Significance The magnitude of CO ₂ efflux from soils (resulting from autotrophic and heterotrophic respiration) is one of the largest uncertainties in projecting future carbon–climate feedbacks. Despite research over several decades, the magnitude, direction, and duration of such feedbacks and their underlying microbial mechanisms are poorly understood, especially in the context of potentially interacting global environmental changes. In a decade-long experiment examining the interactive effects of CO ₂ and N enrichment, N limitation strengthened the stimulatory effects of elevated CO ₂ on soil respiration, primarily via N mining during the decomposition of more recalcitrant organic compounds. This study also provides a strategy for integrating genomics information into ecosystem and Earth system models to improve carbon-cycle predictions.