Methane emissions from sheep pasture,measured with an open‐path eddy covariance system

Methane (CH₄) is an important greenhouse gas, contributing 0.4–0.5 W m^?2 to global warming. Methane emissions originate from several sources, including wetlands, rice paddies, termites and ruminating animals. Previous measurements of methane flux from farm animals have been carried out on animals in unnatural conditions, in laboratory chambers or fitted with cumbersome masks. This study introduces eddy covariance measurements of CH₄, using the newly developed LI‐COR LI‐7700 open‐path methane analyser, to measure field‐scale fluxes from sheep grazing freely on pasture. Under summer conditions, fluxes of methane in the morning averaged 30 nmol m^?2 s^?1, whereas those in the afternoon were above 100 nmol m^?2 s^?1, and were roughly two orders of magnitude larger than the small methane emissions from the soil. Methane emissions showed no clear relationship with air temperature or photosynthetically active radiation, but some diurnal pattern was apparent, probably linked to sheep grazing behaviour and metabolism. Over the measurement period (days 60–277, year 2010), cumulative methane fluxes were 0.34 mol CH₄ m^?2, equating to 134.3 g CO₂ equivalents m^?2. By comparison, a carbon dioxide (CO₂) sink of 819 g CO₂ equivalents m^?2 was measured over the same period, but it is likely that much of this would be released back to the atmosphere during the winter or as off‐site losses (through microbial and animal respiration). By dividing methane fluxes by the number of sheep in the field each day, we calculated CH₄ emissions per head of livestock as 7.4 kg CH₄ sheep^?1 yr^?1, close to the published IPCC emission factor of 8 kg CH₄ sheep^?1 yr^?1.     

Greenhouse gas budget (CO2, CH4 and N2O) of intensively managed grassland following restoration

The first full greenhouse gas (GHG) flux budget of an intensively managed grassland in Switzerland (Chamau) is presented. The three major trace gases, carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) were measured with the eddy covariance (EC) technique. For CO₂ concentrations, an open‐path infrared gas analyzer was used, while N₂O and CH₄ concentrations were measured with a recently developed continuous‐wave quantum cascade laser absorption spectrometer (QCLAS). We investigated the magnitude of these trace gas emissions after grassland restoration, including ploughing, harrowing, sowing, and fertilization with inorganic and organic fertilizers in 2012. Large peaks of N₂O fluxes (20–50 nmol m^?2 s^?1 compared with a <5 nmol m^?2 s^?1 background) were observed during thawing of the soil after the winter period and after mineral fertilizer application followed by re‐sowing in the beginning of the summer season. Nitrous oxide (N₂O) fluxes were controlled by nitrogen input, plant productivity, soil water content and temperature. Management activities led to increased variations of N₂O fluxes up to 14 days after the management event as compared with background fluxes measured during periods without management (<5 nmol m^?2 s^?1). Fluxes of CO₂ remained small until full plant development in early summer 2012. In contrast, methane emissions showed only minor variations over time. The annual GHG flux budget was dominated by N₂O (48% contribution) and CO₂ emissions (44%). CH₄ flux contribution to the annual budget was only minor (8%). We conclude that recently developed multi‐species QCLAS in an EC system open new opportunities to determine the temporal variation of N₂O and CH₄ fluxes, which further allow to quantify annual emissions. With respect to grassland restoration, our study emphasizes the key role of N₂O and CO₂ losses after ploughing, changing a permanent grassland from a carbon sink to a significant carbon source.     

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape‐integrated CH4 budget

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH₄) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH₄. Here, we present results of in situ CH₄ flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH₄ budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open‐top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH₄‐C m^?2 (8.1 ± 1.2 g CO₂‐eq m^?2) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH₄‐C m^?2 (3.9 ± 0.6 g CO₂‐eq m^?2). These uptake rates were subsequently scaled to the entire study area of 0.15 km², a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH₄‐C m^?2 (3.7 ± 1.2 g CO₂‐eq m^?2). The result was a net landscape sink of 12.71 kg CH₄‐C (0.48 tonne CO₂‐eq) during the growing season. A nonsignificant trend was noticed in seasonal CH₄ uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH₄ fluxes. Overall, the net landscape sink of CH₄ tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH₄ budget of the Arctic due to their potential role in counterbalancing CH₄ emissions from wetlands – not the least when taking the future climatic scenarios of the Arctic into account.     

