Strong legacies of emerging trends in winter precipitation on the carbon-climate feedback from Arctic tundra.

Changes in winter precipitation accompanying emerging climate trends lead to a major carbon-climate feedback from Arctic tundra. However, the mechanisms driving the direction, magnitude, and form (CO2 and CH4) of C fluxes and derived climate forcing (i.e. GWP, global warming potential) from Arctic tundra under future precipitation scenarios remain unresolved. Here, we investigated the impacts of 18 years of shallow (SS, -15-30 %) and deeper (IS, +20-45 %; DS, +70-100 %) snow depth on ecosystem C fluxes and GWP in moist acidic tundra over the growing season. The response of Arctic tundra C fluxes to snow accumulation was markedly non-linear. Both shallow- and deeper- winter snow decreased Arctic tundra CO2 emissions relative to ambient (AS), ultimately reducing ecosystem C losses over the growing season. Gross primary productivity (GPP) increased with moderate increases in snow depth and decreased with further snow accumulation closely following transitions in shrub abundance. Photosynthetic uptake, however, was tightly regulated by canopy structure and plant respiration (Raut) to GPP ratio was highly conserved despite substantial transformations of plant community across snow treatments revealing a prominent role of heterotrophic respiration (Rhet) in driving net ecosystem exchange. Consistently, ecosystem C gains responded to constraints on Rhet by temperature limitation within colder soils at SS, and by snow- and thaw-induced increases in soil-water content (SWC) that promoted anaerobic decomposition and dampened the temperature sensitivity of Rhet at IS and DS. Greater CH4 emissions from wetter soils, however, increased the global warming potential (GWP) of Arctic tundra emissions at IS and DS despite decreases in C losses. Overall, our findings indicate the potential of Arctic tussock tundra to reduce C losses over the growing season but also to significantly contribute to the ecosystem GWP under emerging trends in winter precipitation.