Background and aims <p>Long-term nitrogen (N) enrichment can substantially alter fine-root traits, which are central to predicting plant adaptation under global change. However, the multidimensional nature of fine-root morphology, chemistry, and anatomy—and the coordination among these trait dimensions—remains insufficiently understood. We aimed to assess whether long-term nitrogen addition modifies the multidimensional organization of fine-root traits, with particular emphasis on the role of root anatomical structure in trait coordination.</p> Methods <p>We used a 15-year N addition experiment in a mature temperate broadleaf-Korean pine mixed forest and measured morphological, chemical, and anatomical traits of first- to third-order fine roots across three dominant tree species (<i>Pinus koraiensis</i>, <i>Acer ukurunduense</i>, <i>Abies nephrolepis).</i></p> Results <p>Fine-root traits showed a clear multidimensional structure, including a specific root length (SRL)–diameter axis representing a resource acquisition–conservation spectrum, a root tissue density (RTD)–specific root area (SRA) axis reflecting structural investment, and a cortex-to-stele ratio (CSR) axis representing anatomical independence. N addition decreased SRL but increased RTD, SRA, and root nitrogen concentration (RN), indicating a shift toward greater structural investment under N-enriched soil conditions accompanied by reduced P availability. