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Greenhouse gas (GHG) emissions from peatland forest soils are associated with ground water table (WT) level, which controls the vertical distribution of aerobic and anaerobic processes in soil. It has been suggested that transition from rotation forestry with ditch network maintenance (DNM) to selection harvesting would be a feasible alternative to reduce negative GHG and water impacts of peatland forestry as it raises WT and reduces aerobic decomposition of deep peat compared to mature forests. Transpiration from remaining trees would keep WT low enough for stand growth and natural regeneration without DNM. We measured vertical CO2, CH4, N2O and O2 concentration profiles in two peatland forests to provide insights on the controls of processes producing and consuming gases in the soil after harvest-induced hydrological change. The sites located on nutrient-rich peatland soils in Southern Finland, were dominated by Norway spruce, and parts of the sites were selection harvested. Selection harvesting raised WT by 14 cm relative to non-harvested controls, on average. All soil gas concentrations were associated with the proximity to the WT, but their patterns and in-soil fluxes were decoupled. CH4 and CO2 showed remarkable vertical concentration gradients, with very high values in the deepest layer due to low gas permeability of wet peat. However, CH4 was efficiently consumed in the peat layers above the WT where it reached sub-atmospheric concentrations, indicating oxidation of CH4 from both atmospheric and deeper origins. Soils maintained these functions after selection harvest. Surface peat contributed the most to soil-atmosphere CO2 flux, but harvest treatment also modestly increased the source in deeper soil. No consistent differences were observed in N2O emissions, which were the least associated with other gases. Based on our results, selection harvesting in drained nutrient-rich peatland forests without other hydrological measures limitedly reduces soil net emissions compared to non-harvested mature stands.

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.

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.

Riparian forests are known as hot spots of nitrogen cycling in landscapes. Climate warming speeds up the cycle. Here we present results from a multi-annual high temporal-frequency study of soil, stem, and ecosystem (eddy covariance) fluxes of N2O from a typical riparian forest in Europe. Hot moments (extreme events of N2O emission) lasted a quarter of the study period but contributed more than half of soil fluxes. We demonstrate that high soil emissions of N2O do not escape the ecosystem but are processed in the canopy. Rapid water content change across intermediate soil moisture was a major determinant of elevated soil emissions in spring. The freeze-thaw period is another hot moment. However, according to the eddy covariance measurements, the riparian forest is a modest source of N2O. We propose photochemical reactions and dissolution in canopy-space water as reduction mechanisms.

A 15N tracing pot experiment was conducted using two types of wood-based biochars: a regular biochar and a Kon-Tiki-produced nutrient-enriched biochar, at two application rates (1% and 5% (w/w)), in addition to a fertilizer only and a control treatment. Ryegrass was sown in pots, all of which except controls received 15N-labelled fertilizer as either 15NH4NO3 or NH415NO3. We quantified the effect of biochar application on soil N2O emissions, as well as the fate of fertilizer-derived ammonium (NH4+) and nitrate (NO3−) in terms of their leaching from the soil, uptake into plant biomass, and recovery in the soil. We found that application of biochars reduced soil mineral N leaching and N2O emissions. Similarly, the higher biochar application rate of 5% significantly increased aboveground ryegrass biomass yield. However, no differences in N2O emissions and ryegrass biomass yields were observed between regular and nutrient-enriched biochar treatments, although mineral N leaching tended to be lower in the nutrient-enriched biochar treatment than in the regular biochar treatment. The 15N analysis revealed that biochar application increased the plant uptake of added nitrate, but reduced the plant uptake of added ammonium compared to the fertilizer only treatment. Thus, the uptake of total N derived from added NH4NO3 fertilizer was not affected by the biochar addition, and cannot explain the increase in plant biomass in biochar treatments. Instead, the increased plant biomass at the higher biochar application rate was attributed to the enhanced uptake of N derived from soil. This suggests that the interactions between biochar and native soil organic N may be important determinants of the availability of soil N to plant growth.

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.

