Methane Flux Measurements Research Articles

Carbon Dioxide and Methane Release Following Abrupt Thaw of Pleistocene Permafrost Deposits in Arctic Siberia

AbstractThe decomposition of thawing permafrost organic matter (OM) to the greenhouse gases (GHG) carbon dioxide (CO2) and methane forms a positive feedback to global climate change. Data on in situ GHG fluxes from thawing permafrost OM are scarce and OM degradability is largely unknown, causing high uncertainties in the permafrost‐carbon climate feedback. We combined in situ CO2 and methane flux measurements at an abrupt permafrost thaw feature with laboratory incubations and dynamic modeling to quantify annual CO2 release from thawing permafrost OM, estimate its in situ degradability and evaluate the explanatory power of incubation experiments. In July 2016 and 2019, CO2 fluxes ranged between 0.24 and 2.6 g CO2‐C m−2 d−1. Methane fluxes were low, which coincided with the absence of active methanogens in the Pleistocene permafrost. CO2 fluxes were lower three years after initial thaw after normalizing these fluxes to thawed carbon, indicating the depletion of labile carbon. Higher CO2 fluxes from thawing Pleistocene permafrost than from Holocene permafrost indicate OM preservation for millennia and give evidence that microbial activity in the permafrost was not substantial. Short‐term incubations overestimated in situ CO2 fluxes but underestimated methane fluxes. Two independent models simulated median annual CO2 fluxes of 160 and 184 g CO2‐C m−2 from the thaw slump, which include 25%–31% CO2 emissions during winter. Annual CO2 fluxes represent 0.8% of the carbon pool thawed in the surface soil. Our results demonstrate the potential of abrupt thaw processes to transform the tundra from carbon neutral into a substantial GHG source.

Methane flux measurements along a floodplain soil moisture gradient in the Okavango Delta, Botswana.

Data-poor tropical wetlands constitute an important source of atmospheric CH4 in the world. We studied CH4 fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH4 fluxes and controlling environmental factors in the Delta's seasonal floodplains. Net CH4 emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg a−1. Microbial CH4 oxidation of approximately 2.817 × 10−3 ± 0.307 × 10−3 Tg a−1 in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH4 emissions from seasonal floodplains. The observed microbial CH4 sink in the Delta's dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH4 fluxes in both the seasonal and occasional floodplains. The optimum SWC for soil CH4 emissions and oxidation in the Delta were estimated at 50% and 15%, respectively. Electrical conductivity and pH were poorly correlated (r2 ≤ 0.11, p < 0.05) with CH4 fluxes in the seasonal floodplain at Nxaraga.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part1)'.

Methods for instrumental assessment of methane emission in reservoirs

Reservoirs are a significant source of methane in the atmosphere. Different estimations of the total methane emission from reservoirs vary greatly. In order to more accurately estimation of the methane emission from reservoirs, it is necessary to investigate the spatiotemporal variability of CH4 fluxes. To study this variability, detailed instrumental measurements of methane fluxes over a long period are required. Such measurements have been carried out for 6 years at the Mozhaisk Reservoir. These measurements showed that the determining factor of the spatiotemporal methane fluxes variability is the meteorological situation and the water level regime. These factors form the stratification of the water column. The highest values of methane flux into the atmosphere are observed in the middle part of the reservoir. The temporal variation of the methane flux is characterized by a general pattern for all years when the measurements were carried out. The methane flux increases during the summer period and reaches the highest values before the beginning of the autumn mixing stage. Also, high flux values observed during periods of storm events that cause mixing of the water column.

