A spatially-explicit watershed model for tracking nitrogen fate and transport by integration of biogeochemistry and...

Quantifying the contribution of agricultural non-point sources to watershed export of nitrogen (N) depends on the integrated modeling of soil biogeochemistry and watershed hydrology. However, scale mismatch between biogeochemical and hydrological models prevents the reliable explicit tracking of N fate and transport in watersheds. To address this issue, we developed a spatially-explicit watershed model-INtegrated SImulator for bioGeochemical and Hydrological Trackings (INSIGHT)-by embedding a soil biogeochemical model at fine-resolution grids while simultaneously coupling surface and subsurface flowpath models fed by the outputs of water and N fluxes in each grid and layer. An agricultural-forested watershed in the lower Yangtze River basin was used to test if the INSIGHT model is capable of capturing N loss, removal, retention, and exports in different pathways as well as quantifying the contribution of agricultural non-point sources to N exports at high spatiotemporal scales. The model explained the variations in watershed TN export by 79% and enabled the tracking of contributions from different land uses and transport pathways. Surface and subsurface N generation were 26.0 and 206.9 Mg N yr⁻¹, respectively, but 89.6% of surface-derived and 28.3% of subsurface-derived N were only exported to the watershed outlet, because 53.5% of N generation was retained as soil organic N and 11.9% was removed by denitrification during horizontal transport. Tea cropland showed the highest export intensity (31.4 kg N ha⁻¹ yr⁻¹), followed by mixed forest (20.4 kg N ha⁻¹ yr⁻¹) and rice (18.5 kg N ha⁻¹ yr⁻¹). Tea cultivation on steep slopes, with short surface lag time and high fertilization rates, drives high N export intensity. The results suggest that subsurface transport as the dominant mechanism for N retention and export in a watershed, calling for targeted mitigation strategies. These findings highlight that INSIGHT provides a useful tool for tracking N fate and transport in watersheds and supporting targeted agricultural non-point pollution mitigations.