Divergent Regulation of Dissolved Organic Nitrogen Release from Agricultural Wastes: Iron–Aluminum–Organic Carbon Interplay

Abstract

The leaching of dissolved organic nitrogen (DON) from agricultural waste contributes to groundwater pollution, yet the regulatory soil factors and mechanisms—particularly those linked to DON source properties—remain poorly understood. We investogated the transport of wheat-straw-derived DON (WS-DON) and chicken-manure-derived DON (CM-DON) in representative regional Chinese soils using an integrated multiscale approach, including soil column experiments, convection–dispersion modeling, machine learning models (random forest with SHapley Additive exPlanations), partial least squares path modeling, and quantum chemical calculations. CM-DON exhibited 55.6% higher mobility than WS-DON. While total iron (Fe-t), exchangeable aluminum (Al-e), and total organic carbon (TOC) influenced both DON types, their regulatory roles differed. Fe-t (coefficient = 0.65) and TOC (0.42) enhanced WS-DON retention, with Al-e contributing indirectly via synergy with the Fe-TOC system. In contrast, CM-DON retention was mainly driven by the direct effects of Fe-t (0.69) and easily reducible Fe (Fe-ox1, 0.40), modulated by TOC–pH–Fe-ox1 indirect pathways. Quantum chemical analysis revealed that goethite retained hydroxyl-rich WS-DON via Fe–O stable covalent bonds (ΔE = −21.75 kcal mol⁻¹), whereas CM-DON was weakly adsorbed through humic-acid-mediatd clustering. These results highlight the need for differentiated nutrient management strategies to mitigate DON leaching from agricultural residues.