Seasonal Variation of Methane Microseepage in the Dawanqi Oilfield (China): A Possible Climatic Control

AbstractNatural gas microseepage in petroleum‐bearing sedimentary basins is an important complement to geophysical methods in oil‐gas exploration and a natural source of methane (CH4) for the atmosphere. Microseepage, typically occurring in correspondence with petroleum fields throughout the world, is generally lower in summer, due to temperature‐driven methanotrophic consumption, and higher in winter. The global estimates of microseepage methane emission have, however, relatively high uncertainties because of limited amounts of flux data, leading to poor knowledge of the spatial distribution and temporal variability of the gas emission factors. We studied the seasonal variation of microseepage flux to the atmosphere from a petroleum field in China (the Dawanqi oilfield), through methane flux measurements performed in summer 2014, winter 2015, and summer 2019. Winter data refer to frozen soil conditions, with snow cover and ice thickness in the soil exceeding 60 cm. Gas concentration (CH4, CO2, C2+ alkanes) and stable C isotopic composition of CH4 and CO2 in shallow (4 m deep) boreholes confirmed the existence of thermogenic gas seepage. Methane microseepage is higher in summer and lower or nil in winter. This seasonal trend is opposite to what was observed in areas where winter soil is not or poorly frozen. Our data suggest that seasonal microseepage variation may not be univocal worldwide, being strongly dependent on the presence of ice and snow cover in winter. The regional increase of temperature due to climate change, already demonstrated for the Tarim Basin over the last 50 years, could, in the future, reduce winter ice and enhance annual methane emission to the atmosphere.

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH4) fluxes at tree stems – a general phenomenon for plant systems?

Fourier transform infrared spectroscopy and interference of volatile organic compounds on measurements of methane (CH₄) fluxes at tree stems – a general phenomenon for plant systems?

Spatial and temporal distribution of methane emissions from a covered landfill equipped with a gas recollection system

A previously developed surface probe method, which allows for instantaneous methane (CH4) flux measurement, was used to establish CH4 emission maps of a municipal landfill with a final clay cover and equipped with a gas recollection system. In addition to spatial variations, the method was applied at 7 different times over a total timeframe of 65 h and under similar weather conditions to determine the intrinsic temporal variations of CH4 emissions; i.e., the temporal variation related to the dynamic of the landfill rather than the one driven by external factors. Furthermore, continuous CH4 fluxes, with a data acquisition frequency of 1 Hz, were measured during 12 h at a single position, and for one hour at 22 locations of the landfill, spanning a large range of CH4 emission magnitudes. A simple model for the numerical characterization of spatiotemporal variability of the landfill emission was used and allowed us to separately quantify the temporal and spatial variability. This model showed that, in the landfill tested, the temporal distribution of CH4 emissions resulted more homogeneous than the spatial distribution. Other attributes of the temporal and spatial distributions of CH4 emissions were also established including the anisotropic nature of the spatial distribution and, contrastingly, the stochastic temporal variability of such emissions.

Comparison of eddy covariance 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; fluxes from mined and recently rewetted sections in a northwestern German cutover bog

Abstract. With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of these ecosystems. Large parts of the soil profile in natural peatlands are water saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO2) and methane (CH4) flux measurements from a bog site in northwestern Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented 2-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO2 and CH4 flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability, and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from 2 consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH4 and decreased CO2 release. The larger relative difference between cumulative CO2 fluxes from the rewetted (13±6 mol m−2 a−1) and drained (22±7 mol m−2 a−1) section occurred in the second observed year when rewetting apparently reduced CO2 emissions by 40 %. The absolute difference in annual CH4 flux sums was more similar between both years, while the relative difference of CH4 release between the rewetted (0.83±0.15 mol m−2 a−1) and drained (0.45±0.11 mol m−2 a−1) section was larger in the first observed year, indicating a maximum increase in annual CH4 release of 84 % caused by rewetting at this particular site during the study period.

