Soil organic carbon loss from forest road prisms in a mountainous catchment

Forest roads exacerbate water and soil loss in mountainous catchments, significantly impacting forest carbon cycling and ecological functioning. However, limited research has examined the effect of forest roads on carbon loss in these environments. In this study, we investigated soil organic carbon (SOC) loss across different erosion units (upper hillslopes, road surfaces, and road-stream slopes) within road prisms in the Xiangchagou catchment, China. We also identified the significant factors influencing SOC loss and examined their interactions. Our findings revealed that the average SOC concentrations of different erosion units in road prisms were lowest in road surfaces, where SOC losses were significantly higher than on upper hillslopes. Among different flow paths, gullies exhibited the lowest average SOC concentration, but no significant differences were observed between flow path SOC concentrations at different hillslope positions. This suggests that SOC loss is most pronounced on road surface and gullies within road prisms. In contrast, SOC loss varied significantly with hillslope positions in both partially gullied and diffused flow paths, indicating that gully development plays a critical role in determining SOC loss on road-stream slopes. Piecewise structural equation modeling showed that upper hillslope area, road surface slope, flow paths width-to-depth ratio, and roughness were significant factors influencing SOC loss within the road prism (p < 0.05). Additionally, upper hillslope characteristics indirectly influenced SOC loss on road surfaces and flow paths, while road surface characteristics indirectly influenced the SOC loss along flow paths. These findings suggest that SOC loss induced by forest roads is driven not only by road surfaces but also by interactions among different erosion units. To mitigate SOC loss in mountainous catchments, priority should be given to managing road prisms with extensive upper hillslopes and well-developed gullies.