Mineral-associated organic matter (MAOM) holds most of the stabilized carbon stored in soils worldwide, yet the mechanisms that concentrate plant- and microbial-derived material in this stable pool remain poorly resolved. Using a continent-wide soil survey and state-of-the-art molecular characterization, we show that MAOM assembles by two contrasting pathways: adsorption of soluble, plant-derived aromatics onto mineral surfaces and aggregation of insoluble, primarily microbially derived residues within fine microaggregates. The balance between these pathways shifts predictably with soil depth, moisture, and mineralogy—adsorption dominating deep or humid soils and aggregation dominating surface or arid soils. This dual-pathway framework reshapes our understanding of soil-carbon stabilization and offers actionable targets for managing soils to maximize long-term carbon storage under a changing climate.