Winter greenhouse gas fluxes (CO₂, CH₄ and N₂O) from a subalpine grassland

Abstract. Although greenhouse gas emissions during winter contribute significantly to annual balances, their quantification is still highly uncertain in snow-covered ecosystems. Here, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes were measured at a subalpine managed grassland in Switzerland using concentration gradients within the snowpack (CO₂, CH₄, N₂O) and the eddy covariance method (CO₂) during the winter 2010/2011. Our objectives were (1) to identify the temporal and spatial variation of greenhouse gases (GHGs) and their drivers, and (2) to estimate the GHG budget of the site during this specific season (1 December–31 March, 121 days). Mean winter fluxes (December–March) based on the gradient method were 0.77 ± 0.54 μmol m⁻² s⁻¹ for CO₂ (1.19 ± 1.05 μmol m⁻² s⁻¹ measured by eddy covariance), −0.14 ± 0.09 nmol m⁻² s⁻¹ for CH₄ and 0.23 ± 0.23 nmol m⁻² s⁻¹ for N₂O, respectively. In comparison with the CO₂ fluxes measured by eddy covariance, the gradient technique underestimated the effluxes by 50%. While CO₂ and CH₄ fluxes decreased with the progressing winter season, N₂O fluxes did not follow a seasonal pattern. The major variables correlating with the fluxes of CO₂ and CH₄ were soil temperature and snow water equivalent, which is based on snow height and snow density. N₂O fluxes were only explained poorly by any of the measured environmental variables. Spatial variability across the valley floor was smallest for CO₂ and largest for N₂O. During the winter season 2010/2011, greenhouse gas fluxes ranged between 550 ± 540 g CO₂ m⁻² estimated by the eddy covariance approach and 543 ± 247 g CO₂ m⁻², −0.4 ± 0.01 g CH₄ m⁻² and 0.11 ± 0.1 g N₂O m⁻² derived by the gradient technique. Total seasonal greenhouse gas emissions from the grassland were between 574 ± 276 and 581 ± 569 g CO₂ eq. m⁻², with N₂O contributing 5% to the overall budget and CH₄ reducing the budget by 0.1%. Cumulative budgets of CO₂ were smaller than emissions reported for other subalpine meadows in the Swiss Alps and the Rocky Mountains. Further investigations on the GHG exchange of grasslands in winter are needed in order to (1) deepen our currently limited knowledge on the environmental drivers of each GHG, (2) to thoroughly constrain annual balances, and (3) to project possible changes in GHG flux magnitude with expected shorter and warmer winter periods.