Diurnal Oscillations of Gas Production and Effluxes (CO2 and CH4) in Cores from a Peat Bog

Peat cores (15 cm diam X 30 cm deep) from Ellergower Moss, New Galloway, Scotland were kept and monitored at constant temperature (10 ± 0.1ºC) for gas production using a 1.6 mm diam stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. In the headspace, O₂, CO₂ and CH₄ (measured at m/z values 32, 44 and 15 respectively) showed diurnal fluctuations in low-intensity natural daylight and under a light-dark (LD, 12:12) regime. Over the first few cycles O₂ and CO₂ increased together in the dark and decreased in the light, whereas CH₄ showed variations in antiphase with the other two gases. CO₂ and CH₄ also showed diurnal oscillations at 15 cm depth, but these decreased together in the light whereas argon (m/z = 40) was not varying. A highly-damped free-run of the oscillations in gas concentrations at 15cm depth was evident for only 3 cycles in complete darkness and at constant temperature. This might suggest desynchronization between individual plants with different free-running periods. A hydrocarbon signal (m/z = 26) at 15 cm depth also showed diurnal cycles but out of phase with CO 2 and CH 4 . We postulate a circadian control of microbiological activities imposed by the vascular plants (Carex, Eriophorum, Molinia, Calluna, Erica). Under natural conditions the pronounced temperature sensitivity of CO₂ and CH₄ emission results in entrainment to daily temperature cycles. The amplitudes of the rhythms are greatest when temperature and light intensity changes are most pronounced, i.e. when the fluctuations in environmental factors are most potent as synchronizers (zeitgebers) and as masking factors.     

Diurnal Oscillations in Gas Production (O2, CO2, CH4, and N2) in Soil Monoliths

Soil cores (35 cm long, 7 cm diameter) from the Macaulay Land Use Research Institute's Sourhope Research Station in the Scottish Borders were kept and monitored at constant temperature (18± 1°C) for gas production using a 1.6 mm diameter stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. This provided a novel method for on-line, real time monitoring of soil gas dynamics. In closed-system headspace experiments, O₂ and CO₂ (measured at m/z values 32 and 44, respectively) showed anti-phase diurnal fluctuations in low-intensity simulated daylight and under a light-dark (LD, 12:12 h) regime. O₂ increased during periods of illumination and decreased in the dark. The inverse was true for CO₂ production. Ar (m/z = 40) concentration and temperature (°C) remained constant throughout the experiments. The same phase-related oscillations, in CO₂ and O₂ concentrations, were observed at 2 and 5 cm depth in soil cores. The O₂ concentration did not oscillate diurnally at 10 cm depth. In below-ground experiments, CH₄ (m/z = 15) concentration showed diurnal cycles at 2, 5 and 10 cm depth. The CH₄ production had the same diurnal phase cycle as CO₂ but with lower amplitude. Evidence of below-ground diurnal oscillations in N₂ (m/z = 28) concentration was provided at 5 cm depth. The scale of production and consumption of gases associated with soil-atmosphere interactions and below-ground processes, are shown to be a multifaceted output of several variables. These include light, circadian-controlled physiological rhythms of plants and microbes, and the interactions between these organisms.     

Mass spectrometric monitoring of gases (CO2, CH4, O2) in a mesotrophic peat core from Kopparås Mire, Sweden

