatensis forest in south-eastern Australia

Forests are the largest C sink (vegetation and soil) in the terrestrial biosphere and may additionally provide an important soil methane (CH₄) sink, whilst producing little nitrous oxide (N₂O) when nutrients are tightly cycled. In this study, we determine the magnitude and spatial variation of soil–atmosphere N₂O, CH₄ and CO₂ exchange in a Eucalyptus delegatensis forest in New South Wales, Australia, and investigate how the magnitude of the fluxes depends on the presence of N₂-fixing tree species (Acacia dealbata), the proximity of creeks, and changing environmental conditions. Soil trace gas exchange was measured along replicated transects and in forest plots with and without presence of A. dealbata using static manual chambers and an automated trace gas measurement system for 2 weeks next to an eddy covariance tower measuring net ecosystem CO₂ exchange. CH₄ was taken up by the forest soil (?51.8 μg CH₄-C m^?2 h^?1) and was significantly correlated with relative saturation (S_r) of the soil. The soil within creek lines was a net CH₄ source (up to 33.5 μg CH₄-C m^?2 h^?1), whereas the wider forest soil was a CH₄ sink regardless of distance from the creek line. Soil N₂O emissions were small (<3.3 μg N₂O-N m^?2 h^?1) throughout the 2-week period, despite major rain and snowfall. Soil N₂O emissions only correlated with soil and air temperature. The presence of A. dealbata in the understorey had no influence on the magnitude of CH₄ uptake, N₂O emission or soil N parameters. N₂O production increased with increasing soil moisture (up to 50% S_r) in laboratory incubations and gross nitrification was negative or negligible as measured through ¹⁵N isotope pool dilution.The small N₂O emissions are probably due to the limited capacity for nitrification in this late successional forest soil with C:N ratios >20. Soil–atmosphere exchange of CO₂ was several orders of magnitude greater (88.8 mg CO₂-C m^?2 h^?1) than CH₄ and N₂O, and represented 43% of total ecosystem respiration. The forest was a net greenhouse gas sink (126.22 kg CO₂-equivalents ha^?1 d^?1) during the 2-week measurement period, of which soil CH₄ uptake contributed only 0.3% and N₂O emissions offset only 0.3%.     

Role of CH4 oxidation,production and transport in forest soil CH4 flux

Forest soils are an important sink for atmospheric CH₄ but the contribution of CH₄ oxidation, production and transport to the overall CH₄ flux is difficult to quantify. It is important to understand the role these processes play in CH₄ dynamics of forest soils, to enable prediction of how the size of this sink will respond to future environmental change. Methane oxidation, production and transport were investigated for a temperate forest soil, previously shown to be a net CH₄ consumer, to determine the extent to which physical and biological processes contributed to the net flux. The sum of oxidation rates for soil layers were significantly greater (P<0.05) than for the intact soil cores from which the layers were taken. Combined with the immediate inhibition of CH₄ uptake on waterlogging soils, the findings suggested that soil CH₄ diffusion was an important regulator of CH₄ uptake. In support of this, a subsurface maximum for CH₄ oxidation was observed, but the exact depth of the maximum differed when rates were calculated on a mass or on an areal basis. Markedly varying potential CH₄ uptake activities between soil cores were masked in intact core rates. Potential CH₄ oxidation conformed well to Michaelis–Menten kinetics but V_max, K_t and a_A^○ values varied with depth, suggesting different functional methanotrophic communities were active in the profile. The presence of monophasic kinetics in fresh soil could not be used to infer that the soil was exposed only to CH₄ mixing ratios ≤ atmospheric, as challenging soils with 20% CH₄ in air did not induce low-affinity oxidation kinetics. Atmospheric CH₄ oxidation potentials exceeded production potentials by 10–220 times. The results show that the forest soil CH₄ flux was dominated by CH₄ oxidation and transport, methanogenesis played only a minor role.     

