Background Planted Pinus tabuliformis forest, Caragana korshinskii forest, and Medicago sativa grassland are common types of planted vegetation restoration in the loess hill and gully region, with extensive planting history and adaptability. These three vegetation types demonstrate differences in growth habits, root structure, and biomass accumulation, which may lead to varying impacts on soil aggregate stability and organic carbon content. Studying their effects on soil aggregates and organic carbon content may provide a scientific basis for selecting effective vegetation and enhancing soil erosion resistance in the local area.

Methods Representative planted P. tabuliformis forest, C. korshinskii forest, and M. sativa grasslands were selected as study objects, with bare land set as a control. The study consisted of three treatments, and soil samples were collected at depths of 0 – 20 cm, 20 – 40 cm, and 40 – 60 cm following an "S" shaped layout. The samples underwent pretreatment such as air drying and sieving. The Elliot wet sieving method was used to analyze soil aggregate stability, and physicochemical properties such as organic carbon, total nitrogen, bulk density, moisture content, and pH were measured. Finally, statistical software was used for data analysis and graph plotting.

Results 1) Under different vegetations, the aggregates in each soil layer are mainly micro-aggregates of < 0.250 mm, and the content of ≥ 2.000 mm and < 2.000 – 0.250 mm diameter aggregates under the three vegetation restoration measures is significantly higher than that of the control (P < 0.05), with the highest content of > 0.250 mm aggregates (R_>0.25) under the planted P. tabulaeformis forest. 2) Compared with the control, the R_{> 0.25}, MWD, and GMD of the soil under the three vegetation restorations increased by 44.90% – 121.36%, 63.11% – 171.23%, and 31.21% – 194.95% respectively, and the D value reduced by 0.18% – 3.29%. The planted P. tabulaeformis forest showed the best effect on soil stability improvement. 3) After planted vegetation restoration, the organic carbon content in soil aggregates effectively increased by 3.06% – 53.36%, and the planted P. tabulaeformis forest and planted C. korshinskii forest shrubbery showed a higher increase in organic carbon content in aggregates of each grade compared to planted grassland and bare land. 4) The stability of soil aggregates was significantly correlated with the organic carbon content of aggregates ≥ 0.250 mm (P < 0.05), and there was a very significant correlation with the total organic carbon content of the whole soil (P < 0.01). The stability of aggregates is linearly correlated with the content of each grade, and the positive and negative correlation changes at the 0.250 mm grade. This grade also determined the significance of the relationship between the organic carbon content of aggregates of each grade and stability and can be regarded as the key threshold affecting the water stability of soil aggregation.

Conclusions Research results indicates that the planted P. tabulaeformis forest are more beneficial for enhancing soil aggregate stability and soil erosion resistance, making them suitable for promotion and application in vegetation restoration efforts in this region.