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It is proposed that carbon (C) sequestration in response to reactive nitrogen (N_r) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO₂ emissions. While studies have helped clarify the magnitude by which N_r deposition enhances C sequestration by forest vegetation, there remains a paucity of long‐term experimental studies evaluating how soil C pools respond. We conducted a long‐term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha⁻¹ yr⁻¹) in the boreal zone of northern Sweden to understand how atmospheric N_r deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data …