Sink For Atmospheric CH4 Research Articles

Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes

SummaryThis review examines the interactions between soil physical factors and the biological processes responsible for the production and consumption in soils of greenhouse gases. The release of CO2 by aerobic respiration is a non‐linear function of temperature over a wide range of soil water contents, but becomes a function of water content as a soil dries out. Some of the reported variation in the temperature response may be attributable simply to measurement procedures. Lowering the water table in organic soils by drainage increases the release of soil carbon as CO2 in some but not all environments, and reduces the quantity of CH4 emitted to the atmosphere. Ebullition and diffusion through the aerenchyma of rice and plants in natural wetlands both contribute substantially to the emission of CH4; the proportion of the emissions taking place by each pathway varies seasonally. Aerated soils are a sink for atmospheric CH4, through microbial oxidation. The main control on oxidation rate is gas diffusivity, and the temperature response is small. Nitrous oxide is the third greenhouse gas produced in soils, together with NO, a precursor of tropospheric ozone (a short‐lived greenhouse gas). Emission of N2O increases markedly with increasing temperature, and this is attributed to increases in the anaerobic volume fraction, brought about by an increased respiratory sink for O2. Increases in water‐filled pore space also result in increased anaerobic volume; again, the outcome is an exponential increase in N2O emission. The review draws substantially on sources from beyond the normal range of soil science literature, and is intended to promote integration of ideas, not only between soil biology and soil physics, but also over a wider range of interacting disciplines.

Afforestation enhanced soil CH4 uptake rate in subtropical China: Evidence from carbon stable isotope experiments

Afforestation plays an important role in regulating the methane (CH4) exchange between soil and atmosphere. However, it is not fully understood how afforestation affects soil CH4 flux and the carbon isotopic signature of CH4. We conducted a year-long measurement of CH4 in afforested land (woodland and shrubland) and the adjacent cropland using the static chamber-gas chromatographic technique in the Danjiangkou Reservoir of central China. The soil exclusively functioned as a sink for atmospheric CH4 through the entire study period across land use types. Land use types significantly impacted the CH4 uptake rate with the largest average CH4 uptake rate in the shrubland (37.22 μg m−2·h−1), followed by the woodland (27.75 μg m−2·h−1) and the cropland (14.34 μg m−2·h−1). The mean annual CH4 uptake rates increased in the shrubland by 186.3% and the woodland by 93.5%, compared to the cropland. The isotope fractionation factor (αsoil) was lower in the woodland and shrubland, compared to the cropland. The CH4 uptake rates and αsoil exhibited similar seasonal patterns among land use types, with a higher CH4 uptake rates and lower αsoil in spring and summer compared to other seasons. The CH4 uptake rates were positively related to microbial biomass carbon (MBC) and labile C. Meanwhile, the CH4 uptake rate was exponentially correlated with inorganic nitrogen (N) concentration, suggesting the high inorganic N concentration in the cropland possibly inhibited the CH4 uptake rate. In afforested land, CH4 uptake rates positively correlated with soil temperature and negatively correlated with the C: N ratio. The αsoil was negatively related to soil temperature, whereas the δ13C values of CH4 remaining in the chambers were positively related to the δ13C values of soil organic carbon (SOC) and MBC. Our results suggest that the change in soil properties (i.e. high SOC and MBC, low C:N ratio and low inorganic N) following afforestation is a critical control on enhanced CH4 uptake capacity, while a lower αsoil further provides evidence for a high CH4 uptake rate in afforested lands.

Impacts of climate and land use on N2 O and CH4 fluxes from tropical ecosystems in the Mt. Kilimanjaro region, Tanzania.

In this study, we quantify the impacts of climate and land use on soil N2 O and CH4 fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land-use gradients at Mt. Kilimanjaro, combining long-term insitu chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N2 O and CH4 respectively. N2 O emissions correlated positively with soil moisture and total soil nitrogen content. CH4 uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N2 O emissions of tropical montane forests were generally low (<1.2kgNha-1 year-1 ), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10kgNha-1 year-1 ). Forest soils with well-aerated litter layers were a significant sink for atmospheric CH4 (up to 4kgCha-1 year-1 ) regardless of low mean annual temperatures at higher elevations. Land-use intensification significantly increased the soil N2 O source strength and significantly decreased the soil CH4 sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non-CO2 GHG emissions following land-use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N2 O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land-use change.

