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An important question in the context of climate change is to understand how CH₄ production is regulated in anoxic sediments of lakes and reservoirs. The type of organic carbon (OC) present in lakes is a key factor controlling CH₄ production at anoxic conditions, but the studies investigating the methanogenic potential of the main OC types are fragmented. We incubated different types of allochthonous OC (alloOC; terrestrial plant leaves) and autochthonous OC (autoOC; phytoplankton and two aquatic plants species) in an anoxic sediment during 130 d. We tested if (1) the supply of fresh alloOC and autoOC to an anoxic refractory sediment would fuel CH4 production and if (2) autoOC would decompose faster than alloOC. The addition of fresh OC greatly increased CH₄ production and the δ 13C-CH₄ partitioning indicated that CH₄ originated exclusively from the fresh OC. The large CH₄ production in an anoxic sediment fueled by alloOC is a new finding which indicates that all systems with anoxic conditions and high sedimentation rates have the potential to be CH₄ emitters. The autoOC decomposed faster than alloOC, but the total CH₄ production was not higher for all autoOC types, one aquatic plant species having values as low as the terrestrial leaves, and the other one having values as high as phytoplankton. Our study is the first to report such variability, suggesting that the extent to which C fixed by aquatic plants is emitted as greenhouse gases or buried as OC in sediment could more generally differ between aquatic vegetation types.

Meta-analyses show that the temperature dependence of methane fluxes scales consistently across populations of methanogens, microbial communities and whole ecosystems, and that this temperature dependence is higher than for respiration and photosynthesis; this indicates that global warming may impact the relative contributions of CO 2 and CH 4 to total greenhouse gas emissions. Methane emissions highly sensitive to temperature Methane is a potent greenhouse gas with many times the global warming potential of carbon dioxide, so understanding how its emissions might change with increasing temperature is important for climate predictions. These authors conduct a meta-analysis of the temperature dependence of methane emissions from studies of laboratory cultures, environmental samples and whole ecosystems. They find that methane emissions increase with temperature at a greater rate than two other key rate processes in the carbon cycle, respiration and photosynthesis. The effect scales identically from cultures of individual methanogens in the lab up to the whole ecosystems. Methane (CH 4 ) is an important greenhouse gas because it has 25 times the global warming potential of carbon dioxide (CO 2 ) by mass over a century 1 . Recent calculations suggest that atmospheric CH 4 emissions have been responsible for approximately 20% of Earth’s warming since pre-industrial times 2 . Understanding how CH 4 emissions from ecosystems will respond to expected increases in global temperature is therefore fundamental to predicting whether the carbon cycle will mitigate or accelerate climate change. Methanogenesis is the terminal step in the remineralization of organic matter and is carried out by strictly anaerobic Archaea 3 . Like most other forms of metabolism, methanogenesis is temperature-dependent 4 , 5 . However, it is not yet known how this physiological response combines with other biotic processes (for example, methanotrophy 6 , substrate supply 3 , 7 , microbial community composition 8 ) and abiotic processes (for example, water-table depth 9 , 10 ) to determine the temperature dependence of ecosystem-level CH 4 emissions. It is also not known whether CH 4 emissions at the ecosystem level have a fundamentally different temperature dependence than other key fluxes in the carbon cycle, such as photosynthesis and respiration. Here we use meta-analyses to show that seasonal variations in CH 4 emissions from a wide range of ecosystems exhibit an average temperature dependence similar to that of CH 4 production derived from pure cultures of methanogens and anaerobic microbial communities. This average temperature dependence (0.96 electron volts (eV)), which corresponds to a 57-fold increase between 0 and 30°C, is considerably higher than previously observed for respiration (approximately 0.65 eV) 11 and photosynthesis (approximately 0.3 eV) 12 . As a result, we show that both the emission of CH 4 and the ratio of CH 4 to CO 2 emissions increase markedly with seasonal increases in temperature. Our findings suggest that global warming may have a large impact on the relative contributions of CO 2 and CH 4 to total greenhouse gas emissions from aquatic ecosystems, terrestrial wetlands and rice paddies.

Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH₄) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets. Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least 103 teragrams of CH₄ year⁻¹, corresponding to 0.65 petagrams of C as carbon dioxide (CO₂) equivalents year⁻¹, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and freshwaters need to be recognized as important in the global carbon cycle.

Lakes are major sources of methane (CH₄) to the atmosphere that contribute significantly to the global budget. Recent studies have shown that diffusive fluxes, ebullition and surface water CH₄ concentrations can differ significantly within lakes—spatially and temporally. CH₄ fluxes may be affected at longer scales in response to seasons, temperature, lake mixing events, short termweather events like pressure variations, shifting winds and diel cycles. Frequent measurements of fluxes in the same system and integrated assessments of the impacts of the spatiotemporal variability are rare. Thereby, large scale assessments frequently lack information on this variability which can potentially lead to biased estimates. In this study, we analysed the variability of CH₄ fluxes and surface water CH₄ concentrations across open water areas of lakes in a small catchment in southwest Sweden over two annual cycles. Significant patterns in CH₄ concentrations, diffusive fluxes, ebullition and total fluxes were observed in space (between and within lakes) and in time (over diel cycles to years). Differences observed among the lakes can be associated with lake characteristics. The spatial variability within lakes was linked to depth or distance to stream inlets. Temporal variability was observed at diel to seasonal scales and was influenced by weather events. The fluxes increased exponentially with temperature in all three lakes, with stronger temperature dependence with decreasing depth. By comparing subsets of our data with estimates using all data we show that considering the spatio-temporal variability in CH₄ fluxes is critical when making whole lake or annual budgets.

