Objective This study aimed to explore the characteristics of soil water dispersible colloids in forest soils during natural vegetation restoration succession in Ziwuling forest region on the Loess Plateau, which provides scientific data for soil quality assessment and ecological restoration in the area.

Method By using the method of space-for-time substitution, forest soils under Quercus liaodongicae, with vegetation restoration time of 90, 110, 130 and 160 years, respectively, were selected as typical representatives of Ziwuling forest region. Soil water dispersible colloids were extracted using centrifugation, and their surface morphology, clay mineral types, and surface functional groups were characterized. The surface electrochemical properties of the water dispersible colloids were analyzed using the combined determination of surface properties. The total nutrient amounts, available nutrient contents, and nutrient stoichiometry of the soil and water dispersible colloids were determined.

Result As the vegetation restoration time increased, the forest soil organic carbon content and cation exchange capacity increased. The quantity and charge density of soil surface charges gradually increased. The total nitrogen, ammonium nitrogen and nitrate nitrogen contents increased. The ratios of C/N, C/P and N/P showed a trend of initial increase followed by decrease. Nutrient stoichiometry analysis indicated that the soils in the Ziwuling forest were nitrogen-limited. The main clay minerals of water-dispersible colloids in forest soils were hydrous mica, chlorite, and kaolinite. Compared with farmland soil colloids, the mass percentages of forest soil colloids were reduced, and their organic carbon stability was higher. The organic carbon, available potassium, available phosphorus and surface charge quantity of water-dispersible colloids accounted for 26.28%, 45.44%, 68.09% and 79.62% of the bulk soil, indicating that available potassium and phosphorus were mainly concentrated in the soil colloid fraction.

Conclusion Vegetation restoration has increased the nitrogen content in the Ziwuling forest soil, but the soil nutrients are still nitrogen-limited. At the same time, it has reduced the quantity of water-dispersible colloids and the amounts of available phosphorus and potassium, which has mitigated the risk of particle-bound nitrogen and phosphorus migration, and is beneficial for improving the quality of downstream water bodies.