Effect of Biochar on CH4 and N2O Emissions from Lou Soil

In order to investigate the effect of biochar on CH4 and N2O emissions from Lou soil, field plot experiments of winter wheat were conducted with five levels of biochar addition (0, 20, 40, 60, and 80 t·hm-2). The fluxes of CH4 and N2O, wheat production, soil organic carbon, soil water content, and temperature of each soil layer were measured. The results showed that the fluxes of CH4 and N2O changed significantly in different growth periods of winter wheat. Compared with the control, the cumulative CH4 uptake under the biochar amendment increased by 12.88%-71.61%. When the biochar addition was ≥ 40 t·hm-2, the cumulative CH4 uptake was significantly higher and the highest uptake was at the level of 40 t·hm-2. Biochar amendment had no significant effect on cumulative N2O emissions and the global warming potential (GDP). The greenhouse gas intensity (GHGI) decreased by 13.24%-22.14%. The wheat yield increased by 1.72%-32.19% after biochar addition. When the applied biochar level was ≥ 40 t·hm-2, the wheat yield increments were significantly higher. The biochar addition of 40 t·hm-2 was the optimal level for increasing the wheat yield. The soil organic carbon and water content under biochar amendment increased by 1.42-2.69 times and 7.08%-11.96%, respectively. The results suggested that Lou soil was the sink of atmospheric CH4 and the emission source of N2O during the winter wheat growth period, and the biochar level of 40 t·hm-2 was the optimal addition amount.

Responses of CH4 and N2O fluxes to land-use conversion and fertilization in a typical red soil region of southern China

Land-use conversion and fertilization have been widely reported as important management practices affecting CH4 and N2O fluxes; however, few long-term in situ measurements are available after land-use conversion from rice paddies to upland cultivation, especially those including the initial stages after conversion. A 3-year field experiment was conducted in rice paddies and a newly converted citrus orchard to measure CH4 and N2O fluxes in response to land-use conversion and fertilization in a red soil region of southern China. Annual CH4 and N2O emissions averaged 303.9 kg C ha−1 and 3.8 kg N ha−1, respectively, for the rice paddies over three cultivation years. Although annual N2O emissions increased two- to threefold after the conversion of rice paddies to citrus orchard, the substantial reduction in CH4 emissions and even shift into a sink for atmospheric CH4 led to significantly lower CO2-eq emissions of CH4 and N2O in the citrus orchard compared to the rice paddies. Moreover, distinct CH4 emissions were observed during the initial stages and sustained for several weeks after conversion. Our results indicated that the conversion of rice paddies to citrus orchards in this region for higher economic benefits may also lead to lower aggregate CH4 and N2O emissions.

High Temporal and Spatial Variability of Atmospheric-Methane Oxidation in Alpine Glacier Forefield Soils.

Glacier forefield soils can provide a substantial sink for atmospheric CH4, facilitated by aerobic methane-oxidizing bacteria (MOB). However, MOB activity, abundance, and community structure may be affected by soil age, MOB location in different forefield landforms, and temporal fluctuations in soil physical parameters. We assessed the spatial and temporal variability of atmospheric-CH4 oxidation in an Alpine glacier forefield during the snow-free season of 2013. We quantified CH4 flux in soils of increasing age and in different landforms (sandhill, terrace, and floodplain forms) by using soil gas profile and static flux chamber methods. To determine MOB abundance and community structure, we employed pmoA gene-based quantitative PCR and targeted amplicon sequencing. Uptake of CH4 increased in magnitude and decreased in variability with increasing soil age. Sandhill soils exhibited CH4 uptake rates ranging from -3.7 to -0.03 mg CH4 m-2 day-1 Floodplain and terrace soils exhibited lower uptake rates and even intermittent CH4 emissions. Linear mixed-effects models indicated that soil age and landform were the dominating factors shaping CH4 flux, followed by cumulative rainfall (weighted sum ≤4 days prior to sampling). Of 31 MOB operational taxonomic units retrieved, ∼30% were potentially novel, and ∼50% were affiliated with upland soil clusters gamma and alpha. The MOB community structures in floodplain and terrace soils were nearly identical but differed significantly from the highly variable sandhill soil communities. We concluded that soil age and landform modulate the soil CH4 sink strength in glacier forefields and that recent rainfall affects its short-term variability. This should be taken into account when including this environment in future CH4 inventories.IMPORTANCE Oxidation of methane (CH4) in well-drained, "upland" soils is an important mechanism for the removal of this potent greenhouse gas from the atmosphere. It is largely mediated by aerobic, methane-oxidizing bacteria (MOB). Whereas there is abundant information on atmospheric-CH4 oxidation in mature upland soils, little is known about this important function in young, developing soils, such as those found in glacier forefields, where new sediments are continuously exposed to the atmosphere as a result of glacial retreat. In this field-based study, we investigated the spatial and temporal variability of atmospheric-CH4 oxidation and associated MOB communities in Alpine glacier forefield soils, aiming at better understanding the factors that shape the sink for atmospheric CH4 in this young soil ecosystem. This study contributes to the knowledge on the dynamics of atmospheric-CH4 oxidation in developing upland soils and represents a further step toward the inclusion of Alpine glacier forefield soils in global CH4 inventories.

