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The subglacial environment under the Greenland Ice Sheet (GrIS) is an active zone of methane (CH4) production and consumption (1). Recent studies have shown that the meltwaters are a net source of CH4 to the atmosphere (2), although its global significance remains unquantified. It is unknown how CH4 cycling and net emission is linked to the melting of the GrIS, which is expected to increase (3) as the Artic is warming four times faster than the global average. Evaluating the importance of this poorly known source for the atmospheric CH4 budget and its drivers requires a fundamental understanding of the amounts released, the sources and sinks and its age.Traditionally, measurements of the isotopic composition (13CH4 and 12CH3D) are used as fingerprints to identify sources and sinks of CH4. However, this method is limited due to the overlap of source signatures. For example, microbial methanogenesis in some environments can produce stable isotope compositions resembling thermogenic methane (4). Furthermore, substrate isotopic composition, substrate limitation, the kinetics of methane production, transport, and oxidation substantially impact the stable isotope composition of microbially produced CH4. This complicates the interpretation of CH4 cycling and its physicochemical drivers.Clumped isotopes of methane, i.e. molecules of CH4 with two rare isotopes, (13CH3D and 12CH2DD), and its clumping anomaly (the relative difference between the measured value of 13CH3D and 12CH2DD and its stochastic distribution) provide additional insight to constrain CH4 sources and sinks. From the clumping anomaly, it is possible to calculate the formation temperature of methane (i.e. source of methane) if CH4 was formed in thermodynamic equilibrium. In the case of disequilibrium, the clumped signatures can be used to identify various kinetic gas formation and fractionation processes that are impossible to reconstruct from the bulk isotopic composition alone.In this study, we present for the first-time isotopic data of clumped CH4 and traditional isotopes of subglacial CH4 together with radiocarbon measurements (14CH4). These data are related to the isotopic composition of subglacial CO2 and mole fractions of the gases in the air and meltwater. Based on this data set, we will discuss the production and consumption pathways of CH4 in the subglacial environment and how it relates to diurnal and seasonal cycles of meltwater discharge.Reference:Christiansen et al. (2021). DOI: 10.1029/2021JG006308Christiansen, J. R., & Jørgensen, C. J. (2018). DOI: 10.1038/s41598-018-35054-7Ranlanen, et al. (2022), Commun Earth Environ, 2022. DOI: 10.1038/s43247-022-00498-3Valentine et al. (2004). DOI: 10.1016/j.gca.2003.10.012

The impact of rising global temperatures on carbon cycling in some of our most sensitive ecosystems, such as the Arctic, is critical yet challenging to quantify because short- and long-term effects of warming may be different. The observed short-term temperature response of soil organic carbon (SOC) decomposition may be altered in the long-term due to changes in substrate availability and possible acclimation of soil communities. Testing how the temperature response of soil decomposition changes over time with warming under open-air, field-scale conditions presents another obstacle for researchers, as warming experiments at different temperatures and over long measurement periods are expensive and rare. Fortunately, the ForHot grassland site in Iceland has provided us with a unique opportunity to explore soil warming effects by measuring along a naturally occurring geothermal gradient that resulted from an earthquake back in 2008.As part of the FutureArctic project at the ForHot site, we collected five replicate soil cores to a soil depth of 5 cm (100cm3 per sample), from fourteen plots along the geothermal gradient spanning a soil warming gradient from 0°C to +80°C. The samples were incubated in the lab at 5, 15, 25, and 35°C, and flux rates of CO2 and CH4 were measured using a LGR-ICOS M-GGA-918 to produce observational Q10 temperature response relationships. Temperature response rates and curvature appeared to be driven primarily by substrate availability. Samples containing the greatest totals of carbon and nutrients produced the highest rates of CO2 emission and CH4 consumption at all temperatures. This likely being an effect of the 13 years of warming where organic content and spatial proximity to the hotspot are inversely correlated. From this, we can then present an analysis of the potential linearity or nonlinearity that temperature responses can have over a rather extensive temperature gradient and how these responses can change over time.Samples collected from the hotspot, where previous in-situ chamber measurements have shown the highest emissions of CO2 and CH4, had significantly lower CO2 emissions and essentially no flux of CH4 during the laboratory incubations. This suggests a significant contribution of geogenic sourcing to in situ measurements. We present an analysis of the potential use for laboratory incubations at different temperatures to infer geogenic/biogenic flux contributions for in situ measurements where geogenic CO2 and CH4 emissions are present. This will allow us to construct a corrected biogenic carbon budget of the ForHot ecosystem and improve our fundamental understanding of the long-term effects that rising temperatures have on the carbon cycle in subarctic ecosystems.

