Higher Intrinsic Priming Potential in Arctic Tundra Soils During the Cold Season than Growing Season

Arctic tundra experiences marked seasonality, with consequences for belowground processes, including soil carbon (C) cycling. An important component of C cycling is the priming effect, which alters soil C mineralization after C input such as plant root exudates and litter. In this study, we tested whether the intrinsic priming potential of tundra soils varies throughout the year and evaluated whether soil chemical and microbial properties are associated with this seasonality. We collected rooting-zone soils from two sub-arctic tundra heaths with different precipitation regimes every four weeks throughout a full year. We assessed the soil’s intrinsic priming potential in a 60-day incubation experiment at 3 °C after adding a 13 C-labelled cocktail to simulate root exudate input. Cumulative intrinsic priming was positive throughout the year and accounted for up to 42% of total respiration. Its magnitude exhibited significant temporal variations but did not align with broad seasons or narrow seasons defined by the aboveground environment. Instead, it aligned with belowground seasonality, defined by monthly mean soil temperatures below or above 0.5°C, with higher intrinsic priming potential in the belowground cold season than growing season. Structural Equation Modelling revealed that legacy effects of seasonal variation of in-situ soil temperature affected priming potential both directly and via indirect effects on soil nitrogen availability and bacterial:fungal abundance ratio, suggesting that stronger nutrient limitation and seasonal shifts in microbial interactions may contribute to enhanced belowground cold-season priming potential. Together, the higher intrinsic priming potential, higher basal respiration and higher cocktail mineralisation in belowground cold-season soils resulted in a greater intrinsic capacity for C loss. Our results show that when labile carbon input occurs, priming may contribute to soil C losses during the Arctic cold season, reinforcing that soil carbon cycling is not dormant in the cold season.