Net Source Of CH4 Research Articles

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

The hydrothermal caves linked to active faulting can potentially harbour subterranean atmospheres with a distinctive gaseous composition with deep endogenous gases, such as carbon dioxide (CO2) and methane (CH4). In this study, we provide insight into the sourcing, mixing, and biogeochemical processes involved in the dynamic of deep endogenous gas formation in an exceptionally dynamic hypogenic karst system (Vapour Cave, southern Spain) associated with active faulting. The cave environment is characterized by a prevailing combination of rising warm air with large CO2outgassing (>1%) and highly diluted CH4with an endogenous origin. Theδ13CCO2data, which ranges from −4.5 to −7.5‰, point to a mantle-rooted CO2that is likely generated by the thermal decarbonation of underlying marine carbonates, combined with degassing from CO2-rich groundwater. A pooled analysis ofδ13CCO2data from exterior, cave, and soil indicates that the upwelling of geogenic CO2has a clear influence on soil air, which further suggests a potential for the release of CO2along fractured carbonates. CH4molar fractions and theirδD andδ13C values (ranging from −77 to −48‰ and from −52 to −30‰, respectively) suggest that the methane reaching Vapour Cave is the remnant of a larger source of CH4, which was likely generated by microbial reduction of carbonates. This CH4has been affected by a postgenetic microbial oxidation, such that the gas samples have changed in both molecular and isotopic composition after formation and during migration through the cave environment. Yet, in the deepest cave locations (i.e., 30 m below the surface), measured concentration values of deep endogenous CH4are higher than in atmospheric with lighterδ13C values with respect to those found in the local atmosphere, which indicates that Vapour Cave may occasionally act as a net source of CH4to the open atmosphere.

Tundra landscape heterogeneity, not interannual variability, controls the decadal regional carbon balance in the Western Russian Arctic.

Across the Arctic, the net ecosystem carbon (C) balance of tundra ecosystems is highly uncertain due to substantial temporal variability of C fluxes and to landscape heterogeneity. We modeled both carbon dioxide (CO2 ) and methane (CH4 ) fluxes for the dominant land cover types in a ~100-km2 sub-Arctic tundra region in northeast European Russia for the period of 2006-2015 using process-based biogeochemical models. Modeled net annual CO2 fluxes ranged from -300gCm-2 year-1 [net uptake] in a willow fen to 3gCm-2 year-1 [net source] in dry lichen tundra. Modeled annual CH4 emissions ranged from -0.2 to 22.3gCm-2 year-1 at a peat plateau site and a willow fen site, respectively. Interannual variability over the decade was relatively small (20%-25%) in comparison with variability among the land cover types (150%). Using high-resolution land cover classification, the region was a net sink of atmospheric CO2 across most land cover types but a net source of CH4 to the atmosphere due to high emissions from permafrost-free fens. Using a lower resolution for land cover classification resulted in a 20%-65% underestimation of regional CH4 flux relative to high-resolution classification and smaller (10%) overestimation of regional CO2 uptake due to the underestimation of wetland area by 60%. The relative fraction of uplands versus wetlands was key to determining the net regional C balance at this and other Arctic tundra sites because wetlands were hot spots for C cycling in Arctic tundra ecosystems.

Interacting effects of elevated atmospheric CO2 and hydrology on the growth and carbon sequestration of Sphagnum moss

Peatlands are a critical carbon store comprising 30% of the Earth’s terrestrial soil carbon. Sphagnum mosses comprise up to 90% of peat in the northern hemisphere but impacts of climate change on Sphagnum mosses are poorly understood, limiting development of sustainable peatland management and restoration. This study investigates the effects of elevated atmospheric CO2 (eCO2) (800 ppm) and hydrology on the growth of Sphagnum fallax, Sphagnum capillifolium and Sphagnum papillosum and greenhouse gas fluxes from moss–peat mesocosms. Elevated CO2 levels increased Sphagnum height and dry weight but the magnitude of the response differed among species. The most responsive species, S. fallax, yielded the most biomass compared to S. papillosum and S. capillifolium. Water levels and the CO2 treatment were found to interact, with the highest water level (1 cm below the surface) seeing the largest increase in dry weight under eCO2 compared to ambient (400 ppm) concentrations. Initially, CO2 flux rates were similar between CO2 treatments. After week 9 there was a consistent three-fold increase of the CO2 sink strength under eCO2. At the end of the experiment, S. papillosum and S. fallax were greater sinks of CO2 than S. capillifolium and the − 7 cm water level treatment showed the strongest CO2 sink strength. The mesocosms were net sources of CH4 but the source strength varied with species, specifically S. fallax produced more CH4 than S. papillosum and S. capillifolium. Our findings demonstrate the importance of species selection on the outcomes of peatland restoration with regards to Sphagnum’s growth and GHG exchange.

