Experimental warming reshapes soil microbial communities and mineral-associated organic matter formation dynamics in a subarctic mineral horizon

Arctic and subarctic regions are warming faster than the global average, yet carbon dynamics in mineral horizons remain comparatively understudied despite large stocks stabilized as mineral-associated organic matter (MAOM). Clarifying how warming alters mineral-associated carbon (C) and nutrient pools, and the soil microbial communities that mediate MAOM formation and destabilization, is therefore critical for predicting Arctic carbon–climate feedbacks. Here, we experimentally warmed subarctic birch forest soil in Abisko, Sweden, using distinct regimes: chronic (year-round) warming and seasonal warming (summer-only or winter-only). To quantify MAOM formation potential, we developed recoverable Mineral Interface Sampling Probes (MISP) consisting of thin films of Fe- and Al-(hydr)oxides (MISP-Fe and MISP-Al) and coupled them with surface-sensitive spectroscopy techniques (X-ray photoelectron spectroscopy, XPS; Fourier-transform infrared spectroscopy, FTIR). Bacterial and fungal community composition and richness were assessed by high-throughput amplicon sequencing (16S rRNA gene and ITS markers), while microbial abundances were quantified by quantitative PCR (qPCR) as marker-gene copy numbers. Warming increased bacterial and fungal gene copy numbers and the fungal-to-bacterial ratio, while reducing richness in both domains, consistent with a community shift toward fewer warming-tolerant taxa. MISPs showed mineral-type-dependent responses in mineral-associated C formation potential (Fe vs Al (hydr)oxides), whereas mineral-associated N formation potential increased consistently under warmed treatments, yielding newly formed MAOM with a lower C:N ratio. Both microbial community shifts and MISP responses were strongest under summer warming, with comparatively weak responses under chronic or winter warming. Overall, summer warming increased microbial abundance and produced newly formed MAOM with a lower C:N ratio, consistent with soil warming shifting MAOM formation toward a microbial necromass-mediated pathway, where organic matter is processed through microbial biomass before stabilization on mineral surfaces. These findings highlight the sensitivity of MAOM pools and microbial communities in subarctic mineral soil horizons to soil warming.