Methane emission from feather moss stands.

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH4 . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH4 turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH4 cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH4 emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH4 h-1 gdw-1 . Temperature and water content positively influenced CH4 emission. Methanogenic potentials determined under N2 atmosphere in darkness ranged between 22 and 157 pmol CH4 h-1 gdw-1 . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH4 was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 104 and 105 copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH4 was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH4 produced was depleted in 13 C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3-62 mM). The δ13 C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH4 by hydrogenotrophic methanogenesis and causing net emission of CH4 under ambient conditions, albeit at low rates.

Draft Genome Sequence of Uncultured Upland Soil Cluster Gammaproteobacteria Gives Molecular Insights into High-Affinity Methanotrophy.

ABSTRACTAerated soils form the second largest sink for atmospheric CH4. A near-complete genome of uncultured upland soil cluster Gammaproteobacteria that oxidize CH4 at <2.5 ppmv was obtained from incubated Antarctic mineral cryosols. This first genome of high-affinity methanotrophs can help resolve the mysteries about their phylogenetic affiliation and metabolic potential.

Global methane emission estimates for 2000–2012 from CarbonTracker Europe-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; v1.0

Abstract. We present a global distribution of surface methane (CH4) emission estimates for 2000–2012 derived using the CarbonTracker Europe-CH4 (CTE-CH4) data assimilation system. In CTE-CH4, anthropogenic and biospheric CH4 emissions are simultaneously estimated based on constraints of global atmospheric in situ CH4 observations. The system was configured to either estimate only anthropogenic or biospheric sources per region, or to estimate both categories simultaneously. The latter increased the number of optimizable parameters from 62 to 78. In addition, the differences between two numerical schemes available to perform turbulent vertical mixing in the atmospheric transport model TM5 were examined. Together, the system configurations encompass important axes of uncertainty in inversions and allow us to examine the robustness of the flux estimates. The posterior emission estimates are further evaluated by comparing simulated atmospheric CH4 to surface in situ observations, vertical profiles of CH4 made by aircraft, remotely sensed dry-air total column-averaged mole fraction (XCH4) from the Total Carbon Column Observing Network (TCCON), and XCH4 from the Greenhouse gases Observing Satellite (GOSAT). The evaluation with non-assimilated observations shows that posterior XCH4 is better matched with the retrievals when the vertical mixing scheme with faster interhemispheric exchange is used. Estimated posterior mean total global emissions during 2000–2012 are 516 ± 51 Tg CH4 yr−1, with an increase of 18 Tg CH4 yr−1 from 2000–2006 to 2007–2012. The increase is mainly driven by an increase in emissions from South American temperate, Asian temperate and Asian tropical TransCom regions. In addition, the increase is hardly sensitive to different model configurations ( &lt; 2 Tg CH4 yr−1 difference), and much smaller than suggested by EDGAR v4.2 FT2010 inventory (33 Tg CH4 yr−1), which was used for prior anthropogenic emission estimates. The result is in good agreement with other published estimates from inverse modelling studies (16–20 Tg CH4 yr−1). However, this study could not conclusively separate a small trend in biospheric emissions (−5 to +6.9 Tg CH4 yr−1) from the much larger trend in anthropogenic emissions (15–27 Tg CH4 yr−1). Finally, we find that the global and North American CH4 balance could be closed over this time period without the previously suggested need to strongly increase anthropogenic CH4 emissions in the United States. With further developments, especially on the treatment of the atmospheric CH4 sink, we expect the data assimilation system presented here will be able to contribute to the ongoing interpretation of changes in this important greenhouse gas budget.