A cool-temperate young larch plantation as a net methane source - A 4-year continuous hyperbolic relaxed eddy accumulation and chamber measurements

Upland forests are thought to be methane (CH4) sinks due to oxidation by methanotrophs in aerobic soils. However, CH4 budget for upland forests are not well quantified at the ecosystem scale, when possible CH4 sources, such as small wet areas, exists in the ecosystem. Here, we quantified CH4 fluxes in a cool-temperate larch plantation based on four-year continuous measurements using the hyperbolic relaxed eddy accumulation (HREA) method and dynamic closed chambers with a laser-based analyzer. After filling data gaps for half-hourly data using machine-learning-based regressions, we found that the forest acted as a net CH4 source at the canopy scale: 30 ± 11 mg CH4 m−2 yr−1 in 2014, 56 ± 8 mg CH4 m−2 yr−1 in 2015, 154 ± 5 mg CH4 m−2 yr−1 in 2016, and 132 ± 6 mg CH4 m−2 yr−1 in 2017. Hotspot emissions from the edge of the pond could strongly contribute to the canopy-scale emissions. The magnitude of the hotspot emissions was 10–100 times greater than the order of the canopy-scale and chamber-based CH4 fluxes at the dry soils. The high temperatures with wet conditions stimulated the hotspot emissions, and thus induced canopy-scale CH4 emissions in the summer. Understanding and modeling the dynamics of hotspot emissions are important for quantifying CH4 budgets of upland forests. Micrometeorological measurements at various forests are required for revisiting CH4 budget of upland forests.

Recent climatic changes and wetland expansion turned Tibet into a net CH4 source

Methane (CH4) is the second largest contributor to the greenhouse effect. However, it remains unclear to what extent the CH4 cycle acts as a feedback to climate changes, due to insufficient observational constraints and poor knowledge of wetland extent dynamics. The Tibetan Plateau (TP), which has an average elevation of 4000+ m above sea level, contains one-third of China’s natural wetlands. Rapid climate warming (i.e., ~ 0.5 °C per decade since the 1960s) and increasing precipitation in the region have caused wetlands to dry up and then expand, especially since the 2000s. In this study, we assessed the uncertainty and temporal variation of the CH4 budget during 1979–2012 using a biogeochemical model, in situ measurements and dynamic wetland maps. The results showed that the drying up of wetlands from the 1980s to 1990s completely counteracted the rising CH4 emission rates (0.75 ± 0.18 and 0.77 ± 0.19 Tg CH4 year−1 in the 1980s and 1990s, respectively). However, recent precipitation-induced wetland expansion enhanced emissions to 0.96 ± 0.21 Tg CH4 year−1 in the 2000s, which exceeded the rate of CH4 uptake (0.74 ± 0.06 Tg CH4 year−1 in the 2000s). A nonlinear role played by wetland extent in the CH4 budget was revealed, suggesting that there is a need to incorporate wetland extent dynamics over a longer period into model simulations to understand the variation in wetland CH4 release during past decades. Furthermore, the results also indicate that more hydrological components, e.g., wetland shrinkage and expansion under increasing precipitation and glacial melt, should be taken into consideration when projecting wetland CH4 release on the TP.

