Study on Soil Fertility Characteristics of Walnut Orchards with Different Parent Materials and Soil Types in Gyaca County, Tibet

Walnut trees on the Tibetan Plateau have sustainable and nutritious characteristics. They grow naturally without the use of any chemical fertilizers and pesticides. Therefore, the soil fertility of walnut orchards is a key factor influencing walnut yield and quality. This experiment measured and analyzed the basic soil nutrient content, microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and leaf nutrient indicators in five types of representative walnut orchard soil parent materials in Gyaca County to clarify the fertility characteristics of different soil types and their correlations with walnut nutrients, providing a basis for the sustainable development of the plateau walnut industry. The results showed that there were significant differences in soil organic matter (SOM), total nitrogen (TN), available nutrient contents, and microbial activity among soil types with different parent materials. The comprehensive representation of available nutrient levels in the soils is as follows: gray-cinnamon soil and calcareous alluvial soil > mountain meadow soil > plateau prairie soil and brush prairie soil. The average MBC, MBN, and MBP contents in the five soils were 368.8 mg/kg, 28.4 mg/kg, and 23.6 mg/kg, respectively, and the content in the topsoil (0-20 cm) was higher than that of the subsoil (20-40 cm). The total nitrogen (PN), total phosphorus (PP), and total potassium (PK) contents in walnut leaves were found to be closely related to the soil physicochemical properties and microbial activity, among which five indicators, including soil TN, available nitrogen (AN), SOM, MBC, and slowly available potassium (SAK), had the greatest impact on leaf nutrient levels. These findings are valuable for the future management and development of walnut orchards. Therefore, for different soil parent materials and soil types, improving the soil fertility and actively applying nitrogen-rich organic fertilizers should be prioritized to improve the yield and quality of Tibetan Plateau walnuts.