Consumption Of Atmospheric CH4 Research Articles

Arctic soil methane sink increases with drier conditions and higher ecosystem respiration

Arctic wetlands are known methane (CH4) emitters but recent studies suggest that the Arctic CH4 sink strength may be underestimated. Here we explore the capacity of well-drained Arctic soils to consume atmospheric CH4 using >40,000 hourly flux observations and spatially distributed flux measurements from 4 sites and 14 surface types. While consumption of atmospheric CH4 occurred at all sites at rates of 0.092 ± 0.011 mgCH4 m−2 h−1 (mean ± s.e.), CH4 uptake displayed distinct diel and seasonal patterns reflecting ecosystem respiration. Combining in situ flux data with laboratory investigations and a machine learning approach, we find biotic drivers to be highly important. Soil moisture outweighed temperature as an abiotic control and higher CH4 uptake was linked to increased availability of labile carbon. Our findings imply that soil drying and enhanced nutrient supply will promote CH4 uptake by Arctic soils, providing a negative feedback to global climate change.

Variation in CO2 and CH4 fluxes among land cover types in heterogeneous Arctic tundra in northeastern Siberia

Abstract. Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem–atmosphere exchange of carbon (C). However, the estimates of regional carbon dioxide (CO2) and methane (CH4) budgets are highly uncertain. We measured CO2 and CH4 fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71∘ N, 128∘ E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined the net ecosystem exchange (NEE) of CO2, ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg), and CH4 flux during the growing season. We applied a previously developed high-spatial-resolution land cover map over an area of 35.8 km2 for spatial extrapolation. Among the land cover types varying from barren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at the photosynthetically active photon flux density of 800 µmol m−2 h−1 (NEE800 and Pg800) were greatest in the graminoid-dominated habitats, i.e., streamside meadow and fens, with NEE800 and Pg800 of up to −21 (uptake) and 28 mmol m−2 h−1, respectively. Vascular LAI was a robust predictor of both NEE800 and Pg800 and, on a landscape scale, the fens were disproportionately important for the summertime CO2 sequestration. Dry tundra, including the dwarf-shrub and lichen tundra, had smaller CO2 exchange rates. The fens were the largest source of CH4, while the dry mineral soil tundra consumed atmospheric CH4, which on a landscape scale amounted to −9 % of the total CH4 balance during the growing season. The largest seasonal mean CH4 consumption rate of 0.02 mmol m−2 h−1 occurred in sand- and stone-covered barren areas. The high consumption rate agrees with the estimate based on the eddy covariance measurements at the same site. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land cover type. This study highlights the need to distinguish different land cover types including the dry tundra habitats to account for their different CO2 and CH4 flux patterns, especially the consumption of atmospheric CH4, when estimating tundra C exchange on a larger spatial scale.

Physical and hydric factors regulating nitrous oxide and methane fluxes in mountainous Atlantic forest soils in southeastern Brazil

Nitrous oxide (N2O) and methane (CH4) are gases that act in physical and chemical processes related to global climate control and ozone layer stability. Atlantic forest soils are natural emitters of N2O and act in the consumption of CH4 when well aerated. Understanding the dynamics of N2O and CH4 fluxes is of great importance because the exchange of gases between the soil and the atmosphere is susceptible to climate change. The objective of this work was to evaluate N2O and CH4 fluxes using static chambers against different physical and hydric factors (e.g., soil temperature, precipitation, water filled pore space (WFPS), soil texture) in soils of different altitudes and rainfall. The mean annual N2O fluxes obtained at the study sites were: 0.87, 0.25 and 0.58 kg N ha−1 year−1 for altitudes 430, 1150 and 1120 m, respectively. These values are lower than averages reported for tropical forests in the world, including the Amazon rainforest, but they are higher than those reported for cerrado. The soils proved to be sinks of atmospheric CH4 with annual consumption of 3.2 and 2.0 kg C ha−1 year−1 for 430 and 1150 m, respectively. These values are very close to the reported ranges of CH4 uptake by soils of tropical forests in the world, but they are lower than the reported averages for cerrado. Monthly precipitation, WFPS, temperature, and soil texture were the main regulators of N2O fluxes in soils at the study sites. At lower altitude, the consumption of atmospheric CH4 by the soil surface reduces with increasing precipitation. WFPS between 45 and 50% appears to be more favorable for CH4 consumption by the soil surface.

Land use history determines non-native earthworm impacts on atmospheric methane consumption in forest soils, central New York State

Yavitt, J. B. 2015. Land use history determines non-native earthworm impacts on atmospheric methane consumption in forest soils, central New York State. Can. J. Soil Sci. 95: 321–330. I used complementary field and laboratory studies to examine the impact of two types of soil disturbance on net consumption of atmospheric methane (CH4) in forest soils near Ithaca, New York. One type of disturbance is invasion of non-native earthworms that mix soil layers, and the other is agriculture, which reduces the pit-and-mound surface topography to a flat landscape. Measurements of CH4fluxes between soil and the atmosphere were made in autumn before seasonal leaf fall when earthworms had consumed all of the previous year's leaf fall, and revealed no impact by earthworms in the never-tilled soils. Although earthworms did lead to greater consumption of atmospheric CH4in the post-agriculture soils, the mean consumption rate was only −0.2 mg m2d−1. Concentrations of atmospheric CH4in wormed soils were often greater than that in the atmosphere, suggesting that earthworms promote CH4production. In general, earthworms decreased soil permeability to gas diffusion. Post-agriculture soils also had faster CH4oxidation rates when incubated in the laboratory. The results taken together suggest that earthworm impacts on atmospheric CH4consumption depend on the history of soil disturbance.

Effects of Landuse Change on CH4 Soil-Atmospheric Exchange in Alpine Meadow on the Tibetan Plateau

Degradation of shrub meadows and reclamation of alpine meadows may heavily affect the soil sink for atmospheric methane (CH4), but this is poorly understood. Therefore, in situ measurements of atmospheric CH4 consumption were conducted in four landuse types: natural alpine meadow (NM), Elymus nutans pasture (EP), herbaceous meadow in shrub (HS), and a P. fruticosa shrub meadow (PS) within two years. CH4 fluxes were measured using static chambers and gas chromatography. All four types of land use showed atmospheric CH4 sink throughout the two years, with mean soil CH4 consumption rates at 24.6+/-10.9, 33.8+/-15.0, 39.8+/-10.3, and 28.1+/-12.1 mu g CH4.m(-2).hr(-1) for NM, EP, PS, and HS, respectively. Soil CH4 consumption increased by 40% by reclamation from NM to EP, while it decreased by 30% by degradation from PS to HS. Soil CH4 consumption in four types of land use was significantly correlated with temperature at 5 cm depth (P<0.01) and the soil water-filled pore space (WFPS) (P<0.05). Temperature showed stronger effects on soil CH4 consumption than WFPS, except in NM. UV radiation was positively correlated with soil CH4 consumption with increasing temperature and decreasing soil moisture. These findings indicate that a decrease in the grazing pressure in shrub meadows and increase in the area of artificial pasture reclaimed from alpine meadows would enhance the CH4 sink in alpine meadows on the Tibetan Plateau.

Technical Note: Disturbance of soil structure can lead to release of entrapped methane in glacier forefield soils

