BG3.3

Permafrost in the north Polar Regions stores more than 1,500 Pg of organic carbon, which is nearly twice as much as the atmospheric carbon pool. As the Arctic region is experiencing unprecedented warming, accelerated decomposition in permafrost is potentially switching it to a hotspot of carbon emissions. In addition to the widely studies carbon dioxide and methane, permafrost may also be a source of biogenic volatile organic compounds (BVOCs), a reactive group of trace gases which have so far received much less attention. BVOCs can prolong the lifetime of methane through the depletion of hydroxyl radicals, contribute to ozone formation, and lead to the formation of secondary organic aerosol, and thus exert significant impact on climate forcing, especially in unpolluted Arctic region.

Here, we conducted in situ measurements of soil BVOC emissions on an actively degrading permafrost peatland during a growing season. We compared emissions along a gradient of landscape units from soil palsa and vegetated palsa to thaw slump, thaw pond and vegetated thaw pond. BVOC samples were collected onto absorbent cartridges using dynamic enclosure chamber method, and then analyzed with a gas chromatograph coupled with a mass spectrometer (GC/MS), based upon which the emission rates were calculated.

Results suggested that all landscapes units across the peatland showed net emissions of BVOCs during the summertime. Major BVOC groups included monoterpenes, sesquiterpenes, isoprene, hydrocarbons, methanol, acetone, other oxygenated VOCs and other compounds, and these groups were present in all landscape units. All VOC groups also exhibited seasonal and spatial variations across the different sampling months and landscape units. For example, the actively degrading thaw slump showed higher monoterpene emissions that other landscape units, while sesquiterpene emissions were highest from the vegetated thaw ponds. Principal component analysis further revealed temporal and spatial patterns in the relative compositions of the BVOC profiles. Our results show that soil BVOC emissions change in response to active permafrost thaw.

How to cite: Jiao, Y., Davie-Martin, C., Kramshøj, M., Christiansen, C., Lee, H., Althuizen, I., and Rinnan, R.: Volatile carbon emissions from a degrading permafrost peatland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3481, https://doi.org/10.5194/egusphere-egu22-3481, 2022.

Biogenic volatile organic compounds (BVOCs) emitted by plants consist of a broad range of gasses which serve purposes such as protecting against herbivores, communicating with insects and neighboring plants, or increasing the tolerance to environmental stresses. Evidence is accumulating that the composition of BVOC blends plays an important role in fulfilling these purposes. Constitutional emissions give insight into species-specific stress tolerance potentials and are an important first step in linking metabolism and function of co-occurring BVOCs. Here, we investigate the blend composition and interrelations among co-emitted BVOCs in unstressed and drought- and salt stressed seedlings of four broad-leaved tree species, Quercus robur, Fagus sylvatica, Betula pendula, and Carpinus betulus. BVOCs of Q. robur and F. sylvatica were mainly isoprene and monoterpenes, respectively. B. pendula had relatively high sesquiterpene emission; however, it made up only 1.7% of its total emissions while the VOC spectrum was dominated by methanol (∼72%). C. betulus was emitting methanol and monoterpenes in similar amounts compared to other species, casting doubt on its frequent classification as a close-to-zero VOC emitter. Under drought and salt stress the main emitted BVOCs of F. sylvatica and B. pendula slightly decreased, whereas an increase was observed for Q. robur and C. betulus. Beside these major BVOCs, a total of 22 BVOCs could be identified, with emission rates and blend compositions varying drastically between species and treatments. Principal component analyses among species and treatments revealed co-release of multiple compounds. In particular, new links between pathways and catabolites were indicated, e.g., correlated emission rates of methanol, sesquiterpenes (MVA pathway), and green leaf volatiles (hexanal, hexenyl acetate, and hexenal; LOX pathway) during unstressed conditions. Drought stress led to a decrease of all BVOC emissions except for a slight increase of isoprene emissions of Q. robur, which might be due to decoupling from the photosynthesis and led to emptying C storages. Hexenyl acetate (LOX) follows the same pattern as isoprene but might have decreased due to a long droughting period. Salt stress led to an increase of LOX-related BVOCs, and acetaldehyde which supports the hypothesis that acetaldehyde emissions are linked to the oxidation of C₁₈ fatty acids of cell membranes. Our results thus indicate that certain BVOC emissions are highly interrelated, pointing toward the importance to improve our understanding of BVOC blends rather than targeting dominant BVOCs only.

How to cite: Fitzky, A., Peron, A., Kaser, L., Karl, T., Graus, M., Tholen, D., Pesendorfer, M., Mahmoud, M., Trimmel, H., Halbwirth, H., Sandén, H., and Rewald, B.: Diversity and interrelations among the constitutive BVOC emission blends and changes during salt and drought stress of four broad-leaved tree species at seedling stage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4844, https://doi.org/10.5194/egusphere-egu22-4844, 2022.