Emission of CH4 and CO2 was recently discovered at the western margin of the Greenlandic Ice Sheet (GrIS) 1,2. While knowledge on both carbon sources, extent and magnitude of these emissions are still very limited, the previous studies indicate that a primary driver for emission is degassing of dissolved and pressurized gases in the meltwater as it reaches the glacial margin. In this way we suggest that glacial hydrology plays a key role in regulating emission on both temporal and spatial scale.In our studies of subglacial CH4 and CO2 emissions we have so far observed that the seasonal variations in meltwater discharge is correlated to both the magnitude of gas concentrations as well as timing of emissions3,4. We propose that the seasonal variations in the connectivity of subglacial channels to both 1) pockets of sediment with CH4 and CO2 production from both anaerobic and aerobic biological processes and 2) supraglacial meltwater via englacial conduits could be a mechanism, which could explain the overall temporal and seasonal patterns of gas concentrations observed at the glacial margin.We hypothesize that by observing hydrological and geochemical processes at the margin together with CH4 and CO2 in high frequency over the melt season it can be inferred how subglacial hydrological processes regulate biogeochemical and carbon turnover processes. Knowledge on these mechanism and processes are important for future upscaling CH4 and CO2 emission to seasonal periods and larger spatial scales through modeling as well as the assessment of the potential importance of subglacial carbon emissions to the climate system.Here, we will present data that couples meltwater discharge to measurements of dissolved CH4 and gaseous CH4 and CO2 as well as campaign measurements of water geochemistry and its isotopic composition. Preliminary data shows that CH4 and CO2 export display a clear diurnal signal in response to variations in the composition of melt water discharge. EC measurements and isotopic composition of melt water show a dominance of surface meltwater to subglacial meltwater, but clear diurnal trends in the mixing between these two water sources can be deduced from both isotope and elemental geochemistry of the meltwater.1. Christiansen, J. R. & Jørgensen, C. J. First observation of direct methane emission to the atmosphere from the subglacial domain of the Greenland Ice Sheet. Sci. Rep. 8, 16623 (2018).2. Lamarche-Gagnon, G. et al. Greenland melt drives continuous export of methane from its bed. Nature 73–77 (2018). doi:10.1038/s41586-018-0800-03. Jørgensen, C. J., Mønster, J., Fuglsang, K. & Christiansen, J. R. Continuous methane concentration measurements at the Greenland Ice Sheet-atmosphere interface using a low-cost low-power metal oxide sensor system. Atmos. Meas. Tech. Discuss. 13, 3319–3328 (2020).4. Christiansen, J. R., Röckmann, T., Popa, M. E., Sapart, C. J. & Jørgensen, C. J. Carbon emissions from the edge of the Greenland Ice sheet reveal subglacial processes of methane and carbon dioxide turnover. J. Geophys. Res. Biogeosciences 1–13 (2021). doi:10.1029/2021jg006308

Nitric oxide (NO) is a highly reactive trace gas affecting atmospheric chemistry and air quality. Although forest soils have been recognized as an important source of atmospheric NO, there are large uncertainties in global forest soil NO emission inventories (ranged from 0.03 to 8.00 kg N ha(-1), averaged 1.34 +/- 0.28 kg N ha(-1)), partly due to the paucity of high-frequency monitoring from unmanaged forests. In this study, we used an automated sampling system to measure NO fluxes with a high temporal resolution over two years (daily measurements from January 2019 to December 2020) in a mixed forest in Northeast China. We found that the mean annual NO emission was 0.42 +/- 0.04 kg N ha(-1), being 31% of the global forest average. The contribution of NO emission during the growing season was 92% of the yearly NO flux. Soil temperature was the most important edaphic factor in regulating NO emission, explaining 90-92% of the seasonal variation. The apparent temperature sensitivity (Q(10)) of NO flux was 3.67. In the growing season, NO emission was also influenced by soil moisture, with optimum soil moisture of 37% WFPS. By providing a detailed measurement of diurnal, seasonal, and annual dynamics of NO emissions and their environmental controls from forest soils, our data are useful to develop more accurate biogeochemical models that will improve upscaled global NO budgets.

In experimental ecosystem ecology, plot sizes are most often too small to apply eddy flux techniques and estimation of ecosystem gas exchange rates relies on various chamber measurement technologies. Furthermore, drained areas often results in increased growth of trees which complicates application of eddy flux measurements on small plots.We combined ECO2FluX ecosystem-level automatic chambers (prenart.dk) with an LI-8100/LI-8150 multiplexer systems (licor.com) in a range of Danish and Norwegian ecosystems experiments spanning agriculture, grassland/heathlands and peatland ecosystems. The automatic closed, none-steady state chambers each cover an area of 3,117 cm2 (63 cm diameter), are 80 cm tall (volume: 250L), and are capable of switching automatically between transparent and darkened mode, enabling separation of light-sensitive and light-indifferent processes in the ecosystems covered. For CO2 fluxes, net exchange (NEE) was estimated as the flux in transparent mode, ecosystem respiration (RE) in darkened mode, while Gross Ecosystem Productivity (GPP) was estimated as NEE – RE.Chambers were set up to measure gas concentrations every second using enclosure times of 4-5 minutes, first in light mode and 10-30 minutes later in dark mode, with 3-48 repetitions per day. The longest time series spans 5 years of measurements and contain >60,000 point measurements.In this presentation, we will present an analysis of the ability of the light-dark chamber data to infer ecosystem-level rates of gross primary productivity, respiration, net CO2 exchange, and evapotranspiration. In the two Norwegian peatland sites, flux measurements may be compared directly with eddy flux measurements. We also compare the rates of the direct estimates of GPP from the light-dark chamber measurements to estimates inferred from using the light (NEE) measurements only followed by applying methodologies normally used for eddy flux measurements. This comparison may help constrain potential biases in both the closed chamber and eddy flux techniques. Finally, we investigate the ability of using such closed chambers to estimate ecosystem evapotranspiration rates at the plot scale. Such application may be useful for estimating the effects on evapotranspiration in field-scale experiments manipulating the ecosystem water balance either directly or indirectly.