Recent studies have identified the significant role of plants in controlling methane (CH4) emission from peatlands by acting as conduits and further demonstrated that such conduit effect is species-specific. In most studies, species-specific plant-mediated CH4 transport has been estimated indirectly by comparing CH4 flux from surfaces with different plant communities to that from surfaces where plants responsible for CH4 transport are cut, known as the clipping technique. However, the estimation based on the clipping technique has shown large uncertainty due to the plant residual effect. Thus, directly investigating the variation in CH4 transport between different plant species and the factors affecting it is necessary to more precisely assess changes in the CH4 fluxes of peatland ecosystems in a changing environment.We measured CH4 emission directly from shoots of Carex rostrata, Menyanthes trifoliata, Betula nana, and Salix lapponum from the early growing season until the beginning of senescence (June-September 2020 and 2021, three campaigns both years), with three specimens per species and campaign. We also measured CH4 emission from Equisetum fluviatile and Comarum palustre during high summer in 2021 to further shed light on species-specific characteristics of plant-mediated CH4 flux. We monitored abiotic factors such as belowground CH4 concentration, potential CH4 production and oxidation rate, water table level, and peat temperature.During high summer in 2021, C. rostrata had the highest CH4 transport rate per leaf area (6.86 mg m-2 h-1). This value was significantly higher than that from M. trifoliata which was the secondarily important CH4 emitter with the rate of 4.07 mg m-2 h-1. E. fluviatile, C. palustre, B. nana, and S. lapponum had limited CH4 transport rate per leaf area (0.66, 0.02, 0.14, and 0.15 mg m-2 h-1, respectively) and thus were negligible CH4 emitters. CH4 emission from C. rostrata demonstrated the most pronounced seasonal variation (ranging from 0.02 to 24.78 mg m-2 h-1), driven primarily by seasonal vegetation development (phenology) and only secondarily by rhizospheric peat temperature. In contrast, CH4 emission from M. trifoliata, B. nana, and S. lapponum showed little seasonal variation, and no factors that significantly affected the flux from these species were found. Lastly, the sharp decrease in rhizospheric peat CH4 concentration during high summer and the simultaneous increase in emission from C. rostrata, the most dominant species in our site, indicated the conduit effect predominated over the CH4 production and oxidation. The findings highlight the importance of C. rostrata in mediating CH4, which could exacerbate the climatic impact of the thawing permafrost region where C. rostrata can thrive in wet microsites.

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

Funding Information: This study was supported by the Ministry of Education and Science of Estonia (SF0180127s08 grant), the Estonian Research Council (IUT2-16, PRG-352, and MOBERC20), the Czech Science Foundation (17-18112Y), SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797), the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I, grant number LO1415), the EU through the European Regional Development Fund (ENVIRON and EcolChange Centres of Excellence, Estonia, and MOBTP101 returning researcher grant by the Mobilitas Pluss programme), the European Social Fund (Doctoral School of Earth Sciences and Ecology). This work was also supported by Academy of Finland (294088, 288494), from the European Research Council (ERC) under the European Union?s Horizon 2020 research and innovation programme under grant agreement No [757695], and a Department of Energy (DOE) grant to JPM (DE-SC0008165). Funding Information: This study was supported by the Ministry of Education and Science of Estonia ( SF0180127s08 grant), the Estonian Research Council ( IUT2-16 , PRG-352 , and MOBERC20 ), the Czech Science Foundation ( 17-18112Y ), SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions ( CZ.02.1.01/0.0/0.0/16_019/0000797 ), the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I, grant number LO1415 ), the EU through the European Regional Development Fund (ENVIRON and EcolChange Centres of Excellence, Estonia, and MOBTP101 returning researcher grant by the Mobilitas Pluss programme), the European Social Fund (Doctoral School of Earth Sciences and Ecology). This work was also supported by Academy of Finland ( 294088 , 288494 ), from the European Research Council (ERC) under the European Union‘s Horizon 2020 research and innovation programme under grant agreement No [ 757695 ], and a Department of Energy (DOE) grant to JPM ( DE-SC0008165 ). Publisher Copyright: © 2021 Elsevier B.V. The carbon (C) budgets of riparian forests are sensitive to climatic variability. Therefore, riparian forests are hot spots of C cycling in landscapes. Only a limited number of studies on continuous measurements of methane (CH4) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH4 fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH4 fluxes from tree stems using manual chambers for a 1.5 year period from a temperate riparian Alnus incana forest. The results demonstrate that the riparian forest is a minor net annual sink of CH4 consuming 0.24 kg CH4-C ha−1 y−1. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH4 fluxes between the wet and dry periods. During the wet period, 83% of CH4 was emitted from the tree stems while the ecosystem-level emission was equal to the sum of soil and stem emissions. During the dry period, CH4 was substantially consumed in the soil whereas stem emissions were very low. A significant difference between the EC fluxes and the sum of soil and stem fluxes during the dry period is most likely caused by emission from the canopy whereas at the ecosystem level the forest was a clear CH4 sink. Our results together with past measurements of CH4 fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH4 sinks.

