Nitrogen enters the soil surface along with the deposition and destroys the element balance of an ecosystem, which has an important impact on underground ecological processes. As active interfaces with the environment, fine roots play a key role in the processes of underground ecosystems and nutrient cycles. Nitrogen in deposition is mainly in two forms, namely organic nitrogen and inorganic nitrogen, which may have different responses to the ecological balance of fine roots and the soil environment; however, in Picea schrenkiana Fisch. et Mey., as a dominant species in the Tianshan Mountains of Xinjiang, it is not clear how different proportions of nitrogen deposition affect the element balance and interactions between fine roots and soil. In this study, from May 2018 to October 2020, five groups of in situ control experiments with different proportions of exogenous nitrogen addition (different ratios of ON–IN, CK = 0:0, N1 = 10:0, N2 = 7:3, N3 = 5:5, N4 = 3:7, and N5 = 0:10, were mixed and then used with equal total amounts of 10 kg·N·ha−1·a−1) were conducted on Picea schrenkiana. The results showed that inorganic nitrogen had a stronger effect on the carbon, nitrogen, and phosphorus contents of fine roots under different proportions of exogenous nitrogen addition, indicating that the fine roots of Picea schrenkiana had a greater response to inorganic nitrogen sources. In a mixed organic–inorganic nitrogen source with the same proportion of organic and inorganic nitrogen, the reaction between fine-root nitrogen (TN = 7.6 g·kg−1−10.8 g·kg −1) and soil phosphorus (TP = 0.99 g·kg−1−1.93 g·kg−1) was stronger, indicating that the Picea schrenkiana ecosystem may be a nitrogen-limited forest ecosystem. In addition, different proportions of nitrogen source inputs have an indirect impact on the fine-root stoichiometry and biomass of different root sequences through the impact on soil environmental factors and stoichiometry. Therefore, our research provides insights into the impact of increases in nitrogen on the nutrient cycling of mountain forests in arid areas and provides small-scale support for a research database of forest ecosystem responses to nitrogen deposition. © 2023 by the authors.