Exploring the role of surface micro-topography in governing dissolved nitrogen dynamics in agricultural runoff during rainfall.

Understanding the migration characteristics of dissolved nitrogen from farmlands to runoff during rainfall is essential for managing agricultural non-point source pollution. The effects of tillage-induced surface micro-topography on the migration dynamics of dissolved nitrogen from soils to runoff have not been sufficiently explored. In this study, simulated rainfall experiments were conducted to investigate the export pathways, loads, and composition characteristics of dissolved nitrogen, including NO-N, NH-N, and dissolved organic nitrogen (DON). Four typical micro-topography treatments including contour tillage (CT), longitudinal tillage (LT), pit digging tillage (AT), and flat tillage (FT, as control) were tested. All dissolved nitrogen forms in runoff were quantified, and DON was further characterized at the molecular level using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Results showed that the total dissolved nitrogen loads under the higher surface roughness treatments (AT and CT) were about 10 to 50 times greater than those under the lower roughness treatments (FT and LT) (3,681∼15,519 mg m vs. 233∼440 mg m). Surface flow was the primary pathway of dissolved nitrogen export under the FT and LT treatments, while leaching dominated under the AT and CT treatments. The proportions of NO-N, NH-N, and DON loads varied significantly between the two pathways. DON dominated in the surface flow, accounting for 51 %∼77 % of the total dissolved nitrogen pool, while NO-N was the dominant form in the leachate flow, constituting 76 %∼83 % across all four treatments. On a molecular level, DON in leachate flow exhibited significantly higher aromaticity, greater molecular weight, and lower biodegradability compare to that in surface flow. These findings highlight the distinct export behaviors of nitrogen forms under different surface micro-topographies, offering valuable insights for tracing nitrogen loss and improving management strategies in agricultural ecosystems. Our results also provide molecular-level evidence of DON dynamics, contributing to a deeper understanding of the geochemical cycling of soil organic nitrogen.