Effects of Rainfall Intensity and Slope Gradient on Runoff Pathways and Carbon–Nitrogen Transport on Grass-Covered Black Soil Slopes

To elucidate the runoff generation pathways and associated carbon–nitrogen transport characteristics on grass-covered black soil slopes under varying rainfall intensities and slope gradients, a series of simulated rainfall experiments was conducted. Three rainfall intensities (30, 60, and 90 mm·h−1) and four slope gradients (0°, 5°, 10°, and 15°) were investigated. The processes governing surface and subsurface runoff were systematically analyzed, alongside measurements of ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), and total organic carbon (TOC) concentrations and their corresponding fluxes across different flow pathways. At a rainfall intensity of 30 mm·h−1, subsurface runoff preceded surface runoff generation, contributing 55.6% to 79.3% of the total runoff within the initial 60 min. When rainfall intensity increased to 60 and 90 mm·h−1, surface runoff became the dominant pathway across all slope treatments, with the sole exception of the 5° slope. Elevated rainfall intensity substantially increased surface runoff volumes, whereas subsurface runoff exhibited only marginal changes. The influence of slope gradient on runoff pathways was non-monotonic; notably, at moderate to high rainfall intensities, the 5° slope maintained a comparatively higher proportion of subsurface runoff. Ammonium nitrogen was primarily transported via surface runoff, especially under rainfall intensities of 60 and 90 mm·h−1. Nitrate nitrogen was predominantly mobilized through subsurface runoff at 30 mm·h−1, but as rainfall intensity increased, surface runoff gradually assumed dominance. The concentration differences in total organic carbon between surface runoff and subsurface runoff were generally minor; nevertheless, TOC flux was primarily governed by surface runoff. Correlation analyses revealed that ammonium nitrogen export was most closely associated with surface runoff volume, nitrate nitrogen export was jointly influenced by both surface runoff and subsurface runoff, and total organic carbon export exhibited the strongest relationship with total runoff and surface runoff volume. Collectively, these findings demonstrate that rainfall intensity modulates the partitioning between surface and subsurface runoff, thereby profoundly influencing both the pathways and magnitudes of carbon and nitrogen export from grass-covered black soil slopes.