Abstract. Investigations of sources and sinks of atmospheric CH4 are needed to understand the global CH4 cycle and climate-change mitigation options. Glaciated environments might play a critical role due to potential feedbacks with global glacial meltdown. In an emerging glacier forefield, an ecological shift occurs from an anoxic, potentially methanogenic subglacial sediment to an oxic proglacial soil, in which soil-microbial consumption of atmospheric CH4 is initiated. The development of this change in CH4 turnover can be quantified by soil-gas profile analysis. We found evidence for CH4 entrapped in glacier forefield soils when comparing two methods for the collection of soil-gas samples: a modified steel rod (SR) designed for one-time sampling and rapid screening (samples collected ∼1 min after hammering the SR into the soil), and a novel multilevel sampler (MLS) for repetitive sampling through a previously installed access tube (samples collected weeks after access-tube installation). In glacier forefields on siliceous bedrock, sub-atmospheric CH4 concentrations were observed with both methods. Conversely, elevated soil-CH4 concentrations were observed in calcareous glacier forefields, but only in samples collected with the SR, while MLS samples all showed sub-atmospheric CH4 concentrations. Time-series of SR soil-gas sampling (additional samples collected 2, 3, 5, and 7 min after hammering) confirmed the transient nature of the elevated soil-CH4 concentrations, which were decreasing from ∼100 μL L−1 towards background levels within minutes. This hints towards the existence of entrapped CH4 in calcareous glacier forefield soil that can be released when sampling soil-gas with the SR. Laboratory experiments with miniature soil cores collected from two glacier forefields confirmed CH4 entrapment in these soils. Treatment by sonication and acidification resulted in a massive release of CH4 from calcareous cores (on average 0.3–1.8 μg CH4 (g d.w.)−1) (d.w. – dry weight); release from siliceous cores was 1–2 orders of magnitude lower (0.02–0.03 μg CH4 (g d.w.)−1). Clearly, some form of CH4 entrapment exists in calcareous glacier forefield soils, and to a much lesser extent in siliceous glacier forefield soils. Its nature and origin remain unclear and will be subject of future investigations.

The litter layer acts as a moisture-induced bidirectional buffer for atmospheric methane uptake by soil of a subtropical pine plantation

Forest soils are well known sinks for atmospheric methane (CH4), but how the surface litter layer controls gas diffusion into the mineral soil is still unclear. Seasonal rainfall in the humid climate provides a unique opportunity to examine uptake of atmospheric CH4 under a wide range of soil water content (SWC). We studied this question using a litter removal method in a 20-year-old slash pine (Pinus elliottii) plantation in subtropical China during 2005–2007. Soil-atmosphere CH4 fluxes of the control (FCK) and litter-free (FLF) treatments and their differences (litter-affected CH4 flux, FCK–LF = FCK − FLF) were all significantly influenced by SWC and not by soil temperature. Litter layer reduced atmospheric CH4 uptake by soil when SWC was below 15.8 vol%, and increased atmospheric CH4 consumption by soil when SWC was above this value. We concluded that the litter layer acts as a moisture-induced bidirectional buffer for atmospheric CH4 uptake by soils in a subtropical humid pine plantation. However, the removal of the litter layer had a minimal effect (+0.7%) on annual atmospheric CH4 uptake by soil, through compensating effects during the wet and dry seasons. Therefore, in the context of climate change, future changes in SWC will alter the strength of atmospheric CH4 uptake by soils of subtropical pine plantations.

Emissions of greenhouse gases and air pollutants from commercial aircraft at international airports in Korea

The emissions of greenhouse gases (GHGs) and air pollutants from aircraft in the boundary layer at four major international airports in Korea over a two-year period (2009–2010) were estimated using the Emissions and Dispersion Modeling System (EDMS) (i.e. activity-based (Landing/Take-Off (LTO) cycle) methodology). Both domestic and international LTOs and ground support equipment at the airports were considered. The average annual emissions of GHGs (CO2, N2O, CH4 and H2O) at all four airports during the study period were 1.11 × 103, 1.76 × 10−2, −1.85 × 10−3 and 3.84 × 108 kt yr−1, respectively. The emissions of air pollutants (NOx, CO, VOCs and particulate matter) were 5.20, 4.12, 7.46 × 10−1 and 3.37 × 10−2 kt yr−1, respectively. The negative CH4 emission indicates the consumption of atmospheric CH4 in the engine. The monthly and daily emissions of GHGs and air pollutants showed no significant variations at all airports examined. The emissions of GHGs and air pollutants for each aircraft operational mode differed considerably, with the largest emission observed in taxi-out mode.

