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Across the Arctic, the net ecosystem carbon (C) balance of tundra ecosystems is highly uncertain due to substantial temporal variability of C fluxes and to landscape heterogeneity. We modeled both carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes for the dominant land cover types in a ~100-km 2 sub-Arctic tundra region in northeast European Russia for the period of 2006-2015 using process-based biogeochemical models. Modeled net annual CO 2 fluxes ranged from -300 g C m -2  year -1 [net uptake] in a willow fen to 3 g C m -2  year -1 [net source] in dry lichen tundra. Modeled annual CH 4 emissions ranged from -0.2 to 22.3 g C m -2  year -1 at a peat plateau site and a willow fen site, respectively. Interannual variability over the decade was relatively small (20%-25%) in comparison with variability among the land cover types (150%). Using high-resolution land cover classification, the region was a net sink of atmospheric CO 2 across most land cover types but a net source of CH 4 to the atmosphere due to high emissions from permafrost-free fens. Using a lower resolution for land cover classification resulted in a 20%-65% underestimation of regional CH 4 flux relative to high-resolution classification and smaller (10%) overestimation of regional CO 2 uptake due to the underestimation of wetland area by 60%. The relative fraction of uplands versus wetlands was key to determining the net regional C balance at this and other Arctic tundra sites because wetlands were hot spots for C cycling in Arctic tundra ecosystems., (© 2018 John Wiley & Sons Ltd.)