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Summary: Heteropolymer humic substances (HS) are the largest constituents of soil organic matter and are key components that affect plant and microbial growth in maritime Antarctic tundra. We investigated HS decomposition in Antarctic tundra soils from distinct sites by incubating samples at 5°C or 8°C (within a natural soil thawing temperature range of −3.8°C to 9.6°C) for 90 days (average Antarctic summer period). This continuous 3‐month artificial incubation maintained a higher total soil temperature than that in natural conditions. The long‐term warming effects rapidly decreased HS content during the initial incubation, with no significant difference between 5°C and 8°C. In the presence of Antarctic tundra soil heterogeneity, the relative abundance of Proteobacteria (one of the major bacterial phyla in cold soil environments) increased during HS decomposition, which was more significant at 8°C than at 5°C. Contrasting this, the relative abundance of Actinobacteria (another major group) did not exhibit any significant variation. This microcosm study indicates that higher temperatures or prolonged thawing periods affect the relative abundance of cold‐adapted bacterial communities, thereby promoting the rate of microbial HS decomposition. The resulting increase in HS‐derived small metabolites will possibly accelerate warming‐induced changes in the Antarctic tundra ecosystem. [ABSTRACT FROM AUTHOR]