Winter Critical Zone dynamics in the High Arctic: measuring carbon fluxes and geo-biological processes at the Bayelva Critical Zone Observatory during winter

Arctic CO₂ fluxes are a crucial component of the global greenhouse gas balance and are linked with the state and annual dynamics of the tundra. The assessment of the flux magnitude and timing is crucial for quantifying the contribution to climate change generated by the positive feedback induced by Arctic soil respiration. Summer gas flux data are now available for several locations in the Arctic, including those performed by IGG-CNR at the Bayelva Critical Zone Observatory in Ny Ålesund (CO₂ summer fluxes by flux chambers and year-round fluxes by Eddy Covariance) used to implement empirical models of CO₂ fluxes.  Existing data suggest, in fact, that during Winter the tundra could act as a weak carbon source. Winter processes, however, are poorly understood because of a lack of measurements spanning the ‘dark-season’.

The year-round dynamics of Arctic soil ecosystems are barely beginning to be explored, and therefore the soil geo-biological interactions and ecology during polar night are little known. Overall, winter CO₂ flux dynamics is the great unknown in the annual Arctic carbon budget. This is a critical knowledge gap, because the amplified warming of the Arctic is strongest in winter, and yet most studies focus on the summer season. This constrains all models of Arctic soil carbon cycling with the assumption that soil microbes are mostly active during summer and summer-adjacent periods. In fact, while several studies show the active role of microbial communities in releasing greenhouse gases during summer thaw, their contribution to winter fluxes is not well studied.

Recent data show that Winter communities might be more stable than summer ones, and significantly contribute to organic carbon mineralization and continual release of CO₂ even at sub-zero temperatures. Even the burst in greenhouse gases observed at the start of the thaw period might be due to the release of gases accumulated inside the frozen active layer during winter. Despite the potential role of specific winter microbial communities, few studies report their detailed characterization, usually on few winter time points, limiting our ability to link changes in the microbial communities with observed gas fluxes.

Furthermore, winter CO₂ fluxes sometimes show strong flux bursts, associated with rapid changes in local air CO₂ concentration and with strong wind bursts, as revealed by Eddy Covariance. Such processes can have multiple causes: strong winds can induce lateral gas diffusion within the snowpack, forcing CO₂ -rich (or -poor) “bubbles” to emerge. Strong winds can also induce advective transport of CO₂ -rich (-poor) air generated by other processes.

Building on recent work, we then propose to contribute to fill these knowledge gaps by complementing our already-existing flux-measuring infrastructure with a fixed state-of-the art instrumented tower for measuring CO₂ fluxes from the snow (unique in Svalbard), and complemented by the analysis of soil physical-chemical properties, microbial composition and activity. The integrated collected data: a) will feed the new CNR NyA Carbon Flux Observatory, a FAIR data archive that will be created to contribute to the major pan-arctic databases; b) will be used to develop a model to explicitly simulate winter and year-round soil processes and enable the forecasting of biological and physical changes due to climate forcing, thanks to the modelling expertise of the proponents. This project will bring a technological advance of CNR infrastructure providing new services and products (data and models) and a new understanding of crucial processes in Arctic ecosystems and their two-way interaction with climate change.