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Peatlands are significant modulators of biogeochemical cycles and important carbon stocks on a global scale, and they may become large sources of the greenhouse gases (GHG) carbon dioxide and methane because of their vulnerability to management practices and changes in climate. Because gas exchange between peat and the atmosphere occurs primarily via diffusion, a proper knowledge of the gas diffusion rate is essential for correct estimation of the amount of GHG emissions from peatlands. Diffusion is controlled by the structure and connectivity of peat pore space. Pore network modeling (PNM) is an efficient method for the pore-scale description and simulation of transport processes in porous matter and, therefore, a useful tool for the assessment of gas diffusivity in peat, as it explicitly illustrates the relationship between the peat microstructure and the gas transport properties on a macroscopic scale. PNM can also be used to simulate time-dependent soil biogeochemical processes, such as GHG production and consumption.We extracted interconnecting macropore (diameter greater than 0.1 mm) networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples from three depths and simulated steady-state diffusion in the networks using PNM. We then compared the obtained soil gas diffusion coefficients to those determined experimentally from the same samples. The gas diffusivity measurements were made using the diffusion chamber method under different water contents adjusted in a pressure plate extractor. The measured soil gas diffusivity was lower in deeper layers because of decreased air-filled porosity and pore connectivity. Nevertheless, the diffusion rates were not extremely low close to saturation, which may imply that connected air-filled pathways for gas diffusion are present in peat even in wet conditions. The pore network simulations were able to reproduce the experimentally determined gas diffusion dynamics rather well. This also implies that the topology and the dimensions of the pore space of most of the peat samples were adequately represented by the network objects. Therefore, the combination of µCT and PNM can be considered a potential alternative to the assessment of soil gas diffusivity through traditional laboratory measurements. However, further research is needed on gas diffusivity in different peat types over a wide water content range. Furthermore, the presented approach provides a basis for mechanistic simulation of GHG cycling in soils.

Boreal peatlands emit a substantial amount of CH4, a potent greenhouse gas, to the atmosphere. Despite decades of effort made on studying CH4 efflux to the atmosphere, understanding the dynamics of the different co-occurring processes underlying CH4 emission remains a challenge in peatland CH4 modeling, especially during the non-growing seasons. Stable isotope signatures of the soil pore water and emitted CH4 can provide information on these processes. To this end, we conducted a first systematic study using stable isotope methods on in-situ major CH4 turnover processes along a peat profile in a typical boreal peatland (Siikaneva fen) in Southern Finland.We run a cavity ring-down spectrometer continuously in 2022 to capture the dynamics of belowground dissolved CH4 and CO2 concentrations and their δ13C natural abundance signatures at 10, 30 and 50cm. Same variables were measured at 40cm above peatland surface to estimate ecosystem-scale average δ13C value of the emitted CH4 using nocturnal boundary-layer accumulation approach. These data were used to indicate CH4 production pathways, in-situ CH4 oxidation and transport pathways on an annual basis. Additionally, 13C pulse labelling experiments targeting acetoclastic methanogenesis, hydrogenotrophic methanogenesis and methanotrophy were performed both in-situ and in the lab condition to trace all these processes which cannot be separated by the isotope natural abundance approach alone.Preliminary results indicated a successful implementation of these novel methods. Continuous measurement of soil gas showed systematic differences in the vertical profile of soil pore water isotopes between winter and summer. δ13C-CH4 values were highest in the deepest layer during winter but they were the lowest in summer. As expected, CH4 production pathway moved towards acetoclastic methanogenesis through winter-summer transition. In winter, the δ13C-CH4 values from emitted CH4 was higher than those from the soil which indicating CH4 oxidation, while in summer the opposite was found due to CH4 diffusive plant transport. CH4 concentrations were higher in summer than in winter from all the depths, while at -30cm had the highest concentration. In-situ labelling experiments showed a higher rate of acetoclastic methanogenesis at -30cm than at -50cm, while hydrogenotrophic methanogenesis was similar at both depths. Experiments also demostrated that there was substantial methanotrophy in the soil as deep as 50cm belowground.

Spatial heterogeneity in the soil pore network is commonly understood to lead to spatially distinct biogeochemical transformations like the production of methane in anaerobic pockets in unsaturated soils. Yet, demonstrations of this heterogeneity and its linkage to soil structure (e.g., the spatial position in the soil pore network architecture) remains elusive.We therefore developed an assay to elucidate centimeter-scale differences in biogeochemical reactions within and between peat soil cores. For this, we injects a isotope-labeled substrate (sodium 13C2-acetate) at different locations in intact peat samples (10 cm diameter x 10 cm height) and followed its conversion to 13CO2 and 13CH4 over 5 days time in an automated measurement system using a Picarro G2201-i trace gas analyser. We analyse the ratio of 13CH4 and 13CO2 produced from the amended substrate, the fraction of substrate converted to 13CH4/13CO2, and the time course of 13CH4/13CO2 release. To test this approach, we collected seven pairs of peat core samples (15-25cm depths, 10 cm diameter, >30m between apart) at a drained forested peatland in Southern Finland. As one of the goals was to evaluate the effects of water retention hysteresis, half of the samples were set to -15 hPa water potential after draining to -30 hPa water potential, while the other half was set to same water potential after water-saturating the samples. In three experiments per core, we injected 10 nmol sodium acetate in 1mL water at 2, 5, and 8cm depth. We find both fixed effects (of core, injection depth, water treatment) and random effects that might be governed by the position of the injection with the peat core. We find, for example, that while a subset of the peat cores emitted (natural abundance) CH4, these cores showed highly heterogeneous conversions of the injected label into CH4 and CO2 that could not be explained by the fixed effects, demonstrating the spatial heterogeneity of methanogenesis and heterotrophic respiration within the peat core.In our future work, we will explore if pore networks models extracted from microtomographic images can explain these contrasting results.