Most of the organic soils in Denmark are drained and used for agriculture. Greenhouse gas (GHG) emissions from these soils alone account for approximately 6-7 % of the total Danish emission. Rewetting, a relatively new climate change mitigation practice, of organic soils and peatland (i.e. lowland) ecosystems has the potential to reduce the net ecosystem GHG emission and over time turn the ecosystem to a net GHG sink. However, our knowledge of the ecosystem’s mitigation potential 50- or 100 years after abandonment is very limited and related to unknown interactions between soil biogeochemistry, vegetation type and growth and hydrology. Few studies have reported the GHG budgets of CO2 and CH4 for rewetted European peatland, and none includes the long-term response of these fluxes to rewetting. Continuous in-situ measurements of GHG emissions are complicated, time consuming and expensive which explains the data- and knowledge gap on long-term climate effects to some degree. Within the Danish research project RePeat, one of our main study-aims is to investigate how the long-term (0 – 71 years) ecosystem development (from grassland to secondary succession of woody vegetation) after rewetting impact the soil CO2 and CH4 fluxes in relation to changes in soil and biomass carbon stocks and biogeochemistry. We hypothesize that ‘rewetting and secondary succession of forest of farmed peatland will turn the ecosystem into a net carbon sink faster than if the ecosystem is maintained as extensively managed grasslands’. We expect that our research activities in 2021 and 2022 will allow us to test this hypothesis.For this study, we have selected four abandoned/rewetted agricultural sites on organic drained soils that represent a chronosequence spanning 7 decades. Two of the sites have recently been rewetted or will be (site A in 2020 and B in 2021) and the other two sites (C, D) have been abandoned and left for secondary succession since 1994 and 1950, respectively. At each site, two subplots with 5 replicate collars serve to measure the net CO2/CH4 exchange by the static chamber approach and use of the Ultra-Portable Greenhouse Gas Analyzer model 915-0011 (© Los Gatos Research). Soil moisture- and temperature and groundwater depth are measured for each collar. Basic meteorological parameters (precipitation, barometric pressure, photosynthetically active radiation (PAR) and wind- speed and direction) are measured at- or nearby each site. In addition to the GHG, soil physical- and meteorological measurements, we will estimate above- and belowground biomass and collect soil- and groundwater samples for a biogeochemical characterization.Our presentation will show the experimental setup and preliminary findings on CO2/CH4 fluxes and ecosystem hydrology for the four sites.

Terrestrial ecosystems, both natural ecosystems and agroecosystems, generate greenhouse gases (GHGs). The chamber method is the most common method to quantify GHG fluxes from soil–plant systems and to better understand factors affecting their generation and mitigation. The objective of this study was to review and synthesize literature on chamber designs (non‐flow‐through, non‐steady‐state chamber) and associated factors that affect GHG nitrous oxide (N2O) flux measurement when using chamber methods. Chamber design requires consideration of many facets that include materials, insulation, sealing, venting, depth of placement, and the need to maintain plant growth and activity. Final designs should be tailored, and bench tested, in order to meet the nuances of the experimental objectives and the ecosystem under study while reducing potential artifacts. Good insulation, to prevent temperature fluctuations and pressure changes, and a high‐quality seal between base and chamber are essential. Elimination of pressure differentials between headspace and atmosphere through venting should be performed, and designs now exist to eliminate Venturi effects of earlier tube‐type vent designs. The use of fans within the chamber headspace increases measurement precision but may alter the flux. To establish best practice recommendations when using fans, further data are required, particularly in systems containing tall plants, to systematically evaluate the effects that fan speed, position, and mixing rate have on soil gas flux.

Soil moisture and temperature collectively regulate the production and consumption of carbon in soils. With expected changes in both the soil thermal and hydrological regimes globally, experimental data on carbon turnover under these changes in contrasting ecosystems are important for constraining predictive models of soil carbon turnover. We investigated the effect of changes in soil water and temperature on heterotrophic respiration (Rh) and net methane uptake (MU) in soils from grassland ecosystems in Arctic, temperate and subtropical climates.The temperature sensitivity of RH increased with decreasing mean annual temperature, but there was no indication of a site-specific response of Rh to changes in soil moisture. All sites displayed MU, primarily controlled by the soil water content with little temperature dependence. Thus, the optimum temperature for MU did not differ between sites despite the differences in the climate. However, the optimal soil water content for the relative maximum MU decreased with increasing mean annual temperature at the sites.These results point to site-specific adaptation of the microbial community that governs the sensitivity of Rh to temperature, but not soil moisture and the dependency of MU to soil moisture alone. We would also like to discuss how this insight can be used to inform ecosystem models.

A fundamental challenge in experimental ecology is to capture nonlinearities of ecological responses to interacting environmental drivers. Here, we demonstrate that gradient designs outperform replicated designs for detecting and quantifying nonlinear responses. We report the results of (1) multiple computer simulations and (2) two purpose-designed empirical experiments. The findings consistently revealed that unreplicated sampling at a maximum number of sampling locations maximised prediction success (i.e. the R² to the known truth) irrespective of the amount of stochasticity and the underlying response surfaces, including combinations of two linear, unimodal or saturating drivers. For the two empirical experiments, the same pattern was found, with gradient designs outperforming replicated designs in revealing the response surfaces of underlying drivers. Our findings suggest that a move to gradient designs in ecological experiments could be a major step towards unravelling underlying response patterns to continuous and interacting environmental drivers in a feasible and statistically powerful way.

Groundwater depletion occurs at a global scale but requires regional strategies for sustainable management of freshwater resources. In Denmark the groundwater quantity and quality is under pressure, and forested areas are considered to protect groundwater reservoirs. However, little is known on how afforestation or forest conversion impacts the water resource at the catchment scale. We hypothesize that the groundwater formation and streamflow is increased when water consuming conifers are replaced with the less consumptive broadleaf tree species. To test this a distributed hydrological model with an energy-based description of evaporation and transpiration processes (MIKE SHE SWET) was used. Large scale hydrological models were established for two geologically (sandy/clayey) contrasting catchments in Denmark; Skjern and Lejre catchments. Land use scenarios were defined with forest vegetation (conifer/broadleaf) and agricultural crops (grass, maize, wheat and barley) in different areal combinations. Initially, the SWET component was calibrated against plot scale field data from two forest sites to obtain vegetation parameter estimates for conifers and broadleaves. Subsequently, the catchment models were run for 10 years with predefined land use scenarios. MIKE SHE SWET simulated canopy interception and throughfall for conifers and broadleaf forests satisfactorily. The catchment simulations showed that replacing current conifer forests with broadleaves, resulted in a significant increase in groundwater recharge and groundwater level, especially in the Skjern catchment with predominantly sandy soils. Also, doubling the forest area, as intended by national legislations, using only broadleaves did not negatively affect the groundwater generation or minimum stream discharge compared to current conditions at Skjern. However, because the shallow geology in the Lejre catchment are dominated by clayey glacial moraine deposits with low hydraulic conductivity, increased net precipitation in areas covered by broadleaf forests would primarily discharge as overland flow or drainage flow rather than contributing to groundwater formation.