Membrane inlet mass spectrometry was used to monitor dissolved gas concentrations (CO₂, CH₄ and O₂) in a mesotrophic peat core from Kopparås, Sweden. 1 A comparison of depth profiles (down to 22 cm) with an ombrotrophic peat core (Ellergower, SW Scotland) investigated previously, revealed major differences in gas concentrations. Thus methane reached concentrations more than twice as high (800 μM) at depths greater than 12 cm in the Kopparås core. As shown previously, the primary determinant of the depth of the oxic zone is the level of the water table. Whereas in the Scottish cores, mass spectrometric detectability of O₂ was confined to the first 3 cm below this level, in the Swedish core penetration of O₂ was greater (7 cm). CO₂ profiles were similar in cores from both locations. 2 A thick layer of Sphagnum mosses dominated the plant cover of the Swedish peat core. A poorly developed deep root system, as distinct from that of the vascular plant cover in Scottish cores, diminished gas exchange rates, and presumably aerobic methane oxidation at depth around roots. These characteristics may contribute to the development of discontinuities in gas profiles at depths greater 15 cm as upward gas transport is established predominantly by diffusion and/or ebullition in the Swedish core. 3 Monitoring gas concentrations at the peat surface and at 2 cm depth after changing water tables showed a delayed response of approximately 4 days as a result of the high water content and moisture‐regulating capacity of mosses. 4 Recovery processes at 2 cm depth after raising the water table revealed final production rates of dissolved CO₂ and CH₄ in the peat pore water between 0.8 and 4.4 μmol h^?1 L^?1 and between 0.1 and 1.7 μmol h^?1 L^?1, respectively. Higher production rates were found during the day, indicating a diurnal rhythm due to plant photosynthetic activity even at the low values of photosynthetically active radiation (PAR: 110 μmol s^?1 m^?2) used in the experimental set‐up. 5 In the water‐logged mesotrophic Kopparås core changes of dissolved gas concentrations (DGC) at 3 and 14 cm depth were surface temperature‐dependent rather than light dependent. This suggests that changes of air temperature alters the covering vegetation to increase the conductivity for dissolved gases through vascular plants and to facilitate gas transport by diffusion and/or ebullition.     

Response of tundra CH4 and CO2 flux tomanipulation of temperature and vegetation

We conducted plant species removals, air temperaturemanipulations, and vegetation and soil transplants inAlaskan wet-meadow and tussock tundra communities todetermine the relative importance of vegetation typeand environmental variables in controlling ecosystemmethane (CH₄) and carbon dioxide (CO₂) flux. Plastic greenhouses placed over wet-meadow tundraincreased air temperature, soil temperature, and soilmoisture, but did not affect CH₄ or CO₂ flux(measured in the dark). By contrast, removal ofsedges in the wet meadow significantly decreased fluxof CH₄, while moss removal tended to increaseCH₄ emissions. At 15 cm depth, pore-waterCH₄ concentrations were higher in sedge-removalthan in control plots, suggesting that sedgescontribute to CH₄ emissions by transportingCH₄ from anaerobic soil to the atmosphere, ratherthan by promoting methanogenesis. Inreciprocal-ecosystem transplants between thewet-meadow and tussock tundra communities, CH₄and CO₂ emissions were higher overall in thewet-meadow site, but were unrelated to transplantorigin. Methane flux was correlated with localvariation in soil temperature, thaw depth, andwater-table depth, but the relative importance ofthese factors varied through the season. Our resultssuggest that future changes in CH₄ and CO₂flux in response to climatic change will be morestrongly mediated by large-scale changes in vegetationand soil parameters than by direct temperature effects.     

CO2 emissions from an undrained tropical peatland: Interacting influences of temperature,shading and water table depth

Emission of CO₂ from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂ emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het). We made two central observations. First, we found strong diurnal cycles in CO₂ flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_het had a strong linear relationship to the depth of the water table, and under flooded conditions, R_het was small and constant. We used this relationship between R_het and water table depth to estimate carbon export from both R_het and dissolved organic carbon over the course of a year based on water table records. R_het dominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO₂ emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂ flux, water table and temperature: water table and daily average flux data suggest that CO₂ is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂ fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO₂ emissions to increase linearly with water table depth and also have a large diurnal cycle.     