Activity and diversity of methane-oxidizing bacteria in glacier forefields on siliceous and calcareous bedrock

Abstract. The global methane (CH4) cycle is largely driven by methanogenic archaea and methane-oxidizing bacteria (MOB), but little is known about their activity and diversity in pioneer ecosystems. We conducted a field survey in forefields of 13 receding Swiss glaciers on both siliceous and calcareous bedrock to investigate and quantify CH4 turnover based on soil-gas CH4 concentration profiles, and to characterize the MOB community by sequencing and terminal restriction fragment length polymorphism (T-RFLP) analysis of pmoA. Methane turnover was fundamentally different in the two bedrock categories. Of the 36 CH4 concentration profiles from siliceous locations, 11 showed atmospheric CH4 consumption at concentrations of ~1–2 μL L−1 with soil-atmosphere CH4 fluxes of –0.14 to –1.1 mg m−2 d−1. Another 11 profiles showed no apparent activity, while the remaining 14 exhibited slightly increased CH4 concentrations of ~2–10 μL L−1 , most likely due to microsite methanogenesis. In contrast, all profiles from calcareous sites suggested a substantial, yet unknown CH4 source below our sampling zone, with soil-gas CH4 concentrations reaching up to 1400 μL L−1. Remarkably, most soils oxidized ~90 % of the deep-soil CH4, resulting in soil-atmosphere fluxes of 0.12 to 31 mg m−2 d−1. MOB showed limited diversity in both siliceous and calcareous forefields: all identified pmoA sequences formed only 5 operational taxonomic units (OTUs) at the species level and, with one exception, could be assigned to either Methylocystis or the as-yet-uncultivated Upland Soil Cluster γ (USCγ). The latter dominated T-RFLP patterns of all siliceous and most calcareous samples, while Methylocystis dominated in 4 calcareous samples. Members of Upland Soil Cluster α (USCα) were not detected. Apparently, USCγ adapted best to the oligotrophic cold climate conditions at the investigated pioneer sites.

Microbial CH(4) and N(2)O Consumption in Acidic Wetlands.

Acidic wetlands are global sources of the atmospheric greenhouse gases methane (CH4), and nitrous oxide (N2O). Consumption of both atmospheric gases has been observed in various acidic wetlands, but information on the microbial mechanisms underlying these phenomena is scarce. A substantial amount of CH4 is consumed in sub soil by aerobic methanotrophs at anoxic–oxic interfaces (e.g., tissues of Sphagnum mosses, rhizosphere of vascular plant roots). Methylocystis-related species are likely candidates that are involved in the consumption of atmospheric CH4 in acidic wetlands. Oxygen availability regulates the activity of methanotrophs of acidic wetlands. Other parameters impacting on the methanotroph-mediated CH4 consumption have not been systematically evaluated. N2O is produced and consumed by microbial denitrification, thus rendering acidic wetlands as temporary sources or sinks for N2O. Denitrifier communities in such ecosystems are diverse, and largely uncultured and/or new, and environmental factors that control their consumption activity are unresolved. Analyses of the composition of N2O reductase genes in acidic wetlands suggest that acid-tolerant Proteobacteria have the potential to mediate N2O consumption in such soils. Thus, the fragmented current state of knowledge raises open questions concerning methanotrophs and denitrifiers that consume atmospheric CH4 and N2O in acidic wetlands.

Aircraft Emissions of Methane and Nitrous Oxide during the Alternative Aviation Fuel Experiment