Organic soil amendments are proposed to mitigate climate change and support soil fertility by introducing recalcitrant carbon into soil. However, the full impact of recycled biosolids on soil greenhouse gas (GHG) dynamics are still unknown. We conducted a laboratory incubation to assess the climatic effects of two biochars (willow and spruce) and two ligneous biosolids on GHG emissions in controlled moisture conditions. The soil used in the incubation was collected from a soil-amendment field experiment on a clay cropland in South-West of Finland. The soil was sieved, air-dried and then individual samples were adjusted to 25%, 50%, 80% and 120% of water filled pore space (WFPS) before being incubated for 32 days in laboratory conditions. Soil GHG fluxes were measured after 1, 5, 12, 20 and 33 days. The application of 20–40 Mg ha-1 of ligneous amendments, two years prior to our experiment, had increased soil pH, soil organic carbon content and plant available water content. The carbon dioxide (CO2) fluxes were unaffected by the amendment treatments and correlated mainly with soil moisture and microbial biomass. Nitrous oxide (N2O) emissions were reduced by all amendments compared to the un-amended control. Methane (CH4) exchange consisted mostly of slight uptake by the soil but played only a minor role in the total GHG budget overall. The sum of CO2, N2O and CH4 emissions, calculated as CO2-equivalents, exhibited a strong linear relationship with soil moisture. Where the GHG budget was dominated by CO2, it was accompanied by significant N2O emissions at 120% WFPS. The results indicate that soil moisture critically affects the GHG emissions and that while organic soil amendments may have persisting effects on GHG exchange, they primarily occur in water-saturated conditions through N2O dynamics.

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.

Changes in the climate may have unpredictable effects on belowground carbon processes and thus, the carbon balance of boreal forests. To understand the interactions of these processes in soil and to quantify the potential changes in the carbon cycle, partitioning of forest floor respiration is crucial. For this purpose, we used nine different treatments to separate the sources of forest floor carbon dioxide (CO2) emissions in a mature Scots pine (Pinus sylvestris L.) stand in southern Finland. To partition the belowground CO2 emissions, we used two different trenching methods: 1) to exclude roots and mycorrhizal fungal mycelia (mesh with 1-µm pores) and 2) to exclude roots, but not mycorrhizal hyphae (mesh with 50-µm pores). Additionally, we used 3) a control treatment that included roots and fungal hyphae. To partition the CO2 emissions from the forest floor vegetation, we 1) removed it, 2) left only the dwarf shrubs, or 3) left the vegetation intact. The forest floor CO2 emissions were regularly measured with a flux chamber throughout the growing seasons in 2013–2015. The total forest floor respiration was partitioned into respiration of tree roots (contributing 48%), heterotrophic soil respiration (30%) and respiration of ground vegetation other than shrubs (10%), dwarf shrubs (8%), and hyphae of mycorrhizal fungi (4%). Heterotrophic respiration increased in the trenched treatments without ground vegetation over time, due to the so-called ‘Gadgil effect’. In the absence of tree roots, but when hyphal access was allowed, respiration in the dwarf shrub treatment increased throughout the experiment. This indicated that dwarf shrubs had fungal connections to outside the experimental plots via their ericoid mycorrhiza. At the same time, other ground vegetation, such as mosses, suppressed the dwarf shrub respiration in trenched treatments. Our results show that competition on the forest floor is intense between plant roots and soil microbes. publishedVersion

Riparian forests are known as hot spots of N cycling in landscapes and climate warming speeds up the cycle. Here we present results from the first multi-annual high temporal-frequency study of soil, stem and ecosystem (eddy covariance) fluxes of N2O from a typical riparian forest in Europe.Hot moments (extreme events of N2O emission) last a quarter of the study period but contribute more than a half of soil fluxes. For the first time we demonstrate that high soil emissions of N2O do not reach the ecosystem level. During the drought onset, soil N2O emission peaks at intermediate soil water content. Rapid water content change is the main determinant of the emissions. The freeze–thaw period is another hot moment. However, according to the eddy covariance measurements the riparian forest is a modest source of N2O. We propose photochemical reactions and dissolution in canopy-space water as consumption mechanisms.

Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable to emit CH4 under favourable conditions. Soil wetness is a well-known driver of CH4 flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH4 flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH4 flux and soil moisture measurements, complemented with ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a Random Forest model using topography, based on which we upscaled the forest floor CH4 flux – this was performed for two seasons: May–July and August–October. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux, resulting from the soil moisture variability, as well as on the related ground vegetation. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer compared to the autumn period, and overall the CH4 uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH4, however, these wet patches got drier and smaller in size towards the autumn, which was enough for changing their dynamics to CH4 uptake. The results highlight the small-scale spatial variability of the boreal forest floor CH4 flux, and the importance of soil chamber placement in order to obtain spatially representative CH4 flux results. We recommend that a site of similar size and topographical variation would require 15–20 sample points in order to achieve accurate forest floor CH4 flux.

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.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Upland forest soils are typically a net methane (CH4) sink, while trees may act as CH4 sources. Studies on tree CH4 exchange in boreal forests, especially regarding canopies, are rare. We aimed to quantify the contribution of trees to the forest CH4 budget during spring leaf-out period and to reveal the role of microbes in the CH4 exchange of trees. We measured stem and shoot fluxes of two common boreal tree species at a fen and at an upland site at Hyytiälä, southern Finland, together with soil CH4 flux, environmental variables and the abundances methanogens and methanotrophs within the forest. Both birch (Betula pubescens) and spruce (Picea abies) trees emitted CH4 from their aboveground surfaces, with significantly higher stem emissions detected from the birch and higher shoot emissions from the spruce. The shoot CH4 exchange had no clear link to the vertical profile of the canopy or the progress of the leaf-out. The stem CH4 emissions from birches at the fen were high (mean 45 µg h−1 m−2) and decreased drastically with stem height. Their dynamics followed soil temperature, suggesting the emitted CH4 originated from the soil. A lack of similar pattern in the fen spruces and in the upland birch indicates other processes behind the stem CH4 fluxes of these trees. The lack of detection of methanogens or methanotrophs in the aboveground plant tissues suggest that the observed tree-derived CH4 fluxes were not induced by these microbes. The emitted CH4 from the tree stems may, however, be produced microbially in the soil indicating that physiological differences in tree anatomy or adaptation to different belowground conditions might be a key factor explaining the differences between the tree species.Acknowledgements: This research was supported Academy of Finland (288494, 2884941), National Centre of Excellence (272041), ICOS-FINLAND (281255), Helsinki Institute of Life Science (HiLIFE), Czech Science Foundation (17-18112Y) and National Sustainability Program I (LO1415), and the European Research Council (ERC) under Horizon 2020 research and innovation programme, grant agreement No (757695).

Trees have been demonstrated to play a role in the methane (CH4) and nitrous oxide (N2O) cycling in forests. Emissions of these two greenhouse gases have been observed from tree stems and shoots. The stem emissions of both CH4 and N2O have been suggested to originate from the soil, however, their transportation mechanisms might differ, and furthermore, at least the stem-emitted CH4 can also be produced within tree tissue. Boreal forests are considered a sink of CH4 due to predominant soil oxidation, but when CH4 is taken up by the roots, it bypasses the CH4-oxidation zone in the surface soil. The stem N2O fluxes at the boreal zone have been shown to follow seasonal physiological activity of trees. However, studies on tree CH4 and N2O fluxes are scarce in the boreal zone.We studied the tree stem CH4 and N2O exchange from the stems of Scots pine, downy birch, and Norway spruce – in total 47 trees, growing at six study plots with naturally different soil moisture and ground vegetation conditions (6–9 trees per plot). The measurements were performed during July–August 2017 at the Hyytiälä SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations) ICOS (Integrated Carbon Observation System) research site, in southern Finland. In addition to the stems, we measured forest floor CH4 and N2O fluxes at all the plots, and shoot CH4 fluxes from birch and pine at one plot. The stem chambers were installed at the tree bases, ca. 30 cm above the soil surface. Additionally, from the trees with the shoot measurements, we measured the stem fluxes from several heights in order to study the flux variation in the stem vertical profile. All the flux measurements were conducted with closed chambers – the stem and forest floor measurements were performed by using manual sampling and gas chromatography, while a portable greenhouse gas analyser was used for the shoot measurements. Soil moisture and soil temperature were monitored at the study plots throughout the measurement period.The results show that all the studied tree species emit both CH4 and N2O from stems. Birches growing at one plot with waterlogging conditions stand out with the highest stem CH4 emissions. Concerning the N2O emissions, birch stems showed significantly higher emissions than pine stems. The results of the shoot measurements indicate that both birch and pine emit small amounts of CH4 from their shoots, but the driving factors of the emissions may be different for the two species. Our aim is to model the spatial variability of the stem CH4 and N2O fluxes at the site, and to develop an upscaling method combining the stem and forest floor CH4 and N2O exchange, based on an existing modelling work on the forest floor CH4 fluxes at the site. Acknowledgements: This research was supported by the Academy of Finland (288494, 2884941), National Centre of Excellence (272041), ICOS-FINLAND (281255), Czech Science Foundation (17-18112Y) and National Sustainability Program I (LO1415), and the European Research Council (ERC) under Horizon 2020 research and innovation programme, grant agreement No (757695).

