Dual and divergent formation pathways govern the composition and origins of mineral-associated organic carbon

Mineral-associated organic matter (MAOM) is the largest and most stable soil carbon reservoir, playing a central role in soil health and climate mitigation. Yet, quantitative understanding is lacking for the two fundamental processes forming MAOM— adsorption of dissolved organic matter and aggregation of insoluble organic particles—and how each pathway incorporates plant- versus microbial-derived carbon. Analyzing soils from diverse ecosystems across North America, we found that both pathways substantially contributed to MAOM, with adsorption accounting for 11–96% and aggregation for 5–89% of total MAOM carbon, depending strongly on environmental context. Adsorbed MAOM was consistently dominated by plant-derived aromatic compounds and exhibited Langmuir-type saturation dynamics, whereas aggregated MAOM was consistently enriched in non-aromatic, microbial residues and did not saturate. On average, plant-derived carbon constitutes 59% of total MAOM carbon. Despite substantial environmental shifts in pathway dominance, the molecular composition within each fraction remained remarkably consistent across ecosystems, indicating strong mineral and microbial filtration effects. These findings provide pathway-specific parameters for refining soil-carbon models and offer practical guidance for management strategies targeting long-term carbon sequestration.