Given the predicted growth of aviation and the recent developments of alternative aviation fuels, quantifying methane (CH(4)) and nitrous oxide (N(2)O) emission ratios for various aircraft engines and fuels can help constrain projected impacts of aviation on the Earth's radiative balance. Fuel-based emission indices for CH(4) and N(2)O were quantified from CFM56-2C1 engines aboard the NASA DC-8 aircraft during the first Alternative Aviation Fuel Experiment (AAFEX-I) in 2009. The measurements of JP-8 fuel combustion products indicate that at low thrust engine states (idle and taxi, or 4% and 7% maximum rated thrusts, respectively) the engines emit both CH(4) and N(2)O at a mean ± 1σ rate of 170 ± 160 mg CH(4) (kg Fuel)(-1) and 110 ± 50 mg N(2)O (kg Fuel)(-1), respectively. At higher thrust levels corresponding to greater fuel flow and higher engine temperatures, CH(4) concentrations in engine exhaust were lower than ambient concentrations. Average emission indices for JP-8 fuel combusted at engine thrusts between 30% and 100% of maximum rating were -54 ± 33 mg CH(4) (kg Fuel)(-1) and 32 ± 18 mg N(2)O (kg Fuel)(-1), where the negative sign indicates consumption of atmospheric CH(4) in the engine. Emission factors for the synthetic Fischer-Tropsch fuels were statistically indistinguishable from those for JP-8.

Reduced net atmospheric CH4 consumption is a sustained response to elevated CO2 in a temperate forest

We compared, from 2004 through 2006, rates of soil–atmosphere CH4 exchange at permanently established sampling sites in a temperate forest exposed to ambient (control plots; ∼380 μL L−1) or elevated (ambient + 200 μL L−1) CO2 since August 1996. A total of 880 observations showed net atmospheric CH4 consumption (flux from the atmosphere to the soil) from all static chambers most of the time at rates varying from 0.02 mg m−2 day−1 to 4.5 mg m−2 day−1. However, we infrequently found net CH4 production (flux from the soil to the atmosphere) at lower rates, 0.01 mg m−2 day−1 to 0.08 mg m−2 day−1. For the entire study, the mean (±SEM) rate of net CH4 consumption in control plots was higher than the mean for CO2-enriched plots, 0.55 (0.03) versus 0.51 (0.03) mg m−2 day−1. Annual rates of 184, 196, and 197 mg m−2 for net CH4 consumption at control plots during the three calendar years of this study were 19, 10, and 8% higher than comparable values for CO2 enriched plots. Differences between treatments were significant in 2004 and 2005 and nearly significant in 2006. Volumetric soil water content was consistently higher at CO2-enriched sites and a mixed-effects model identified a significant soil moisture x CO2 interaction on net atmospheric CH4 consumption. Increased soil moisture at CO2-enriched sites likely increases diffusional resistance of surface soils and the frequency of anaerobic microsites supporting methanogenesis, resulting in reduced rates of net atmospheric CH4 consumption. Our study extends our observations of reduced net atmospheric CH4 consumption at CO2-enriched plots to nearly five continuous years, suggesting that this is likely a sustained negative feedback to increasing atmospheric CO2 at this site.

Responses of ethylene and methane consumption to temperature and pH in temperate volcanic forest soils

SummaryThere is limited knowledge about the consumption and interaction of methane (CH4) and ethylene (C2H4) in forest soils under disturbances of temperature and acidification. Temperate volcanic forest topsoils (0‐5 cm) sampled under different tree species (e.g. Pinus sylvestris, Cryptomeria japonica and Quercus serrata) were used to study the capacities for CH4 and C2H4 consumption and their sensitivity to temperature and pH. We also studied the responses of soil nitrogen (N) transformations to temperature and relationships to consumption of both CH4 and C2H4. The C2H4 consumption rates increased with temperature up to 35oC, whereas the optimum temperature for CH4 consumption rates was approximately 25oC. Both Q10 values and activation energies for CH4 consumption rates over the range 5 to 25oC were larger than corresponding values for C2H4 consumption rates. The rates of nitrous oxide (N2O) and nitric oxide (NO) evolution and net N mineralization in the soils increased exponentially with temperature up to 35oC, with relatively large Q10 values and activation energies for NO evolution. In these forest topsoils, rates of CH4 and C2H4 consumption at pH &lt; 4.0 were negligible, and the pH optimum for both consumptions varied from 5.5 to 6.2. Most of the tested forest soils had an optimum pH for CH4 and C2H4 consumption that was above natural pH values, which indicated that soil acidification would inhibit CH4 and C2H4 consumption in situ. There was a high rate of net C2H4 evolution from forest soils acidified experimentally to pH &lt; 4.0, particularly from Cryptomeria japonica forest soil, and 67% of the variation in C2H4 evolution rates could be accounted for by the increase in soil water‐soluble organic carbon concentrations. Previous studies have shown that addition of C2H4 in headspace gases can inhibit atmospheric CH4 consumption in such forest soils. Hence, the evolution of C2H4 from temperate volcanic forest soils at decreasing pH can exacerbate inhibition of the soil atmospheric CH4 consumption in situ.