Annual methane uptake from different land uses in an agro-pastoral ecotone of northern China

The semi-arid grasslands in northern China are subjected to accelerating land use change due to population growth and food demand. Considerable uncertainty exists on annual methane (CH4) fluxes of different land uses because most measurements have only been conducted during the growing season. Using static chamber – gas chromatographic technique, we quantified and characterized annual CH4 fluxes (from 2012 to 2015) from four land uses common in an agro-pastoral ecotone of northern China: summer-grazed grassland (SumGrazed), winter-grazed grassland (WinGrazed), ungrazed grassland since 1997 (Ungrazed) and oat cropland (OatCrop). The soil at all land uses functioned exclusively as a sink for atmospheric CH4 through the entire three years. Annual CH4 uptake rates averaged 1.42, 2.36, 1.12 and 2.57kgCha−1 yr−1 for SumGrazed, WinGrazed, Ungrazed and OatCrop, respectively, during 2012–2015. Compared to Ungrazed, OatCrop and WinGrazed increased annual CH4 uptake by 129 and 111%, respectively. Non-growing season (October–April) contributed 28–43% of the annual CH4 uptake at all land uses. Across all four land uses, annual cumulative CH4 uptake decreased with increasing soil water-filled pore space (WFPS) explaining 81% of the variance in annual CH4 uptake. WFPS negatively correlated to CH4 uptake in the growing season (R2=0.16-0.35, P<0.001). CH4 uptake increased with soil temperature through the entire observed period (R2=0.38-0.63, P<0.001) and the non-growing season (R2=0.51–0.74, P<0.001). We conclude that grazing has the potential to increase CH4 uptake from the atmosphere and consequently, contribute positively to CH4 related part of C budget.

Nitrous Oxide and Methane Emissions in Spring Maize Field in the Semi‐Arid Regions of Loess Plateau

A 2‐year field study was conducted to measure nitrous oxide (N2O) and methane (CH4) in a rain‐fed spring maize cropland in the Loess Plateau, P. R. China, and to determine the effects of optimized nitrogen (N) fertilization practices on urea‐derived N2O emission factor (EF), grain yield, net greenhouse gas (NGHG) emission, and net greenhouse gas intensity (NGHGI). Five treatments were considered, including control (CK), conventional N fertilization (Con), optimal N fertilization (Opt), optimal N fertilization plus nitrification inhibitor (Opt + DCD), and optimal N fertilization with slow release urea (Opt + SR). Soil acted as a small sink for atmospheric CH4. Nitrogen fertilization and heavy rainfall events (&gt;40 mm) were the main factors controlling N2O emissions. The annual mean EF ranged from 0.12 to 0.55%. Compared to conventional N fertilizer, nitrification inhibitor decreased the annual cumulative N2O, NGHG, and NGHGI emissions by 45, 52, and 48%, respectively, without decreasing grain yield. In conclusion, nitrification inhibitor addition was the most effective practice to reduce N2O emissions in the rain‐fed regions of Loess Plateau.

Warmer and drier conditions and nitrogen fertilizer application altered methanotroph abundance and methane emissions in a vegetable soil.

Methane (CH4) is a potent greenhouse gas, and soil can both be a source and sink for atmospheric CH4. It is not clear how future climate change may affect soil CH4 emissions and related microbial communities. The aim of this study was to determine the interactive effects of a simulated warmer and drier climate scenarios and the application of different nitrogen (N) sources (urea and manure) on CH4 emissions and related microbial community abundance in a vegetable soil. Greenhouses were used to control simulated climate conditions which gave 2.99°C warmer and 6.2% lower water content conditions. The field experiment was divided into two phases. At the beginning of phase II, half of the greenhouses were removed to study possible legacy effects of the simulated warmer and drier conditions. The responses in methanogen and methanotroph abundance to a simulated climate change scenario were determined using real-time PCR. The results showed that the simulated warmer and drier conditions in the greenhouses significantly decreased CH4 emissions largely due to the lower soil moisture content. For the same reason, CH4 emissions of treatments in phase I were much lower than the same treatments in phase II. The abundance of methanotrophs showed a more significant response than methanogens to the simulated climate change scenario, increasing under simulated drier conditions. Methanogenic community abundance remained low, except where manure was applied which provided a source of organic C that stimulated methanogen growth. Soil moisture content was a major driver for methanotroph abundance and strongly affected CH4 emissions. The application of N source decreased CH4 emissions probably because of increased methanotrophic activity. CH4 emissions were positively correlated to methanogenic abundance and negatively correlated to methanotrophic abundance. These results demonstrate that projected future climate change conditions can have a feedback impact on CH4 emissions from the soil by altering soil conditions (particularly soil moisture) and related microbial communities.