Methane emission from northern mires shows typically strong spatial and seasonal variations. These variations have been assigned to e.g. differences in methane production due to variation in substrate input, transport pathways, methane oxidation in aerobic peat layers, and temperature variations. Stable isotope signatures of the emitted methane and methane in pore water can help us to constrain our hypotheses of these variations.We have measured δ13C of methane emission in Mycklemossen mire in Sweden by automated chamber system for two years. We also have measured δ13C of methane in pore water in three depths in Siikaneva mire in Finland by an automated diffusion tube system for one seasonal cycle. At both sites ecosystem scale δ13C of emitted methane was measured using nocturnal boundary layer accumulation (NBLA) approach.We observed systematic spatial variation in δ13C of emitted methane at Mycklemossen site, which mostly indicated the importance of substrate availability in explaining the spatial variability. At Siikaneva we observe systematic differences in the depth distribution of δ13C of pore water methane. Interestingly, this distribution is different in summer and winter. The ecosystem scale δ13C of emitted methane derived by chambers and NBLA approach very close to each other. We will discuss the observations, their implications, and future integration of the data and new measurement.

In drained peatland forests, drainage ditches cover ca. 3% of the area, but contribute to up to 100% of methane (CH4) emission. The drained peat soils, in contrast, can act as a CH4 sink especially under efficient drainage. Therefore, emissions from ditches will impact whether drained peatland is a net CH4 sink or source. The net CH4 flux is likely to be impacted by the conditions in the ditches such as the extent and type of plant cover and the time since drainage. In order to provide more accurate ditch CH4 emission factors for national greenhouse gas (GHG) inventory, we examined the fluxes and the underlying CH4 cycling processes in a nutrient rich peatland forest in Ränskälänkorpi, Southern Finland during May-October 2021. We compared the ecosystem-atmosphere CH4 fluxes and their δ13C values from moss dominated and open water ditches. We determined CH4 and CO2 mixing ratios and their δ13C values in water and in sediment by gas chromatography and cavity ring-down spectroscopy (Picarro G2201-i), respectively. We also assessed the role of CH4 as a carbon source for Sphagnum mosses growing in ditches by analyzing δ13C values in submerged and partly submerged Sphagnum using elemental analysis - isotope ratio mass spectrometry. We found that mean seasonal CH4 emissions from moss dominated ditches were 90% lower than from open water surfaces. In this dry summer, moss-dominated ditches were occasionally net sinks of atmospheric CH4. These results can be explained by CH4 consuming microbes inhabiting surface water, moss layer or sediment below the moss layer and using CH4 as a source of carbon and energy. Isotopic mass balance calculations accounting for the measured δ13C values of Sphagnum moss, dissolved CO2 and CH4 as well as fractionation against 13C during (mass-transfer-limited) moss CO2 fixation indicated that 10-28% of carbon in ditch Sphagnum mosses potentially originated from oxidized CH4. Ditch network maintenance, including removing mosses, is likely to decrease along with changing peatland forest management, e.g., continuous cover forestry. Our results suggest that ditches overgrown by mosses have potential to reduce CH4 emissions from drained peatland forests and could serve as an additional GHG mitigation measure to management practices that maintain a continuous forest cover, attenuate the changes in water table level and thus reduce CH4 emissions from peat soils.

Recent evidence shows that the shoots of Scots pine trees are a source of small emissions of methane (CH4), but it remains unclear if these emissions significantly reduce the CH4 sink strength of boreal forests. Progress in the shoot methane research is hindered by the technical challenges of shoot-level trace-gas flux measurements requiring closed-loop, non-steady-state chamber systems, as well as the difficulties in producing sufficient measurement data from hard-to-reach canopies in remote field sites. We have identified possible sources of disturbance by VOCs and water vapour in spectral, laser-based online measurement systems, and developed ways to manage them (Kohl et al., 2019; Tenhovirta et al., 2022). Extended measurement campaigns in ambient conditions were made possible by conducting the measurements on tree saplings instead of mature trees, resulting in the finding that CH4 emissions from shoots increase with solar radiation and are enhanced by temperature (Tenhovirta et al., 2022). Our automated chamber measurement system in the greenhouse (Kohl, Koskinen et al., 2021) has enabled continuous measurements and control/treatment experiments. These measurements reveal that the CH4 emissions from Scots pine shoots follow diel cycles (Kohl, Tenhovirta, et al., in prep.), and rule out soil production or photosynthesis as a source for these emissions (Tenhovirta et al., in prep). In addition, developing this system has provided solutions to common issues associated to chamber measurements, including excessive heating, water vapour accumulation and leakage. In this contribution we show how some of these issues are solved in manual and automatic chamber systems to achieve reliable measurements, how to choose detection limits, and what solutions we propose for the remaining challenges. References Kohl L, Koskinen M, Polvinen T, Tenhovirta SAM, Rissanen K, Patama M, Zanetti A, Pihlatie M. 2021. An automated system for trace gas flux measurements from plant foliage and other plant compartments. Atmospheric Measurement Techniques 14: 4445–4460. Kohl L, Koskinen M, Rissanen K, Haikarainen I, Polvinen T, Hellén H, Pihlatie M. 2019. Technical note: Interferences of volatile organic compounds (VOC) on methane concentration measurements. Technical note: Interferences of volatile organic compounds (VOC) on methane concentration measurements: 1–18. Tenhovirta SAM, Kohl L, Koskinen M, Patama M, Lintunen A, Zanetti A, Lilja R, Pihlatie M. 2022. Solar radiation drives methane emissions from the shoots of Scots pine. New Phytologist 235: 66–77.

Peatlands are globally significant carbon stocks and may become major sources of the greenhouse gases (GHGs) carbon dioxide and methane in a changing climate and under anthropogenic management pressure. Diffusion is the dominant gas transport mechanism in peat; therefore, a proper knowledge of the soil gas diffusion coefficient is important for the estimation of GHG emissions from peatlands. Pore network modeling (PNM) is a potential tool for the determination of gas diffusivity in peat, as it explicitly connects the peat microstructure and the characteristics of the peat pore network to macroscopic gas transport properties. In the present work, we extracted macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and simulated gas diffusion in these networks using PNM. These results were compared to the soil gas diffusion coefficients determined from the same samples in the laboratory using the diffusion chamber method. The measurements and simulations were conducted for peat samples from three depths. The soil gas diffusion coefficients were determined under varying water contents adjusted in a pressure plate apparatus. We also assessed the applicability of commonly used gas diffusivity models to peat. The laboratory measurements showed a decrease in gas diffusivity with depth due to a decrease in air-filled porosity and pore space connectivity. However, gas diffusivity was not extremely low close to saturation, which may indicate that the structure of the macropore network is such that it enables the presence of connected diffusion pathways through the peat matrix, even in wet conditions. The traditional gas diffusivity models were not very successful in predicting the soil gas diffusion coefficient. This may indicate that the microstructure of peat differs considerably from the structure of mineral soils and other kinds of porous materials for which these models have been constructed and calibrated. By contrast, the pore network simulations reproduced the laboratory-determined soil gas diffusion coefficients rather well. Thus, the combination of the µCT and PNM methods may offer a promising alternative to the traditional estimation of soil gas diffusivity through laboratory measurements.

Peatlands are important natural sources of atmospheric methane (CH4) emissions. The production and emission of CH4 are strongly influenced by the diffusion of oxygen into the soil and of CH4 from the soil to the atmosphere, respectively. This diffusion, in turn, is controlled by the structure of macropore networks. The characterization of peat pore structure and connectivity through complex network theory approaches can give conceptual insight into how the relationship between the microscale pore space properties and CH4 emissions on a macroscopic scale is shaped. The evolution of the pore space that is connected to the atmosphere can also be conceptualized through a pore network modeling approach. Pore regions isolated from the atmosphere may further develop into anaerobic pockets, which are local hotspots of CH4 production in unsaturated peat. In this study, we extracted interconnecting macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and evaluated local and global connectivity metrics for the networks. We also simulated the water retention characteristics of the peat samples using a pore network modeling approach and compared the simulation results with measured water retention characteristics. The results showed large differences in peat macropore structure and pore network connectivity between vertical soil layers. The macropore space was more connected and the flow paths through the peat matrix were less tortuous near the soil surface than at deeper depths. In addition, macroporosity, structural anisotropy, and average pore throat diameter decreased with depth. Narrower and more winding air-filled diffusion channels may reduce the rate of gas transport as the distance from the peat layer to the soil–air interface increases. The network analysis also suggests that both local and global network connectivity metrics, such as the network average clustering coefficient and closeness centrality, might serve as proxies for assessing the efficiency of gas diffusion in air-filled pore networks. However, the applicability of the network metrics was restricted to the high-porosity near-surface layer. The spatial extent and continuity of the pore network and the spatial distribution of the pores may be reflected in different network metrics in contrasting ways. The hysteresis of peat water content between wetting and drying was found to affect the evolution of the volume of connected air-filled pore space in unsaturated peat. Thus, the formation of anaerobic pockets may occur in a smaller soil volume and methanogenesis may be slower when the peat is wetting compared to in drying conditions. This hysteretic behavior might explain the hotspots and episodic spikes of CH4 emissions, and therefore, it should be taken into account in biogeochemical models.

The role of boreal upland forests in the global methane cycle remains poorly constrained. While chamber-based measurements clearly show that the soils of upland forest act as methane sinks, micrometeorological measurements indicate that the same forests are methane-neutral at the ecosystem level. We conducted a measurement campaign covering soil, tree stem, tree shoot, and ecosystem-level flux measurements to test whether upscaled methane fluxes from tree stems and shoots can close the observed gap between the soil and ecosystem fluxes.The campaign was conducted in a Scots pine dominated upland forest in southern Finland at the SMEAR II Hyytiälä research station between July 1 - Aug 15 2017. It included weekly measurement of methane fluxes at 15 soil locations, 47 stem chambers at the three tree species (Pinus sylvestris, Picea abies, Betula sp.), and 6 shoot chambers, as well as micrometeorological measurement of methane fluxes at 23 m height with two methods, eddy covariance (EC) and true eddy accumulation (TEA). Soil and stem methane fluxes were further upscaled based on a topographical statistical model (Vainio et al., 2021).Our results show a persistent gap between chamber- and micrometeorological flux measurements. While the soil acted as a moderate methane sink (-1.71 nmol m-2 s-1 ,95% confidence interval -2.03 to -1.39), micrometeorological measurements indicated that the forest was near methane neutral (EC: -0.29±0.24 nmol m-2 s-1; TEA: -0.25±0.16 nmol m-2 s-1). Spatial heterogeneity was a significant factor for soil methane uptake, as the median methane location in the tower footprint showed an approximately 0.5 nmol m-2 s-1 greater uptake than the footprint average. Methane exchange from stems (-0.035 to 0.083 nmol m-2 ground area s-1) and shoots (0.025 to 0.075 nmol m-2 ground area s-1) were at least an order of magnitude smaller than the gap between the soil and ecosystem measurements. While these estimates are associated with significant uncertainties primarily stemming from the upscaling model, it is unlikely that the stem and shoot fluxes act as the missing methane source in this ecosystem.Overall, results indicate that the gap between soil and ecosystem fluxes results either from a systematic error in micrometeorological flux measurements or from too high uncertainties related to measured fluxes very close to the detection limit of the EC/TEA system. It is also possible that an unidentified methane source exists in these forests. We were, for example, not able to conduct shoot flux measurements at moist sites within the flux tower footprint. We further note that our campaign was conducted during the peak summer months when stem and soil fluxes are expected to be relatively small due to low soil moisture. Nevertheless, our data suggests that a difference between trace gas fluxes at the soil and ecosystem level are not necessarily indicative of stem or canopy processes, and that such differences need to be interpreted with great care.References:Vainio, E., Peltola, O., Kasurinen, V., Kieloaho, A.-J., Tuittila, E.-S., Pihlatie, M.: Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux, Biogeosciences, 18, 2003–2025, https://doi.org/10.5194/bg-18-2003-2021, 2021.

