Long‐Term Impact of Cropping System and Nitrogen Input Quality on Nitrogen Release and Organic Matter Chemistry

Abstract

Long‐term integrated soil management strategies are essential for enhancing soil organic matter (SOM) quality and promoting the availability of mineralizable nitrogen (N). This study employed a tripartite analytical framework to investigate how crop rotations and organic N amendments influence soil N availability and SOM chemistry. A twelve‐week incubation assay quantified potentially mineralizable nitrogen (Min N) across five cropping systems: continuous corn (CC), continuous soybean (Soy), continuous clover (Cl), corn‐soybean (CS), and corn‐clover (CCl), under four N fertility regimes: no N input (N0), synthetic N as Urea (NF), uncomposted manure (UCM), and composted manure (CM). To evaluate the chemical stability of soil N, pyrolysis was used to calculate NH 3 ‐T50 (the temperature at which 50% of ammonia is released), serving as an indicator of thermal N stability. In parallel, X‐ray Absorption Near Edge Structure (XANES) spectroscopy characterized the chemical speciation of carbon and N in SOM. The Min N assay revealed significantly enhanced N availability in legume‐inclusive systems, particularly Cl and CCl treatment amended with CM or UCM. The NH 3 ‐T50 exhibited a significant negative correlation with Min N, indicating that more labile, biologically accessible N forms are associated with lower thermal stability. The XANES spectroscopy suggested that CC systems were characterized by amide and pyrrole N structures: markers of proteinaceous residues and limited microbial decomposition. In contrast, CCl treatments were enriched with pyridinic, aromatic, and aliphatic compounds, indicative of advanced microbial processing and stabilization pathways. Notably, UCM and CM treatments clustered closely in multivariate space, suggesting comparable SOM transformation trajectories despite compost maturity differences. Divergence in SOM chemistry under NF and N0 treatments reflected shifts toward labile or recalcitrant N pools, modulated by rotation context and input type. These findings highlight that integrating legumes and organic N amendments fosters a more microbially accessible soil N pool while supporting the development of chemically complex and stable SOM. The combined use of Min N assays, thermal analysis, and molecular spectroscopy provides a robust framework for understanding the interactions between crop rotation, N management, and SOM quality in sustainable agroecosystems.