Microbial contributions to subterranean methane sinks.

Sources and sinks of methane (CH4 ) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH4 concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH4 . Recently, it was hypothesized that this CH4 depletion may be caused by radiolysis, an abiotic process whereby CH4 is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH4 concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH4 in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH4 at a rate of 1.3-2.7mgCH4 ·m-2 ·d-1 . These CH4 oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH4 in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH4 not only in caves, but also in smaller-size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH4 oxidation scales up to affect local, regional, and global CH4 cycling.

Consumption of atmospheric methane in a limestone cave in Indiana, USA

Recent observations suggest that karst landscapes may be an unaccounted sink for atmospheric CH4, but questions remain about the processes contributing to sub-atmospheric CH4 mole fractions in caves. The CH4 dynamics associated with karst environments were studied over 18months at 6 locations in Buckner Cave, Southern Indiana by measuring the mole fractions and stable isotopic composition of CH4 and carbon dioxide (CO2). CO2 mole fractions were used to infer seasonal changes in airflow. Samples were obtained on a monthly basis. CH4 mole fractions ranged from 1.9±0.1ppm near the cave entrance to 0.1±0.1ppm in the more remote parts of the cave. The carbon and hydrogen stable isotopic compositions of CH4 in the cave ranged from −58.7 to +7 ‰ (VPDB) and −170 to +10 ‰ (VSMOW), respectively. The isotopic data suggest that the CH4 dynamics of Buckner Cave can be described by a seasonally variable mixing system in which atmospheric CH4 enters primarily through the main entrance and is subsequently consumed by methanotrophs. Additionally, at least two smaller CH4 sources are evident when air has been stagnant. The results suggest that subterranean karst cavities are an important sink for atmospheric CH4.

Greenhouse gas emissions in a spring wheat–field pea sequence under different tillage practices in semi-arid Northwest China

Greenhouse gas emissions from agriculture production are predicted to increase as the world’s population and demand for food increases. This research assessed the influence of tillage systems on CO2, N2O and CH4 fluxes and soil properties in spring wheat–field pea sequence in a rain-fed semi-arid environment. The tillage practices included; conventional tillage with straw removed (T), no–till with straw removed (NT), no-till with straw retention on the soil surface (NTS) and conventional tillage with straw incorporated (TS). Greenhouse gas emissions were monitored during the 2013, 2014 and 2015 cropping seasons using a carbon dioxide analyzer and static chamber-gas chromatography. Although the results showed that all treatments served as sources of atmospheric CO2 and N2O, and a sink of atmospheric CH4, CO2 and N2O emissions significantly decreased by 21 and 34 % in NTS sites, and CH4 uptake increased by 22 % compared with T fields. Soil organic carbon (0–30 cm) increased by 24, 19 and 7 % in NTS compared to T, NT and TS respectively. In addition, the contribution to increase soil organic carbon was much higher under straw retention treatments. T plots significantly increased global warming potential by 37 and 30 % in N2O and CH4 versus NTS soils. NTS increased straw and grain yield compared with straw removal treatments (NT and T), but that had no effect compared with TS treatment. NTS farming practices demonstrated increases in crop productivity and reduced greenhouse gas under dryland cropping systems.