Plants can emit methane (CH4) produced by an unknown aerobic, non-enzymatic process, driven by plant stressors like UV-radiation, elevated temperatures and wounding. In ambient spring conditions in Finland, CH4 emissions from the shoots of Scots Pine (Pinus sylvestris) correlated with solar radiation independently of temperature (Tenhovirta et al., in revision). The spring-time shoot CH4 emissions also had a diurnal pattern with the highest emissions during noon. It remains unknown whether these emissions are driven directly by solar radiation or indirectly via its effect on tree physiological processed such as photosynthesis or stomatal conductance. Characterizing the ecophysiology of the CH4 fluxes of tree canopies is a crucial step in order to understand the role of forests in the global CH4 cycle.To test whether shoot CH4 emissions are driven by tree physiological activity (e.g. stomatal conductance), we conducted a measurement campaign in greenhouse conditions during which Scots pine saplings were exposed to drought. During this 3-month-long campaign, CH4, carbon dioxide (CO2) and water vapour (H2O) fluxes from tree shoots were measured with an automated shoot trace gas flux measurements system (ShoTGa-FluMS)(Kohl, Koskinen et al., 2021). This system is capable of replacing the CO2 assimilated by the shoots, removing transpired water and cooling the chambers to near ambient temperatures. The experimental setup consisted of six 2-3 year old nursery saplings each with a shoot enclosed inside an automated shoot chamber, alternating (a) in closed loop with a Picarro G2301 cavity ring-down spectroscopy (CRDS) greenhouse gas concentration analyser (CH4 and CO2 measurements), (b) in a flow-through setup with a Li-cor 850 CO2-H2O analyser (photosynthesis and transpiration measurements), or (c) flushed with ambient air. The saplings were exposed to a daily 9-hour photoperiod of ~ 600-800 µmol s-1 m-2 photosynthetically active radiation (PAR), and irrigated automatically. Drought was induced by stopping the irrigation and continued to the point where net uptake of CO2 no longer occurred.Our experiment produced a unique dataset of continuous measurements of shoot-level CH4, CO2 and H2O fluxes over a period of several weeks. Our preliminary results show small but consistent CH4 emissions from the shoots of Scots Pine during daylight, supporting our earlier findings of the dependency of shoot CH4 emissions on light. The data furthermore allows to analyse the effects of drought on tree physiological activity and shoot CH4 fluxes providing much needed process understanding of shoot CH4 emissions from boreal trees.ReferencesKohl, Koskinen et al. 2021. An automated system for trace gas flux measurements from plant foliage and other plant compartments. Atmospheric Measurement Techniques 14: 4445–4460.

Peatlands are globally significant modulators of biogeochemical cycles and important natural sources of methane. The emissions are strongly influenced by the diffusion of oxygen into the peat and the diffusion of methane from the peat to the atmosphere. The structure of peat macropore networks controls the gas transport. The characterization of peat pore structure and connectivity using complex network theory methods can give important conceptual insight into the relationship between the microscale pore space characteristics and methane emissions on a macroscopic scale. Both gas transfer in unsaturated peat and the evolution of the connected air-filled pore space can be conceptualized through a pore network modeling approach. Pores that become isolated from the atmosphere may eventually develop into anaerobic pockets, which are local hotspots of methane production in unsaturated peat.We extracted macropore (diameter greater than 0.1 mm) networks from three-dimensional X-ray micro-computed tomography (micro-CT) images of peat samples collected from a boreal forested peatland and evaluated local and global connectivity metrics for the networks. We also simulated the soil-water retention curves of the peat samples using pore network modeling and compared the results with measured water retention characteristics. There were fundamental differences in macropore structure and connectivity between vertical peat layers. Macropore connectivity was higher and the flow routes through the peat matrix were less tortuous in the near-surface peat than in the deeper layers. Furthermore, the number and volume of macropores, the average width of pore throats, and the structural anisotropy of peat decreased with depth. Therefore, gas exchange with the atmosphere may be slowed down because of narrower and more tortuous air-filled diffusion channels as the distance between the peat layer and the soil-atmospheric interface increases.The network analysis also suggests that local and global network connectivity metrics, such as the network average clustering coefficient and closeness centrality, might be proxies for gas diffusion capability in air-filled pore networks. However, the applicability of the metrics was restricted to the topmost peat layer with high porosity. The spatial extent and larger-scale connectivity of the network and the spatial distribution of the pores within the network may be reflected in different network metrics in contrasting ways.The hysteresis of peat water content was found to affect the evolution of the interconnected air-filled pore volume in unsaturated peat. Therefore, the volume available for the formation of anaerobic pockets may be smaller and methane production may be slower in wetting conditions than in drying conditions. This hysteretic behavior might be one of the reasons behind observed hotspots and episodic spikes of methane emissions, and therefore hysteresis should be included in biogeochemical models describing methane dynamics in peat.