Carbon Dioxide and Methane Fluxes From Tree Stems,Coarse Woody Debris,and Soils in an Upland Temperate Forest

Forest soils and canopies are major components of ecosystem CO₂ and CH₄ fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO₂ and CH₄ fluxes. We measured CO₂ and CH₄ fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO₂ sources (4.58 ± 2.46 µmol m^?2 s^?1, mean ± 1 SD) and net sinks of CH₄ (?2.17 ± 1.60 nmol m^?2 s^?1). Coarse woody debris was a CO₂ source (4.23 ± 3.42 µmol m^?2 s^?1) and net CH₄ sink, but with large uncertainty (?0.27 ± 1.04 nmol m^?2 s^?1) and with substantial differences depending on wood decay status. Stems were CO₂ sources (1.93 ± 1.63 µmol m^?2 s^?1), but also net CH₄ sources (up to 0.98 nmol m^?2 s^?1), with a mean of 0.11 ± 0.21 nmol m^?2 s^?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH₄, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO₂ fluxes, whereas CH₄ emissions from living stems offset net soil and CWD CH₄ uptake by 3.5%. Our results demonstrate the importance of CH₄ emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO₂ and CH₄ dynamics.     

Carbon fluxes in forested bog margins along a human impact gradient: could vegetation structure be used as an indicator of peat carbon emissions?

Bog ecosystems are sensitive to anthropogenic disturbance, including drainage and air pollution. Carbon (C) balance measurements to determine the effect of disturbance on bog functioning are laborious; therefore reliable proxies for C fluxes that could facilitate upscaling from single studies to a larger scale would be valuable. We measured peat CO₂ emissions (R _s), CH₄ efflux and vegetation characteristics in four bog areas that formed a gradient from pristine to severely disturbed peatlands, affected by drainage, peat mining, alkaline air pollution and underground oil-shale mining. We expected that sites experiencing higher human impact (i.e., the vegetation was more distinct from that of a natural bog) would have higher R _s and lower CH₄ emissions, but differences in peat C emissions between the most disturbed and pristine sites were not significant. Growing period median R _s ranged from 0.5 to 2.2 g C m^?2 day^?1 for our plots; methane emissions, measured from July to December were an order of magnitude lower, ranging from ?5.9 to 126.7 mg C m^?2 day^?1. R _s and CH₄ emissions were primarily determined by water table depth, as was tree stand productivity. Therefore, stand structural parameters could potentially be good indicators of soil C emissions from poorly drained forested bogs.     

Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄ and CO₂ fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄ and CO₂ in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂ but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄ production and limit CO₂ release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m^?2 d^?1 and absorbed 1.6 ± 0.2 g CO₂‐C m^?2 d^?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄ emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m^?2 d^?1) but remained a net source, while the net CO₂ flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄ fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂ and CH₄ were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m^?2 (as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m^?2in aerobic phases), high CH₄ effluxes during the anaerobic flooded phase and modest CH₄ effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.     

Methane emissions from soils: synthesis and analysis of a large UK data set

Nearly 5000 chamber measurements of CH₄ flux were collated from 21 sites across the United Kingdom, covering a range of soil and vegetation types, to derive a parsimonious model that explains as much of the variability as possible, with the least input requirements. Mean fluxes ranged from ?0.3 to 27.4 nmol CH₄ m^?2 s^?1, with small emissions or low rates of net uptake in mineral soils (site means of ?0.3 to 0.7 nmol m^?2 s^?1) and much larger emissions from organic soils (site means of ?0.3 to 27.4 nmol m^?2 s^?1). Less than half of the observed variability in instantaneous fluxes could be explained by independent variables measured. The reasons for this include measurement error, stochastic processes and, probably most importantly, poor correspondence between the independent variables measured and the actual variables influencing the processes underlying methane production, transport and oxidation. When temporal variation was accounted for, and the fluxes averaged at larger spatial scales, simple models explained up to ca. 75% of the variance in CH₄ fluxes. Soil carbon, peat depth, soil moisture and pH together provided the best sub‐set of explanatory variables. However, where plant species composition data were available, this provided the highest explanatory power. Linear and nonlinear models generally fitted the data equally well, with the exception that soil moisture required a power transformation. To estimate the impact of changes in peatland water table on CH₄ emissions in the United Kingdom, an emission factor of +0.4 g CH₄ m^?2 yr^?1 per cm increase in water table height was derived from the data.     

