Riikka Rinnan

Biogenic volatile organic compounds (BVOCs) are gases that influence atmospheric chemistry, nutrient cycling and species interactions, yet the contribution of heterotrophic marine bacteria to marine BVOC emissions remains poorly constrained. In addition, the extent to which the volatilome is linked to bacterial phylogeny is unknown. Here, we characterize the volatilome of 16 heterotrophic bacterial strains isolated from Baltic Sea surface water, spanning Alphaproteobacteria, Gammaproteobacteria, Betaproteobacteria, Bacteroidota and Actinomycetes. Headspace BVOCs were quantified under standardized growth conditions using Proton Transfer Reaction Time‐of‐Flight Mass Spectrometry (PTR‐TOF‐MS). A broadly overlapping bacterial volatilome was identified, with compound composition and proportional abundance similar across many strains, irrespective of phylogeny. Namely, most strains shared a set of core compounds with a subset of strain‐specific, low abundance compounds. Acetone accounted for more than 50% of the emissions in most volatilomes. The remaining fraction of emissions was primarily comprised of other low‐molecular‐weight oxygenated compounds. Interestingly, two strains demonstrated strain‐specific emission patterns, significantly diverging from the group in their emission rate and compound composition. Together, these findings suggest that marine heterotrophic bacteria may contribute a broadly conserved collection of BVOCs to the ocean–atmosphere interface, highlighting their role as a widespread source of trace gases in marine ecosystems.

Geometrid moth outbreaks are important natural disturbances in mountain birch ( Betula pubescens ssp. czerepanovii ) forests of northern Fennoscandia and are expected to intensify with climate change. Impacts of these outbreaks and moth herbivory on biogenic volatile organic compound (VOC) emissions of birch remain unquantified despite the importance of VOCs in plant-herbivore interactions and atmospheric chemistry. Here, we quantify in situ VOC emission responses of mountain birch ( n = 256) to naturally increasing geometrid herbivory across three consecutive growing seasons. We show that VOC emissions increased with rising herbivore pressure across the years, culminating in the outbreak, during which early-season VOC emissions were higher than at peak herbivory. At peak herbivory, birch emitted more complex VOC blends, indicating seasonal shifts in composition. Following the outbreak, a second leaf flush sustained emissions of some VOCs. Our findings demonstrate that VOC emission rates and compositions are dynamically linked to leaf phenology and herbivory intensity across multiple years.

Insect outbreaks are intensifying across high-latitude forests, yet their effects on biogenic volatile organic compound (BVOC) emissions from understory vegetation remain poorly resolved under natural outbreak conditions. Here, we examine how a severe geometrid moth outbreak alters understory microclimate and BVOC emissions in a subarctic mountain birch forest, using field measurements from two growing seasons spanning increasing herbivory to the peak outbreak year. We find that understory BVOC emissions decline markedly during the outbreak (38-70%), despite more emission-favourable abiotic conditions. We suggest that this decline results from the combined effects of direct herbivory on susceptible understory species and canopy-mediated feedbacks following mountain birch defoliation. These feedbacks likely intensify radiative, thermal, and drought stress in the understory, leading to widespread loss of BVOC-emitting green vegetation. Our findings demonstrate that severe geometrid outbreaks can suppress, rather than enhance, BVOC emissions by shifting emission control from abiotic stimulation to green biomass limitation, while highlighting that BVOC responses to herbivory are context-dependent and species-specific.

Measurements of surface-atmosphere carbon dioxide (CO ₂ ) and methane (CH ₄ ) fluxes have been relatively sparse across the Arctic tundra and boreal biomes, causing significant uncertainties in carbon budget estimates from the region. While the availability of Arctic-boreal carbon flux data has increased substantially over the past decade, the data have remained spread across different repositories, scientific articles, and unpublished sources, making it difficult to leverage. Here we present a new dataset of monthly Arctic-boreal carbon fluxes (ABCFlux v2) across terrestrial (wetlands and uplands) and freshwater (lakes and rivers) ecosystems compiled from previous syntheses including the Arctic-boreal CO ₂ flux database (ABCFlux v1), the Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4), and the Global River Methane Database (GRiMeDB). In addition, we consider data from general-purpose (e.g., Zenodo) and flux network repositories, literature, and site principal investigators. The dataset includes surface-atmosphere CO ₂ fluxes of gross primary production (GPP), ecosystem respiration (Reco), and net ecosystem exchange (NEE), alongside CH ₄ fluxes. For aquatic ecosystems, we split CH ₄ fluxes into diffusive and ebullitive flux pathways, and included potential emissions from transient storage in the water column (“storage fluxes”), alongside CO ₂ and CH ₄ concentrations dissolved in the surface water. Fluxes are measured through a variety of methods including chamber and eddy covariance techniques alongside bubble traps, ice-surveys, and concentration-based turbulence-driven modelling in aquatic ecosystems. The monthly flux data are reported together with supporting methodological and environmental metadata. The resulting ABCFlux v2 has 23 847 flux site-months, 8182 concentration site-months, and 199 seasonal observations from 1024 sites, and includes 56 139 reported fluxes (i.e. sum of GPP, Reco, NEE, and CH ₄ fluxes) from the years 1984 to 2024. The majority of monthly observations occurred after 1999. Wetlands had the highest number of site-month observations (8758), followed by boreal forest (6981), lotic ecosystems (6275), lentic ecosystems (3799) and upland tundra (3308). Measurements of CO ₂ dominated the dataset across most ecosystem types (25 222) except for lentic ecosystems, where CH ₄ flux site-months (3098) were more frequent than CO ₂ flux site-months (2915). Overall, ABCFlux v2 includes 160 % more site-months for terrestrial CO ₂ flux data compared to ABCFlux v1. Integrating and updating BAWLD-CH4 flux data from growing season averages to monthly fluxes resulted in 5671 site-months of chamber CH ₄ data compared to 762 site-years. This collaborative initiative, involving contributions from over 260 researchers, provides a comprehensive overview of the current state of the Arctic-boreal carbon flux network and its data, and serves as an important step in reducing uncertainties in Arctic-boreal carbon budgets and in enhancing our understanding of climate feedbacks. The data can be accessed at ORNL DAAC at https://doi.org/10.3334/ORNLDAAC/2448 (Virkkala et al., 2026).