Methane production in plant foliage under aerobic conditions remains a cryptic and poorly constrained component of the global methane cycle. While several in-vitro studies reported light-dependent production of methane from plant biomolecules, thus far no studies have investigated methane fluxes at plant shoots during diel cycles. Here, we show that methane emissions from Scots pine shoots follow a distinct diurnal pattern and we demonstrate how these cycles allow estimating an upper limit of shoot methane emissions from ecosystem-atmosphere methane fluxes measured by the eddy covariance method. We present data from three measurement campaigns in forest, garden, and greenhouse settings that quantified methane fluxes of the shoots of Scots pine saplings and adult trees using manual and automated shoot chamber flux measurements systems, two distinct of trace gas analysers (Los Gatos Research UGGA, Picarro G2301). Despite the methodological differences, all campaigns found average methane flux rates between 0.05 and 0.20 nmol g-1 foliar dry weight h-1 in all campaigns. In the garden and greenhouse campaigns, where 24-hour measurement campaigns were possible, shoot methane fluxes exhibited pronounced diurnal cycles with a strong light dependent emission during daytime and low fluxes (mostly below the detection limit) during nighttime. Based on these strong light-dependent diurnal cycles, we were able to calculate an upper limit for shoot methane emissions at the ecosystem level. For this, we quantified the light-dependent and light-independent components of ecosystem-atmosphere methane fluxes measured by eddy covariance, with the light-dependent component tentatively indicating shoot-level methane fluxes. The monthly averages of the so-quantified light-dependent component accounted for 0.0-0.4 nmol methane m-2 sec-1 (range of monthly averages), which corresponds to ~0-1 nmol methane g-1 foliar dry weight h-1. This component is approximately 10-fold higher than shoot-level fluxes, indicating that other processes beside shoot emissions may contribute to light-dependent methane emissions. Nevertheless, even this higher estimate of shoot methane emissions correspond with the low end of the range reported by Keppler et al. (2006; 0.75–55 nmol g-1 d.w. h-1) and fall within the range reported by Fraser et al. (2015; 0.03–2 nmol g-1 d.w. h-1). Taken together, our results show how combining shoot and ecosystem level measurements can help constraining shoot emissions sufficiently for incorporating these fluxes in regional and global methane budgets. Taken together, our results show how combining shoot and ecosystem level measurements can help constraining shoot emissions sufficiently for incorporating these fluxes in regional and global methane budgets.References:Keppler, F., Hamilton, J., Braß, M. et al. Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187–191 (2006). https://doi.org/10.1038/nature04420Fraser, W. T., Blei, E., Fry, S. C., et al.. Emission of methane, carbon monoxide, carbon dioxide and short-chain hydrocarbons from vegetation foliage under ultraviolet irradiation. Plant, cell & environment, 38(5), 980–989 (2015). https://doi.org/10.1111/pce.12489

Plant shoots can emit methane (CH4) which is produced by an unknown aerobic, non-enzymatic process within the plant. Only a few publications report shoot CH4 fluxes outside a laboratory setting, and those of boreal trees come to contradictory results (Machacova et al., 2016; Sundqvist et al., 2012). Resolving the CH4 fluxes of boreal trees is needed in order to understand the role of boreal forests in the global methane budget.We conducted shoot chamber measurements on Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) between April and May 2019, to find out if the shoots of boreal conifer trees are a source of aerobic CH4 during the early growing season. The experiment was done with potted 2-3 year old nursery saplings in a common garden experiment, to enable regular measurements over a period of six weeks. CH4 fluxes were measured 2-3 times per day, on two days per week from seven saplings (four P. sylvestris and three P. abies, respectively). We also conducted two around the clock campaigns where we measured the saplings hourly throughout the day and night. The CH4 and carbon dioxide (CO2) exchange were quantified with a portable LGR online greenhouse gas analyser connected in closed loop to custom-made, transparent shoot chambers. Photosynthetically active radiation (PAR) was measured concurrently with a PP Systems EGM-4 monitor.Our measurements show emissions of CH4 from both tree species, ranging from 0.25 to 7.64 and -0.45 to 6.42 g-1 needle dry weight h-1 (inter-quartile range) from P. sylvestris and P. abies shoots, respectively. The shoot CH4 emissions from both species correlated positively with PAR. During the around the clock measurements the emissions showed a diurnal pattern. Our experiment demonstrates that the shoots of both P. sylvestris and P. abies can be a source of CH4 in the spring and that the source process is likely driven by solar irradiation. ReferencesMachacova, K., Bäck, J., Vanhatalo, A. et al. 2016. Pinus sylvestris as a missing source of nitrous oxide and methane in boreal forest. Scientific Reports, 6(September 2015), 1–8.Sundqvist, E., Crill, P., Mlder, M. et al. 2012. Atmospheric methane removal by boreal plants. Geophysical Research Letters, 39(21), 10–15.

Methane (CH4) emissions from a peatland are a balance between CH4-producing and consuming processes in the peat.Certain graminoid and other herbaceous plant species provide a pathway for CH4 to escape from the peat profile without becoming available to CH4-consuming microbes (eg. Riutta et al. 2020). Whether also shrubs such as Betula nana and trees such as B. pubescens that are abundant in peatlands provide such a pathway is at least partly an open question.We measured CH4 fluxes of B. nana shoots both at a field site on a north boreal fen and in a climate-controlled (temperature, PAR) cabinet setup. In the field, sporadic measurements were made with manual chambers. Both chamber setups enclosed one shoot of the plant. In the climate-controlled setup, flux was measured multiple times per hour with an automated chamber system, and CH4 concentration in the soil was monitored with gas samples. PAR and temperature were monitored by the automated setup. 13C-labelled CH4 was injected into the soil in the climate-controlled setup to confirm that the source of the CH4 in the chamber was the soil and not possible aerobic CH4 production in the shoot, nor external contamination. Potential microbial CH4 production within B. nana is evaluated through PCR detection and sequencing of the methanogenic mcrA gene (in progress). In the climate-controlled setup, fluxes were measured similarly also from saplings of B. pubescens and B. pendula – a related species that does not grow in peatlands. A computerized tomography (CT) scan of the plants was performed to assess the presence or absence of aerenchymatic tissues, which could act as a pathway for the CH4 in the plant. CH4 emission from B. nana shoots was observed both in the field and in the climate-controlled setup. No PAR dependence of the emission was found, excluding the possibility of aerobic production of CH4 in the shoot. Further, the labelling experiment confirmed that B. nana acted as a conduit for CH4 originating from the soil. B. pubescens showed little flux and B. pendula no flux, even though CH4 was present in the soil. The CT scan confirmed presence of aerenchymatic tissue in the shoots B. nana and to a lesser extent in B. pubescens but not in B. pendula. ReferencesRiutta, T., Korrensalo, A., Laine, A.M., Laine, J., Tuittila, E.-S., 2020. Interacting effects of vegetation components and water level on methane dynamics in a boreal fen. Biogeosciences 17, 727–740.