Methane and nitrous oxide fluxes of soils in pure and mixed stands of European beech and Norway spruce

Tree species can affect the sink and source strength of soils for atmospheric methane and nitrous oxide. Here we report soil methane (CH₄) and nitrous oxide (N₂O) fluxes of adjacent pure and mixed stands of beech and spruce at Solling, Germany. Mean CH₄ uptake rates ranged between 18 and 48 μg C m^?2 hour^?1 during 2.5 years and were about twice as great in both mixed and the pure beech stand as in the pure spruce stand. CH₄ uptake was negatively correlated with the dry mass of the O horizon, suggesting that this diminishes the transport of atmospheric CH₄ into the mineral soil. Mean N₂O emission was rather small, ranging between 6 and 16 μg N m^?2 hour^?1 in all stands. Forest type had a significant effect on N₂O emission only in one mixed stand during the growing season. We removed the O horizon in additional plots to study its effect on gas fluxes over 1.5 years, but N₂O emissions were not altered by this treatment. Surprisingly, CH₄ uptake decreased in both mixed and the pure beech stands following the removal of the O horizon. The decrease in CH₄ uptake coincided with an increase in the soil moisture content of the mineral soil. Hence, O horizons may maintain the gas diffusivity within the mineral soil by storing water which cannot penetrate into the mineral soil after rainfall. Our results indicate that conversion of beech forests to beech–spruce and pure spruce forests could decrease soil CH₄ uptake, while the long‐term effect on N₂O emissions is expected to be rather small.     

Methane concentrations and methanotrophic community structure influence the response of soil methane oxidation to nitrogen content in a temperate forest

Methane oxidation in forest soils removes atmospheric CH₄. Many studies have determined methane uptake rates and their controlling variables, yet the microorganisms involved have rarely been assessed simultaneously over the longer term. We measured methane uptake rates and the community structure of methanotrophic bacteria in temperate forest soil (sandy clay loam) on a monthly basis for two years in South Korea. Methane uptake rates at the field site did not show any seasonal patterns, and net uptake occurred throughout both years. In situ uptake rates and uptake potentials determined in the laboratory were 2.92 ± 4.07 mg CH₄ m⁻² day⁻¹ and 51.6 ± 45.8 ng CH₄ g⁻¹ soil day⁻¹, respectively. Contrary to results from other studies, in situ oxidation rates were positively correlated with soil nitrate concentrations. Short-term experimental nitrate addition (0.20-1.95 μg N g⁻¹ soil) significantly stimulated oxidation rates under low methane concentrations (1.7-2.0 ppmv CH₄), but significantly inhibited oxidation under high methane concentrations (300 ppmv CH₄). We analyzed the community structures of methanotrophic bacteria using a DNA-based fingerprinting method (T-RFLP). Type II methanotrophs dominated under low methane concentrations while Type I methanotrophs dominated under high methane concentrations. Nitrogen addition selectively inhibited Type I methanotrophic bacteria. Overall, the results of this study indicate that the effects of inorganic N on methane uptake depend on methane concentrations and that such a response is related to the dissimilar activation or inhibition of different types of methanotrophic bacteria.     

The inhibiting effect of nitrate fertilisation on methane uptake of a temperate forest soil is influenced by labile carbon

Upland soils are the most important terrestrial sink for the greenhouse gas CH₄. The oxidation of CH₄ is highly influenced by reactive N which is increasingly added to many ecosystems by atmospheric deposition and thereby also alters the labile C pool in the soils. The interacting effects of soil N availability and the labile C pool on CH₄ oxidation are not well understood. We conducted a laboratory experiment with soil columns consisting of homogenised topsoil material from a temperate broad-leaved forest to study the net CH₄ flux under the combined or isolated addition of NO ₃ ^? and glucose as a labile C source. Addition of NO ₃ ^? and glucose reduced the net CH₄ uptake of the soil by 86% and 83%, respectively. The combined addition of both agents led to a nearly complete inhibition of CH₄ uptake (reduction by 99.4%). Our study demonstrates a close link between the availability of C and N and the rate of CH₄ oxidation in temperate forest soils. Continued deposition of NO ₃ ^? has the potential to reduce the sink strength of temperate forest soils for CH₄.     

The influence of land use and pesticides on methane oxidation in some Belgian soils

 In a first experiment, the effect of land use on the uptake rate of atmospheric CH₄ was studied in laboratory incubations of intact soil cores. A soil under deciduous forest showed the highest CH₄ oxidation. Its overall CH₄ uptake during the measuring period (202 days) was 1.03 kg CH₄ ha^–1. Natural grassland showed the second highest CH₄ oxidizing capacity (0.71 kg CH₄ ha^–1). The overall amount of CH₄ uptake by fertilized pasture was 0.33 kg CH₄ ha^–1. CH₄ oxidation in arable soils with different fertilizer treatments varied between 0.34 and 0.37 kg CH₄ ha^–1. Undisturbed soils had a higher CH₄ uptake capacity than agricultural soils. The moisture content of the soil was found to be an important parameter explaining temporal variations of CH₄ oxidation. Different methods of fertilization which had been commenced 10 years previously were not yet reflected in the total CH₄ uptake rate of the arable soil. In a second experiment, a number of frequently used pesticides were screened for their possible effect on CH₄ oxidation. In a sandy arable soil lenacil, mikado and oxadixyl caused significantly reduced CH₄ oxidation compared to the control. Under the same conditions, but in a clayey arable soil, mikado, atrazine and dimethenamid caused a reduction of the CH₄ uptake. In a landfill cover soil, with a 100-fold higher CH₄ oxidation rate, no inhibition of CH₄ oxidation was observed, not even when the application rate of pesticides was tenfold higher than usual. Received: 1 December 1998     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

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.     