In a changing climate, understanding how soil hydrology impacts greenhouse gas dynamics will be important for the future management of the soils in the forests on the Canadian Pacific west coast. In a laboratory study, the impact of soil hydrology on potential net methane (CH4) exchange rates and the abundance of methanotrophs (CH4oxidation) and methanogens (CH4production) in upland and water-saturated wet soils were investigated. CH4oxidation and production rates were highest in the wet soils, which corresponded to higher numbers of methanotrophs and methanogens, indicating a link between the microbial abundance and CH4exchange rates. Also, CH4production was induced in the upland soils, indicating the presence of methanogens. The optimum soil moisture content for CH4oxidation was highest in upland soils and the wet soils sustained higher CH4oxidation rates over a broader range of soil moisture. These results underline the importance of the soil hydrological controls of CH4oxidation in contrasting soils and forest types, which deserves further attention in field-based studies.

In this paper, the performance of a low-cost and low-power methane (CH4) sensing system prototype based on a metal oxide sensor (MOS) sensitive to CH4 is tested in a natural CH4-emitting environment at the Greenland ice sheet (GrIS). We investigate if the MOS could be used as a supplementary measurement technique for monitoring CH4 emissions from the GrIS with the scope of setting up a CH4 monitoring network along the GrIS. The performance of the MOS is evaluated on the basis of simultaneous measurements using a cavity ring-down spectroscopy (CRDS) reference instrument for CH4 over a field calibration period of approximately 100 h. Results from the field calibration period show that CH4 concentrations measured with the MOS are in very good agreement with the reference CRDS. The absolute concentration difference between the MOS and the CRDS reference values within the measured concentration range of approximately 2–100 ppm CH4 was generally lower than 5 ppm CH4, while the relative concentration deviations between the MOS and the CRDS were generally below 10 %. The calculated root-mean-square error (RMSE) for the entire field calibration period was 1.69 ppm (n=37 140). The results confirm that low-cost and low-power MOSs can be effectively used for atmospheric CH4 measurements under stable water vapor conditions. The primary scientific importance of the study is that it provides a clear example of how the application of low-cost technology can enhance our future understanding on the climatic feedbacks from the cryosphere to the atmosphere.

Laser scanning, also known as light detection and ranging (LiDAR), is an active remote sensing technology, which provides 3D information of surfaces. This information can be used for a broad range of applications, such as assessment of topography and structural information about the vegetation cover, including canopy density and canopy architecture, which are relevant to hydrological studies. In this chapter we present and discuss how LiDAR data can be applied in studies of forest-water interactions. LiDAR-derived digital terrain models (DTMs) are an integral part of distributed hydrological models and enable us to assess the entire catchment areas. Assessments of vegetation properties can be assisted by LiDAR data, thereby increasing the detail and accuracy of how vegetation is represented in hydrological models. LiDAR applications to forest-water interactions in riparian zones present a special case in this chapter, where the components of terrain, river bed and stream cross-section, and vegetation assessment are combined to describe and quantify the status of the riparian biosphere. With the analyses of LiDAR data, estimates of variables, which are relevant for hydrological modelling, can be improved. These analyses can be performed on an area-wide scale and enable analyses that would not be feasible using manual field techniques.

Ice sheets and glaciers play an important role for the global carbon cycle through the exchange of their subglacial carbon with the proglacial aquatic environment and the atmosphere in the form of CH4 and CO2. However, the subglacial environment below ice sheets and glaciers is largely inaccessible from the surface and hence we know very little about the carbon turnover processes in these extreme habitats that lead to this carbon export.Biological CH4 production and oxidation has been found in subglacial sediments across Canada, Antarctica, west Greenland and at the center of the Greenland Ice sheet. This points at a common glacial process for gaseous CH4 and CO2 emissions, but this knowledge is backed by very few direct field observations from two locations in Greenland and one in Iceland. The lack of field based studies is the single most-limiting factor for increasing our understanding of the magnitude and extent of subglacial carbon emission to the atmosphere and its relevance for the global carbon budget.We present new field measurements suggesting that it is possible to quantify the carbon turnover processes in the subglacial environment using high frequency concentration measurements and stable isotope composition of CH4 and CO2 in gaseous and dissolved form sampled at a subglacial meltwater outlet.During three field campaigns in the early, mid and late melt season in 2018 and 2019 we measured significantly elevated CH4 and CO2 concentrations in the air and water exiting a subglacial cave system. We devised a field sampling program for retrieval of discrete gas and water samples that allow identification of the original (common) source of the gaseous and dissolved CH4 and CO2 by quantifying the d13C and dH signature of the source.Our field measurements are amongst the first to directly quantify the emission of CH4 and CO2 to the atmosphere and our isotopic investigations clearly show a biological source of CH4 and its oxidation to CO2 in the subglacial environment and point to a hydrological control on the release of both CH4 and CO2.These types of data are instrumental to improve the understanding of subglacial carbon processes and design future field investigations to assess its climatic relevance and to narrow the uncertainty of emission estimates.

Microbial gene markers are hypothesized to be mediating factors between environment factors and methane flux, but mediation is not typically modeled directly in ecosystem studies using graphic models. Structural equation modelling (SEM) was used to test if mcrA and 16S markers were mediating the effects of soil moisture on methane flux in two ecosystem types, Upland and Wetland Forest. SEM results indicated that mcrA functional marker was a mediator in the Upland Forest but not in the Wetland Forest. In the Upland Forest, the 16S marker indirectly effected methane flux though its effect on the mcrA marker. The results suggest that functional genes are mediating drivers in ecosystems where environmental factors are weak drivers of methane fluxes. Our results highlight the importance of testing for microbial indirect pathways in assessing drivers of methane cycling and provide a basis for more complex modelling of mediated pathways in analysis of ecosystem processes.