Organic and inorganic volatile compounds containing one or two carbon atoms (C1, C2), such as carbon dioxide, methane, methanol, formaldehyde, carbon monoxide, chloromethane, formic acid, acetic acid, ethane and ethene are ubiquitous in the environment and play an important role in atmospheric physics and chemistry as they act as greenhouse gases, destroy stratospheric and tropospheric ozone and control the atmospheric oxidation capacity. Furthermore, these compounds play an important role in global carbon cycling. Up to now, most C1 and C2 compounds in the environment were associated to complex metabolic and enzymatic pathways in organisms or combustion processes of biomass. So far, it was not recognized that many C1 and C2 compounds in the geobiosphere might also have a common origin in methyl groups from methyl-substituted substrates that are cleaved by the iron-catalysed formation of methyl radicals.

We performed a set of laboratory experiments containing methyl-substituted substances, an iron species (e.g. hematite, ferrihydrite or bispidine-iron complexes for the better understanding of the mechanism), H₂O₂ for the activation of the iron species and ascorbic acid as a radical scavenger. The experiments were conducted under ambient conditions (atmospheric pressure and 22°C) and variable parameters such as pH value, substrate concentration and O₂ saturation.

We show that a range of organic and inorganic C1 and C2 compounds can be produced by environmentally important methyl-substituted substances such as dimethyl sulfoxide (DMSO), methionine, choline, trimethylamine, synapyl alcohol (lignin component) and galacturonic acid methyl ester (pectin component). Applying isotopically labelled (²H/¹³C) methyl groups from DMSO and methionine we unambiguously demonstrate that labelled methane, ethane, methanol, formaldehyde and acetic acid are produced from methyl-substituted substances.

Based on our preliminary results we hypothesise that formation of methyl radicals by abiotic and possibly also by biochemical processes is ubiquitous in the environment with various heteroatom-methylated substrates. We propose that by generating methyl radicals formation of the entire set of C1 compounds with carbon oxidation states of -IV to +IV but also formation of C2 compounds is possible. The relative amounts of the formed individual C1 species might depend on the redox milieu and biogeochemical conditions such as the availability of methyl radical donors, iron species, pH, O₂ concentration and possibly a range of other parameters.  To thoroughly understand, the chemistry behind these processes and to verify mechanistic scenarios, we also performed computational modeling based on density functional theory and ab-initio quantum-chemical studies.

The investigated methyl moieties are ubiquitous in the terrestrial and marine biosphere. Thus, for future studies we will put our assembled knowledge into practice and study these reactions in water and soil samples collected from the field.

How to cite: Hädeler, J., Lauer, R., Gunasekaran, V., Rheinberger, K., Comba, P., and Keppler, F.: Iron catalysed formation of methyl radicals as a common source of environmentally important volatile carbon compounds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6138, https://doi.org/10.5194/egusphere-egu22-6138, 2022.

Wetlands are well-known for their high emissions of methane to the atmosphere, but emissions of volatile organic compounds (VOCs) are also reported from wetlands. Wetlands cover about 2 % of the total land surface area and most of these wetlands are found in the boreal and tundra regions. A class of compounds called terpenes that include isoprene, monoterpenes, sesquiterpenes, and diterpenes make up 80% of the global biogenic volatile organic compound (BVOC) emissions. These compounds are highly reactive towards oxidants like ozone (O₃), hydroxyl radicals (OH), and nitrate radicals (NO₃) and form secondary organic aerosols in the atmosphere. Hence, quantifying the BVOC emissions accurately is crucial in determining the organic aerosol budget and constraining their contribution to climate-relevant processes such as new-particle formation and cloud formation.

In this study we performed ecosystem scale eddy covariance (EC) measurements of BVOCs and their oxidation products at Siikaneva, a southern Finnish boreal wetland (61^o48' N, 24^o09' E, 160 m a.s.l.), from 19th May 2021 to 28th June 2021 using a Vocus-proton transfer reaction mass spectrometer (Vocus-PTR) co-located with a sonic anemometer (METEK USA-1) at 10 Hz.  BVOCs were sampled from a platform, 2.5 m above the wetland using a high flow main inlet (5000 lpm), with core sampling of 5 lpm into the Vocus-PTR, which substantially reduced the wall losses of less volatile compounds such as sesquiterpenes, diterpenes, and oxygenated VOCs. The EC data were analyzed following standard correction procedures such as lag correction, coordinate rotation, and uncertainty analysis using the InnFLUX tool by Striednig et al. (2020). The high frequency attenuations of the fluxes were corrected using transfer functions estimated using the sensible heat flux cospectra.