Phosphorus addition mitigates N<sub>2</sub>O and CH<sub>4</sub> emissions in N-saturated subtropical forest, SW China

Abstract. Chronically elevated nitrogen (N) deposition has led to severe nutrient imbalance in forest soils. Particularly in tropical and subtropical forest ecosystems, increasing N loading has aggravated phosphorus (P) limitation of biomass production, and has resulted in elevated emissions of nitrous oxide (N2O) and reduced uptake of methane (CH4), both of which are important greenhouse gases. Yet, the interactions of N and P and their effects on greenhouse gas emissions remain elusive. Here, we report N2O and CH4 emissions together with soil N and P data for a period of 18 months following a single P addition (79 kg P ha−1, as NaH2PO4 powder) to an N-saturated, Masson pine-dominated forest soil at TieShanPing (TSP), Chongqing, south-western (SW) China. We observed a significant decline in both nitrate (NO3−) concentrations in soil water (5 and 20 cm depths) and in soil N2O emissions, following P application. We hypothesise that enhanced N uptake by plants in response to P addition, resulted in less available NO3− for denitrification. By contrast to most other forest ecosystems, TSP is a net source of CH4. P addition significantly decreased CH4 emissions and turned the soil from a net source into a net sink. Based on our observation and previous studies in South America and China, we believe that P addition relieves N inhibition of CH4 oxidation. Within the 1.5 years after P addition, no significant increase of forest growth was observed and P stimulation of forest N uptake by understorey vegetation remains to be confirmed. Our study indicates that P fertilisation of N-saturated, subtropical forest soils may mitigate N2O and CH4 emissions, in addition to alleviating nutrient imbalances and reducing losses of N through NO3− leaching.

Temperate forest methane sink diminished by tree emissions.

Global budgets ascribe 4-10% of atmospheric methane (CH4 ) sinks to upland soils and have assumed until recently that soils are the sole surface for CH4 exchange in upland forests. Here we report that CH4 is emitted from the stems of dominant tree species in a temperate upland forest, measured using both the traditional static-chamber method and a new high-frequency, automated system. Tree emissions averaged across 68 observations on 17 trees from May to September were 1.59±0.88μmolCH4 m-2 stemh-1 (mean±95% confidence interval), while soils adjacent to the trees consumed atmospheric CH4 at a rate of -4.52±0.64μmolCH4 m-2 soilh-1 (P<0.0001). High-frequency measurements revealed diurnal patterns in the rate of tree-stem CH4 emissions. A simple scaling exercise suggested that tree emissions offset 1-6% of the growing season soil CH4 sink and may have briefly changed the forest to a net CH4 source.

Eddy covariance measurements of the net turbulent methane flux in the city centre – results of 2-year campaign in Łódź, Poland

Abstract. To investigate temporal variability of methane (CH4) fluxes in an urban environment, air–surface exchange fluxes of CH4 were continuously measured using eddy covariance techniques at a city-centre site in Łódź, Poland, from July 2013 to August 2015. In the immediate vicinity of the measurement site, potential methane sources include vehicle traffic, dense sewerage infrastructure and natural gas networks. Sensible and latent heat fluxes have also been measured since 2000 and carbon dioxide fluxes since 2007 at this site. Upward CH4 fluxes dominated during the measurement period, indicating that the city centre is a net source of CH4 to the troposphere. The highest monthly fluxes were observed in winter (2.0 to 2.7 g m−2 month−1) and the lowest in summer (0.8 to 1.0 g m−2 month−1). Fluxes on working days were around 6 % higher than on weekends. The cumulative flux indicates that the city centre emitted a net quantity of nearly 18 g m−2 of CH4 in 2014. Stable values of the FCO2∕ FCH4 ratio in months (minimum 2.41 × 10−3, maximum 5.3 × 10−3) and the lack of a clear annual course suggest comparable magnitude of both fluxes.

