<b>Impact of Magnitude and Duration of Soil Warming on Bacterial Communities and Nitrogen Cycling in the Rhizosphere and Bulk Soil of Subarctic Grasslands</b>

Global warming driven by anthropogenic activities has significantly altered terrestrial ecosystems in northern latitudes, including subarctic grasslands. This study examines how the magnitude and duration of soil warming influence bacterial community structure, abundance, and nitrogen cycling in the rhizosphere and bulk soil compartments of two geothermally warmed Icelandic subarctic grasslands (GN: 11–13 years and GO: &gt;60 years warmed). These sites serve as models for understanding global warming effects, employing temporal sampling across soil warming gradients ranging from ambient to +15.0℃. The results revealed that bacterial abundances showed significant warming thresholds at lower temperature increases in the rhizospheres (+0.83℃ in GN, +1.34℃ in GO) compared to bulk soils (+1.42℃ in GN; no threshold detected in GO), with abundances declining below these thresholds, indicating greater warming sensitivity of rhizosphere communities. Bacterial community composition changed mainly through species replacement, likely influenced by increased dispersal limitation under increasing temperature. Warming structured microbial co-occurrence networks into distinct modules, with genera that increased under warming dominating positive associations and genera that declined dominating negative associations, particularly in both compartments of GO. This suggests that long-term exposure led to microbial adaptations, resulting in site-specific responses. Although elevated temperature did not alter the overall abundance of organisms involved in major nitrogen cycling processes, species replacement was detectable at genera and phylotype levels across all groups. Regardless of duration, <i>nosZ</i>-type denitrifiers decreased with warming. However, warming did not affect the abundance of total ammonia oxidizers and increased <i>nirS</i>-type denitrifiers in both soil compartments of GO, potentially enhancing nitrogen loss and nitrous oxide emissions. These findings emphasize the importance of long-term studies and compartment-specific analyses to understand microbial community dynamics and their impact on ecosystem processes under global warming.