Soil Compartment-Specific Bacterial Communities and Nitrogen Cycling Responses to Warming Magnitude in Subarctic Grasslands with Contrasting Thermal Histories

High-latitude soils are warming rapidly, yet the effects of soil warming magnitude on microbial communities across soil compartments remain unclear. We leveraged a geothermal warming chronosequence in Icelandic subarctic grasslands to examine bacterial community dynamics and nitrogen-cycling potential in rhizosphere and bulk soil under relatively stable soil moisture. Two adjacent grasslands with contrasting warming histories, 11–13 years (GN) and >60 years (GO), and differing baseline soil properties, were studied independently along continuous soil warming gradients of up to +15 °C using plant root ingrowth soil cores, with five sampling events between 2019 and 2021. Bacterial 16S rRNA gene abundance declined linearly with warming in GN across both soil compartments, whereas in GO this pattern was observed only in the rhizosphere. Community structural shifts occurred at lower temperature thresholds in GN than in GO; these cross-site patterns are consistent with warming-history legacies but may also reflect site differences. Warming increased beta-diversity across soil groups, mainly through species turnover, with reduced homogeneous selection and ecological drift and increased dispersal limitation. Across both grasslands, rhizosphere communities showed greater warming sensitivity than bulk soil, with stronger abundance responses and lower temperature thresholds for community reorganization. In GO, central microbial taxa shifted, particularly in the rhizosphere, while overall co-occurrence network structure remained stable. Nitrogen-cycling gene abundances were primarily structured by sampling occasion, whereas warming effects varied by grassland and soil compartment. In GO, rhizosphere communities showed reduced microbial nitrogen retention potential through strong suppression of nifH and nrfA gene abundances relative to amoA nitrification and nir-type denitrification genes, whereas bulk soil functional profiles remained comparatively buffered. Together, these results indicate compartment-specific, site-contingent microbial reorganization under sustained soil warming, with patterns across GN and GO consistent with warming-history legacies superimposed on baseline soil differences. Incorporating site-specific context, warming duration, and rhizosphere–bulk soil contrasts into future studies may improve predictions of microbial responses to prolonged temperature increases in subarctic grasslands, although direct process measurements are needed to quantify ecosystem feedbacks.