Soil Greenhouse Gas Emissions and Carbon Dynamics of a No-Till, Corn-Based Cellulosic Ethanol Production System

Crop residues like corn (Zea mays L.) stover perform important functions that promote soil health and provide ecosystem services that influence agricultural sustainability and global biogeochemical cycles. We evaluated the effect of corn stover removal from a no-till, corn-soybean (Glycine max (L.) Merr) rotation on soil greenhouse gas (GHG; CO2, N2O, CH4) fluxes, crop yields, and soil organic carbon (SOC) dynamics. We conducted a 4-year study using replicated field plots managed with two levels of corn stover removal (none; 55 % stover removal) for four complete crop cycles prior to initiation of ground surface gas flux measurements. Corn and soybean yields were not affected by stover removal with yields averaging 7.28 Mg ha−1 for corn and 2.64 Mg ha−1 for soybean. Corn stover removal treatment did not affect soil GHG fluxes from the corn phase; however, the treatment did significantly increase (107 %, P = 0.037) N2O fluxes during the soybean phase. The plots were a net source of CH4 (∼0.5 kg CH4-C ha−1 year−1 average of all treatments and crops) during the generally wet study duration. Soil organic carbon stocks increased in both treatments during the 4-year study (initiated following 8 years of stover removal), with significantly higher SOC accumulation in the control plots compared to plots with corn stover removal (0–15 cm, P = 0.048). Non-CO2 greenhouse gas emissions (945 kg CO2-eq ha−1 year−1) were roughly half of SOC (0–30 cm) gains with corn stover removal (1.841 Mg CO2-eq ha−1 year−1) indicating that no-till practices greatly improve the viability of biennial corn stover harvesting under local soil-climatic conditions. Our results also show that repeated corn stover harvesting may increase N loss (as N2O) from fields and thereby contribute to GHG production and loss of potential plant nutrients.

Nitrous oxide and methane fluxes from cattle excrement on C3 pasture and C4-dominated shortgrass steppe

Cattle play a major role in nutrient cycling of grassland ecosystems through biomass removal and excrement deposition (urine and feces). We studied the effects of cattle excrement patches (urine at 430 and feces at 940kgNha−1) on nitrous oxide (N2O) and methane (CH4) fluxes using semi-static chambers on cool-season (C3), Bozoisky-select (Psathyrostachys juncea) pasture, and warm-season (C4)-dominated native rangeland of the shortgrass steppe (SGS) in northeastern Colorado. Nitrous oxide emission factors (EF; i.e., percent of added N emitted as N2ON) did not differ between urine and feces on the C4-dominated native rangeland (0.11 and 0.10%) and C3 pasture (0.13 and 0.10%). These EFs are substantially less than the Intergovernmental Panel on Climate Change (IPCC) Tier 1 Default EF (2%) for manure deposited on pasture, indicating that during dry years the IPCC Tier 1 Default EF would result in a significant overestimation of emissions from excrement patches deposited on SGS C4-dominated native rangeland and C3 pasture. Over the first year of the study (19 June 2012–18 June 2013), cumulative CH4 uptake was 38% greater for urine (−1.49 vs. −1.08kg CH4C ha−1) and 28% greater for control plots (−2.09 vs. −1.63kg CH4C ha−1) on C4-dominated native rangeland compared to C3 pasture. In contrast, feces patches were net sources of CH4 with emissions from the C3 pasture (0.64kg CH4C ha−1) 113% greater than the C4-dominated native rangeland (0.30kg CH4C ha−1). Conversion of C4-dominated native rangeland to C3 pasture can have long term effects on CH4 uptake; therefore consideration should be taken before implementing this management practice.

Combining two complementary micrometeorological methods to measure CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O fluxes over pasture

Abstract. New Zealand's largest industrial sector is pastoral agriculture, giving rise to a large fraction of the country's emissions of methane (CH4) and nitrous oxide (N2O). We designed a system to continuously measure CH4 and N2O fluxes at the field scale on two adjacent pastures that differed with respect to management. At the core of this system was a closed-cell Fourier transform infrared (FTIR) spectrometer, which measured the mole fractions of CH4, N2O and carbon dioxide (CO2) at two heights at each site. In parallel, CO2 fluxes were measured using eddy-covariance instrumentation. We applied two different micrometeorological ratio methods to infer the CH4 and N2O fluxes from their respective mole fractions and the CO2 fluxes. The first is a variant of the flux-gradient method, where it is assumed that the turbulent diffusivities of CH4 and N2O equal that of CO2. This method was reliable when the CO2 mole-fraction difference between heights was at least 4 times greater than the FTIR's resolution of differences. For the second method, the temporal increases of mole fractions in the stable nocturnal boundary layer, which are correlated for concurrently emitted gases, are used to infer the unknown fluxes of CH4 and N2O from the known flux of CO2. This method was sensitive to “contamination” from trace gas sources other than the pasture of interest and therefore required careful filtering. With both methods combined, estimates of mean daily CH4 and N2O fluxes were obtained for 56 % of days at one site and 73 % at the other. Both methods indicated both sites as net sources of CH4 and N2O. Mean emission rates for 1 year at the unfertilised, winter-grazed site were 8.9 (±0.79) nmol CH4 m−2 s−1 and 0.38 (±0.018) nmol N2O m−2 s−1. During the same year, mean emission rates at the irrigated, fertilised and rotationally grazed site were 8.9 (±0.79) nmol CH4 m−2 s−1 and 0.58 (±0.020) nmol N2O m−2 s−1. At this site, the N2O emissions amounted to 1.21 (±0.15) % of the nitrogen inputs from animal excreta and fertiliser application.

