This study investigates how warming and changes in precipitation may affect the cycling of carbon (C) in tundra soils, and between high Arctic tundra and the atmosphere. We quantified ecosystem respiration (R_eco) and soil pore space CO₂ in a polar semi-desert in northwestern Greenland under current and future climate conditions simulated by long-term experimental warming (+2 °C, +4 °C), water addition (+50% summer precipitation), and a combination of both (+4 °C × +50% summer precipitation). We also measured the ¹⁴C content of R_eco and soil CO₂ to distinguish young C cycling rapidly between the atmosphere and the ecosystem from older C stored in the soil for centuries to millennia.

We identified changes in the amount and timing of precipitation as a key control of the magnitude, seasonality and sources of R_eco in a polar semi-desert. Throughout each summer, small (<4 mm) precipitation events during drier periods triggered the release of very old C pulses from the deep soil, while larger precipitation events (>4 mm), more winter snow and experimental irrigation were associated with higher R_eco fluxes and the release of recently fixed (young) C. Warmer summers and experimental warming also resulted in higher R_eco fluxes (+2 °C > +4 °C), but coincided with losses of older C.

We conclude that in high Arctic, dry tundra systems, future magnitudes and patterns of old C emissions will be controlled as much by the summer precipitation regime and winter snowpack as by warming. The release of older soil C is of concern, as it may lead to net C losses from the ecosystem. Therefore, reliable predictions of precipitation amounts, frequency, and timing are required to predict the changing C cycle in the high Arctic.