Elevated graminoid cover co-occurs with Ascomycota-dominated soils in Longyearbyen, Svalbard.

Arctic warming has coincided with dramatic changes in plant cover, but the impact that aboveground biomass shifts have on soil microbial communities and processes remains poorly understood. To address this, we investigated spatial patterns of soil microbes in relation to vegetation changes using a space-for-time approach in the high Arctic region of Longyearbyen, Svalbard. We collected and characterized 31 topsoil samples from three sites that differed in nutrient input, CO2 flux, soil chemistry, and plant cover. Pronounced vegetation differences were observed at fine spatial scales, including a highly localized graminoid-dominated hotspot within areas of mixed plant communities. This graminoid-rich hotspot coincided with locally elevated soil fertility and exhibited particularly high CO2 fluxes. In areas that transitioned from dwarf shrub- to graminoid-dominated vegetation, we observed an increase in estimated fungal abundance, a shift from heterogeneous to Ascomycota-dominated fungal communities, and a greater abundance of r-strategist prokaryotes. Multiple regression on biotic and abiotic distance matrices revealed that soil fungi may be especially sensitive to changes compared to prokaryotes and plants. These findings highlight the need for future experiments investigating fungi in high Arctic tundra to better understand feedback between biotic and abiotic factors under warming.