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Carbon dioxide exchange, soil C and N, leaf mineral nutrition and leaf carbon isotope discrimination (LCID-Delta) were measured in three High Arctic tundra ecosystems over 2 years under ambient and long-term (9 years) warmed (similar to2degreesC) conditions. These ecosystems are located at Alexandra Fiord (79degreesN) on Ellesmere Island, Nunavut, and span a soil water gradient; dry, mesic, and wet tundra. Growing season CO2 fluxes (i.e., net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (R-e)) were measured using an infrared gas analyzer and winter C losses were estimated by chemical absorption. All three tundra ecosystems lost CO2 to the atmosphere during the winter, ranging from 7 to 12 g CO2-C m(-2) season(-1) being highest in the wet tundra. The period during the growing season when mesic tundra switch from being a CO2 source to a CO2 sink was increased by 2 weeks because of warming and increases in GEP. Warming during the summer stimulated dry tundra GEP more than R-e and thus, NEE was consistently greater under warmed as opposed to ambient temperatures. In mesic tundra, warming stimulated GEP with no effect on R-e increasing NEE by similar to10%, especially in the first half of the summer. During the similar to70 days growing season (mid-June-mid-August), the dry and wet tundra ecosystems were net CO2-C sinks (30 and 67 g C m(-2) season(-1), respectively) and the mesic ecosystem was a net C source (58 g C m(-2) season(-1)) to the atmosphere under ambient temperature conditions, due in part to unusual glacier melt water flooding that occurred in the mesic tundra. Experimental warming during the growing season increased net C uptake by similar to12% in dry tundra, but reduced net C uptake by similar to20% in wet tundra primarily because of greater rates of R-e as opposed to lower rates of GEP. Mesic tundra responded to long-term warming with similar to30% increase in GEP with almost no change in R-e reducing this tundra type to a slight C source (17 g C m(-2) season(-1)). Warming caused LCID of Dryas integrafolia plants to be higher in dry tundra and lower in Salix arctic plants in mesic and wet tundra. Our findings indicate that: (1) High Arctic ecosystems, which occur in similar mesoclimates, have different net CO2 exchange rates with the atmosphere; (2) long-term warming can increase the net CO2 exchange of High Arctic tundra by stimulating GEP, but it can also reduce net CO2 exchange in some tundra types during the summer by stimulating R-e to a greater degree than stimulating GEP; (3) after 9 years of experimental warming, increases in soil carbon and nitrogen are detectable, in part, because of increases in deciduous shrub cover, biomass, and leaf litter inputs; (4) dry tundra increases in GEP, in response to long-term warming, is reflected in D. integrifolia LCID; and (5) the differential carbon exchange responses of dry, mesic, and wet tundra to similar warming magnitudes appear to depend, in part, on the hydrologic (soil water) conditions. Annual net ecosystem CO2-C exchange rates ranged from losses of 64 g C m(-2) yr(-1) to gains of 55 g C m(-2) yr(-1). These magnitudes of positive NEE are close to the estimates of NPP for these tundra types in Alexandra Fiord and in other High Arctic locations based on destructive harvests.