Steep gully slopes on China's Loess Plateau experience severe soil erosion and nutrient loss via runoff and sediment, due to steep topography and sparse vegetation. However, prior researches on nutrient loss have focused predominantly on sloping farmland, leaving limited understanding of how plant communities regulate such loss on steep gully slopes. To address this, seven micro-runoff plots (six plant communities, one bare land) were established to measure surface runoff, sediment, and associated carbon (C), nitrogen (N), and phosphorus (P) losses, together with environmental factors from June 2021 to October 2021. Results demonstrated bare land experienced the greatest average nutrient loss C, N, and P nutrients via runoff (128.22 mg, 10.05 mg, 0.77 mg) and sediment (5947.86 mg, 244.39 mg, 173.77 mg,).. Grasslands exhibited 18.02 mg, 3.47 mg, and 0.16 mg runoff-induced average C, N, and P losses, respectively, higher compared to shrublands. Sediment-associated average C, N, and P losses from grasslands were 1.17, 1.23, and 1.58 times greater than that from shrublands. Rainfall characteristics, particularly precipitation, the maximum rainfall intensity of 30-min and 60-min, total rainfall kinetic energy, and rainfall erosivity, significantly promoted runoff-mediated nutrient losses (p < 0.05). Vegetation significantly reduced runoff-related total carbon (TC), total nitrogen (TN), and total phosphorus (TP) losses (p < 0.05), with a more pronounced effect on restraining sediment-associated TN and TP losses. Vegetation coverage, litter thickness, and root mass density were key mitigating factors. Soil TC, organic carbon, and TN indirectly suppressed runoff-mediated nutrient losses primarily by enhancing plant growth, while soil TC strongly inhibited sediment nutrient losses. Hippophae rhamnoides Linn. and Bothriochloa ischcemum (Linn.) Keng. demonstrated the highest efficacy in suppressing nutrient loss. This study clarifies nutrient loss characteristics and dominant factors controlling nutrient loss through runoff and sediment, providing scientific guidance for mitigation strategies on steep gully slopes.