Mineral associated organic matter fractions have an outsized role in nitrogen mineralization and denitrification

• More N was mineralized from MAOM than POM. • Compost increased the stability of MAOM. • MAOM and POM dynamics varied between soils. • MAOM is an important source of plant-available N. The mineral-associated (MAOM) and particulate (POM) fractions of soil organic matter demonstrate distinct dynamics and biochemical properties, but it is unknown how these fractions contribute to soil fertility and respond to soil health management practices, such as compost application. Given their distinct physical and biochemical characteristics, these fractions contain differing amounts of potentially mineralizable nitrogen (N). In this study, we aimed to determine N flux from MAOM and POM fractions. Following a growth chamber study where soils of two different textures were treated with 15 N-labeled urea fertilizer, with or without compost, the soils were fractionated into MAOM and POM and added back to their respective original soils as a substrate-addition experiment. Throughout the 52-day incubation, net N mineralization was assessed, and nitrous oxide (N 2 O) flux and CO 2 production were measured using cavity ring down spectroscopy. In addition, we performed denitrification assays to assess the contribution of each fraction to denitrification under anaerobic conditions. We found that the MAOM addition increased N 2 O production in the denitrification assays and during the incubation with the sandy soils. Across both soils, more N was mineralized from MAOM than from POM. Compost-application history reduced the rate of N mineralization from MAOM and decreased N 2 O production throughout the incubation in the sandy soils. Our study demonstrates that isolated MAOM-sized fractions were an active source of N in these incubated soils, and that the specific N-cycling dynamics of MAOM and POM can vary considerably depending on soil texture and management history, such as prior compost application.