Nitrogen (N) fertilization and mechanical site preparation are used to increase productivity of forests and prevent paludification in wet climates. However, these practices may also alter soil microbial community structure and rates of decomposition and respiration. The effects of mounding, drainage and fertilization were investigated at a mounded hybrid spruce site and a drained coastal western redcedar – western hemlock site in British Columbia (B.C.), Canada. CO2 efflux was measured at four dates over a one-year period using static closed chambers. Total bacterial and fungal ribosomal marker abundance and structure were assessed using quantitative PCR and terminal-restriction fragment polymorphism (T-RFLP), respectively. Mounding reduced soil water content of planting sites, but also removed much of the forest floor layer from the rooting zone and led to a significant decrease in CO2 efflux rates, as well as bacterial and fungal abundance and diversity. Drainage increased CO2 fluxes and led to significant differences in T-RFLP profiles in drained and undrained samples. Fertilization had variable effects on the microbial community and CO2 fluxes at the two sites. Site preparation and fertilization can improve regeneration of wet forests that would otherwise be subject to paludification, but can lead to shifts in carbon fluxes and microbial community structure. Major disturbances to soil structure and composition (e.g., following mounding) can alter the microbial community structure in a way that suppresses microbial diversity and function for up to one year following treatment.

Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO2), whereas methane (CH4) has not received the same attention. Soil hydrology is a key driver of CH4 dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH4 cycling and the resultant net CH4 exchange is not well understood at this scale. We studied the growing season variations of in situ CH4 fluxes, microbial gene abundances of methanotrophs (CH4 oxidizers) and methanogens (CH4 producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH4 displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH4 m−2 h−1) to a net emission in the wetter soils (0–90 μmol CH4 m−2 h−1). Seasonal variations of CH4 fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH4 emission was inversely related to the water table depth in the wet soils. Spatial variability of CH4 exchange was related to the abundance of genes involved in CH4 oxidation and production, but there was no indication of a temporal link between microbial groups and CH4 exchange. Our data show that the abundances of genes involved in CH4 oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type.

A laboratory and field experiment compared fluxes of CO 2 , CH 4 and N 2 O measured with cavity ring-down spectroscopy (CRDS) and gas chromatography (GC). The comparison between CRDS and GC showed that average CO 2 fluxes were significantly higher for CRDS in both the laboratory and field, but the same experimental treatments effects were detected for both techniques. Compared to CRDS, the GC technique was severely limited in detecting CH 4 fluxes in both the laboratory and field. Thus, only 16% of measured GC fluxes were detectable in the laboratory and none in the field whereas CRDS could detect 65% and 97% of the CH 4 fluxes in the laboratory and field. In contrast, N 2 O fluxes measured with CRDS and GC were not different for both the laboratory and field. It was observed that a lower proportion of N 2 O fluxes could be detected with CRDS (73%) than GC (92%) in the laboratory and similar recovery (65% and 68%) for the field. Thus, the same treatment effects were observed for both CRDS and GC. Furthermore, the comparison between CRDS and GC showed that enclosure times as short as 600 s for our field study site are suitable to estimate the same treatment effects, but not necessarily flux magnitude. We conclude that CRDS and GC can provide the same level of information regarding treatment effects in both laboratory and field experiments for CO 2 and N 2 O, but not for CH 4 and it is possible to reduce enclosure time without comprising comparability between the two techniques.

Forests contribute to improve water quality, affect drinking water resources, and therefore influence water supply on a regional level. The forest canopy structure affects the retention of precipitation (Pr) in the canopy and hence the amount of water transferred to the forest floor termed canopy throughfall (TF). We investigated the possibilities of estimating TF based on bulk Pr and canopy structure estimated from airborne light detection and ranging (LiDAR) data. Bulk Pr and TF fluxes combined with airborne LiDAR data from 11 locations representing the most common forest types (mono-species broadleaf/coniferous and mixed forests) in Denmark were used to develop empirical models to estimate TF on a monthly, seasonal, and annual basis. This new approach offers the opportunity to greatly improve predictions of TF on catchment wide scales. Overall, results show that TF can be estimated by Pr and a canopy density metric derived from LiDAR data. In all three types of TF data sets Pr was the variable explaining the majority of the variance in TF. The proportion of explained variance adhering to the LiDAR variable increased from 1.7% for the monthly data set to 12.2% and 19.5% for seasonal and annual data sets, respectively. Although the analysis was limited to Denmark the model was successful in estimating TF for contrasting tree species (broadleaf vs. conifers) and points to a potential for extending our model approach to other similar regions. Our approach can help to assess how forest cover impacts water resources on a large scale in regions where forests play a major role in water resource management.

