Distinct Potential Priming under Snow-Covered and Snow-Free Conditions in Subarctic Tundra

Sub-arctic tundra ecosystems experience strong seasonality, which could potentially influence the belowground processes, including the soil carbon cycling. Priming effect is a significant component in carbon cycling and describes the change of soil carbon mineralization after fresh carbon input. In this study, we investigated the effect of seasonality on the potential priming and the driving factors of the effect. We collected soils (0-10cm) from two tundra heath ecosystems with different precipitation regimes (Abisko and Vassijaure). We sampled soils every four weeks throughout a year and conducted 60-day incubation experiments at 3°C. To simulate the root exudate input effect, a ¹³C labelled artificial cocktail was added to assess the potential priming. Cumulative priming was positive throughout the whole year with fluctuations across sampling periods, accounting for significant portions of total respiration. While no significant seasonal variation on priming was observed, we found a significant snow state effect (snow-covered state vs snow-free state) on priming at both sites, with a trend of increasing priming potential during snow-covered periods. Surprisingly, the relationships between the driving factors – soil N availability (mineral N [NH4⁺ and NO3^-], total dissolved N [DON]), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) – and priming remained consistent across snow states. Structural Equation Modeling (SEM) revealed that soil temperature and snow state influenced soil N availability (mineral N and dissolved organic N [DON]), DOC, and abundance of microbial communities (Bacteria and Archaea). DOC and MBC, along with bacterial abundance, positively influenced cumulative cocktail mineralization. Subsequently, cumulative cocktail mineralization strongly enhanced cumulative priming, whereas mineral nitrogen (N) availability had a suppressive effect. These results underscore the critical role of snow state in shaping potential priming, revealing consistent underlying drivers.