Many biochemical processes and dynamics are strongly controlled by terrain topography, making digital elevation models (DEM) a fundamental dataset for a range of applications. This study investigates the quality of four pan-Arctic DEMs (Arctic DEM, ASTER DEM, ALOS DEM and Copernicus DEM) within the Kalix River watershed in northern Sweden, with the aim of informing users about the quality when comparing these DEMs. The quality assessment focuses on both the vertical accuracy of the DEMs and their abilities to model two fundamental elevation derivatives, including topographic wetness index (TWI) and landform classification. Our results show that the vertical accuracy is relatively high for Arctic DEM, ALOS and Copernicus and in our study area was slightly better than those reported in official validation results. Vertical errors are mainly caused by tree cover characteristics and terrain slope. On the other hand, the high vertical accuracy does not translate directly into high quality elevation derivatives, such as TWI and landform classes, as shown by the large errors in TWI and landform classification for all four candidate DEMs. Copernicus produced elevation derivatives with results most similar to those from the reference DEM, but the errors are still relatively high, with large underestimation of TWI in land cover classes with a high likelihood of being wet. Overall, the Copernicus DEM produced the most accurate elevation derivatives, followed by slightly lower accuracies from Arctic DEM and ALOS, and the least accurate being ASTER.

Methane is an important greenhouse gas 1 , but the role of trees in the methane budget remains uncertain 2 . Although it has been shown that wetland and some upland trees can emit soil-derived methane at the stem base 3 , 4 , it has also been suggested that upland trees can serve as a net sink for atmospheric methane 5 , 6 . Here we examine in situ woody surface methane exchange of upland tropical, temperate and boreal forest trees. We find that methane uptake on woody surfaces, in particular at and above about 2 m above the forest floor, can dominate the net ecosystem contribution of trees, resulting in a net tree methane sink. Stable carbon isotope measurement of methane in woody surface chamber air and process-level investigations on extracted wood cores are consistent with methanotrophy, suggesting a microbially mediated drawdown of methane on and in tree woody surfaces and tissues. By applying terrestrial laser scanning-derived allometry to quantify global forest tree woody surface area, a preliminary first estimate suggests that trees may contribute 24.6–49.9 Tg of atmospheric methane uptake globally. Our findings indicate that the climate benefits of tropical and temperate forest protection and reforestation may be greater than previously assumed. Studies of in situ woody surface methane exchange in upland tropical, temperate and boreal forest trees find that methane uptake can result in a net tree methane sink that is globally significant and demonstrates an additional climate benefit provided by trees.

Lakes as carbon sinks Inland water sediments are important, but commonly disregarded long-term carbon sinks — in fact, the annual burial of organic carbon in lakes and reservoirs exceeds that of ocean sediments. Gudasz et al . now show that for several different types of lake in subarctic Sweden, the mineralization of carbon in lake sediments significantly increases as temperatures increase. Assuming that future organic carbon delivery to the lake sediments will be similar to present-day conditions, this could act as a positive feedback to global warming. The annual burial of organic carbon in lakes and reservoirs exceeds that of ocean sediments, but inland waters are components of the global carbon cycle that receive only limited attention. Here the authors find that the mineralization of organic carbon in lake sediments exhibits a strong positive relationship with temperature, suggesting that warmer water temperatures lead to more mineralization and less organic carbon burial. Peatlands, soils and the ocean floor are well-recognized as sites of organic carbon accumulation and represent important global carbon sinks 1 , 2 . Although the annual burial of organic carbon in lakes and reservoirs exceeds that of ocean sediments 3 , these inland waters are components of the global carbon cycle that receive only limited attention 4 , 5 , 6 . Of the organic carbon that is being deposited onto the sediments, a certain proportion will be mineralized and the remainder will be buried over geological timescales. Here we assess the relationship between sediment organic carbon mineralization and temperature in a cross-system survey of boreal lakes in Sweden, and with input from a compilation of published data from a wide range of lakes that differ with respect to climate, productivity and organic carbon source. We find that the mineralization of organic carbon in lake sediments exhibits a strongly positive relationship with temperature, which suggests that warmer water temperatures lead to more mineralization and less organic carbon burial. Assuming that future organic carbon delivery to the lake sediments will be similar to that under present-day conditions, we estimate that temperature increases following the latest scenarios presented by the Intergovernmental Panel on Climate Change 7 could result in a 4–27 per cent (0.9–6.4 Tg C yr −1 ) decrease in annual organic carbon burial in boreal lakes.