House dust is an important medium for exposure to persistent pollutants, such as metals. Detailed characterization of metal composition is needed to identify sources and potential health impacts of exposure. In this study we show that specific metals in dust dominate in different locations within residential homes in a mid-size Canadian city (Fort McMurray, Alberta), up to two years after a major wildfire event in 2016. Dust samples were collected in high-traffic (e.g. bedroom, N = 186), low-traffic (e.g. basement, N = 158), and entranceway areas (N = 171) of residential homes (N = 125), and analyzed for 25 trace metal elements using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The profile of metal concentrations in the entranceway resembled that of outdoor soils, especially for crustal elements. On the other hand, Cu, Zn, and Pb concentrations in dust sampled in indoor living areas were on average three to six times higher than in other indoor locations indicating indoor sources for these elements. In general, Pb concentrations were similar or lower than in an average Canadian residence, but a substantial fraction showed anomalously high concentrations in the low-traffic areas, particularly on concrete surfaces in basements. Notably, the 2016 wildfires showed limited influence on metal concentrations in indoor dust, despite the widespread concerns in the community about long term exposure. Enrichment factor ratio calculations and principal component analysis showed two classes of sources of metals in dust that represent geogenic-outdoor sources and anthropogenic-indoor sources. Overall, we demonstrate that outdoor and indoor sources of dust impact its composition, and these influences are reflected in the different areas of a home.

Plant litter chemistry is altered during decomposition but it remains unknown if these alterations, and thus the composition of residual litter, will change in response to climate. Selective microbial mineralization of litter components and the accumulation of microbial necromass can drive litter compositional change, but the extent to which these mechanisms respond to climate remains poorly understood. We addressed this knowledge gap by studying needle litter decomposition along a boreal forest climate transect. Specifically, we investigated how the composition and/or metabolism of the decomposer community varies with climate, and if that variation is associated with distinct modifications of litter chemistry during decomposition. We analyzed the composition of microbial phospholipid fatty acids (PLFAs) in the litter layer and measured natural abundance δ13CPLFA values as an integrated measure of microbial metabolisms. Changes in litter chemistry and δ13C values were measured in litterbag experiments conducted at each transect site. A warmer climate was associated with higher litter nitrogen concentrations as well as altered microbial community structure (lower fungi:bacteria ratios) and microbial metabolism (higher δ13CPLFA). Litter in warmer transect regions accumulated less aliphatic‐C (lipids, waxes) and retained more O‐alkyl‐C (carbohydrates), consistent with enhanced 13C‐enrichment in residual litter, than in colder regions. These results suggest that chemical changes during litter decomposition will change with climate, driven primarily by indirect climate effects (e.g., greater nitrogen availability and decreased fungi:bacteria ratios) rather than direct temperature effects. A positive correlation between microbial biomass δ13C values and 13C‐enrichment during decomposition suggests that change in litter chemistry is driven more by distinct microbial necromass inputs than differences in the selective removal of litter components. Our study highlights the role that microbial inputs during early litter decomposition can play in shaping surface litter contribution to soil organic matter as it responds to climate warming effects such as greater nitrogen availability.

Permafrost thaw, as a consequence of climate warming, liberates large quantities of frozen organic carbon in the Arctic regions. The response of gaseous carbon release upon permafrost thaw might play a crucial role in the future evolution of atmosphere-land fluxes of biogenic gases such as volatile organic compounds (VOCs), a group of reactive gases and the dominant modulator of tropospheric oxidation capacities. Here, we examine the response of volatile release from Finnish Lapland permafrost soils to temperature increase in a series of laboratory incubation experiments. The experiments show that when the temperature rises from 0 °C to 15 °C, various VOC species are significantly emitted from the gradually thawing soils. The VOC fluxes from thawing permafrost are on average four times as high as those from active layer. Acetic acid and acetone dominate the total volatile emissions from both permafrost and active layer, with significant amounts of aromatics and terpenes detected as well. The emission rate and the composition of volatile release from thawing soils are highly responsive to temperature variations. As temperature increases, more less volatile compounds are released, i.e., sesquiterpenes and diterpenes. Collectively, these results demonstrate the highly overlooked volatile production from thawing permafrost, which will create a stronger permafrost carbon-climate feedback.

