There is limited information regarding biogeochemical pools and fluxes in maritime tundra ecosystems along the Antarctic Peninsula. To collect baseline information on biogeochemical processes in a tundra ecosystem dominated by two vascular plant species (Colobanthus quitensis and Deschampsia antarctica) at Biscoe Point off the coast of Anvers Island, we measured pools and fluxes of C and N in transplanted tundra microcosm cores, complemented with sampling of precipitation and surface runoff. Snow and snowmelt from the tundra collection site and soil leachates from the cores were enriched with N and dissolved organic carbon compared to precipitation and snowmelt samples collected at Palmer Station, indicating high loading of N and organic matter from the penguin colonies adjacent to the tundra site. Relatively high values of δ15N in the live and dead biomass of D. antarctica and C. quitensis (5.6-25.1‰) indicated an enrichment of N in this tundra ecosystem, possibly through N inputs from adjacent penguin colonies. Stepwise multiple linear regressions found that ecosystem respiration and gross primary production were best predicted by live biomass of D. antarctica, suggesting a disproportionately high contribution of D. antarctica to CO2 fluxes. The cores with higher δ15N and lower δ13C in the soil organic horizon exhibited higher CO2 fluxes. The results suggest that abundant N inputs from penguin colonies and the competitive balance between plant species might play a critical role in the response of tundra ecosystems along the Antarctic Peninsula to projected climate change.