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.