Impact of Grazing Intensity and Seasons on Greenhouse Gas Emissions in Tropical Grassland

Greenhouse gases (GHG) can be affected by grazing intensity, soil, and climate variables. This study aimed at assessing GHG emissions from a tropical pasture of Brazil to evaluate (i) how the grazing intensity affects the magnitude of GHG emissions; (ii) how season influences GHG production and consumption; and (iii) what are the key driving variables associated with GHG emissions. We measured under field conditions, during two years in a palisade-grass pasture managed with 3 grazing intensities: heavy (15 cm height), moderate (25 cm height), and light (35 cm height) N₂O, CH₄ and CO₂ fluxes using static closed chambers and chromatographic quantification. The greater emissions occurred in the summer and the lower in the winter. N₂O, CH₄, and CO₂ fluxes varied according to the season and were correlated with pasture grazing intensity, temperature, precipitation, % WFPS (water-filled pores space), and soil inorganic N. The explanatory variables differ according to the gas and season. Grazing intensity had a negative linear effect on annual cumulative N₂O emissions and a positive linear effect on annual cumulative CO₂ emissions. Grazing intensity, season, and year affected N₂O, CH₄, and CO₂ emissions. Tropical grassland can be a large sink of N₂O and CH₄. GHG emissions were explained for different key driving variables according to the season.     

塔克拉玛干沙漠腹地冬季土壤呼吸及其驱动因子

利用Li-8150系统测定了塔克拉玛干沙漠腹地冬季(1月)土壤呼吸,分析了环境驱动因子对极端干旱区荒漠生态系统土壤呼吸的影响。结果表明:(1)冬季土壤呼吸日变化呈现出显著的单峰曲线,土壤呼吸速率最大值出现在12:00,为0.0684μmol CO2m-2s-1,凌晨04:00附近出现最小值,为-0.0473μmol CO2m-2s-1;(2)土壤呼吸速率与各层气温,0cm地表温度均存在着极其显著或显著的线性关系,且都具有正相关性;(3)土壤呼吸速率与5cm土壤湿度存在着较为明显的线性关系,该层湿度能够解释土壤呼吸的69.5%;(4)0cm地表温度对土壤呼吸贡献最大,其次是5cm土壤湿度;(5)以0cm地表温度、5cm土壤湿度为变量,通过多元回归分析表明:土壤温度-湿度构成的多变量模型能够解释大于86.9%的土壤呼吸变化情况;(6)研究时段内土壤呼吸速率的平均值是-1.45mg CO2m-2h-1。     

The hyporheic zone as a source of dissolved organic carbon and carbon gases to a temperate forested stream

The objective of this study was to examine chemical changes in porewaters that occur over small scales (cm) as groundwater flows through the hyporheic zone and discharges to a stream in a temperate forest of northern Wisconsin. Hyporheic-zone porewaters were sampled at discrete depths of 2, 10, 15, 61, and 183 cm at three study sites in the study basin. Chemical profiles of dissolved organic carbon (DOC), CO₂, CH₄, and pH show dramatic changes between 61 cm sediment depth and the water-sediment interface. Unless discrete samples at small depth intervals are taken, these chemical profiles are not accounted for. Similar trends were observed at the three study locations, despite each site having very different hydraulic-flow regimes. Increases in DOC concentration by an order of magnitude from 61 to 15 cm depth with a corresponding decrease in pH and rapid decreases in the molecular weight of the DOC suggest that aliphatic compounds (likely organic acids) are being generated in the hyporheic zone. Estimated efflux rates of DOC, CO₂, and CH₄ to the stream are 6.2, 0.79, 0.13 moles m² d^-1, respectively, with the vast majority of these materials produced in the hyporheic zone. Very little of these materials are accounted for by sampling stream water, suggesting rapid uptake and/or volatilization.     