Interactive effects of tree species and soil moisture on methane consumption

Methane consumption by temperate forest soils is a major sink for this important greenhouse gas, but little is known about how tree species influence CH₄ uptake by soils. Here, we show that six common tree species in Siberian boreal and temperate forests significantly affect potential CH₄ consumption in laboratory microcosms. Overall, soils under hardwood species (aspen and birch) consumed CH₄ at higher rates than soils under coniferous species and grassland. While NH₄⁺ addition often reduces CH₄ uptake, we found no effect of NH₄⁺ addition, possibly because of the relatively high ratio of CH₄-to-NH₄⁺ in our incubations. The effects of soil moisture strongly depended on plant species. An increase in soil moisture enhanced CH₄ consumption in soils under spruce but had the opposite effect under Scots pine and larch. Under other species, soil moisture did not affect CH₄ consumption. These results could be explained by specific responses of different groups of CH₄-oxidizing bacteria to elevated moisture.     

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 (145 ppmv CH₄) and a high (20,200 ppmv CH₄) K_m value and V_max values of 16.8 and 839 nmol gdw⁻¹ h⁻¹, respectively. The activity with the low K_m 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 10 mm. Atmospheric CH₄ (1.8 ppmv) was consumed in soil aggregates <6 mm, but soil aggregates >6 mm 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.     

Methane flux characteristics in forest soils under an East Asian monsoon climate

Methane (CH₄) uptake by soil can possibly be suppressed more in regions with heavy summer precipitation, such as those under the East Asian monsoon climate, as compared to that in regions with a dry summer. In order to determine how precipitation patterns affect seasonal and spatial variations in CH₄ fluxes in temperate forest soils, such fluxes and selected environmental variables were measured on different parts of a hill slope in a cypress forest in central Japan. On the upper and middle parts of the slope, CH₄ uptake was observed throughout the year, and the uptake rates increased slightly with soil temperature and decreased with soil water content. The CH₄ flux predicted using data for the middle and upper parts of the slope ranged from −1.12 to −0.83 kg-CH₄ ha⁻¹ y⁻¹ (i.e. CH₄ uptake by soil) and from −2.30 to −2.04 kg-CH₄ ha⁻¹ y⁻¹, respectively. In contrast, in the relatively wet lower part of the slope near an in-stream wetland, large CH₄ emissions (>2 mg-CH₄ m⁻¹ d⁻¹) were observed during the rainy summer. In this wetter plot, the soil functioned as a net annual CH₄ source in a rainy year. Hence the variation in CH₄ flux with a change in soil water conditions and soil temperature on the lower part of the slope contrasted to that on the upper and middle parts of the slope. The predicted CH₄ flux for this lower plot ranged from −0.45 kg-CH₄ ha⁻¹ y⁻¹ in a dry year to 1.80 kg-CH₄ ha⁻¹ y⁻¹ in a rainy year. Our results suggest that consideration of the soil water conditions across a watershed is important for estimating the CH₄ budgets for entire forest watershed, particularly in regions subject to a wet summer.     

Severe soil frost reduces losses of carbon and nitrogen from the forest floor during simulated snowmelt: A laboratory experiment