Current atmospheric models show that we still lack the knowledge of all possible sinks of nitrous oxide (N2O). One potential, yet not studied sink could be the phyllospheric microbiota of trees. We studied Norway spruce (Picea abies) branches collected from three locations in Finland; from southern (Helsinki), central (Kuopio) and from northern (Kittilä) parts of the country. In our study, we differentiated the functioning and diversity of microbes living on the plant surfaces (epiphytic) and inside the plant (endophytic). Our preliminary data from incubation experiment suggest that the epiphytic microbes of spruce branches from central and southern parts of Finland have the potential to reduce N2O although the variation within sites was notable. We isolated microbial clones from both epiphytic and endophytic samples and amplified their nitrous oxide reductase enzyme coding gene, nosZ. N2O consumption potential study of clones in oxic and anoxic circumstances showed that N2O reduction on phyllosphere has a microbial origin. We are going to determine the original microbial population of spruce phyllospheric samples with targeted metagenomic sequencing of nitrogen cycling genes and with nosZ amplicon sequencing. As a conclusion, phyllospheric N2O consuming microbes can potentially cause sink for atmospheric and internally produced N2O in trees.

In agriculture, organic soil amendments are envisioned to mitigate climate change through carbon sequestration. However, the full impact of the organic amendments on soil physico-chemical dynamics is still poorly understood. We conducted a laboratory incubation to assess the net climate effect of four ligneous organic amendments: two biochars (willow and spruce) and two fiber byproducts of paper and pulp industry. Soil samples were collected from a soil-amendment field experiment at Qvidja farm, South-West of Finland. Soil samples were sieved, air-dried and adjusted to 30%, 50%, 80% and 110% of water holding capacity (WHC), and incubated for 32 days in standard laboratory conditions. Greenhouse gas (GHG) emissions were measured after 1, 5, 12, 20 and 33 days. The carbon dioxide (CO2) emissions were highest at 80% WHC, and lowest at severely water stressed conditions at 30% WHC. The organic amendments did not have an observable effect on CO2 dynamics. The CO2 emissions correlated linearly with soil moisture and microbial biomass nitrogen. Nitrous oxide (N2O) emissions were systematically lower in the amended soils compared to the control soil, and independent of soil nitrate concentrations. Without organic amendments, N2O emissions increased exponentially with soil moisture content. Methane (CH4) emissions fluctuated throughout the incubation, exhibiting mostly negative values. Consequently, CH4 emissions played only a minor role in the GHG budget.CO2, N2O and CH4 emissions, calculated as CO2 equivalent, exhibited a linear correlation with the moisture gradient. CO2 dominated the GHG budget up to a moisture of 80% WHC, but was superseded by N2O emissions at 110% WHC. The results indicate that soil moisture content is critically affecting the GHG emissions and that while organic soil amendments may have persisting effects on GHG exchange, they primarily occur through N2O dynamics.

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.

We have analyzed decade-long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July-August with average value of 73 nmol m(-2) s(-1). Wintertime fluxes were small but positive, with January-March average of 6.7 nmol m(-2) s(-1). Daily average methane emission correlated best with peat temperatures at 20-35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable-slope generalized linear model (r(2) = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process-based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m(-2) and was 2.7 +/- 0.4% of annual GPP. Over 10-year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space-to-time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales.

The contribution of boreal forest plants to the methane (CH4) cycle is still uncertain. We studied the above and belowground CH4 fluxes of common boreal plants, and assessed the possible contribution of CH4 producing and oxidizing microbes (methanogens and methanotrophs, respectively) to the fluxes. We measured the CH4 fluxes and the amounts of methanogens and methanotrophs in the above- and belowground parts of Vaccinium myrtillus, Vaccinium vitis-idaea, Calluna vulgaris and Pinus sylvestris seedlings and in non-planted soil in a microcosm experiment. The shoots of C. vulgaris and P. sylvestris showed on average emissions of CH4, while the shoots of the Vaccinium species indicated small CH4 uptake. All the root-soil-compartments consumed CH4, however, the non-rooted soils showed on average small CH4 emission. We found methanotrophs from all the rooted and non-rooted soils. Methanogens were not detected in the plant or soil materials. The presence of plant roots seem to increase the amount of methanotrophs and thus CH4 uptake in the soil. The CH4 emissions from the shoots of C. vulgaris and P. sylvestris demonstrate that the plants have an important contribution to the CH4 exchange dynamics in the plant-soil systems.

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 (N

The impact of atmospheric reactive nitrogen (Nr) deposition on carbon (C) sequestration in soils and biomass of unfertilised, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC / dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of Nr deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2019) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet Nr deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and Nr inputs and losses, these data were also combined with in situ flux measurements of NO, N2O and CH4 fluxes, soil NO3− leaching sampling, as well as results of soil incubation experiments for N and greenhouse gas (GHG) emissions, surveys of available data from online databases and from the literature, together with forest ecosystem (BASFOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from −70 to 826 g (C) m−2 yr−1 at total wet + dry inorganic Nr deposition rates (Ndep) of 0.3 to 4.3 g (N) m−2 yr−1; and from −4 to 361 g (C) m−2 yr−1 at Ndep rates of 0.1 to 3.1 g (N) m−2 yr−1 in short semi-natural vegetation (moorlands, wetlands and unfertilised extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO2 exchange, while CH4 and N2O exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Nitrogen losses in the form of NO, N2O and especially NO3− were of the order of 10–20 % of Ndep at sites with Ndep 3 g (N) m−2 yr−1, indicating that perhaps one third of the sites were in a state of early to advanced N saturation. Net ecosystem productivity increased with Nr deposition up to 2–2.5 g (N) m−2 yr−1, with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP / GPP ratio). At elevated Ndep levels (> 2.5 g (N) m−2 yr−1), where inorganic Nr losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate Ndep levels was partly the result of geographical cross-correlations between Ndep and climate, indicating that the actual mean dC / dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. Ndep.

Studies that quantify plant methane (CH4) emission rely on the accurate measurement of small changes CH4 concentrations that coincide with much larger changes in the concentration of volatile organic compounds (VOCs). Here, we assessed if 11 commonly occurring VOCs (e.g., methanol, - and -pinene, ∆3-carene) interfered with CH4 concentrations measurements by five laser absorption spectroscopy and Fourier-transformed infrared spectroscopy (FTIR) based CH4 analysers, and quantified the interference of seven compounds on three instruments. Our results showed widespread interfere of 5 VOCs with FTIR based CH4 analysers, but only minimal interference with laser absorption spectroscopy based analysers. VOCs not included in the spectral library exerted a strong strong (641800ppb apparent CH4/ppm VOC) bias on FTIR based measurements, which can lead to substancial over- and underestimations of CH4 fluxes. Minor (0.7126ppb/ppm) interferences with FTIR based measurements where also detected when the spectrum of the interfering VOC was included in the library. In contrast, we detected only minor (4 emissions. However, our results also suggest that FTIR can provided precisely quantify VOC concentrations, and could therefore provide a method complementary to proton-transfer-reaction mass spectrometry (PTR-MS).

The majority of nitrogen (N) in boreal forest soils is bound to soil organic matter (SOM) in forms not readily available to plants. Northern boreal forest ecosystems are often N limited, despite atmospheric N deposition, and the utilization of organic N from SOM is of crucial importance to the site productivity. The effect of microbial produced oxidative SOM degrading enzymes (laccase and manganese peroxidases) and proteases on soil N forms and availability was studied in a pot experiment with or without a Scots pine (Pinus sylvestris L.) seedling. The combination of SOM degrading enzymes and proteases decreased the total soil N content and increased the N losses significantly in the absence of the Scots pine seedlings. The total soil N content also decreased in the presence of the Scots pine seedlings, irrespective of the enzyme treatment. Most of the other N parameters studied were not sensitive to enzyme additions, and differed only between planted and non-planted treatments. Our results show that the alkylamine content of boreal forest soil are at the same levels as nitrate. We showed that SOM decomposition, stimulated by oxidative enzyme additions, is a key step in soil organic N utilization, while proteases alone do not increase N use from SOM. Plants stimulate N losses from SOM highlighting the importance of rhizosphere processes in soil C and N cycling.