Arctic soils are known to be important methane (CH4) consumers and sources. This study integrates in situ fluxes of CH4 between upland and wetland soils with potential rates of CH4 oxidation and production as well as abundance and diversity of the methanotrophs and methanogens measured with pyrosequencing of 16S DNA and rRNA fragments in soil and permafrost layers. Here, the spatial patterns of in situ CH4 fluxes for a 2,000 years old Arctic landscape in West Greenland reveal similar CH4 uptake rates (−4 ± 0.3 μmol m−2 h−1) as in other Arctic sites, but lower CH4 emissions (14 ± 1.5 μmol m−2 h−1) at wetland sites compared to other Arctic wetlands. Potential CH4 oxidation was similar for upland and wetland soils, but the wetter soils produced more CH4 in active and permafrost layers. Accordingly, the abundance of methanogenic archaea was highest in wetland soils. The methanotrophic community also differed between upland and wetland soils, with predominant activity of Type II methanotrophs in the active layer for upland soils, but only Type I methanotrophs for the wetland. In the permafrost of upland and wetland soils, activity of the methanotrophs belonging to Type I and Type II as well as methanogens were detected. This study indicates that the magnitude of CH4 oxidation and the direction of the flux, i.e. uptake or emission, are linked to different methanotrophic communities in upland and wetland soils. Also, the observed link between production/consumption rates and the microbial abundance and activity indicates that the age of an Arctic landscape is not important for the CH4 consumption but can be very important for CH4 production. Considering the prevalence of dry landscapes and contrasting ages of high Arctic soils, our results highlight that well-drained soils should not be overlooked as an important component of Arctic net CH4 budget.

We investigated CH4 oxidation in afforested soils over a 200-year chronosequence in Denmark including different tree species (Norway spruce, oak and larch) and ages. Samples of the top mineral soil (0–5 cm and 5–15 cm depth) were incubated and analyzed for the abundance of the soil methane-oxidizing bacteria (MOB) and ammonia-oxidizing bacteria (AOB) and archaea (AOA) based on quantitative PCR (qPCR) on pmoA and amoA genes. Our study showed that CH4 oxidation rates and the abundance of MOB increased simultaneously with time since afforestation, suggesting that the methanotrophic activity is reflected in the abundance of this functional group. The development of forest soils resulted in increased soil organic carbon and reduced bulk density, and these were the two variables that most strongly related to CH4 oxidation rates in the forest soils. For the top mineral soil layer (0–5 cm) CH4 oxidation rates did not differ between even aged stands from oak and larch, and were significantly smaller under Norway spruce. Compared to the other tree species Norway spruce caused a decrease in the abundance of MOB over time that could explain the decreased oxidation rates. However, the cause for the lower abundance remains unclear. The abundance of ammonia-oxidizers along the chronosequence decreased over time, oppositely to the MOB. However, our study did not indicate a direct link between CH4 oxidation rates and ammonia-oxidizers. Here, we provide evidence for a positive impact of afforestation of former cropland on CH4 oxidation capacity in soils most likely caused by an increased population size and activity of MOB.

We investigated the influence of tree species on the natural 15N abundance in forest stands under elevated ambient N deposition. We analysed δ15N in litter, the forest floor and three mineral soil horizons along with ecosystem N status variables at six sites planted three decades ago with five European broadleaved tree species and Norway spruce. Litter δ15N and 15N enrichment factor (δ15Nlitter–δ15Nsoil) were positively correlated with N status based on soil and litter N pools, nitrification, subsoil nitrate concentration and forest growth. Tree species differences were also significant for these N variables and for the litter δ15N and enrichment factor. Litter from ash and sycamore maple with high N status and low fungal mycelia activity was enriched in 15N (+0.9 delta units) relative to other tree species (European beech, pedunculate oak, lime and Norway spruce) even though the latter species leached more nitrate. The δ15N pattern reflected tree species related traits affecting the N cycling as well as site fertility and former land use, and possibly differences in N leaching. The tree species δ15N patterns reflected fractionation caused by uptake of N through mycorrhiza rather than due to nitrate leaching or other N transformation processes.

Recent research has shown that wet or hydromorphic soils in forests are hotspots for greenhouse gas (GHG) emission of methane (CH4) and nitrous oxide (N2O), and that emission of these gases may offset the mitigation potential from carbon (C) sequestration. However, quantitative evidence at the forest scale is limited. We investigated the role of hydromorphic soils for N2O and CH4 fluxes at the forest district level (Barritskov, 348 ha) by mapping the distribution of upland and hydromorphic soils, measuring the soil carbon and nitrogen stocks and field fluxes of N2O and CH4 for a period of 2 years as well as in laboratory experiments. Field exchange rates of N2O (mean ± standard error of the mean(SE), μg N2O–N m−2 h−1) were similar for hydromorphic (3.8 ± 1.2) and upland soils (3.8 ± 0.4). Although both soil types displayed net CH4 oxidation the average rate (μg CH4–C m−2 h−1) was significantly lower in hydromorphic soils (−5.8 ± 2) compared to the upland soils (−23 ± 1.2). Soil water content (SWC) was, as expected, higher in hydromorphic soils which was consistent with lower uptake of CH4 as well as significantly larger soil carbon stocks in O horizon plus 0–30 cm mineral soil (86 ± 6 versus 66 ± 5 Mg C ha−1 in hydromorphic versus upland). Oxidation rates of CH4 in laboratory incubations at ambient concentration (2 μL L−1) were similar in the two soil types, but the hydromorphic soils oxidised CH4 fastest when incubated at 10,000 μL L−1 CH4: only hydromorphic soils produced CH4. Potential N2O production did not differ between soil types and N2 production was significantly higher in hydromorphic soils, which also had a higher pH > 6. Based on four scenarios, we assessed how reduced ditching might affect the emissions of N2O and CH4 from upland soils. The CH4 sink of the soil decreased in all four reduced ditching scenarios from 1.3 to 7 Mg CO2-equivalent (eqv) y−1. The emissions of N2O and CH4 in the current state and all scenarios comprised only a minute fraction ( We conclude that hydromorphic soils are potential hotspots for CH4 production and reduced uptake of atmospheric CH4, but their limited area covered by such soils at Barritskov implies that upland soils are most important in terms of soil C stock and the non-CO2 GHG budget. Ceased drainage activities in upland soils are expected to increase the likelihood of CH4 emissions and reduce soil CH4 uptake.