Upscaling of methane exchange in a boreal forest using soil chamber measurements and high-resolution LiDAR elevation data

Forest soils are generally considered to be net sinks of methane (CH4), but CH4 fluxes vary spatially depending on soil conditions. Measuring CH4 exchange with chambers, which are commonly used for this purpose, might not result in representative fluxes at site scale. Appropriate methods for upscaling CH4 fluxes from point measurements to site scale are therefore needed. At the boreal forest research site, Norunda, chamber measurements of soils and vegetation indicate that the site is a net sink of CH4, while tower gradient measurements indicate that the site is a net source of CH4. We investigated the discrepancy between chamber and tower gradient measurements by upscaling soil CH4 exchange to a 100ha area based on an empirical model derived from chamber measurements of CH4 exchange and measurements of soil moisture, soil temperature and water table depth. A digital elevation model (DEM) derived from high-resolution airborne Light Detection and Ranging (LiDAR) data was used to generate gridded water table depth and soil moisture data of the study area as input data for the upscaling. Despite the simplistic approach, modeled fluxes were significantly correlated to four out of five chambers with R>0.68. The upscaling resulted in a net soil sink of CH4 of −10μmolm−2h−1, averaged over the entire study area and time period (June–September, 2010). Our findings suggest that additional contributions from CH4 soil sources outside the upscaling study area and possibly CH4 emissions from vegetation could explain the net emissions measured by tower gradient measurements.

Ecosystem carbon (CO2 and CH4) fluxes of a Populus dettoides plantation in subtropical China during and post clear-cutting

Poplar plantations have widely spread around the world due to its high productivity and adaptability. Clear-cutting is the primary harvesting method for poplar plantation management in southern China. However, the effect of harvesting on ecosystem carbon fluxes limits our ability to estimate its carbon sequestration. A consecutive, three-year observation on ecosystem CO2 and CH4 flux (FCO2 and FCH4) of a Populus dettoides plantation on the floodplain of Yangtze River was made prior and post to the clear-cutting using an eddy-covariance system. We found that clear-cutting turned the ecosystem from a strong carbon sink to a mediate carbon source only in several months, July to next January, after the harvest. The ecosystem turned to a net carbon sink at the beginning of the first growing season following clear-cutting due to the large productivity of understory vegetation in this region. The annual carbon budget was −424.3±52.5g-Cm−2 (95% confidence interval) in the harvesting year, with −53.6±22.8g-Cm−2 the first year and −290.7±34.2g-Cm−2 the second year after clear-cutting. Clear-cutting turned the ecosystem from a net CH4 sink to a net CH4 source after the third month, but during the three years the CH4 emission only balanced out a very small portion (0.3%) of FCO2. In non-inundation periods, FCH4 varied from −0.01 to 0.24mmolm−2d−1, with a mean (±SD) of 0.11±0.08mmolm−2d−1, while it ranged from 0.33 to 4.39mmolm−2d−1 during inundation, with a mean (±SD) of 2.17±1.16mmolm−2d−1. Daily and weekly FCH4 during non-inundation period were highly correlated with ground water table, soil moisture, and friction velocity, while FCH4 during inundation depended on inundation depth.