Significant methane ebullition from alpine permafrost rivers on the East Qinghai–Tibet Plateau

Inland waters are large sources of methane to the atmosphere. However, considerable uncertainty exists in estimating the emissions of this potent greenhouse gas from global streams and rivers due, in part, to a lack of direct measurements in the high-altitude cryosphere and poor accounting for ebullition. Here we present methane concentrations and fluxes over three years in four basins on the East Qinghai–Tibet Plateau. Methane ebullition rates decrease exponentially whereas diffusion declines linearly with increasing stream order. Nonetheless, the average ebullition rate (11.9 mmolCH4 m−2 d−1) from these streams and rivers—which have large organic stocks in surrounding permafrost, abundant cold-tolerant methanogens, shallow water depths, and experience low air pressure—were six times greater than the global average and reached a maximum of 374.4 mmolCH4 m−2 d−1. Upscaled total emissions from sampled third- to seventh-order waterways of the East Qinghai–Tibet Plateau are estimated to be 0.20 TgCH4 yr−1, 79% of which was attributed to ebullition. These methane emissions are approximately 20% of CO2 emissions (2.70 TgCO2 yr−1) in terms of carbon release and two times greater in terms of CO2-equivalent emissions. When upscaled to first- to seventh-order waterways, we estimate emissions of 0.37–1.23 TgCH4 yr−1. Our findings demonstrate that high-elevation rivers on the Qinghai–Tibet Plateau are hotspots of methane delivery to the atmosphere. The large ebullitive fluxes, which constitute a substantial fraction of global fluvial methane emissions, reveal a positive feedback between climate warming, permafrost thaw and methane emissions. High-elevation rivers in permafrost of the East Qinghai–Tibet Plateau are hotspots of methane emissions, according to measurements of methane fluxes in the region.

A Small Nimble In Situ Fine-Scale Flux Method for Measuring Tree Stem Greenhouse Gas Emissions and Processes (S.N.I.F.F)

Tree stem methane emissions are gaining increasing attention as an overlooked atmospheric source pathway. Existing methods for measuring tree stem greenhouse gas fluxes and isotopes may provide robust integrated emission estimates, but due to their coarse resolution, the capacity to derive insights into fine-scale dynamics of tree stem emissions is limited. We demonstrate and field test an alternative method that is Small, Nimble, In situ and allows for Fine-scale Flux (‘SNIFF’) measurements, on complex and contrasting stem surfaces. It is lightweight and therefore suitable to remote field locations enabling real-time data observations allowing for field-based, data driven sampling regimes. This method facilitated novel results capturing fine-scale vertical and radial methane flux measurements (5 cm increments) and revealed: (1) 86–89% of methane emissions emanated from the lower 30 cm of sampled wetland tree species; (2) clear vertical and horizontal trends in δ13C-CH4 possibly due to fractionation associated with oxidation and/or mass-dependant fractionation during diffusive transport; and (3) the occurrence of substantial radial heterogeneity. We also compared a variety of up-scaling approaches to estimate methane flux per tree, including novel smartphone 3D photogrammetry that resulted in substantially higher stem surface area estimations (> 16 to 36%) than traditional empirical methods. Utilising small chambers with high radial and vertical resolution capabilities may therefore facilitate more robust future assessments into the drivers, pathways, oxidation sinks and magnitude of tree stem greenhouse gas emissions, and compliment previous broad-scale sampling techniques.

Methane flux measurements in rice by static flux chamber and eddy covariance

AbstractUnderstanding methane (CH4) fluxes from rice (Oryza sativa L.) at the field scale is paramount to reducing environmental impacts while ensuring global food security. Greenhouse gas (GHG) measurements at the plot scale using static flux chambers (SFC) have long informed the understanding of flux dynamics and have largely been the basis of global flux estimates. However, in many parts of the world, the landscapes where agricultural fluxes are generated come from larger fields. Eddy covariance (EC) can measure trace gases on larger fields, but there are few studies available quantifying CH4 emissions under typical practices at a field scale. Furthermore, few of these studies are from the U.S. Midsouth, the largest producer of U.S. rice. This study compares and quantifies field‐scale SFC and EC flux measurements on a large commercial system in northeastern Arkansas during the 2015 and 2016 growing seasons, following typical producer practices. Daily measured SFC CH4 fluxes did not differ from EC‐daily CH4 fluxes (p = .108). Total season CH4 emissions, calculated as the sum of daily fluxes ranged from 50 to 156 kg CH4 ha−1 season−1, with SFC reporting greater emissions than EC. Although SFC and EC‐daily flux measurements were similar early (p = .382) and late (p = .543) in the season, they differed mid‐season (p &lt; .001) with SFC consistently reporting greater fluxes than EC. The findings of this study help unify season long plot‐scale and field‐scale flux measurements and signify an advancement of our understanding of GHG fluxes from rice systems.