Lateral CH 4 inputs to Arctic lakes through groundwater discharge could be substantial and constitute an important pathway that links CH 4 production in thawing permafrost to atmospheric emissions via lakes. Yet, groundwater CH 4 inputs and associated drivers are hitherto poorly constrained because their dynamics and spatial variability are largely unknown. Here, we unravel the important role and drivers of groundwater discharge for CH 4 emissions from Arctic lakes. Spatial patterns across lakes suggest groundwater inflows are primarily related to lake depth and wetland cover. Groundwater CH 4 inputs to lakes are higher in summer than in autumn and are influenced by hydrological (groundwater recharge) and biological drivers (CH 4 production). This information on the spatial and temporal patterns on groundwater discharge at high northern latitudes is critical for predicting lake CH 4 emissions in the warming Arctic, as rising temperatures, increasing precipitation, and permafrost thawing may further exacerbate groundwater CH 4 inputs to lakes. CH 4 inputs to Arctic lakes via groundwater discharge are an important pathway that links CH 4 production in thawing permafrost to emission via lakes. Here the authors unravel the role and drivers of groundwater inflows for CH 4 emissions from Arctic lakes.

Methane fluxes from the stems of Amazonian floodplain trees indicate that the escape of soil gas through wetland trees is the dominant source of methane emissions in the Amazon basin. Missing methane in the Amazon Wetlands are the single largest global source of the greenhouse gas methane, but the contribution of the Amazon floodplain, the largest natural geographic source of methane in the tropics, remains poorly understood. Methane emission inventories underestimate the atmospheric burden of methane determined via remote sensing and inversion modelling. This paper reports on methane fluxes from the stems of Amazonian floodplain trees and finds that gas leaving the soil through wetland trees is the dominant source of regional methane emissions. The authors also provide an estimate of methane emission for the Amazon basin based on atmospheric methane profiles and find that it can be reconciled with the combined emission estimate from floodplain trees and other regional methane sources. Overall, the findings suggest that the large methane emission from trees could be what was missing from the Amazon budget. Wetlands are the largest global source of atmospheric methane (CH 4 ) 1 , a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain 2 , 3 , the largest natural geographic source of CH 4 in the tropics, consistently underestimate the atmospheric burden of CH 4 determined via remote sensing and inversion modelling 4 , 5 , pointing to a major gap in our understanding of the contribution of these ecosystems to CH 4 emissions. Here we report CH 4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH 4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests 6 and tropical peat swamp forests 7 , representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ 13 C) of −66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH 4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a ‘top-down’ regional estimate of CH 4 emissions of 42.7 ± 5.6 teragrams of CH 4 a year for the Amazon basin, based on regular vertical lower-troposphere CH 4 profiles covering the period 2010–2013. We find close agreement between our ‘top-down’ and combined ‘bottom-up’ estimates, indicating that large CH 4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH 4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH 4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH 4 source when trees are combined with other emission sources.

Freshwaters bring a notable contribution to the global carbon budget by emitting both carbon dioxide (CO2) and methane (CH4) to the atmosphere. Global estimates of freshwater emissions traditionally use a wind-speed-based gas transfer velocity, kCC (introduced by Cole and Caraco, 1998), for calculating diffusive flux with the boundary layer method (BLM). We compared CH4 and CO2 fluxes from BLM with kCC and two other gas transfer velocities (kTE and kHE), which include the effects of water-side cooling to the gas transfer besides shear-induced turbulence, with simultaneous eddy covariance (EC) and floating chamber (FC) fluxes during a 16-day measurement campaign in September 2014 at Lake Kuivajärvi in Finland. The measurements included both lake stratification and water column mixing periods. Results show that BLM fluxes were mainly lower than EC, with the more recent model kTE giving the best fit with EC fluxes, whereas FC measurements resulted in higher fluxes than simultaneous EC measurements. We highly recommend using up-to-date gas transfer models, instead of kCC, for better flux estimates. BLM CO2 flux measurements had clear differences between daytime and night-time fluxes with all gas transfer models during both stratified and mixing periods, whereas EC measurements did not show a diurnal behaviour in CO2 flux. CH4 flux had higher values in daytime than night-time during lake mixing period according to EC measurements, with highest fluxes detected just before sunset. In addition, we found clear differences in daytime and night-time concentration difference between the air and surface water for both CH4 and CO2. This might lead to biased flux estimates, if only daytime values are used in BLM upscaling and flux measurements in general. FC measurements did not detect spatial variation in either CH4 or CO2 flux over Lake Kuivajärvi. EC measurements, on the other hand, did not show any spatial variation in CH4 fluxes but did show a clear difference between CO2 fluxes from shallower and deeper areas. We highlight that while all flux measurement methods have their pros and cons, it is important to carefully think about the chosen method and measurement interval, as well as their effects on the resulting flux.