Below- and above-canopy methane and nitrous oxide fluxes in a subalpine spruce forest

Our understanding of forests as methane (CH 4 ) and nitrous oxide (N 2 O) sinks or sources remains limited, contributing to large uncertainties in terrestrial greenhouse gas (GHG) budgets. We investigated the CH 4 and N 2 O exchange in a subalpine spruce forest in Davos (Switzerland) at multiple scales over seven years. Measurements included forest-floor fluxes with automatic chambers, GHG concentrations along a vertical canopy profile, and eddy covariance (EC) fluxes below and above the canopy. The forest floor was a small net CH 4 sink (-0.5 ± 0.05 g CH 4 C m −2 yr −1 ; mean ± standard deviation), while the forest was a small net CH 4 source (0.54±0.22 g CH 4 C m −2 yr −1 ). CH 4 concentrations near the forest floor were low, increased with height, and stayed relatively constant within and above the forest canopy (10 to 35 m). While supporting the observed forest-floor sink, the profiles could not explain the discrepancy to the ecosystem CH 4 budget. Forest-floor CH 4 fluxes measured by chambers and below-canopy EC showed comparable magnitudes and seasonal dynamics, driven by snow depth, soil temperature, soil moisture, and below-canopy photosynthetic photon flux density. These findings suggest a coupling between above-ground CO 2 assimilation and below-ground CH 4 dynamics, possibly mediated by plant-soil interactions. Above-canopy N 2 O fluxes were negligible, with annual net ecosystem N 2 O budgets close to zero (-0.012 to 0.035 g N 2 O-N m −2 yr −1 ). Considering the global warming potentials of CO 2 , CH 4 and N 2 O over a 100-year period, the subalpine spruce forest remained a net GHG sink, though CH 4 and N 2 O emissions offset its sink strength by 3–12%.