PSXI-4 Composting management to reduce greenhouse gas and ammonia emissions from feedlot manure

Abstract Management of livestock manure may recycle nutrients and decrease greenhouse gas (GHG) and ammonia (NH3) emissions. The objectives were to ascertain effects of environmental conditions and turning on methane (CH4), nitrous oxide (N2O), and NH3 emissions and if treatment with 8.5 g of dicyandiamide (DCD), a denitrification agent, altered GHG emissions. Manure and bedding were collected from feedlot pens and used to construct 3 piles (~1.9 m3 volume) each in winter (WI) and spring (SP). WI piles were turned once, and SP piles were turned twice. Methane, N2O, and NH3 emissions were collected. Methane and N2O flux measurements were collected from SP piles using a static chamber (3.7m L x 2.2m W x 0.9m H). Initial dry matter and nitrogen contents were 33.2 and 30.0% and 20.1 and 17.7 g/kg in WI and SP piles, respectively. Average ambient temperatures and wind speeds were 0.3oC and 10.7oC and 1.76 m/s and 1.97 m/s during WI and SP, respectively. Internal temperatures reached 51±3.9oC on d 4–11 and gradually decreased. Normalized CH4 averaged 2.19 mg٠s٠m-4 and N2O emissions averaged 0.84 mg٠s٠m-4, and were not different between the WI and SP piles. Turning did not affect CH4 emissions from WI piles, but were 55% greater (P &amp;lt; 0.05) when SP piles were turned a second time. Emissions of N2O increased 51% when WI and SP piles were turned (P &amp;lt; 0.05). Ammonia emissions were 83.5% greater from WI piles due to their higher initial concentrations of NH4+-N (2.21 vs. 1.11 g/kg; P &amp;lt; 0.05). Turning did not influence CH4 and N2O fluxes. Addition of DCD at pile formation appears to decrease N2O emissions and fluxes 3 and 10 d later. Turning management and season impacted overall CH4, N2O, and NH3 emissions. Fine-tuning manure handling and management during different seasons may effectively reduce GHG and NH3 emissions.

Application of an open-path eddy covariance methane flux measurement system to a larch forest in eastern Siberia

Canopy-Scale methane (CH4) flux measurement over a larch forest in eastern Siberia was conducted by eddy covariance method using an open-path CH4 gas analyzer. Though the uncorrected flux showed strong CH4 uptake in the daytime, this changed to CH4 emission after density and spectroscopic effects were corrected. Random flux errors calculated from cross-covariance functions suggested that CH4 flux was nearly the same as the upper boundary of the limit of flux detection at the 95th percentile, being barely resolved by the measurement system; and that most of the daytime CH4 flux remained positive even after uncertainties due to random flux errors were taken into consideration. CH4 flux showed clear diurnal variation, representing emission in the daytime and near-zero in the nighttime, irrespective of wind direction. The daytime CH4 flux was dependent on both air temperature and volumetric soil water content. The CH4 flux from May 29, to June 12, was calculated as net emissions of 4.9–13.8 nmol m−2 s−1 in daily average, ranging between the forest floor and a mesotrophic fen near this site measured by static chambers in a previous study.