模拟水位变化对杭州湾芦苇湿地夏季温室气体日通量的影响

水位是影响湿地温室气体排放的重要因子。采用静态箱-气相色谱法研究了模拟条件下不同水位(0、5、10 cm和20 cm)对芦苇湿地温室气体(CO_2、CH_4、N_2O)夏季昼夜通量变化的影响。结果表明,1)4种不同水位CO_2通量日变化均表现为昼低夜高,且白天为汇,夜间为源,整体均表现为CO_2的汇;不同水位CH_4通量日变化则均表现为昼高夜低,且整体上均表现为CH_4的源;N_2O通量总体上水淹后均表现为昼高夜低而0cm水位表现为昼低夜高;2)随着水位的增加CH_4和CO_2平均通量呈现先增加后降低的趋势,且10cm水位下CH_4和CO_2平均通量最高,N_2O通量则在5cm水位最高;3)通过相关性和主成分分析表明,气温、水温是土壤CH_4、N_2O通量日变化的主导因子,而土壤温度是CO_2日变化通量的主导因子,同时,土壤p H、Eh及水体p H、Eh是CO_2通量日变化的重要因子之一。     

Spatial and temporal variations of dissolved gases (CH4, CO2, and O2) in peat cores

Spatial and temporal variations in the concentrations of dissolved gases (CH₄, CO₂, and O₂) in peat cores were studied using membrane inlet mass spectrometry (MIMS). Variations in vertical gas profiles were observed between random peat cores taken from hollows on the same peat bog. Methane concentrations in profiles (0–30 cm) generally increased with depth and reached maximum values in the range of 200–450 m CH₄ below about 13-cm depth. In some profiles, a peak of dissolved methane was observed at 7-cm depth. Oxygen penetrated to approximately 2-cm depth in the hollows. The sampling probe was used to continuously monitor CH₄, CO₂, and O₂ concentrations at fixed depths in peat cores over periods of several days. The concentration of dissolved CO₂ and O₂ at 1-cm depth oscillated over a 24-h period with the maximum of CO₂ concentration corresponding with the minimum of 0₂. Diurnal variations in CO₂ but not CH₄ were measured at 15-cm depth; dissolved CO₂ levels decreased during daylight hours to a constant minimum concentration of 4.85 mm. This report also describes the application of MIMS for the measurement of gaseous diffusion rates in peat using an inert gas (argon); the value of D, the diffusion coefficient, was 2.07 × 10^–8 m² s^–1. Correspondence to: D. Lloyd     

Diurnal cycling of dissolved gas concentrations in a willow vegetation filter treating landfill leachate

Membrane inlet mass spectrometry (MIMS) was used to monitor continuously and simultaneously the concentrations of dissolved gases (O₂, CO₂, CH₄), in the willow root zone at the laboratory scale, and within the treatment bed of a willow vegetation filter treating leachate at a landfill site in mid Wales. These results demonstrate that willows are able to release oxygen into the root zone which accumulates during daylight. Diurnal cycles of oxygen, carbon dioxide, and methane were observed, whereby CO₂ and CH₄ varied reciprocally in relation to O₂. The intensity of these cycles appeared to be related to light intensity and temperature. Oxygen was shown to fluctuate between completely anaerobic and fully aerated (300 μM), between day and night in sunny conditions.     

The Impact of Nitrogen Placement and Tillage on NO, N2O, CH4 and CO2 Fluxes from a Clay Loam Soil

