<p>In cropland soils, the majority of organic nitrogen (N) is stored in the mineral associated organic matter (MAOM) fraction of soil organic matter (SOM). Because strong bonds with mineral surfaces are thought to limit microbial access to MAOM, particulate organic matter (POM) is often assumed to drive N mineralization and nutrient cycling, despite being a smaller and more N-poor pool. Emerging evidence suggests that N in MAOM can be mobilized into mineralization pathways, challenging this assumption. Yet, when and how MAOM contributes to N mineralization remains unclear, particularly how SOM quantity, composition, and stoichiometry influence its contribution. To examine these dynamics, we reconstructed soils from physically isolated POM and MAOM fractions, systematically varying SOM composition. The MAOM was isolated from a soil previously amended with ¹⁵N fertilizer, which retained a distinct isotopic signature relative to the POM fraction. Leveraging this difference in isotopic composition, we applied a two-pool isotopic mixing model, with POM and MAOM as the endmembers, to quantify the contribution of both fractions to mineral N production. Across treatments, we observed strong effects of SOM content, POM C:N ratio, and POM:MAOM ratio on mineral N production. In the treatment most representative of agricultural soil conditions, ~ 75% of mineralized N originated from the MAOM. Higher POM C:N ratios increased the proportional contribution of MAOM to mineralized N, consistent with hypotheses around microbial N mining. While POM mineralized more rapidly on a per-unit basis, MAOM consistently contributed more total N to the mineralized pool. These findings demonstrate that MAOM can be a dominant source of mineralized N across a range of SOM conditions, even in the presence of high-quality POM. As MAOM represents the largest pool of organic N in cropland soils, understanding the factors governing its turnover is essential for improving nutrient management.</p>