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.

Chamber measurements of trace gas fluxes between the land surface and the atmosphere have been conducted for almost a century. Different chamber techniques, including static and dynamic, have been used with varying degrees of success in estimating greenhouse gases (CO2, CH4, N2O) fluxes. However, all of these have certain disadvantages which have either prevented them from providing an adequate estimate of greenhouse gas exchange or restricted them to be used under limited conditions. Generally, chamber methods are relatively low in cost and simple to operate. In combination with the appropriate sample allocations, chamber methods are adaptable for a wide variety of studies from local to global spatial scales, and they are particularly well suited for in situ and laboratory-based studies. Consequently, chamber measurements will play an important role in the portfolio of the Pan-European long-term research infrastructure Integrated Carbon Observation System. The respective working group of the Integrated Carbon Observation System Ecosystem Monitoring Station Assembly has decided to ascertain standards and quality checks for automated and manual chamber systems instead of defining one or several standard systems provided by commercial manufacturers in order to define minimum requirements for chamber measurements. The defined requirements and recommendations related to chamber measurements are described here.

The productivity of boreal forests is considered to be limited by low nitrogen (N) availability. Increased atmospheric N deposition has altered the functioning and N cycling of these N-sensitive ecosystems by increasing the availability of reactive nitrogen. The most important components of N pools and fluxes were measured in a boreal Scots pine stand in Hyytiälä, Southern Finland. The measurements at the site allowed direct estimations of nutrient pools in the soil and biomass, inputs from the atmosphere and outputs as drainage flow and gaseous losses from two micro-catchments. N was accumulating in the system, mainly in woody biomass, at a rate of 7 kg N ha−1 yr−1. Nitrogen input as atmospheric deposition was 7.4 kg N ha−1 yr−1. Dry deposition and organic N in wet deposition contributed over half of the inputs in deposition. Total outputs were 0.4 kg N ha−1 yr−1, the most important outputs being N2O emission to the atmosphere and organic N flux in drainage flow. Nitrogen uptake and retranslocation were equally important sources of N for plant growth. Most of the assimilated N originated from decomposition of organic matter, and the fraction of N that could originate directly from deposition was about 30%. In conclusion, atmospheric N deposition fertilizes the site considerably, but there are no signs of N saturation. Further research is needed to estimate soil N2 fluxes (emission and fixation), which may amount up to several kg N ha−1 yr−1.

The role of trees in the nitrous oxide (N2O) balance of boreal forests has been neglected despite evidence suggesting their substantial contribution. We measured seasonal changes in N2O fluxes from soil and stems of boreal trees in Finland, showing clear seasonality in stem N2O flux following tree physiological activity, particularly processes of CO2 uptake and release. Stem N2O emissions peak during the vegetation season, decrease rapidly in October, and remain low but significant to the annual totals during winter dormancy. Trees growing on dry soils even turn to consumption of N2O from the atmosphere during dormancy, thereby reducing their overall N2O emissions. At an annual scale, pine, spruce and birch are net N2O sources, with spruce being the strongest emitter. Boreal trees thus markedly contribute to the seasonal dynamics of ecosystem N2O exchange, and their species-specific contribution should be included into forest emission inventories., Forest soil is known to be a source of the greenhouse gas N2O, but the impact of what is planted in that soil has long been overlooked. Here Machacova and colleagues quantify seasonal N2O fluxes from common boreal tree species in Finland, finding that all trees are net sources of this gas.

Alkylamines are important precursors in secondary aerosol formation in the boreal forest atmosphere. To better understand the behavior and sources of two alkylamines, dimethylamine (DMA) and diethylamine (DEA), we estimated the magnitudes of soil–atmosphere fluxes of DMA and DEA using a gradient-diffusion approximation based on measured concentrations in soil solution and in the canopy air space. The ambient air concentration of DMA used in this study was a sum of DMA and ethylamine. To compute the amine fluxes, we first estimated the soil air space concentration from the measured soil solution amine concentration using soil physical (temperature, soil water content) and chemical (pH) state variables. Then, we used the resistance analogy to account for gas transport mechanisms in the soil, soil boundary layer, and canopy air space. The resulting flux estimates revealed that the boreal forest soil with a typical long-term mean pH 5.3 is a possible source of DMA (170 ± 51 nmol m−2 day−1) and a sink of DEA (−1.2 ± 1.2 nmol m−2 day−1). We also investigated the potential role of fungi as a reservoir for alkylamines in boreal forest soil. We found high DMA and DEA concentrations both in fungal hyphae collected from field humus samples and in fungal pure cultures. The highest DMA and DEA concentrations were found in fungal strains belonging to decay and ectomycorrhizal fungal groups, indicating that boreal forest soil and, in particular, fungal biomass may be important reservoirs for these alkylamines.