Mosses dominate ground cover in many ecosystems and are central to carbon and nutrient cycling, water regulation, and surface energy balance. Their poikilohydric physiology, without stomatal control or vascular transport, couples metabolism closely to moisture. While climate extremes affecting moisture become more frequent, how drought and rewetting affect volatile organic compound (VOC) emissions from mosses remains poorly understood. This study examined how a heatwave-associated drought and subsequent rewetting affected VOC emission rates and blend compositions in the moss Tomentypnum nitens. Subarctic heath mesocosms containing T. nitens were maintained in climate chambers during a pre drought period and exposed sequentially to a 3-week heatwave-associated drought, and 1 week rewetting treatment. Moss shoots detached from mesocosms were enclosed in a gas-exchange cuvette to measure VOC emissions analysed by GC-MS, net photosynthesis, and evaporation rates. Tissue water content was also determined. Drought strongly reduced water content and evaporation and shifted net photosynthesis from positive to negative values. Total VOC emissions declined by 90% in response to drought and recovered after rewetting (P = .06). Drought also shifted the VOC blend towards hydrocarbon dominance (nearly 80% of total). After rewetting, the blend contained a greater proportion of oxygenated VOCs and a smaller proportion of sesquiterpenes than before drought. The legacy effects of drought on VOC blend may influence both the ecological roles of moss VOCs and their contribution to biosphere–atmosphere interactions.

Arctic tundra ecosystems have experienced marked changes in temperature and precipitation patterns in recent decades. The impact of these changes on the physical and biogeochemical mechanisms governing soil respiration and CO2 emissions remains poorly understood. We investigate how seasonal thermo–hydrological dynamics impact soil freezing/thawing, soil respiration, and O2 and CO2 transport and exchange in a typical well-drained Arctic tundra ecosystem by combining laboratory experiments and field observations with process-based modeling. Our results demonstrate that low temperatures constrain soil CO2 production and efflux during most of the year, but rising temperatures and large snowmelt infiltration in early spring reduce thermal constraints and induce relatively short yet important periods of soil water-saturated conditions. During these periods, depth-resolved measurements show noticeable limitations in oxygen transport and corresponding CO2 production, which leads to the reduction of CO2 effluxes. The snowmelt infiltration also reduces CO2 efflux by promoting downward CO2 migration and delays its emission by limiting diffusive transport to the atmosphere until the water level recedes. Comparing the simulations with a no-snowmelt scenario, we find that these physical and biogeochemical controls during such short periods collectively reduce annual CO2 efflux by 7%, highlighting the importance of climate-driven forcing in modulating soil greenhouse gas emissions.

Summary: Forest soils are an important source of volatile organic compounds (VOCs), yet little is known about how different tree species influence soil VOC emissions and the role of rhizosphere processes in mediating VOC release form roots. We analysed soil VOC emissions from the soil surface and bulk soil as well as from roots with intact rhizosphere and washed roots of Picea abies and Fagus sylvatica . Tree saplings were grown on natural forest soil, and VOC emissions and gas exchange of soils and roots were measured under controlled conditions using online gas analysers integrated into an automated system. To assess the contribution of rhizosphere soil and microbial communities to root VOC emissions, roots were analysed (a) without washing, preserving the rhizosphere, (b) water-washed, and (c) ethanol-washed (70 vol%) to minimize microbial contributions. Species-specific VOC emission patterns were observed in both soils and roots. P. abies showed higher total emission rates and a more diverse, terpenoid-rich VOC profile dominated by α-pinene, β-pinene, β-myrcene, and α-phellandrene than F. sylvatica . Notably, these differences were evident not only at the soil surface but also in root and litter free bulk soil. Root washing further revealed that the rhizosphere is a hotspot of terpenoid production in P. abies , with significantly higher monoterpenoid emissions from unwashed roots than from water or ethanol-washed roots. This study demonstrates how tree species shape net soil VOC emissions, potentially leading to cascading effects on atmospheric VOC concentrations, and highlights the importance of the rhizosphere in regulating belowground VOC production.

Empirical in-situ measurements of ecosystem carbon dioxide respiration ( R _eco ) in high-latitude ecosystems remain limited, yet they are essential for understanding how tundra carbon cycling responds to climate warming across different environmental contexts and for reducing uncertainties in upscaled carbon budgets and carbon–climate feedbacks. Here, we present the TundraFlux Database, which to date is the most comprehensive synthesis of tundra R _eco responses to experimental warming. The database compiles over 24 000 daily-aggregated in-situ R _eco measurements from control and warmed plots with open-top chambers at 64 Arctic and alpine tundra sites across 12 countries. By coupling R _eco measurements with extensive metadata on climate, vegetation, and soil characteristics, the TundraFlux Database enables the integration of field-scale ecological processes into large-scale models, offering new opportunities to refine global carbon budgets and test predictions of tundra ecosystem responses to warming. Open access to the TundraFlux Database will empower the research community to better quantify and predict how warming alters carbon cycling in Arctic and alpine tundra ecosystems. The data can be accessed on Zenodo (https://doi.org/10.5281/zenodo.17976235, Schwieger, 2026a).