Carbon dioxide,methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

We investigated soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH₄ and N₂O, we observed uptake and emission ranging from ?160 to 245 μg CH₄ m^?2 h^?1 and ?52 to 21 μg N₂O m^?2 h^?1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO₂ fluxes (259, 246, 220, and 250 mg CO₂ m^?2 h^?1, respectively), without pattern for N₂O fluxes (?3.7, 1.5, ?2.2, and ?7.6 μg N₂O m^?2 h^?1, respectively), whereas the uptake rates of CH₄ increased with stand age (6.4, ?7.9, ?10.8, and ?23.3 μg CH₄ m^?2 h^?1, respectively). For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated soil exchanges of CH₄ and N₂O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO₂ fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH₄ and N₂O fluxes, but CH₄ emission was observed following summer rainfall events. LFH layer removal reduced CO₂ emissions by 43%, increased CH₄ uptake during dry and warm soil conditions by more than twofold, but did not affect N₂O flux. We suggest that significant alternating sink and source potentials for both CH₄ and N₂O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.     

Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO₂]. Net ecosystem CO₂ exchange (NEE) was measured in the eighth year of CO₂ enrichment at the Nevada Desert Free‐Air CO₂ Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO₂ m^?2 s^?1) of ecosystems exposed to elevated atmospheric CO₂ were similar to those maintained at current ambient CO₂ levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO₂] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO₂]. Mean daily NEE varied seasonally across both CO₂ treatments, increasing from about 0.1 μmol CO₂ m^?2 s^?1 in December to a maximum of 0.5–0.6 μmol CO₂ m^?2 s^?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO₂ occurred 1 month earlier than it did in ecosystems exposed to ambient CO₂, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO₂ m^?2 s^?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO₂ m^?2 s^?1), November (0.28±0.03 μmol CO₂ m^?2 s^?1), and December (0.54±0.06 μmol CO₂ m^?2 s^?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO₂ uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO₂ treatments were net CO₂ sinks in 2004, but exposure to elevated CO₂ reduced CO₂ sink strength by 30% (positive net ecosystem productivity=127±17 g C m^?2 yr^?1 ambient CO₂ and 90±11 g C m^?2 yr^?1 elevated CO₂, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO₂– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.     

Modelling carbon balances of coastal arctic tundra under changing climate

Rising air temperatures are believed to be hastening heterotrophic respiration (R_h) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO₂) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil‐water contents (θ). The model was then tested with CO₂ and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998–1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO₂ fixation, and so no significant effect on net CO₂ exchange was modelled or measured. An increase in growth chamber θ from field capacity to saturation caused substantial reductions in respiration but not in CO₂ fixation, and so an increase in net CO₂ exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2–4 g C m^?2 d^?1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (～1 g C m^?2 d^?1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m^?2 y^?1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m^?2 y^?1) during 1999 when air temperatures were close to long‐term normals. During 100 years under rising atmospheric CO₂ concentration (C_a), air temperature and precipitation driven by the IS92a emissions scenario, modelled R_h rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH₄) emissions were predicted to rise substantially in coastal tundra with IS92a‐driven climate change (from ～20 to ～40 g C m^?2 y^?1), causing a substantial increase in the emission of CO₂ equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (～1 kg C m^?2) would have been modelled after 100 years, including additional emissions of CH₄.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH₄ and CO₂ flux from a small shallow pond during a period of 4 months. Mean CH₄ flux and surface water CH₄ concentration were 8.0 [3.3–15.1] ± 3.1 mmol m^?2 day^?1 (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO₂ flux was 1.1 [?9.8 to 16.0] ± 6.9 mmol m^?2 day^?1. Substantial diel changes in CO₂ flux and surface water CH₄ concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH₄ emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO₂ flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO₂ consumption by plants was higher relative to CO₂ production by respiration during warm sunny days. Interestingly, CH₄ fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH₄ and CO₂ summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH₄ emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.     