Tropical forest soils serve as substantial and persistent methane sinks

Although tropical forest soils contributed substantially global soil methane uptake, observations on soil methane fluxes in tropical forests are still sparse, especially in Southeast Asia, leading to large uncertainty in the estimation of global soil methane uptake. Here, we conducted two-year (from Sep, 2016 to Sep, 2018) measurements of soil methane fluxes in a lowland tropical forest site in Hainan island, China. At this tropical forest site, soils were substantial methane sink, and average annual soil methane uptake was estimated at 2.00 kg CH4-C ha−1 yr−1. The seasonality of soil methane uptake showed strong methane uptake in the dry season (−1.00 nmol m−2 s−1) and almost neutral or weak soil methane uptake in the wet season (−0.24 nmol m−2 s−1). The peak soil methane uptake rate was observed as −1.43 nmol m−2 s−1 in February, 2018, the driest and coolest month during the past 24 months. Soil moisture was the dominant controller of methane fluxes, and could explain 94% seasonal variation of soil methane fluxes. Soil temperature could not enhance the explanation of seasonal variation of soil methane fluxes on the top of soil moisture. A positive relationship between soil methane uptake and soil respiration was also detected, which might indicate co-variation in activities of methanotroph and roots and/or microbes for soil heterotrophic respiration. Our study highlights that tropical forests in this region acted as a methane sink.

Gap-filling approaches for eddy covariance methane fluxes: A comparison of three machine learning algorithms and a traditional method with principal component analysis.

Methane flux (FCH4 ) measurements using the eddy covariance technique have increased over the past decade. FCH4 measurements commonly include data gaps, as is the case with CO2 and energy fluxes. However, gap-filling FCH4 data are more challenging than other fluxes due to its unique characteristics including multidriver dependency, variabilities across multiple timescales, nonstationarity, spatial heterogeneity of flux footprints, and lagged influence of biophysical drivers. Some researchers have applied a marginal distribution sampling (MDS) algorithm, a standard gap-filling method for other fluxes, to FCH4 datasets, and others have applied artificial neural networks (ANN) to resolve the challenging characteristics of FCH4 . However, there is still no consensus regarding FCH4 gap-filling methods due to limited comparative research. We are not aware of the applications of machine learning (ML) algorithms beyond ANN to FCH4 datasets. Here, we compare the performance of MDS and three ML algorithms (ANN, random forest [RF], and support vector machine [SVM]) using multiple combinations of ancillary variables. In addition, we applied principal component analysis (PCA) as an input to the algorithms to address multidriver dependency of FCH4 and reduce the internal complexity of the algorithmic structures. We applied this approach to five benchmark FCH4 datasets from both natural and managed systems located in temperate and tropical wetlands and rice paddies. Results indicate that PCA improved the performance of MDS compared to traditional inputs. ML algorithms performed better when using all available biophysical variables compared to using PCA-derived inputs. Overall, RF was found to outperform other techniques for all sites. We found gap-filling uncertainty is much larger than measurement uncertainty in accumulated CH4 budget. Therefore, the approach used for FCH4 gap filling can have important implications for characterizing annual ecosystem-scale methane budgets, the accuracy of which is important for evaluating natural and managed systems and their interactions with global change processes.

Enhanced efficiency nitrogen fertilizers maintain yields and mitigate global warming potential in an intensified spring wheat system

Enhanced efficiency nitrogen fertilizers (EENFs), including nitrification inhibitors (NIs) and slow-release fertilizers (SRFs), are considered a feasible pathway for improving grain yield and mitigating greenhouse gas (GHG) emissions. However, the usage of EENFs in an intensified spring wheat system has not been well documented. The combined application of EENFs with water and fertilizer management (EENFs-WFM) was investigated in an irrigated spring wheat cropping system over two years. Measurements of soil nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) fluxes were taken across five treatments: no N fertilizer as a control (CK), conventional N fertilization and irrigation (Con), optimum N fertilization and irrigation (Opt), optimum N fertilization and irrigation plus nitrification inhibitor (Opt + NI), and optimum N fertilization as slow-release fertilizer and irrigation (Opt-SRF). The cumulative N2O emissions in both growing seasons were significantly (P < 0.05) higher than the fallow seasons and accounted for 56–83% of total emissions. The Opt, Opt-SRF, and Opt + NI treatments significantly reduced the cumulative N2O emissions by 25%, 34%, and 45%, respectively, relative to the Con treatment, while the fertilizer N input decreased by 36%. The soil acted as a tiny sink for atmospheric CH4, with no significant effect in any treatment. Moreover, global warming potential (GWP) and greenhouse gas intensity (GHGI) declined by ∼45% and ∼33%, respectively, in the Opt + NI treatment and ∼46% and ∼34%, respectively, in the Opt-SRF treatment, relative to the Con treatment, with almost no effect on grain yield. Our results highlight that EENFs-WFM is a promising management system for maintaining yield while minimizing GWP and GHGI.