Drivers of atmospheric methane uptake by montane forest soils in the southern Peruvian Andes

Abstract. The soils of tropical montane forests can act as sources or sinks of atmospheric methane (CH4). Understanding this activity is important in regional atmospheric CH4 budgets given that these ecosystems account for substantial portions of the landscape in mountainous areas like the Andes. We investigated the drivers of net CH4 fluxes from premontane, lower and upper montane forests, experiencing a seasonal climate, in south-eastern Peru. Between February 2011 and June 2013, these soils all functioned as net sinks for atmospheric CH4. Mean (standard error) net CH4 fluxes for the dry and wet season were −1.6 (0.1) and −1.1 (0.1) mg CH4-C m−2 d−1 in the upper montane forest, −1.1 (0.1) and −1.0 (0.1) mg CH4-C m−2 d−1 in the lower montane forest, and −0.2 (0.1) and −0.1 (0.1) mg CH4-C m−2 d−1 in the premontane forest. Seasonality in CH4 exchange varied among forest types with increased dry season CH4 uptake only apparent in the upper montane forest. Variation across these forests was best explained by available nitrate and water-filled pore space indicating that nitrate inhibition of oxidation or diffusional constraints imposed by changes in water-filled pore space on methanotrophic communities may represent important controls on soil–atmosphere CH4 exchange. Net CH4 flux was inversely related to elevation; a pattern that differs to that observed in Ecuador, the only other extant study site of soil–atmosphere CH4 exchange in the tropical Andes. This may result from differences in rainfall patterns between the regions, suggesting that attention should be paid to the role of rainfall and soil moisture dynamics in modulating CH4 uptake by the organic-rich soils typical of high-elevation tropical forests.

Environmental and biological controls on CH4 exchange over an evergreen Mediterranean forest

Methane (CH4) fluxes in a Mediterranean holm oak (Quercus ilex L.) forest were measured over 19 months, using eddy covariance technique and soil chambers measurements, complemented by microbiological characterization of CH4-cycling microorganisms along a vertical soil profile.CH4 budget at the site was close to the neutrality, although periodical switching of the ecosystem from sink to source was observed at daily and seasonal scale. Forest soil represented a net sink for atmospheric CH4 throughout the whole study period (mean uptake rate of 1.64×10−3μmolm−2s−1).Different environmental parameters influenced CH4 fluxes at different time scales.At half-hour timescale, CH4 fluxes followed a half-sinusoid curve with high emission recorded during the central hour of day (mean emission rate of 1.05×10−2μmolm−2s−1). Stomatal conductance, solar radiation (global and UV) and latent heat flux were the main drivers controlling ecosystem CH4 emissions, suggesting the occurrence of a plant-mediated transport through xylem as a way of CH4 emissions from canopy and supporting the hypothesis of UV-induced production of CH4 from leaves at the site.At daily scale, CH4 fluxes were strongly connected to temperature and precipitation which promote CH4-oxidizing microorganisms activity during the cold season and methanogens activity during the dry season.This study is the first long-term CH4 eddy covariance measurement above a Mediterranean forest and demonstrates that, at present, the ecosystem is a small sink of CH4 when considering both the soil and vegetation processes together, and this sink capacity is strictly connected to the water availability. Future changes in temperature and precipitation patterns may increase CH4 emissions, turning the ecosystem to a source of CH4.

Influence of run of river dams on floodplain sediments and carbon dynamics

Quantifying the biophysical impacts of management on river and stream ecosystems is an important issue that requires understanding of ecological, hydrological and geomorphological processes. We conducted a year-long ecogeomorphological experiment to determine sedimentation and carbon cycling differences between run-of-river (ROR) dams in a 200year old impounded floodplain and a floodplain that was formerly impounded >65years ago. Our study shows that ROR dams do not necessarily enhance floodplain sedimentation or carbon storage, but promote brief periods of sediment CH4 flux (up to 2.91nmolCH4m−2s−1) to the atmosphere. Removal of a ROR dam may result in channel widening, and removal by lateral transport (i.e., erosion) of nearly 14 MgC per floodplain. We did not find significant differences in mean sediment CO2 fluxes or temperature sensitivity (Q10=2.1±0.4) of CO2 efflux among floodplains. All floodplains were likely an annual net source of sediment CO2 flux (annual mean of 2.12±0.974μmolCO2m−2s−1) to the atmosphere, and a sink for atmospheric CH4 (annual mean of −0.221±0.163nmolCH4m−2s−1). We provide a conceptual model on the management consequences on ROR dam structures for floodplain sedimentation/erosion, and sediment carbon cycling.