We quantified the changes in the soil properties and fluxes of soil CO2, CH4 and volatile organic compound (VOC) fluxes following a clear-cut (CC) and prescribed burning of slash (BCC) over a three year time period in a mature spruce forest. Clear-cutting increased soil moisture, soil temperature, pH as well as NH4–N and NO3–N concentrations in the soil. PH and soil temperature in the BCC site were even higher than in the CC site. Probably due to decreased tree root respiration, the soil CO2 efflux decreased only slightly in the first growing season following the clear-cut. During the following two years, the CO2 efflux at the CC site was significantly higher than in the mature control forest due to increased decomposition, which was stimulated by higher soil moisture and temperature. The temperature dependencies of the CO2 efflux did not differ between these sites. The clear-cut and burning of slash, however, decreased the CO2 efflux and its temperature response in BCC for two years but in the third year, the differences between control and BCC were no longer significant. Soil was a sink for CH4 in all treatments. After the clear-cutting and burning of slash, the net CH4 uptake was immediately decreased, but one year later the uptake was comparable to that of the mature control forest. The CC treatment decreased the soil CH4 uptake, however, it did not significantly differ from that of the control. The soil VOC emissions measured at the BCC site were 100-fold compared to those measured before clear-cutting, but the emissions decreased rapidly during the three months following the burning. Although the CO2 effluxes from the BCC site were lower for more than 2.5 years compared to the CC site, the total amount of carbon released from prescribed burning was higher due to the immediate carbon losses. Moreover, the clear-cut and burning of slash temporarily decreased the ability of the forest to bind atmospheric CH4 and increased the VOC emissions significantly.

A prescribed fire experiment was conducted on 26 June 2009 in Hyytiälä, Finland, to study aerosol and trace gas emissions from prescribed fires of slash fuels and the effects of fire on soil properties in a controlled environment. A 0.8 ha forest near the SMEAR II measurement station (Station for Measuring Ecosystem-Atmosphere Relations) was cut clear; some tree trunks, all tree tops and branches were left on the ground and burned. The amount of burned organic material was ~46.8 tons (i.e., ~60 tons ha−1). The flaming phase lasted 2 h 15 min, the smoldering phase 3 h. Measurements were conducted on the ground with both fixed and mobile instrumentation, and in the air from a research aircraft. In the middle of the burning area, CO2 concentration peaked around 2000–3000 ppm above the baseline, and peak vertical flow velocities were ~9 m s−1, as measured with a 10 Hz 3-D sonic anemometer placed within the burn area. In the mobile measurements the peak particle number concentrations were approximately 1–2 × 106 cm−3 in the plume at a distance of 100–200 m from the burn area. On the ground at the SMEAR II station the geometric mean diameter of the mode with the highest concentration was 80 ± 1 nm during the flaming phase and in the middle of the smoldering phase, but, at the end of the smoldering phase, the largest mode was 122 nm. In the volume size distributions, geometric mean diameter of the largest volume mode was 153 nm during the flaming phase and 300 nm during the smoldering phase. The lowest single-scattering albedo of the ground-level measurements was 0.7 in the flaming-phase plume and ~0.9 in the smoldering phase. Elevated concentrations of several volatile organic compounds (VOC) (including acetonitrile, a biomass burning marker) were observed in the smoke plume at ground level. Measurements at the forest floor (i.e., a richly organic layer of soil and debris, characteristic of forested land) showed that VOC fluxes were generally low and consisted mainly of monoterpenes, and VOC flux peaked after the burning. After one year, the fluxes had nearly stabilized close to the level before the burning. The clear-cutting and burning of slash increased the total long-term CO2 release from the soil, and altered the physical, chemical and biological properties of the soil, such as increased the available nitrogen contents of the soil, which in turn, affected the long-term fluxes of greenhouse gases.