Quantifying the impact of emission outbursts and non-stationary flow on eddy-covariance CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; flux measurements using wavelet techniques

Abstract. Methane flux measurements by the eddy-covariance technique are subject to large uncertainties, particularly linked to the partly highly intermittent nature of methane emissions. Outbursts of high methane emissions, termed event fluxes, hold the potential to introduce systematic biases into derived methane budgets, since under such conditions the assumption of stationarity of the flow is violated. In this study, we investigate the net impact of this effect by comparing eddy-covariance fluxes against a wavelet-derived reference that is not negatively influenced by non-stationarity. Our results demonstrate that methane emission events influenced 3 %–4 % of the flux measurements and did not lead to systematic biases in methane budgets for the analyzed summer season; however, the presence of events substantially increased uncertainties in short-term flux rates. The wavelet results provided an excellent reference to evaluate the performance of three different gap-filling approaches for eddy-covariance methane fluxes, and we show that none of them could reproduce the range of observed flux rates. The integrated performance of the gap-filling methods for the longer-term dataset varied between the two eddy-covariance towers involved in this study, and we show that gap-filling remains a large source of uncertainty linked to limited insights into the mechanisms governing the short-term variability in methane emissions. With the capability for broadening our observational methane flux database to a wider range of conditions, including the direct resolution of short-term variability on the order of minutes, wavelet-derived fluxes hold the potential to generate new insight into methane exchange processes with the atmosphere and therefore also improve our understanding of the underlying processes.

ENSO‐Influenced Drought Drives Methane Flux Dynamics in a Tropical Wet Forest Soil

AbstractGlobal atmospheric methane growth rates have wildly fluctuated over the past three decades, which may be driven by the proportion of tropical land surface saturated by water. The El Niño/Southern Oscillation Event (ENSO) cycle drives large‐scale climatic trends globally, with El Niño events typically bringing drier weather than La Niña. In a lowland tropical wet forest in Costa Rica, we measured methane flux bimonthly from March 2016 to June 2017 and using an automated chamber system. We observed a strong drying trend for several weeks during the El Niño in 2016, reducing soil moisture below normal levels. In contrast, soil conditions had high water content prior to the drought and during the moderate La Niña that followed. Soil moisture varied across the period studied and significantly impacted methane flux. Methane consumption was greater during the driest part of the El Niño period, while during La Niña and other time periods, soils had lower methane consumption. The mean methane flux observed was −0.022 mg CH4‐C m−2 hr−1, and methane was consumed at all timepoints, with lower consumption in saturated soils. Our data show that month studied, and the correlation between soil type and month significantly drove methane flux trends. Our data indicate that ENSO cycles may impact biogenic methane fluxes, mediated by soil moisture conditions. Climate projections for Central America show dryer conditions and increased El Niño frequency, further exacerbating predicted drought. These trends may lead to negative climate feedbacks, with drier conditions increasing soil methane consumption from the atmosphere.

Characteristics of annual greenhouse gas flux and NO release from alpine meadow and forest on the eastern Tibetan Plateau