Considerable progress has been made in understanding the impacts of soil frost on carbon (C) and nitrogen (N) cycling, but the effects of soil frost on C and N fluxes during snowmelt remain poorly understood. We conducted a laboratory experiment to determine the effects of soil frost on C and N fluxes from forest floor soils during snowmelt. Soil cores were collected from a sugar maple (Acer saccharum)–American beech (Fagus grandifolia) and a red spruce (Picea rubens)–balsam fir (Abies balsamea) forest at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A. Soils were exposed to one of three temperature treatments, including severe (?15 °C), mild (?0.5 °C), and no soil frost (+5 °C) conditions. After one week the soils were incubated at +5 °C and snow was placed on top of the soils to simulate spring snowmelt. NO₃^? losses were up to 5.5 mg N kg^?1 soil greater in the mild soil frost treatment than the severe soil frost treatment. Net losses of NH₄⁺ and DON in leachate were up to 19 and 18 mg N kg^?1 soil greater in the no soil frost and mild soil frost treatments, respectively, than the severe soil frost treatment. In contrast, soil frost did not have a significant impact on dissolved organic C or cumulative gaseous fluxes of C and N throughout the snowmelt period. However, the total cumulative flux of C (i.e. dissolved organic C + CO₂ + CH₄) and N (i.e. dissolved organic N + NH₄ + NO₃ + N₂O) in the severe soil frost treatment were between one quarter and one half that observed in the no soil frost treatment for both forest types. Together, the results of this study show that total fluxes of N in leachate, as well as total cumulative C and N fluxes (gases + leachate), were significantly reduced following severe soil frost. We conclude that the extent to which C and N cycling during snowmelt is altered in response to changes in winter climate depend on both the presence and severity of soil frost.     

Fluxes of CO2, CH4 and N2O from drained organic soils in deciduous forests

We examined net greenhouse gas exchange at the soil surface in deciduous forests on soils with high organic contents. Fluxes of CO₂, CH₄ and N₂O were measured using dark static chambers for two consecutive years in three different forest types; (i) a drained and medium productivity site dominated by birch, (ii) a drained and highly productive site dominated by alder and (iii) an undrained and highly productive site dominated by alder. Although the drained sites had shallow mean groundwater tables (15 and 18 cm, respectively) their average annual rates of forest floor CO₂ release were almost twice as high compared to the undrained site (1.9±0.4 and 1.7±0.3, compared to 1.0±0.2 kg CO₂ m⁻² yr⁻¹). The average annual CH₄ emission was almost 10 times larger at the undrained site (7.6±3.1 compared to 0.9±0.5 g CH₄ m⁻² yr⁻¹ for the two drained sites). The average annual N₂O emissions at the undrained site (0.1±0.05 g N₂O m⁻² yr⁻¹) were lower than at the drained sites, and the emissions were almost five times higher at the drained alder site than at the drained birch site (0.9±0.35 compared to 0.2±0.11 g N₂O m⁻² yr⁻¹). The temporal variation in forest floor CO₂ release could be explained to a large extent by differences in groundwater table and air temperature, but little of the variation in the CH₄ and N₂O fluxes could be explained by these variables. The measured soil variables were only significant to explain for the within-site spatial variation in CH₄ and N₂O fluxes at the undrained swamp, and dark forest floor CO₂ release was not explained by these variables at any site. The between-site spatial variation was attributed to variations in drainage, groundwater level position, productivity and tree species for all three gases. The results indicate that N₂O emissions are of greater importance for the net greenhouse gas exchange at deciduous drained forest sites than at coniferous drained forest sites.     

Potential fluxes of N2O and CH4 from soils of three forest types in Eastern Canada

We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N₂O) and methane (CH₄) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH₄NO₃ amendment and an elevated headspace CH₄ concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N₂O and CH₄ fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N₂O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N₂O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH₄ when incubated under an elevated headspace CH₄ concentration, except the poorly drained soils in the deciduous forest, which emitted CH₄. CH₄ consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N₂O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH₄ unlike the well-drained soils, which are net sinks of atmospheric CH₄. Because wetland soils can produce a substantial amount of CH₄ and N₂O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.     

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.     

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO₂, CH₄ and N₂O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N₂O and CH₄ exchange. We measured soil CO₂, CH₄ and N₂O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1-2 years). There was no seasonal change and no fire effect upon soil N₂O exchange. Soil N₂O fluxes were very low, generally between −1.0 and 1.0 μg N m⁻² h⁻¹, and often below the minimum detection limit. There was an increase in soil NH₄⁺ in the months after the 2008 fire event, but no change in soil NO₃⁻. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH₄ sink that equated to between −2.0 and −1.6 kg CH₄ ha⁻¹ y⁻¹ with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH₄ uptake. There were short periods of soil CH₄ emission, up to 20 μg C m⁻² h⁻¹, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO₂ fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH₄ fluxes, but a much stronger relationship with soil CO₂ fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N₂O source, and possibly even a sink. Annual soil CH₄ flux measurements suggest that the 1.9 million km² of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO₂-e per year. This sink estimate would offset potentially 10% of Australian transport related CO₂-e emissions. This CH₄ sink estimate does not include concurrent CH₄ emissions from termite mounds or ephemeral wetlands in Australian savannas.     

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.