Environmental controls of temporal and spatial variability in CO2 and CH4 fluxes in a neotropical peatland

Tropical peatlands play an important role in the global storage and cycling of carbon (C) but information on carbon dioxide (CO₂) and methane (CH₄) fluxes from these systems is sparse, particularly in the Neotropics. We quantified short and long‐term temporal and small scale spatial variation in CO₂ and CH₄ fluxes from three contrasting vegetation communities in a domed ombrotrophic peatland in Panama. There was significant variation in CO₂ fluxes among vegetation communities in the order Campnosperma panamensis > Raphia taedigera > Cyperus. There was no consistent variation among sites and no discernible seasonal pattern of CH₄ flux despite the considerable range of values recorded (e.g. ?1.0 to 12.6 mg m^?2 h^?1 in 2007). CO₂ fluxes varied seasonally in 2007, being greatest in drier periods (300–400 mg m^?2 h^?1) and lowest during the wet period (60–132 mg m^?2 h^?1) while very high emissions were found during the 2009 wet period, suggesting that peak CO₂ fluxes may occur following both low and high rainfall. In contrast, only weak relationships between CH₄ flux and rainfall (positive at the C. panamensis site) and solar radiation (negative at the C. panamensis and Cyperus sites) was found. CO₂ fluxes showed a diurnal pattern across sites and at the Cyperus sp. site CO₂ and CH₄ fluxes were positively correlated. The amount of dissolved carbon and nutrients were strong predictors of small scale within‐site variability in gas release but the effect was site‐specific. We conclude that (i) temporal variability in CO₂ was greater than variation among vegetation communities; (ii) rainfall may be a good predictor of CO₂ emissions from tropical peatlands but temporal variation in CH₄ does not follow seasonal rainfall patterns; and (iii) diurnal variation in CO₂ fluxes across different vegetation communities can be described by a Fourier model.     

Land use and season affect fluxes of CO2, CH4, CO,N2O,H2 and isotopic source signatures in Panama: evidence from nocturnal boundary layer profiles

Conversion of tropical rainforests to pastures and plantations is associated with changes in soil properties and biogeochemical cycling, with implications for carbon cycling and trace gas fluxes. The stable isotopic composition of ecosystem respiration (δ¹³C_R and δ¹⁸O_R) is used in inversion models to quantify regional patterns of CO₂ sources and sinks, but models are limited by sparse measurements in tropical regions. We measured soil respiration rates, concentrations of CO₂, CH₄, CO, N₂O and H₂ and the isotopic composition of CO₂, CH₄ and H₂ at four heights in the nocturnal boundary layer (NBL) above three common land‐use types in central Panama, during dry and rainy seasons. Soil respiration rates were lowest in Plantation (average 3.4 μmol m^?2 s^?1), highest in Pasture (8.3 μmol m^?2 s^?1) and intermediate in Rainforest (5.2 μmol m^?2 s^?1). δ¹³C_R closely reflected land use and increased during the dry season where C₃ vegetation was present. δ¹⁸O_R did not differ by land use but was lower during the rainy than the dry season. CO₂ was correlated with other species in approximately half of the NBL profiles, allowing us to estimate trace gas fluxes that were generally within the range of literature values. The Rainforest soil was a sink for CH₄ but emissions were observed in Pasture and Plantation, especially during the wet season. N₂O emissions were higher in Pasture and Plantation than Rainforest, contrary to expectations. Soil H₂ uptake was highest in Rainforest and was not observable in Pasture and Plantation during the wet season. We observed soil CO uptake during the dry season and emissions during the wet season across land‐use types. This study demonstrated that strong impacts of land‐use change on soil–atmosphere trace gas exchange can be detected in the NBL, and provides useful observational constraints for top‐down and bottom‐up biogeochemistry models.     