The importance of the world’s largest plateau (i.e., the Tibetan Plateau) as a source/sink for atmospheric greenhouse gases (GHGs) still remains uncertain as field observations covering entire years and contrasting ecosystems are scarce. Here we report on year-round measurements of methane (CH4), nitrous oxide (N2O) and nitric oxide (NO) fluxes from two representative ecosystems (cattle-grazed meadow and natural forest, >3300 m a.s.l) on the eastern Tibetan Plateau. In our study, the alpine soils generally functioned as sinks for atmospheric CH4. The pronounced seasonality of CH4 uptake, with higher uptake in the growing season and lower uptake in the non-growing season, was mainly regulated by the combined changes in soil moisture and temperature. Soil CH4 uptake was lower at the meadow site due to high soil moisture and grazing-induced soil compaction reducing gas diffusivity and hampering CH4 uptake. For the alpine meadow pulses of N-oxide fluxes were observed during the freeze-thaw period, while freeze-thaw related pulses did not occur at the forest site, where the seasonality of N-oxide fluxes was primarily regulated by changes in soil temperature (Q10 = 3.0). Annual cumulative fluxes for the meadow and forest ecosystems ranged between −1.15 and −2.24 kg CH4-C ha−1 yr−1, 0.13-0.65 kg N2O-N ha−1 yr−1 and 0.022-0.047 kg NO-N ha−1 yr−1. Non-growing season fluxes contributed 34–45% and 21–51% to the annual budgets of CH4 and N-oxides, respectively. Multiple regression analyses showed that soil N-oxide fluxes were significantly positively correlated to concentrations of soil dissolved organic C (DOC) and inorganic N, with DOC and inorganic N concentrations being higher at the forest site. Our study also shows that alpine forest is a stronger source for non-CO2 GHGs relative to alpine meadow (higher by 195 kg CO2-eq ha−1 yr−1). This difference in GHG fluxes significantly offsets the net climate benefits due to increased C-sequestration associated with afforestation/reforestation.

Methane diffusive fluxes from sediment exposed in a Brazilian tropical reservoir drawdown zone

Between 2012 and 2015, a severe drought and the consequent decrease in water supply in the southeastern region of Brazil reached concerning levels, leading to a water crisis. This situation caused aerial exposure of some reservoirs drawdown zone. This study aims to measure methane fluxes emitted by this drawdown zone along the Funil hydropower reservoir, Rio de Janeiro, Brazil. It was observed that there was a temporal variation of the methane flux in the sediments exposed in the three scenarios studied: drought, flooding and full (0.43, −0.31 and −0.01 mg CH4 m-2 h-1, respectively). The CH4 flux by area in the scenario with the highest exposed sediment area was 0.195 ton CH4 d-1. Considering the values recorded, the contribution of these emissions must become considered in the overall balance of methane in the atmosphere as a new pathway by reservoirs, mainly with the expected forecasts and possible cycles of droughts that may occur frequently. However, the temporal variability of these fluxes indicated the need for future monitoring initiatives that seek to understand how climate change affects the biochemical processes within the exposed sediments of drawdown zones.

Using high-frequency soil oxygen sensors to predict greenhouse gas emissions from wetlands

Understanding the mechanisms that lead to greenhouse gas (GHG) emissions requires knowledge of short-term fluctuations in drivers of fluxes. To better understand how variation and lags in abiotic conditions affect soil GHG fluxes, we coupled weekly methane (CH4) and nitrous oxide (N2O) flux measurements with time-series soil sensor data for oxygen (O2), moisture and temperature. Including time-series data improved models for soil CH4 and N2O fluxes compared to models which did not contain time series data. Fluctuations in soil O2 drove CH4 fluxes, where rapid changes in redox conditions led to high fluxes. N2O fluxes occurred when soils were warm and dry. Soil O2 was the best predictor and thus sensor for understanding CH4 fluxes, whereas soil moisture best predicted N2O fluxes. Combining data from multiple sensors improved models for both gases, underscoring the relative importance of interactions among drivers. Overall, our top models explained 15% and 30% of the variance in N2O and CH4 fluxes, respectively. Fluxes predicted using linear interpolation between measured fluxes were lower than time-series model predictions for both N2O and CH4 fluxes. This suggests linear interpolation may fail to capture “hot moments” or episodic events which lead to high fluxes. Long-term, continuous data from sensors, which accounts for short-term variation in abiotic drivers, may improve our ability to predict the timing and intensity of GHG emissions from wetland soils.