Long-term observations of CH4 and N2O fluxes in a subalpine Norway spruce forest using chamber and eddy covariance methods

Methane (CH₄) and nitrous oxide (N₂O) substantially contribute to global greenhouse gas (GHG) emissions together with carbon dioxide (CO₂). To understand their impact on future climate change, prioritizing the study of CH₄ and N₂O fluxes becomes critical. Forest ecosystems, primarily investigated for CO₂ exchange, are less explored concerning their exchange of CH₄ and N₂O. Forests are known to be sinks for CH₄, while their role in N₂O fluxes varies, acting as either sources or sinks. However, comprehensive studies that concurrently examine CH₄ and N₂O fluxes in forests, particularly over extended periods and at high elevation, remain scarce. At high altitudes, measuring GHG fluxes with chambers during snowy periods is challenging, leading to a lack of winter flux data which are crucial for understanding flux dynamics related to freeze-thaw cycles and snow patterns. This study addresses this gap by investigating long-term CH₄ and N₂O fluxes in a subalpine Norway spruce forest (Davos, CH-Dav, ICOS Class 1 Ecosystem station, Switzerland), encompassing both soil and canopy interactions with the atmosphere.

Over five years (2017, 2020-2023 for CH₄; 2017, 2020 for N₂O), we employed automatic chambers to measure forest-floor fluxes, complemented by below-canopy eddy covariance CH₄ flux measurements starting from May 2023, as well as static chamber measurements in 2023. Our research objectives were to 1) characterize the magnitude and seasonal dynamics of CH₄ and N₂O forest-floor fluxes, and 2) compare CH₄ fluxes using chamber and eddy covariance techniques to better understand the interaction of soil and vegetation with the atmosphere.

We hypothesized that the forest floor primarily acts as a net sink for CH₄, with soil temperature and snow dynamics being important drivers due to their impact on microbial activity and diffusion rates between soil and atmosphere. Given the low nitrogen availability at the study site, we anticipated very low N₂O emissions. Additionally, we hypothesized that comparing CH₄ fluxes from chambers and eddy covariance would reveal small differences in their magnitudes, attributable to the distinct measurement scales and scopes of these two techniques. Our results confirmed the forest floor as a consistent CH₄ sink, exhibiting substantial short-term fluctuations driven predominantly by air temperature and snow cover. N₂O fluxes were negligible over the two-year observation period. Our study contributes to a deeper understanding of how environmental drivers and seasonal dynamics influence CH₄ and N₂O fluxes in high-elevation forests.