High carbon emissions from thermokarst lakes and their determinants in the Tibet Plateau

Thermokarst lakes are potentially important sources of methane (CH₄) and carbon dioxide (CO₂). However, considerable uncertainty exists regarding carbon emissions from thermokarst lakes owing to a limited understanding of their patterns and motivators. In this study, we measured CH₄ and CO₂ diffusive fluxes in 163 thermokarst lakes in the Qinghai–Tibet Plateau (QTP) over 3 years from May to October. The median carbon emissions from the QTP thermokarst lakes were 1440 mg CO₂ m⁻² day⁻¹ and 60 mg CH₄ m⁻² day⁻¹, respectively. The diffusive rates of CO₂ and CH₄ are related to the catchment land cover type. Sediment microbial abundance and hydrochemistry explain 51.9% and 38.3% of the total variance in CH₄ diffusive emissions, respectively, while CO₂ emissions show no significant relationship with environmental factors. When upscaling carbon emissions from the QTP thermokarst lakes, the annual average CH₄ release per lake area is equal to that of the pan-Arctic region. Our findings highlight the importance of incorporating in situ observation data with different emission pathways for different land cover types in predicting carbon emissions from thermokarst lakes in the future.     

Regional-scale measurements of CH4 exchange from a tall tower over a mixed temperate/boreal lowland and wetland forest

The biosphere–atmosphere exchange of methane (CH₄) was estimated for a temperate/boreal lowland and wetland forest ecosystem in northern Wisconsin for 1997–1999 using the modified Bowen ratio (MBR) method. Gradients of CH₄ and CO₂ and CO₂ flux were measured on the 447‐m WLEF‐TV tower as part of the Chequamegon Ecosystem–Atmosphere Study (ChEAS). No systematic diurnal variability was observed in regional CH₄ fluxes measured using the MBR method. In all 3 years, regional CH₄ emissions reached maximum values during June–August (24±14.4 mg m^?2 day^?1), coinciding with periods of maximum soil temperatures. In 1997 and 1998, the onset in CH₄ emission was coincident with increases in ground temperatures following the melting of the snow cover. The onset of emission in 1999 lagged 100 days behind the 1997 and 1998 onsets, and was likely related to postdrought recovery of the regional water table to typical levels. The net regional emissions were 3.0, 3.1, and 2.1 g CH₄ m^?2 for 1997, 1998, and 1999, respectively. Annual emissions for wetland regions within the source area (28% of the land area) were 13.2, 13.8, and 10.3 g CH₄ m^?2 assuming moderate rates of oxidation of CH₄ in upland regions in 1997, 1998, and 1999, respectively. Scaling these measurements to the Chequamegon Ecosystem (CNNF) and comparing with average wetland emissions between 40°N and 50°N suggests that wetlands in the CNNF emit approximately 40% less than average wetlands at this latitude. Differences in mean monthly air temperatures did not affect the magnitude of CH₄ emissions; however, reduced precipitation and water table levels suppressed CH₄ emission during 1999, suggesting that long‐term climatic changes that reduce the water table will likely transform this landscape to a reduced source or possibly a sink for atmospheric CH₄.     

Fluxes of carbon dioxide and water vapour over an undisturbed tropical forest in south-west Amazonia

• 1 Carbon dioxide and water vapour fluxes were measured for 55 days by eddy covariance over an undisturbed tropical rain forest in Rondonia, Brazil. Profiles of CO₂ inside the canopy were also measured.
• 2 During the night, CO₂ concentration frequently built up to 500 ppm throughout the canopy as a result of low rates of exchange with the atmosphere. In the early morning hours, ventilation of the canopy occurred.
• 3 Ecosystem gas exchange was calculated from a knowledge of fluxes above the canopy and changes of CO₂ stored inside the canopy. Typically, uptake by the canopy was 15 μmol m^?2 s^?1 in bright sunlight and dark respiration was 6-7 μmol m^?2 s^?1 The quantum requirement at low irradiance was: 40 mol photons per mol of CO₂.
• 4 Bulk stomatal conductance of the ecosystem was maximal in the early morning (0.4-1.0 mol m^?2 s^?1) and declined over the course of the day as leaf-to-air vapour pressure difference increased.