Effects of an experimental drought and recovery on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest

AbstractChanges in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climate. Here, we report the final results of a 5‐year, large‐scale (1 ha) throughfall exclusion experiment, followed by 1 year of recovery with natural throughfall, conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N2O emissions in four out of five treatment years (a natural drought year being the exception), and then recovered during the first year after the drought treatment stopped. Similarly, consumption of atmospheric CH4 increased under drought treatment, except during a natural drought year, and it also recovered to pretreatment values during the first year that natural throughfall was permitted back on the plot. No treatment effect was detected for NO emissions during the first 3 treatment years, but NO emissions increased in the fourth year under the extremely dry conditions of the exclusion plot during a natural drought. Surprisingly, there was no treatment effect on soil CO2 efflux in any year. The drought treatment provoked significant tree mortality and reduced the allocation of C to stems, but allocation of C to foliage and roots were less affected. Taken together, these results suggest that the dominant effect of throughfall exclusion on soil processes during this 6‐year period was on soil aeration conditions that transiently affected CH4, N2O, and NO production and consumption.

Ethylene oxidation, atmospheric methane consumption, and ammonium oxidation in temperate volcanic forest soils

Temperate volcanic forest surface soils under different forest stands (e.g., Pinus sylvestris L., Cryptomeria japonica, and Quercus serrata) were sampled to study the kinetics of ethylene (C2H4) oxidation and the C2H4 concentrations that effectively inhibit oxidation of atmospheric methane (CH4) and nitrification. The kinetics of C2H4 oxidation in temperate volcanic forest soils was biphasic, indicating that at least two different microbial populations, one with low and another with high apparent K m values, were responsible for ethylene oxidation. Methane consumption activity and ammonium oxidation of soil were inhibited by adding ethylene. Added C2H4 at concentrations of 3, 10, and 20 μl C2H4 per liter in the headspace gas respectively reduced by 20%, 50%, and 100% atmospheric CH4 consumption by soil, and these values were much smaller than those inhibiting ammonium oxidation in these forest soils; thus, the CH4 consumption activity was more sensitive to the addition of C2H4 than ammonium oxidation. Previous studies have shown that accumulation of C2H4 in such volcanic forest soils within 3 days of aerobic and anaerobic incubations can reach a range from 0.2 to 0.3 and from 1.0 to 3.0 μl C2H4 per liter in the headspace gas, respectively. It is suggested that C2H4 production beneath forest floors, particularly after heavy rain, can to some extent affect the capacity of forest surface soils to consume atmospheric CH4, but probably, it has no impact on ammonium oxidation.

Biochemical and molecular characterization of methanotrophs in soil from a pristine New Zealand beech forest

Methane (CH4) oxidation and the methanotrophic community structure of a pristine New Zealand beech forest were investigated using biochemical and molecular methods. Phospholipid-fatty acid-stable-isotope probing (PLFA-SIP) was used to identify the active population of methanotrophs in soil beneath the forest floor, while terminal-restriction fragment length polymorphism (T-RFLP) and cloning and sequencing of the pmoA gene were used to characterize the methanotrophic community. PLFA-SIP suggested that type II methanotrophs were the predominant active group. T-RFLP and cloning and sequencing of the pmoA genes revealed that the methanotrophic community was diverse, and a slightly higher number of type II methanotrophs were detected in the clone library. Most of the clones from type II methanotrophs were related to uncultured pmoA genes obtained directly from environmental samples, while clones from type I were distantly related to Methylococcus capsulatus. A combined data analysis suggested that the type II methanotrophs may be mainly responsible for atmospheric CH4 consumption. Further sequence analysis suggested that most of the methanotrophs detected shared their phylogeny with methanotrophs reported from soils in the Northern Hemisphere. However, some of the pmoA sequences obtained from this forest had comparatively low similarity (<97%) to known sequences available in public databases, suggesting that they may belong to novel groups of methanotrophic bacteria. Different methods of methanotrophic community analysis were also compared, and it is suggested that a combination of molecular methods with PLFA-SIP can address several shortcomings of stable isotope probing.

Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest

AbstractChanges in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N2O emissions by &gt;40% and increased rates of consumption of atmospheric CH4 by a factor of &gt;4. No treatment effect has yet been detected for NO and CO2 fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N2O and CO2 concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N2O, whereas significant CO2 production occurs down to 100 cm in both seasons, and small production of CO2 occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO2 production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H2O and CO2.

Unexpected results of a pilot throughfall exclusion experiment on soil emissions of CO 2 , CH 4 , N 2 O, and NO in eastern Amazonia

The eastern Amazon Basin may become drier as a result of less regional recirculation of water in a largely deforested landscape and because of increased frequency and intensity of El Nino events induced by global warming. Drier conditions may affect several plant and soil microbial processes, including soil emissions of CO2, CH4, NO, and N2O. We report here unanticipated results of a pilot study that was initiated to test the feasibility of a larger-scale throughfall exclusion experiment. In particular, soil drying caused a switch from net consumption of atmospheric CH4 by soils in the control plot to net CH4 emission from soils in the experimentally dried plot. This result is surprising because production of CH4 requires anaerobic microsites, which are uncommon in dry soil. A plausible explanation for increased CH4 emissions in the dried plot is that dry soil conditions favor termite activity and increased coarse root mortality provides them with a substrate. Another surprise was that both NO and N2O fluxes were elevated several years after initiation of the drying experiment. Apparently, a pulse of N availability caused by experimental drying persisted for at least 3 years. As expected, CO2 emissions were lower in the dried plots, which is consistent with lower rates of root growth observed in root in-growth cores placed in the dried plots. More work is needed to test these explanations and to confirm these phenomena, but these results demonstrate that changes in climate could have unanticipated effects on biogeochemical processes in soils that we do not adequately understand.

Influence of atmospheric CO2 enrichment on methane consumption in a temperate forest soil

AbstractRates of atmospheric CH4 consumption of soils in temperate forest were compared in plots continuously enriched with CO2 at 200 µL L−1 above ambient and in control plots exposed to the ambient atmosphere of 360 µL CO2 L−1. The purpose was to determine if ecosystem atmospheric CO2 enrichment would alter soil microbial CH4 consumption at the forest floor and if the effect of CO2 would change with time or with environmental conditions. Reduced CH4 consumption was observed in CO2‐enriched plots relative to control plots on 46 out of 48 sampling dates, such that CO2‐enriched plots showed annual reductions in CH4 consumption of 16% in 1998 and 30% in 1999. No significant differences were observed in soil moisture, temperature, pH, inorganic‐N or rates of N‐mineralization between CO2‐enriched and control plots, indicating that differences in CH4 consumption between treatments were likely the result of changes in the composition or size of the CH4‐oxidizing microbial community. A repeated measures analysis of variance that included soil moisture, soil temperature (from 0 to 30 cm), and time as covariates indicated that the reduction of CH4 consumption under elevated CO2 was enhanced at higher soil temperatures. Additionally, the effect of elevated CO2 on CH4 consumption increased with time during the two‐year study. Overall, these data suggest that rising atmospheric CO2 will reduce atmospheric CH4 consumption in temperate forests and that the effect will be greater in warmer climates. A 30% reduction in atmospheric CH4 consumption by temperate forest soils in response to rising atmospheric CO2 will result in a 10% reduction in the sink strength of temperate forest soils in the atmospheric CH4 budget and a positive feedback to the greenhouse effect.

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.