The knowledge of tree species effects on soil organic carbon (C) turnover based on rigorous experimental designs is limited for common European deciduous tree species. We assessed soil respiration, and rates of C turnover in six tree species in a more than 30-year-old common garden experiment replicated at six sites in Denmark. The studied tree species were the broadleaves beech (Fagus sylvatica L.), pedunculate oak (Quercus robur L.), lime (Tilia cordata L.), sycamore maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.) and the conifer Norway spruce (Picea abies (L.) Karst.). Rates of C turnover were estimated by (i) the ratio of estimated soil heterotrophic respiration (Rh) to C stock in forest floor and top mineral soil, (ii) the ratio of litterfall C to forest floor C, (iii) foliar mass loss in litterbags, and (iv) mineral soil C turnover assessed by laboratory incubation. Soil respiration differed significantly among several species and increased in the order beech

Afforestation of former agricultural land is a means to mitigate anthropogenic greenhouse gas emissions. The objectives of this study were (1) to assess the effect of oak (Quercus robur) and Norway spruce (Picea abies [L.] Karst.) stands of different stand ages (13–17 and 40 years after afforestation, respectively) on N2O and CH4 exchange from the soil under these species and (2) identify the environmental factors responsible for the differences in gas exchange between tree species of different ages. N2O and CH4 fluxes (mean ± SE) were measured for two years at an afforested site. No species difference was documented for N2O emission (oak: 4.2 ± 0.7 μg N2O-N m−2 h−1, spruce: 4.0 ± 1 μg N2O-N m−2 h−1) but the youngest stands (1.9 ± 0.3 μg N2O-N m−2 h−1) emitted significantly less N2O than older stands (6.3 ± 1.2 μg N2O-N m−2 h−1). CH4 exchange did not differ significantly between tree species (oak: −8.9 ± 0.9, spruce: −7.7 ± 1) or stand age (young: −7.3 ± 0.9 μg CH4-C m−2 h−1, old: −9.4 ± 1 μg CH4-C m−2 h−1) but interacted significantly; CH4 oxidation in the soil increased with stand age in oak and decreased with age for soils under Norway spruce. We conclude that the exchange of N2O and CH4 from the forest soil undergoes a quick and significant transition in the first four decades after planting in both oak and Norway spruce. These changes are related to (1) increased soil N availability over time as a result of less demand for N by trees in turn facilitating higher N2O production in older stands and (2) decreasing bulk density and increased gas diffusivity in the top soil over time facilitating better exchange of N2O and CH4 with the atmosphere.

The magnitude of greenhouse gas (GHG) flux rates may be important in wet and intermediate wet forest soils, but published estimates are scarce. We studied the surface exchange of methane (CH4) and nitrous oxide (N2O) from soil along toposequences in two temperate deciduous forest catchments: Strodam and Vestskoven. The soil water regime ranged from fully saturated to aerated within the catchments. At Strodam the largest mean flux rates of N2O (15 μg N2O-N m−2 h−1) were measured at volumetric soil water contents (SWC) between 40 and 60% and associated with low soil pH compared to smaller mean flux rates of 0-5 μg N2O-N m−2 h−1 for drier (SWC 80%). At Vestskoven the same response of N2O to soil water content was observed. Average CH4 flux rates were highly variable along the toposequences (−17 to 536 μg CH4-C m−2 h−1) but emissions were only observed above soil water content of 45%. Scaled flux rates of both GHGs to catchment level resulted in emission of 322 and 211 kg CO2-equivalents ha−1 year−1 for Strodam and Vestskoven, respectively, with N2O contributing the most at both sites. Although the wet and intermediate wet forest soils occupied less than half the catchment area at both sites, the global warming potential (GWP) derived from N2O and CH4 was more than doubled when accounting for these wet areas in the catchments. The results stress the importance of wet soils in assessments of forest soil global warming potentials, as even small proportions of wet soils contributes substantially to the emissions of N2O and CH4.

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30-year-old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small-leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallo during 2 years. Mean annual percolation below the root zone (mm yr−1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)≫ spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha−1 yr−1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha−1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha−1 yr−1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)≫ maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallo. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short-term perspective (30 years) of these stands.

The area of broadleaved forests is projected to increase in Denmark as well as in the rest of Europe. However, studies of the N leaching response to elevated N deposition have focused on coniferous stands and considerable uncertainty still remains on whether broadleaved and coniferous forests respond differently to elevated N. We studied N input–output relations for eight stands intensively monitored during 2002–2005 and literature data for 37 additional stands which together formed a comprehensive dataset on Danish forests including 26 broadleaved stands and 19 coniferous stands. Nitrate leaching was significantly higher in first generation stands on former arable land with mineral soil C/N ratios 10–15, but both low and high rates were observed independent of the N input. A net N loss was observed in some of these stands even though they are in the aggrading phase and accumulate N in the biomass. Broadleaved stands had significantly lower throughfall N deposition than coniferous stands and this seems to be the main process where forest type exerts an influence on the N cycle. Lower soil C/N ratios offset the effect of throughfall N deposition and thus N leaching did not differ between the two forest types. The best regression models for prediction of nitrate leaching included throughfall N deposition and C/N ratio, but only a minor part of the variability was explained. The C/N ratio of the upper mineral soil was more generally applicable than that of the organic layer. The N retention of the soil was reasonably well predicted above a C/N ratio of 25, but below this threshold the importance is not known. We suggest focusing future efforts on quantifying the relative retention functions (sink strength) of the vegetation and the soil organic matter to improve the predictions of N retention and N leaching.

At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N2O), carbon dioxide (CO2) and methane (CH4) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N2O and CO2, but take up less atmospheric CH4 than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N2O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N2O ranged from 2.0 to 4.4 kg N ha−1 year−1, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N2O emissions in the laboratory incubated soils varied with slope positions. Soil CO2 release and CH4 uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N2O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling.

Two contrasting forest ecosystems located in close proximity to each other were selected for evaluating the importance of tree species and afforestation in relation to the water balance and the qua ...