     

Spatial variability of N2O, CH4 and CO2 fluxes within the Xilin River catchment of Inner Mongolia, China: a soil core study

In order to identify the effects of land-use/cover types, soil types and soil properties on the soil-atmosphere exchange of greenhouse gases (GHG) in semiarid grasslands as well as provide a reliable estimate of the midsummer GHG budget, nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) fluxes of soil cores from 30 representative sites were determined in the upper Xilin River catchment in Inner Mongolia. The soil N₂O emissions across all of the investigated sites ranged from 0.18 to 21.8 μg N m^-2 h^-1, with a mean of 3.4 μg N m^-2 h^-1 and a coefficient of variation (CV, which is given as a percentage ratio of one standard deviation to the mean) as large as 130%. CH₄ fluxes ranged from -88.6 to 2,782.8 μg C m^-2 h^-1 (with a CV of 849%). Net CH₄ emissions were only observed from cores taken from a marshland site, whereas all of the other 29 investigated sites showed net CH₄ uptake (mean: -33.3 μg C m^-2 h^-1). CO₂ emissions from all sites ranged from 3.6 to 109.3 mg C m^-2 h^-1, with a mean value of 37.4 mg C m^-2 h^-1 and a CV of 66%. Soil moisture primarily and positively regulated the spatial variability in N₂O and CO₂ emissions (R²?=?0.15–0.28, P?<?0.05). The spatial variation of N₂O emissions was also influenced by soil inorganic N contents (P?<?0.05). By simply up-scaling the site measurements by the various land-use/cover types to the entire catchment area (3,900 km²), the fluxes of N₂O, CH₄ and CO₂ at the time of sampling (mid-summer 2007) were estimated at 29 t CO₂-C-eq d^-1, -26 t CO₂-C-eq d^-1 and 3,223 t C d^-1, respectively. This suggests that, in terms of assessing the spatial variability of total GHG fluxes from the soils at a semiarid catchment/region, intensive studies may focus on CO₂ exchange, which is dominating the global warming potential of midsummer soil-atmosphere GHG fluxes. In addition, average GHG fluxes in midsummer, weighted by the areal extent of these land-use/cover types in the region, were approximately -30.0 μg C m^-2 h^-1 for CH₄, 2.4 μg N m^-2 h^-1 for N₂O and 34.5 mg C m^-2 h^-1 for CO₂.     

Carbon exchange of grazed pasture on a drained peat soil

Land‐use changes have contributed to increased atmospheric CO₂ concentrations. Conversion from natural peatlands to agricultural land has led to widespread subsidence of the peat surface caused by soil compaction and mineralization. To study the net ecosystem exchange of carbon (C) and the contribution of respiration to peat subsidence, eddy covariance measurements were made over pasture on a well‐developed, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water table fluctuated between 0.02 m in winter 2002 to 0.75 m during late summer and early autumn 2003. Peat soil moisture content varied between 0.6 and 0.7 m³ m^?3 until the water table dropped below 0.5 m, when moisture content reached 0.38 m³ m^?3. Neither depth to water table nor soil moisture was found to have an effect on the rate of night‐time respiration (ranging from 0.4–8.0 μmol CO₂ m^?2 s^?1 in winter and summer, respectively). Most of the variance in night‐time respiration was explained by changes in the 0.1 m soil temperature (r²=0.93). The highest values for daytime net ecosystem exchange were measured in September 2002, with a maximum of ?17.2 μmol CO₂ m^?2 s^?1. Grazing events and soil moisture deficiencies during a short period in summer reduced net CO₂ exchange. To establish an annual C balance for this ecosystem, non‐linear regression was used to model missing data. Annually integrated (CO₂) C exchange for this peat–pasture ecosystem was 45±500 kg C ha^?1 yr^?1. After including other C exchanges (methane emissions from cows and production of milk), the net annual C loss was 1061±500 kg C ha^?1 yr^?1.