Plant shoots can emit methane (CH4) from multiple source processes (microbial methanogenesis in soils and core wood, aerobic CH4 production in foliage). We constructed a chamber system to isolate these processes and study how leaf level CH4 emissions respond to environmental factors like dark-light-cycles, temperature, drought, or CO2 concentrations. Tree samplings are located in a FITOCLIMA D 1200 plant growth chamber for PAR, temperature and humidity control and equipped with a measurement chamber to quantify CH4 exchange in a closed loop setup with a Picarro G2301 CH4 analyser. The system was further customized to control temperature, CO2, and humidity in the measurement chamber. The system allows the detection of CH4 flux rates of on the order of 1 nmol CH4 h-1 and can conduct high frequency (< 15 min) measurements of CH4 emissions rates from small shoots (These experiments demonstrated that the shoots of different tree species emit CH4 from distinct sources. Scots pine shoots emitted CH4 produced within the shoot, likely through aerobic CH4 production, which showed a strong diurnal cycles that follows irradiation and photosynthesis rates. Shoot from some birch species, in contrast, showed emissions of soil-borne CH4 that remained constant throughout day and nighttime. We expect that future experiment with this unique setup will allow to further disentangle shoot CH4 emissions and characterize their response to environmental conditions including light, temperature, and relative humidity.

Volatile organic compounds (VOCs) play an essential role in climate change and air pollution by modulating tropospheric oxidation capacity and providing precursors for ozone and aerosol formation. Arctic permafrost buries large quantities of frozen soil carbon, which could be released as VOCs with permafrost thawing or collapsing as a consequence of global warming. However, due to the lack of reported studies in this field and the limited capability of the conventional measurement techniques, it is poorly understood how much VOCs could be emitted from thawing permafrost and the chemical speciation of the released VOCs. Here we apply a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) in laboratory incubations for the first time to examine the release of VOCs from thawing permafrost peatland soils sampled from Finnish Lapland. The warming-induced rapid VOC emissions from the thawing soils were mainly attributed to the direct release of old, trapped gases from the permafrost. The average VOC fluxes from thawing permafrost were four times as high as those from the active layer (the top layer of soil in permafrost terrain). The emissions of less volatile compounds, i.e. sesquiterpenes and diterpenes, increased substantially with rising temperatures. Results in this study demonstrate the potential for substantive VOC releases from thawing permafrost. We anticipate that future global warming could stimulate VOC emissions from the Arctic permafrost, which may significantly influence the Arctic atmospheric chemistry and climate change.

Warmer climates have been associated with reduced bioreactivity of soil organic matter (SOM) typically attributed to increased diagenesis; the combined biological and physiochemical transformation of SOM. Additionally, cross site studies have indicated that ecosystem regime shifts, associated with long-term climate warming, can affect SOM properties through changes in vegetation and plant litter production thereby altering the composition of soil inputs. The relative importance of these two controls, diagenesis and inputs, on SOM properties as ecosystems experience climate warming, however, remains poorly understood. To address this issue we characterized the elemental, chemical (nuclear magnetic resonance spectroscopy and total hydrolysable amino acids analysis), and isotopic composition of plant litter and SOM across a well-constrained mesic boreal forest latitudinal transect in Atlantic Canada. Results across forest sites within each of three climate regions indicated that (1) climate history and diagenesis affect distinct parameters of SOM chemistry, (2) increases in SOM bioreactivity with latitude were associated with elevated proportions of carbohydrates relative to plant waxes and lignin, and (3) despite the common forest type across regions, differences in SOM chemistry by climate region were associated with chemically distinct litter inputs and not different degrees of diagenesis. The observed climate effects on vascular plant litter chemistry, however, explained only part of the regional differences in SOM chemistry, most notably the higher protein content of SOM from warmer regions. Greater proportions of lignin and aliphatic compounds and smaller proportions of carbohydrates in warmer sites’ soils were explained by the higher proportion of vascular plant relative to moss litter in the warmer relative to cooler forests. These results indicate that climate change induced decreases in the proportion of moss inputs not only impacts SOM chemistry but also increases the resistance of SOM to decomposition, thus significantly altering SOM cycling in these boreal forest soils. This article is protected by copyright. All rights reserved.

Wildfires are increasing in prevalence and intensity and emit large quantities of persistent organic and inorganic pollutants. Recent fires have caused elevated concerns that residual pollutants in indoor environments pose a long‐term health hazard to residents, however, to date no studies have investigated how long fire‐derived pollutants are retained in indoor environments. We quantified polycyclic aromatic hydrocarbons (PAHs) and toxic trace elements in ground ashes from the 2016 wildland‐urban interface fires in Fort McMurray (Alberta, Canada) and in house dust from 64 homes. We document residual arsenic pollution from local building fires, but found no evidence that forest fire ash remained in households 14 months after the fire. Overall, house dust pollutant concentrations were equal or lower than in other locations unaffected by wildfires. Given the current and future concerns over wildfire impacts, this study provides importance evidence on the degree of their long‐term effects on the residential environment.

Present-day serpentinization generates groundwaters with conditions (pH > 11, Eh

Soil microbial biomass is a key source of soil organic carbon (SOC), and the increasing propor- tion of microbially derived SOC is thought to drive the enrichment of 13 C during SOC decomposition. Yet, little is known about how the d 13 C of soil microbial biomass differs across space or time, or with the composition of the microbial community. Variation in soil microbial d 13 C may occur due to variation in substrates used by soil microorganisms, and variation in how different microorganisms synthesize biomass. Understanding these variations in soil microbial d 13 C would enable more accurate interpretation of patterns in the d 13 C of SOC. Here, we report the variation in d 13 C values of individual phospholipid fatty acids

Second Deep Carbon Observatory Summer School; Yellowstone National Park, Montana and Wyoming, 23–28 July 2016

Fluvial ecosystems process large quantities of dissolved organic matter as it moves from the headwater streams to the sea. In particular, hyporheic sediments are centers of high biogeochemical reactivity due to their elevated residence time and high microbial biomass and activity. However, the interaction between organic matter and microbial dynamics in the hyporheic zone remains poorly understood. We evaluated how variance in resource chemistry affected the microbial community and its associated activity in experimentally grown hyporheic biofilms. To do this we fed beech leaf leachates that differed in chemical composition to a series of bioreactors filled with sediment from a sub-alpine stream. Differences in resource chemistry resulted in differences in diversity and phylogenetic origin of microbial proteins, enzyme activity, and microbial biomass stoichiometry. Specifically, increased lignin, phenolics, and manganese in a single leachate resulted in increased phenoloxidase and peroxidase activity, elevated microbial biomass carbon:nitrogen ratio, and a greater proportion of proteins of Betaproteobacteria origin. We used this model system to attempt to link microbial form (community composition and metaproteome) with function (enzyme activity) in order to better understand the mechanisms that link resource heterogeneity to ecosystem function in stream ecosystems.

