Arctic soil CO release during freeze-thaw cycles modulated by silicon and calcium.

Arctic soils are the largest pool of soil organic carbon worldwide. Temperatures in the Arctic have risen faster than the global average during the last decades, decreasing annual freezing days and increasing the number of freeze-thaw cycles (temperature oscillations passing through zero degrees) per year as the temperature is expected to fluctuate more around 0 °C. At the same time, proceeding deepening of seasonal thaw may increase silicon (Si) and calcium (Ca) concentrations in the active layer of Arctic soils as the concentrations in the thawing permafrost layer might be higher depending on location. We analyzed the importance of freeze-thaw cycles for Arctic soil CO fluxes. Furthermore, we tested how Si (mobilizing organic C) and Ca (immobilizing organic C) interfere with the soil CO fluxes in the context of freeze-thaw cycles. Our results show that with each freeze-thaw cycle the CO fluxes from the Arctic soils decreased. Our data revealed a considerable CO emission below 0 °C. We also show that pronounced differences emerge in Arctic soil CO fluxes with Si increasing and Ca decreasing CO fluxes. Furthermore, we show that both Si and Ca concentrations in Arctic soils are central controls on Arctic soil CO release, with Si increasing Arctic soil CO release especially when temperatures are just below 0 °C. Our findings could provide an important constraint on soil CO emissions upon soil thaw, as well as on the greenhouse gas budget of high latitudes. Thus we call for work improving understanding of freeze-thaw cycles as well as the effect of Ca and Si on carbon fluxes, as well as for increased consideration of those factors in wide-scale assessments of carbon fluxes in the high latitudes.