To evaluate the impact of N placement depth and no-till (NT) practice on the emissions of NO, N₂O, CH₄ and CO₂ from soils, we conducted two N placement experiments in a long-term tillage experiment site in northeastern Colorado in 2004. Trace gas flux measurements were made 2–3 times per week, in zero-N fertilizer plots that were cropped continuously to corn (Zea mays L.) under conventional-till (CT) and NT. Three N placement depths, replicated four times (5, 10 and 15 cm in Exp. 1 and 0, 5 and 10 cm in Exp. 2, respectively) were used. Liquid urea–ammonium nitrate (UAN, 224 kg N ha⁻¹) was injected to the desired depth in the CT- or NT-soils in each experiment. Mean flux rates of NO, N₂O, CH₄ and CO₂ ranged from 3.9 to 5.2 μg N m⁻² h⁻¹, 60.5 to 92.4 μg N m⁻² h⁻¹, −0.8 to 0.5 μg C m⁻² h⁻¹, and 42.1 to 81.7 mg C m⁻² h⁻¹ in both experiments, respectively. Deep N placement (10 and 15 cm) resulted in lower NO and N₂O emissions compared with shallow N placement (0 and 5 cm) while CH₄ and CO₂ emissions were not affected by N placement in either experiment. Compared with N placement at 5 cm, for instance, averaged N₂O emissions from N placement at 10 cm were reduced by more than 50% in both experiments. Generally, NT decreased NO emission and CH₄ oxidation but increased N₂O emissions compared with CT irrespective of N placement depths. Total net global warming potential (GWP) for N₂O, CH₄ and CO₂ was reduced by deep N placement only in Exp. 1 but was increased by NT in both experiments. The study results suggest that deep N placement (e.g., 10 cm) will be an effective option for reducing N oxide emissions and GWP from both fertilized CT- and NT-soils.     

Carbon quality and nutrient status drive the temperature sensitivity of organic matter decomposition in subtropical peat soils

Estimates of gaseous carbon (C) fluxes in wetlands are heavily based on temperature. However, isolating specific effects of temperature on anaerobic C processing from other controls (C quality and nutrients) has proven difficult. Here, we test the hypothesis that temperature sensitivity of soil organic matter (SOM) decomposition is more influenced by C quality than nutrient availability in subtropical freshwater, sawgrass (Cladium jamaicense)-based peats. Carbon age (characterized by depth: 0–10 and 10–20 cm) was used as a surrogate of C quality while two sites were selected with contrasting levels of nutrient (P) availability. In anaerobic laboratory incubations temperature was increased in 5 °C steps to assess the proportion of C available at a given temperature (i.e. thermo-labile C) as productions of gaseous (CO₂ and CH₄) and dissolved organic C (DOC) fractions. Thermo-labile C increased 3.1–3.6 times from 15 °C to 30 °C in all soils. Disproportionate increase in the production of gaseous forms versus DOC as well as CH₄:CO₂ was observed with warming. Observed Q₁₀ values followed the trend of CH₄ (~14) ? CO₂ (~2.5) > DOC (~1.7) and temperature sensitivity was more dependent on C quality than nutrient availability over the entire temperature range. Spectral analysis indicated more bio-available DOC production at higher temperature. Regression analysis also indicated that C quality primarily influenced SOM decomposition at lower temperature, while at higher temperature nutrient limitation dominantly controlled SOM decomposition. These findings confirm the role of C quality in temperature sensitivity of warm peat soils, but also indicate an increased importance of nutrient limitation at higher temperature.     

Diurnal and seasonal variation in methane emissions in a northern Canadian peatland measured by eddy covariance

Eddy covariance measurements of methane (CH₄) net flux were made in a boreal fen, typical of the most abundant peatlands in western Canada during May–September 2007. The objectives of this study were to determine: (i) the magnitude of diurnal and seasonal variation in CH₄ net flux, (ii) the relationship between the temporally varying flux rates and associated changes in controlling biotic and abiotic factors, and (iii) the contribution of CH₄ emission to the ecosystem growing season carbon budget. There was significant diurnal variation in CH₄ emission during the peak of the growing season that was strongly correlated with associated changes in solar radiation, latent heat flux, air temperature and ecosystem conductance to water vapor. During days 181–215, nighttime average CH₄ efflux was only 47% of the average midday values. The peak value for daily average CH₄ emission rate was approximately 80 nmol m^?2 s^?1 (4.6 mg CH₄ m^?2 h^?1), and seasonal variation in CH₄ flux was strongly correlated with changes in soil temperature. Integrated over the entire measurement period [days 144–269 (late May–late September)], the total CH₄ emission was 3.2 g CH₄ m^?2, which was quite low relative to other wetland ecosystems and to the simultaneous high rate of ecosystem net CO₂ sequestration that was measured (18.1 mol CO₂ m^?2 or 217 g C m^?2). We estimate that the negative radiative forcing (cooling) associated with net carbon storage over the life of the peatland (approximately 2200 years) was at least twice the value of positive radiative forcing (warming) caused by net CH₄ emission over the last 50 years.     