Ultra-basic reducing springs at continental sites of serpentinization act as portals into the biogeochemistry of a subsurface environment with H2 and CH4 present. Very little, however, is known about the carbon substrate utilization, energy sources, and metabolic pathways of the microorganisms that live in this ultra-basic environment. The potential for microbial methanogenesis with bicarbonate, formate, acetate, and propionate precursors and carbon monoxide (CO) utilization pathways were tested in laboratory experiments by adding substrates to water and sediment from the Tablelands, NL, CAD, a site of present-day continental serpentinization. Microbial methanogenesis was not observed after bicarbonate, formate, acetate, or propionate addition. CO was consumed in the live experiments but not in the killed controls and the residual CO in the live experiments became enriched in 13C. The average isotopic enrichment factor resulting from this microbial utilization of CO was estimated to be 11.2 ± 0.2‰. Phospholipid fatty acid concentrations and δ13C values suggest limited incorporation of carbon from CO into microbial lipids. This indicates that in our experiments, CO was used primarily as an energy source, but not for biomass growth. Environmental DNA sequencing of spring fluids collected at the same time as the addition experiments yielded a large proportion of Hydrogenophaga-related sequences, which is consistent with previous metagenomic data indicating the potential for these taxa to utilize CO. ISSN:1664-302X

Glucans like cellulose and starch are a major source of carbon for decomposer food webs, especially during early- and intermediate-stages of decomposition. Litter quality has previously been suggested to notably influence decomposition processes as it determines the decomposability of organic material and the nutrient availability to the decomposer community. To study the impact of chemical and elemental composition of resources on glucan decomposition, a laboratory experiment was carried out using beech (Fagus sylvatica, L.) litter from four different locations in Austria, differing in composition (concentration of starch, cellulose and acid unhydrolyzable residue or AUR fraction) and elemental stoichiometry (C:N:P ratio). Leaf litter was incubated in mesocosms for six months in the laboratory under controlled conditions. To investigate the process of glucan decomposition and its controls, we developed an isotope pool dilution (IPD) assay using 13C-glucose to label the pool of free glucose in the litter, and subsequently measured the dilution of label over time. This enabled us to calculate gross rates of glucose production through glucan depolymerization, and glucose consumption by the microbial community. In addition, potential activities of extracellular cellulases and ligninases (peroxidases and phenoloxidases) were measured to identify effects of resource chemistry and stoichiometry on microbial enzyme production. Gross rates of glucan depolymerization and glucose consumption were highly correlated, indicating that both processes are co-regulated and intrinsically linked by the microbial demand for C and energy and thereby to resource allocation to enzymes that depolymerize glucans. At early stages of decomposition, glucan depolymerization rates were correlated with starch content, indicating that starch was the primary source for glucose. With progressing litter decomposition, the correlation with starch diminished and glucan depolymerization rates were highly correlated to cellulase activities, suggesting that cellulose was the primary substrate for glucan depolymerization at this stage of decomposition. Litter stoichiometry did not affect glucan depolymerization or glucose consumption rates early in decomposition. At later stages, however, we found significant negative relationships between glucan depolymerization and litter C:N and AUR:N ratio and a positive relationship between glucan depolymerization and litter N concentration. Litter C:N and C:P ratios were negatively related to cellulase, peroxidase and phenoloxidase activities three and six months after incubation, further corroborating the importance of resource stoichiometry for glucan depolymerization after the initial pulse of starch degradation., Highlights ► We studied effects of litter stoichiometry and chemistry on glucan decomposition. ► A mesocosm experiment of litter decomposition was conducted over 6 months. ► Glucan decomposition was enzyme-limited and was promoted in high N litter. ► Substrates of glucan decomposition shifted from starch to cellulose with time.

Plant shoots can act as sources or sinks of trace gases including methane and nitrous oxide. Accurate measurementsof these trace gas fluxes require enclosing of shoots in closed non-steady state chambers. Due to plant physiological activity, this type of enclosures, however, lead to CO2 depletion in the enclosed air volume, condensation of transpired water, and warmingof the enclosures exposed to sunlight, all of which may bias the flux measurements. Here, we present PlasTraGAS, ab novel measurement system designed for continuous and automated measurements of trace gas and volatile organic compound (VOC) fluxes from plant shoots. The system uses transparent shoot enclosures equipped with Peltier cooling elements and automatically replaces fixated CO2 and removes transpired water from the enclosure. The system is designed for measuring trace gasfluxes over extended periods, capturing diurnal and seasonal variations and linking trace gas exchange to plant physiologicalfunctioning and environmental drivers. Initial measurements show daytime CH4 emissions two pine shoots of 0.056 and 0.089 nmol g−1 foliage d.w.h−1or 7.80 and 13.1 nmol m−2 h−1. Simultaneously measured CO2 uptake rates were 9.2 and 7.6 mmol m−2 sec−1 and transpiration rates of 1.24 and 0.90 mol m−2 h−1. Concurrent measurement of VOC emissionsdemonstrated that potential effects of spectral interferences on CH4 flux measurements were at least ten-fold smaller than themeasured CH4 fluxes. Overall, this new system solves multiple technical problems that so far prevented automated plant shoottrace gas flux measurements, and holds the potential for providing important new insights into the role of plant foliage in the global CH4 and N2O cycles.