Mineral-associated organic nitrogen pool size, composition, and accessibility mediated by agricultural management and soil geochemical characteristics

• MAOM contained the majority of N in agricultural soils spanning a range of geochemical characteristics. • Across a range of geochemical characteristics, cropland soils contained 50% less MAOM-N compared to grassland soils. • MAOM-N molecular diversity was lower in cropland soils than in grassland soils. • 20% of MAOM-N was mobilized by extractions that may mimic rhizosphere conditions. • Soil geochemical characteristics were not strongly predictive of MAOM-N decline due to management intensity. Mineral-associated organic matter (MAOM) contains a substantial portion of soil nitrogen (N). MAOM-N could serve as an important N source for crops, but its availability and response to agricultural management across different soil types remains largely unknown. We characterized MAOM-N isolated from nine paired soils spanning a range of geochemical characteristics and maintained under two land uses—more intensively managed annual cropping systems or less intensively managed grasslands. On average, we found that MAOM contained approximately two-thirds of total soil N. Across all soil types, more intensive agricultural management resulted in a 50% decline in MAOM-N stocks, as well as a reduction in MAOM-N molecular diversity. Although clay content and extractable metals were positively correlated with MAOM-N stocks, none of the geochemical characteristics measured were strongly predictive of MAOM-N decline due to land use. This suggests that more intensive crop management limited the formation or caused the disruption of a broad suite of associations between a variety of soil minerals and organic N compounds. Under both agricultural land uses, approximately 20% of MAOM-N was mobilized through sequential extractions that may mimic conditions in the rhizosphere, suggesting that a significant portion of MAOM-N may be available to plants. Together, these findings help to quantify MAOM’s potential to supply crops with N across different soil types and agricultural systems. This can inform agricultural management recommendations and supports a growing understanding of MAOM as a dynamic N source and sink.