Can we explain the observed methane variability after the Mount Pinatubo eruption?

Abstract. The CH4 growth rate in the atmosphere showed large variations after the Pinatubo eruption in June 1991. A decrease of more than 10 ppb yr−1 in the growth rate over the course of 1992 was reported, and a partial recovery in the following year. Although several reasons have been proposed to explain the evolution of CH4 after the eruption, their contributions to the observed variations are not yet resolved. CH4 is removed from the atmosphere by the reaction with tropospheric OH, which in turn is produced by O3 photolysis under UV radiation. The CH4 removal after the Pinatubo eruption might have been affected by changes in tropospheric UV levels due to the presence of stratospheric SO2 and sulfate aerosols, and due to enhanced ozone depletion on Pinatubo aerosols. The perturbed climate after the eruption also altered both sources and sinks of atmospheric CH4. Furthermore, CH4 concentrations were influenced by other factors of natural variability in that period, such as El Niño–Southern Oscillation (ENSO) and biomass burning events. Emissions of CO, NOX and non-methane volatile organic compounds (NMVOCs) also affected CH4 concentrations indirectly by influencing tropospheric OH levels.Potential drivers of CH4 variability are investigated using the TM5 global chemistry model. The contribution that each driver had to the global CH4 variability during the period 1990 to 1995 is quantified. We find that a decrease of 8–10 ppb yr−1 CH4 is explained by a combination of the above processes. However, the timing of the minimum growth rate is found 6&amp;amp;nash;9 months later than observed. The long-term decrease in CH4 growth rate over the period 1990 to 1995 is well captured and can be attributed to an increase in OH concentrations over this time period. Potential uncertainties in our modelled CH4 growth rate include emissions of CH4 from wetlands, biomass burning emissions of CH4 and other compounds, biogenic NMVOC and the sensitivity of OH to NMVOC emission changes. Two inventories are used for CH4 emissions from wetlands, ORCHIDEE and LPJ, to investigate the role of uncertainties in these emissions. Although the higher climate sensitivity of ORCHIDEE improves the simulated CH4 growth rate change after Pinatubo, none of the two inventories properly captures the observed CH4 variability in this period.

Impacts of prescribed burning on soil greenhouse gas fluxes in a suburban native forest of south-eastern Queensland, Australia

Abstract. Prescribed burning is a forest management practice that is widely used in Australia to reduce the risk of damaging wildfires. Prescribed burning can affect both carbon (C) and nitrogen (N) cycling in the forest and thereby influence the soil-atmosphere exchange of major greenhouse gases, i.e. carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). To quantify the impact of a prescribed burning (conducted on 27 May 2014) on greenhouse gas exchange and the potential controlling mechanisms, we carried out a series of field measurements before (August 2013) and after (August 2014 and November 2014) the fire. Gas exchange rates were determined in four replicate plots which were burned during the combustion and in another four adjacent unburned plots located in green islands, using a set of static chambers. Surface soil properties including temperature, pH, moisture, soil C and N pools were also determined either by in situ measurement or by analysing surface 10 cm soil samples. All of the chamber measurements indicated a net sink of atmospheric CH4, with mean CH4 uptake ranging from 1.15 to 1.99 mg m−2 d−1. Prescribed burning significantly enhanced CH4 uptake as indicated by the significant higher CH4 uptake rates in the burned plots measured in August 2014. In the following 3 months, the CH4 uptake rate was recovered to the pre-burning level. Mean CO2 emission from the forest soils ranged from 2721.76 to 7113.49 mg m−2 d−1. The effect of prescribed burning on CO2 emission was limited within the first 3 months, as no significant difference was observed between the burned and the adjacent unburned plots in both August and November 2014. The CO2 emissions showed more seasonal variations, rather than the effects of prescribed burning. The N2O emission in the plots was quite low, and no significant impact of prescribed burning was observed. The changes in understory plants and litter layers, surface soil temperature, C and N substrate availability and microbial activities, following the prescribed burning, were the factors that controlled the greenhouse gas exchanges. Our results suggested that the low-intensity prescribed burning would decrease soil CO2 emission and increase CH4 uptake, but this effect would be present within a relatively short period. Only slight changes in the surface soil properties during the combustion and very limited impacts of prescribed burning on the mineral soils supported the rapid recovery of the greenhouse gas exchange rates.