Deeper winter snow reduces ecosystem C losses but increases the global warming potential of Arctic tussock tundra over the growing season.

Abstract:

Arctic winter precipitation is projected to increase globally over the next decades, spatial variability encompassing areas with increases and decreases in winter snow. Changes in winter precipitation strongly affect C dynamics in Arctic systems and may lead to major positive climate forcing feedbacks. However, impacts of predicted changes in snowfall and accumulation on the rate and form of C fluxes (CO₂ and CH₄) and associated forcing feedbacks from Arctic tundra remain uncertain. We investigated how changes in winter precipitation affect net ecosystem CO₂ and CH₄ fluxes and budgets of moist acidic tundra in an 18-yrs snow fence experiment over a complete growing season at Toolik Lake, AK. Arctic tundra under ambient winter precipitation (CTL) was a net source of CO₂ and CH₄, yielding net C losses over the growing season. Reduced snow (–15-30% snow depth; RS) switched the system to a net CO₂ sink mostly by limiting SOC decomposition within colder soils. Snow additions progressively reduced net ecosystem CO₂ losses compared to CTL, switching the system into a weaker net CO₂ source with medium additions (+20-45% snow depth; MS) and into a small net CO₂ sink with high additions (+70-100% snow depth; HS). Increasingly wetter soils with snow additions constrained the temperature sensitivity of aerobic decomposition and favored the anaerobic metabolism, buffering ecosystem CO₂ losses despite substantial soil warming. Accordingly, Arctic tundra switched from a sustained CH₄ sink at RS site to an increasingly stronger CH₄ source with snow additions. Accounting for both CO₂ and CH₄, the RS site became a net C sink over the growing season, overall reducing the global warming potential (CO₂ equiv.; GWP) of the system relative to CTL. Snow additions progressively reduced net C losses at the MS site compared to CTL and the system transitioned into a net C sink at HS plots, partly due to the slower metabolism of anaerobic decomposition. However, given the greater radiative forcing of CH₄, GWP was similar in MS and CTL sites, whereas it was higher in HS than in CTL sites. Our study suggests that changes in snowfall and accumulation are a critical forcing element in future climate scenarios.