Elucidating soil dissolved organic nitrogen mineralization across soil depths under contrasting...

Nitrate nitrogen (NO₃⁻) contamination of groundwater is an increasingly critical global environmental issue. Mineralization of dissolved organic nitrogen (DON) is a key step in nitrogen transformation, influencing both plant nitrogen-use efficiency and groundwater pollution. However, quantitative assessments of DON mineralization in soils are hindered by the inability to distinguish DON fractions with different mineralization rates. To address this gap, we investigated two agricultural sites with contrasting fertilization levels. Soil samples were collected from multiple depths, and the molecular composition of DON was identified using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Mineralization incubation experiments were conducted to obtain mineralization potentials and rate constants, which were subsequently used to develop a two-component mineralization model. DON concentrations declined over time, with substantially lower mineralization in deeper soils. Mineralization rate constant of the easily decomposable plant materials (DPM) of DON was consistently higher than that of the resistant/recalcitrant plant materials (RPM). Furthermore, the mineralization rates of both DPM and RPM declined with increasing soil depth. Overall, the two-component model accurately captured soil nitrogen mineralization kinetics and differentiated the mineralization behaviors of DPM and RPM, providing valuable implications for nitrogen management and groundwater pollution control in agricultural systems.