Net exchanges of methane and carbon dioxide on the Qinghai-Tibetan Plateau from 1979 to 2100

Methane (CH4) is a potent greenhouse gas (GHG) that affects the global climate system. Knowledge about land–atmospheric CH4 exchanges on the Qinghai-Tibetan Plateau (QTP) is insufficient. Using a coupled biogeochemistry model, this study analyzes the net exchanges of CH4 and CO2 over the QTP for the period of 1979–2100. Our simulations show that the region currently acts as a net CH4 source with 0.95 Tg CH4 y−1 emissions and 0.19 Tg CH4 y−1 soil uptake, and a photosynthesis C sink of 14.1 Tg C y−1. By accounting for the net CH4 emission and the net CO2 sequestration since 1979, the region was found to be initially a warming source until the 2010s with a positive instantaneous radiative forcing peak in the 1990s. In response to future climate change projected by multiple global climate models (GCMs) under four representative concentration pathway (RCP) scenarios, the regional source of CH4 to the atmosphere will increase by 15–77% at the end of this century. Net ecosystem production (NEP) will continually increase from the near neutral state to around 40 Tg C y−1 under all RCPs except RCP8.5. Spatially, CH4 emission or uptake will be noticeably enhanced under all RCPs over most of the QTP, while statistically significant NEP changes over a large-scale will only appear under RCP4.5 and RCP4.6 scenarios. The cumulative GHG fluxes since 1979 will exert a slight warming effect on the climate system until the 2030s, and will switch to a cooling effect thereafter. Overall, the total radiative forcing at the end of the 21st century is 0.25–0.35 W m−2, depending on the RCP scenario. Our study highlights the importance of accounting for both CH4 and CO2 in quantifying the regional GHG budget.

River geochemistry, chemical weathering, and atmospheric CO2 consumption rates in the Virunga Volcanic Province (East Africa)

AbstractWe report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope, and vegetation. High total suspended matter (289–1467 mg L−1) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in nonvolcanic subcatchments. Stable carbon isotope signatures (δ13C) of POC and DOC mean δ13C of −22.5‰ and −23.5‰, respectively, are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH4 to the atmosphere (4–5052 nmol L−1). Most rivers show N2O concentrations close to equilibrium, but some rivers showed high N2O concentrations related to denitrification in groundwaters. δ13C signatures of dissolved inorganic carbon suggested magmatic CO2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO2‐mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0–2779.9 t km−2 yr−1) and atmospheric CO2 consumption (0.4–37.0 × 106 mol km−2 yr−1) rates were higher than previously reported in the literature for basaltic terrains.

Nitrous oxide and methane dynamics in a coral reef lagoon driven by pore water exchange: Insights from automated high‐frequency observations

AbstractAutomated cavity ring down spectroscopy was used to make continuous measurements of dissolved methane, nitrous oxide, and carbon dioxide in a coral reef lagoon for 2 weeks (Heron Island, Great Barrier Reef). Radon (222Rn) was used to trace the influence of tidally driven pore water exchange on greenhouse gas dynamics. Clear tidal variation was observed for CH4, which correlated to 222Rn in lagoon waters. N2O correlated to 222Rn during the day only, which appears to be a response to coupled nitrification‐denitrification in oxic sediments, fueled by nitrate derived from bird guano. The lagoon was a net source of CH4 and N2O to the atmosphere and a sink for atmospheric CO2. The estimated pore water‐derived CH4 and N2O fluxes were 3.2‐fold and 24.0‐fold greater than the fluxes to the atmosphere. Overall, pore water and/or groundwater exchange were the only important sources of CH4 and major controls of N2O in the coral reef lagoon.

Methane exchange in a boreal forest estimated by gradient method

Forests are generally considered to be net sinks of atmospheric methane (CH4) because of oxidation by methanotrophic bacteria in well-aerated forests soils. However, emissions from wet forest soils, and sometimes canopy fluxes, are often neglected when quantifying the CH4 budget of a forest. We used a modified Bowen ratio method and combined eddy covariance and gradient methods to estimate net CH4 exchange at a boreal forest site in central Sweden. Results indicate that the site is a net source of CH4. This is in contrast to soil, branch and leaf chamber measurements of uptake of CH4. Wetter soils within the footprint of the canopy are thought to be responsible for the discrepancy. We found no evidence for canopy emissions per se. However, the diel pattern of the CH4 exchange with minimum emissions at daytime correlated well with gross primary production, which supports an uptake in the canopy. More distant source areas could also contribute to the diel pattern; their contribution might be greater at night during stable boundary layer conditions.

CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; in sea ice from a subarctic fjord under influence of riverine input

Abstract. We present the CH4 concentration [CH4], the partial pressure of CO2 (pCO2) and the total gas content in bulk sea ice from subarctic, land-fast sea ice in the Kapisillit fjord, Greenland. Fjord systems are characterized by freshwater runoff and riverine input and based on δ18O data, we show that &gt; 30% of the surface water originated from periodic river input during ice growth. This resulted in fresher sea-ice layers with higher gas content than is typical from marine sea ice. The bulk ice [CH4] ranged from 1.8 to 12.1 nmol L−1, which corresponds to a partial pressure ranging from 3 to 28 ppmv. This is markedly higher than the average atmospheric methane content of 1.9 ppmv. Evidently most of the trapped methane within the ice was contained inside bubbles, and only a minor portion was dissolved in the brines. The bulk ice pCO2 ranged from 60 to 330 ppmv indicating that sea ice at temperatures above −4 °C is undersaturated compared to the atmosphere (390 ppmv). This study adds to the few existing studies of CH4 and CO2 in sea ice, and we conclude that subarctic seawater can be a sink for atmospheric CO2, while being a net source of CH4.

Forest ecosystem changes from annual methane source to sink depending on late summer water balance

AbstractForests dominate the global carbon cycle, but their role in methane (CH4) biogeochemistry remains uncertain. We analyzed whole‐ecosystem CH4fluxes from 2 years, obtained over a lowland evergreen forest in Maine, USA. Gross primary productivity provided the strongest correlation with the CH4flux in both years, with an additional significant effect of soil moisture in the second, drier year. This forest was a neutral to net source of CH4in 2011 and a small net sink in 2012. Interannual variability in the summer hydrologic cycle apparently shifts the ecosystem from being a net source to a sink for CH4. The small magnitude of the CH4fluxes and observed control or CH4fluxes by forest productivity and summer precipitation provide novel insight into the CH4cycle in this globally important forest ecosystem.

Interannual, seasonal, and retrospective analysis of the methane and carbon dioxide budgets of a temperate peatland

AbstractThree years (2009–2011) of near‐continuous methane (CH4) and carbon dioxide (CO2) fluxes were measured with the eddy covariance (EC) technique at a temperate peatland located within the Marcell Experimental Forest, in northern Minnesota, USA. The peatland was a net source of CH4 and a net sink of CO2 in each year with annual carbon budgets of −26.8 (±18.7), −15.5 (±14.8), and −14.6 (±21.5) g C m−2 yr−1 for 2009–2011, respectively. Differences in the seasonal hydrometeorological conditions among the three study years were most pronounced during 2011, which was considerably warmer (+1.3°C) and wetter (+40 mm) than the 30 year average. The annual CH4 budget was +11.8 (±3.1), +12.2 (±3.0), and +24.9 (±5.6) g C m−2 yr−1 for the respective years and accounted for 23%–39% of the annual carbon budget. The larger CH4 emission in 2011 is attributed to significant warming of the peat column coupled with a high water table position throughout the entire growing season. Historical (1991–2011) CH4 emissions were estimated based on long‐term hydrometeorological records and functional relationships derived from our 3 year field study. The predicted historical annual CH4 budget ranged from +7.8 to +15.2 (±2.7) g CH4‐C m−2 yr−1. Recent (2007–2011) increases in temperature, precipitation, and rising water table at this site suggest that CH4 emissions have been increasing, but were generally greater from 1991 to 1999 when average soil temperature and precipitation were higher than in recent years. The global warming potential (considering CO2 and CH4) for this peatland was calculated based on a 100 year time horizon. In all three study years, the peatland had a net positive radiative forcing on climate and was greatest (+187 g C m−2) in 2011. The interannual variability in CH4 exchange at this site suggests high sensitivity to variations in hydrometeorological conditions.