Effects of a transient oxic period on mineralization of organic matter to CH4 and CO2 in anoxic peat incubations

Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH₄ and CO₂, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH₄ and CO₂ to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH₄ production, oxic conditions during period 2 did not result in a lower potential CH₄ production rate during period 3, although production was delayed ～1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH₄, oxic conditions during period 2 resulted in zero production of CH₄ during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH₄ production, the oxic period had a negligible effect on anoxic CO₂ production during period 3, in surface as well as deep peat. In both surface and deep peat, CO₂ production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO₂ production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.     

Greenhouse gas dynamics in boreal, littoral sediments under raised CO2 and nitrogen supply

SUMMARY 1. The effects of increasing CO₂ and nitrogen loading and of a change in water table and temperature on littoral CH₄, N₂O and CO₂ fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at ?15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO₂ for 18 weeks. The experiment included fertilisation with NO₃^– and NH₄⁺ (to a total 3 g N m^?2). 2. Changes in the water table and temperature strongly regulated sediment CH₄ and cCO₂ fluxes (community CO₂ release), but did not affect N₂O emissions. Increase in the water table increased CH₄ emissions but reduced cCO₂ release, while increase in temperature increased emissions of both CO₂ and CH₄. 3. The raised CO₂ increased carbon turnover in the sediments, such that cCO₂ release was increased by 16–26%. However, CH₄ fluxes were not significantly affected by raised CO₂, although CH₄ production potential (at 22 °C) of the sediments incubated at high CO₂ was increased. In the boreal region, littoral CH₄ production is more likely to be limited by temperature than by the availability of carbon. Raised CO₂ did not affect N₂O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO₃^– did severely limit N₂O production. The NO₃^– addition caused up to a 100‐fold increase in the fluxes of N₂O. The NH₄⁺ addition did not increase N₂O fluxes, indicating low nitrification capacity in the sediments.     

Diurnal Cycle of Oxygen and Sulfide Microgradients and Microbial Photosynthesis in a Cyanobacterial Mat Sediment

The diurnal variation in the microgradients of O₂, H₂S, and Eh were studied in the benthic cyanobacterial mats of a hypersaline desert lake (Solar Lake, Sinai). The results were related to light intensity, light penetration into the mat, temperature, pH, NH₄⁺, photosynthetic activity, pigments, and the zonation of the microbial community. Extreme diurnal variation was found, with an O₂ peak of 0.5 mM at 1 to 2 mm of depth below the mat surface during day and a H₂S peak of 2.5 mM at 2 to 3 mm of depth at night. At the O₂-H₂S interface, the two compounds coexisted over a depth interval of 0.2 to 1 mm and with a turnover time of a few minutes. The photic zone reached 2.5 mm into the mat in summer, and the main ¹⁴CO₂ light fixation took place at 1 to 2 mm of depth. During winter, light and photosynthesis were restricted to the uppermost 1 mm. The quantitative dynamics of O₂ and H₂S were calculated from the chemical gradients and from the measured diffusion coefficients.     