International audience; The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 μg CH4 m−2 h−1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m3 in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (Rlin), whereas the chamber fluxes calculated by the exponential flux calculation method (Rexp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.

Land-terminating glaciers in Greenland and Iceland are sources of methane (CH4) to the atmosphere1,2,3. CH4 is produced through microbial methanogenesis underneath the ice, transported dissolved in subglacial meltwater to the margin where the gas is emitted to the atmosphere via degassing4. However, sparse empirical data exist about the spatial distribution of subglacial CH4 production and emission in other glaciated regions of the world, limiting our understanding of its regional and global importance in atmospheric carbon budgets and its possible role in the climate system.In August 2022, we conducted fieldwork at three outlet glaciers - Dusty, Kluane and Donjek glaciers - of the St. Elias Icefields in Yukon, Canada, to investigate if these alpine glaciers are also sources of CH4 emissions to the atmosphere. The glaciers were chosen due to the absence of proglacial lakes and the presence of meltwater upwellings at the glacier termini, which were accessed via helicopter.In-situ extracted dissolved CH4 and CO2 concentrations were measured in the field with a portable greenhouse gas analyzer. Additionally, extracted gas was collected in exetainers for concentration measurements via gas chromatography and in Tedlar gas bags for stable carbon and hydrogen isotope analyses of CH4 to decipher its origin. Further, water samples were collected for geochemical analyses. At Dusty glacier, we performed a high-intensity sampling campaign over 10 hours and continuous measurements of dissolved CH4 concentrations with a custom-made low-cost and low-power dissolved CH4 sensor5 to study changes in dissolved gas concentrations, stable isotopic signatures and water chemistry during the rising limb of the diurnal discharge curve.In-situ measured CH4 and CO2 concentrations yielded significantly elevated CH4 and depleted CO2 levels in the meltwater of all three glaciers. Discrete gas samples confirmed dissolved CH4 concentrations 45x, 135x and 250x above the atmospheric equilibrium concentration (3.6 nmol L-1) in the meltwater of Dusty, Kluane and Donjek glaciers, respectively. First measurements of stable carbon and hydrogen isotope values of CH4 showed enrichment in 13C, while 2H was depleted compared to atmospheric CH4, at all sites, likely originating from a thermogenic source or caused by bacterial CH4 oxidation. Water analyses showed an alkaline environment enriched in carbonates and DOC, in contrast to more acidic waters from glaciers in Greenland and Iceland.These first measurements demonstrate that the subglacial meltwaters from glaciers in the St. Elias Icefields are net sources of CH4 and net sinks of CO2 to the atmosphere. Our findings indicate that CH4 emissions from subglacial environments under alpine glaciers may be a more common phenomenon than previously thought, and a potential cause for remotely sensed CH4 concentrations anomalies over glaciated regions. However, more alpine glaciers and outlets from the Greenland Ice Sheet need to be studied to evaluate this link and provide the needed ground truthing for satellite sensors in high latitudes.1. Christiansen & Jørgensen (2018) DOI 10.1038/s41598-018-35054-72. Lamarche-Gagnon et al. (2019) DOI 10.1038/s41586-018-0800-03. Burns et al. (2018) DOI 10.1038/s41598-018-35253-24. Christiansen et al. (2021) DOI 10.1029/2021JG0063085. Sapper et al. (2022) DOI:10.5194/egusphere-egu22-9972

An unknown source of methane (CH4) was recently discovered under the Kangerlussuaq sector of the Greenland Ice Sheet (GrIS). CH4 is transported dissolved in meltwater from the subglacial environment to the margin of the ice sheet, where it rapidly degasses to the atmosphere. Existing knowledge gaps concern the magnitude of emissions, seasonal patterns and spatial variations along the margin of the GrIS, which require long-term monitoring and large-scale measurement campaigns at multiple meltwater outlets. A limiting factor for such studies in remote areas is that CH4 analysers (laser spectroscopy) are power-hungry, maintenance-intensive, and expensive. To overcome these obstacles, we are developing a low-cost, low power sensor for measuring dissolved CH4 in subglacial meltwater systems in the MetICE project: the WaterWorm.The WaterWorm is based on a metal oxide sensor (MOS) designed for CH4 detection (Figaro TGS2611-E00), which is highly sensitive to variations in relative humidity (RH) and temperature. In the WaterWorm, the MOS is encased in a hydrophobic but gas-permeable silicone tube, ensuring a stable and fully saturated headspace (100% RH) during submergence. We calibrated the analogue output (in mV) of the submerged WaterWorm against a reference CH4 analyser (μGGA, GLA-331, LGR Research) connected to a dissolved gas extraction system (DGES, LGR Research) in temperature-controlled laboratory experiments by stepwise enrichment of the water with CH4. These calibration tests showed that the sensor output (set at two readings per minute) is proportional to dissolved CH4 at constant humidity and temperature.During fieldwork near Kangerlussuaq, Greenland, in summer 2021, a field baseline calibration was performed in a meltwater stream on the surface of the GrIS at ambient CH4 concentrations. WaterWorms were deployed for ten weeks in the meltwater of a small outlet of the Isunnguata Sermia glacier with known CH4 export and stable meltwater temperatures (0.0 - 0.1°C) to test the sensor under field conditions. Throughout this period, the WaterWorms measured elevated dissolved CH4 concentrations with diurnal variations that corresponded to similar diurnal variation in gaseous CH4 measurements performed with the reference CH4 analyser.The WaterWorm is a promising and cost-efficient option for the seasonal monitoring of dissolved CH4 in glacial meltwater. With material costs of only 150€, the WaterWorm can be left unattended in the field and positioned directly at the ice edge. This makes the sensor suitable for a large-scale CH4 monitoring network along the margin of the GrIS. The next steps involve material tests to build WaterWorms for applications in other aquatic environments and at different water depths.