Developmental Stage-Dependent Stoichiometric Characteristics of Fine Roots and Their Relationship with Soil Nutrients in Caragana tibetica Shrubs in a Desert Steppe Ecosystem

Fine roots serve as the primary organs for nutrient and water uptake in plants while also representing a critical pathway for the return of soil nutrients. Caragana tibetica is widely recognized as an indicator species marking the transition from grassland to desert. Therefore, investigating the stoichiometric characteristics of its fine roots is of great significance for elucidating plant–soil nutrient cycling in the desert steppe ecosystem. In this study, taking C. tibetica shrubs at different developmental stages as the research objects, the methods of in situ sampling in the field and determination of indoor indicators are adopted to study the stoichiometric characteristics of C, N, and P in fine roots and soils at different developmental stages, as well as the relationship between them. The results indicate significant differences in the stoichiometric characteristics of fine roots across different developmental stages (p < 0.05). The fine root C:N and N:P ratios are significantly higher in the decline stage, and the C:P ratio is significantly higher in the stable stage. Along with shrub development, soil nutrient accumulation capacity is significantly greater in the growth and stable stages, whereas no significant difference is observed in the soil total phosphorus content. The soil organic carbon is the greatest in the growth stage, and the total nitrogen is greatest in the decline stage. In addition, in the incipient stage, the correlation between total phosphorus in the soil and the stoichiometric characteristics of fine roots is relatively strong. In the stable stage, the correlation between soil organic carbon and total nitrogen and the stoichiometric characteristics of fine roots is relatively weak. In the decline stage, the correlation between total nitrogen in fine roots and soil nutrients in the growth stage is the strongest. The results of this study reveal the differences in stoichiometric characteristics between fine roots and soil across different developmental stages of the C. tibetica shrub, highlighting the distinct adaptation strategies of fine roots and soil to environmental changes in each stage.