Spatial and temporal variability in methane emissions from tree stems of <Emphasis Type="Italic">Fraxinus mandshurica</Emphasis> in a cool-temperate floodplain forest

We performed field measurements on the spatial and temporal variability in CH₄ emissions from stem surfaces of mature Fraxinus mandshurica Rupr. trees in a floodplain forest of northern Japan. Stem CH₄ fluxes were measured by a static closed-chamber method at ca. 15 cm above ground on ten selected trees to test among-individual variability, and the diurnal and seasonal changes in three representative trees. Daytime stem CH₄ emission rates varied between 81 and 1,305 µg CH₄ m^?2 h^?1 among individual trees, and showed a spatial gradient apparently corresponding to the difference in water table depth at the experimental site. Stem CH₄ fluxes were quite stable throughout a 24 h period for foliated trees in August and were similar for defoliated trees in November. Large differences were observed in the magnitude of seasonal changes in stem CH₄ flux among individual trees; one sampled tree showed no clear seasonal changes in stem CH₄ flux, while another tree exhibited drastic seasonal changes ranging larger than one order of magnitude. Results demonstrated the high variability in stem CH₄ emissions in space and time, and suggested the importance of soil temperature, water table depth and porewater CH₄ concentration as possible environmental factors controlling stem CH₄ emissions from temperate forested wetlands.     

Dissolved gases in frozen basal water from the NGRIP borehole: implications for biogeochemical processes beneath the Greenland Ice Sheet

Little information exists on biogeochemical transformations in aquatic ecosystems beneath polar ice sheets (i.e., water-saturated sediments, streams, rivers, and lakes) and their role in global elemental cycles. Subglacial environments may represent important sources of atmospheric CO₂ and/or CH₄ during deglaciation, thus acting as amplifiers in the climate system. However, the role of subglacial environments in global climate processes has been difficult to assess given the absence of biogeochemical data from the basal zones of inland polar ice sheets. Here, we report on the concentrations of CO₂, CH₄, and H₂ in samples of refrozen basal water recovered at a depth of ~3,042 meters below the surface during the North Greenland Ice Core Project (NGRIP). CH₄ and H₂ concentrations in the NGRIP samples were approximately 60- and 700-fold higher, respectively, relative to air-equilibrated water, whereas CO₂ was ~fivefold lower. Metabolic pathways such as (1) methanogenesis, (2) organic matter fermentation, carboxydotrophic, and/or methylotrophic activity, and (3) CO₂ fixation provide plausible biotic explanations for the observed CH₄, H_2, and CO₂ concentrations, respectively.     

Vertical profiles of CH4 concentrations,dissolved substrates and processes involved in CH4 production in a flooded Italian rice field

Vertical profiles were measured in soil cores taken from flooded rice fields in the Po valley during July and August 1990. Methane concentrations generally increased with depth and reached maximum values of 150–500 μM in 5–13 cm depth. However, the shape of the profiles was very different when studying different soil cores. The CH₄ content of gas bubbles showed a similar variability which apparently was due to spatial rather than temporal inhomogeneities. Similar inhomogeneities were observed in the vertical profiles of acetate, propionate, lactate, and formate which showed maximum values of 1500, 66, 135, and 153, μM, respectively. However, maxima and minima of the vertical profiles of the different substates usually coincided in one particular soil core. Large inhomogeneities in the vertical profiles were also observed for the rates of total CH₄ production, however, the percentage contribution of H₂/CO₂ to CH₄ production was relatively homogeneous at 24 ± 7% (SD). Similarly, the H₂ content of gas bubbles was relatively constant at 93.3 ± 9.6 ppmv when randomly sampled in the rice field at different times of the day. A small contribution (6%) of H₂/CO₂ to acetate production was also observed. Vertical profiles of the respiratory index (RI) for [2-¹⁴C] acetate showed that acetate was predominantly degraded by methanogenesis in 5–11 cm depth, but by respiration in the surface soil (3 cm depth) and in soil layers below 13–16 cm depth which coincided with a transition of the colour (grey to reddish) and the physical characteristics (porosity, density) of the soil. The observations indicate that the microbial community which degrades organic matter to CH₄ is in itself relatively homogenous, but operates at highly variable rates within the soil structure. Author for correspondence