We observed high emissions of isoprene, monoterpenes, sesquiterpenes and the first-ever emission fluxes of diterpenes from a wetland. The average normalized standard emission factor (EF) at standard photosynthetically active radiation of 1000 μmols m^-2 s^-1 and standard temperature of 30 ^oC for isoprene using the emission algorithm by Guenther et al. (2012) was determined as 1200 μmols m^-2 day^-1. For comparison, a relaxed eddy accumulation (REA) flux measurement study at the same site by Haapanala et al. (2006) had reported much lower EF of 240 μmols m^-2 day^-1. We observed sesquiterpene emissions reaching up to 50% of monoterpene emissions on average and occasionally even higher than monoterpenes emissions. For diterpenes, we found mean emissions of 0.4 μmols m^-2 day^-1.

During the campaign, the temperature peaked at 32 ^oC which is abnormally high for boreal environments and all the terpenoid emissions showed an exponential temperature dependence. The derived exponential temperature coefficient (Q10) value for isoprene was 4 times higher than the values used in the widely used MEGAN model. Our study reveals that VOC emissions from boreal environment are very sensitive to temperature change and since temperature is one of the main drivers of BVOC emission, anthropogenic global warming can induce much higher BVOC emissions in the future.

How to cite: Vettikkat, L., Miettinen, P., Buchholz, A., Rantala, P., Yu, H., Schallhart, S., and Schobesberger, S.: Eddy covariance measurements reveal high emissions of terpenes from a boreal fen, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6938, https://doi.org/10.5194/egusphere-egu22-6938, 2022.

Insect herbivory amplifies the biogenic volatile organic compound (BVOCs) emissions into the atmosphere, where BVOCs participate in atmospheric chemistry processes. In the high latitudes, herbivory induced BVOCs are considered as a major contribution to the total plant BVOC emissions during periods of active insect herbivore feeding. However, current BVOC models do not quantify BVOC emissions upon insect herbivory. Including effects of herbivory in models would be especially relevant in order to model BVOC emissions in the Arctic, where insect herbivore pressure is expected to increase with climate change.

We gathered data from enclosure-based field studies conducted in the Subarctic, that assessed the effects of outbreak-causing geometrid moth larvae (Operophtera brumata and Epirrita autumnata) feeding on the BVOC emissions of the dominant tree species, mountain birch (Betula pubescens var. pumila (L.)). The feeding damage ranged from background herbivory to up to 100% defoliation, thus mimicking local insect outbreak conditions.

The leaf area based BVOC emissions from mountain birch increased linearly with increasing feeding damage up to a maximum of 15 %, depending on the BVOC group. After this maximum, BVOC emissions declined as the leaf area decreased.

These results provide quantitative relationships between leaf area eaten and the emission rate of atmospherically important BVOC groups in the Subarctic mountain birch forest. Our results have practical implications for incorporating the modelling of herbivory induced BVOC emissions into the mainstream VOC models such as MEGAN (Model of Emissions of Gases and Aerosols from Nature) or LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator).

How to cite: Rieksta, J., Li, T., Lauge Borchmann, R., and Rinnan, R.: Quantitative relationships between insect herbivory severity and BVOC emissions in a Subarctic mountain birch forest, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11680, https://doi.org/10.5194/egusphere-egu22-11680, 2022.

Volatile organic compounds (VOCs) are vigorously cycled by microbes as metabolic substrates and products and as signaling molecules. Yet, current microbial metabolomic studies predominantly focus on nonvolatile metabolites and overlook VOCs, which therefore represent a missing component of the metabolome. In metabolomic studies, it is important to know which compounds within metabolic pathways may be considered volatile to predict potentially overlooked compounds and potentially include VOC measurement approaches to capture them.

In this study, we adapted and automated an atmospheric vapor pressure predictive model for metabolomic research to calculate relative volatility indices (RVIs) for compounds in a metabolic pathway through identification of the compound’s functional groups. We then evaluated the importance of considering compound volatility in soil metabolomic studies by comparing the ability of a suite of complementary analytical tools (nuclear magnetic resonance (NMR) spectroscopy, gas chromatography-mass spectrometry (GC-MS), and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS)) to capture complete metabolic pathways in soil.

We found that the metabolites that were not detected by NMR, GC-MS, and FT-ICR-MS within metabolic pathways had significantly higher volatility than those that were detected, revealing a bias against volatile metabolites in standard metabolomics pipelines. Moreover, we show that including VOC-resolving measurements (proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS)) captured the volatile compounds missed by other metabolomic techniques, and together, the combined approaches captured more complete microbial metabolic processes in soil.  Our results demonstrate the importance and prevalence of VOCs as metabolites in soil. Including volatile metabolites in metabolomics, both conceptually and in practice, will build a more comprehensive understanding of microbial processes across ecological communities.

How to cite: Meredith, L., Tfaily, M., Geffre, P., Graves, K., Riemer, K., Honeker, L., and Krechmer, J.: Microbial volatile organic compounds: important but overlooked in microbial systems studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12851, https://doi.org/10.5194/egusphere-egu22-12851, 2022.