Factors and thresholds determining sediment delivery pathways between forest road and stream in mountainous watershed

Road erosion is a global environmental issue. The transport of eroded sediment from road to stream can devastatingly impact water quality. Research efforts to combat this issue have prioritized maintaining roads and preventing erosion. Little research has explored the fate of eroded road materials in their transport from road to stream. Therefore, taking the Xiangchagou watershed in the Dabie Mountains of China as a case, this study investigated how local geography determines the formation and thresholds of different delivery pathways (gullied, partially gullied, and diffuse pathways) for road-eroded sediment through runoff, based on the binary logistic regression analysis, random forest model, and the lowest point data fitting methods. It explored how these different pathways have variable impacts on local plant and soil communities. Our results showed that soil D-50, bulk density, shrub-grass coverage, and local plant diversity varied significantly (p < 0.05) between different delivery pathways. In particular, soil bulk density and local plant diversity varied significantly (p < 0.05) among different position of these delivery pathways. Specifically, the diffuse pathways are characterized by lower soil bulk density, looser soil structure, and higher species diversity, likely explained by the deposition of sediment-loaded runoff. Gullied pathways, conversely, had significantly more compacted surface soil and lower local plant diversity, likely as a result of scouring by sediment-loaded runoff. Slope gradient of upslope catchment (Us), road segment (Rs), and lower hillslope (Ds) and effective contributing area (Eca), which determine the intensity and kinetic energy of sediment-loaded runoff, are the main external drivers that determine the occurrence of gullied pathways. By contrast, the soil median particle size, surface roughness, and shrub-grass cover of lower hillslope, which contribute to erosion resistance of hillslope, are the primary intrinsic drivers promoting development of pathways from partial to full gully. Furthermore, we found that the critical topographic thresholds for the initiation of the gully pathways and the continued development of gully from partial to complete gully pathways could be expressed as D-s = 0.35E(ca)(-213); 0.13 and D-s = 0.41E(ca)(-0.17), respectively. These results are expected to provide basis for understanding the formation and development mechanism of sediment delivery pathways from road to stream.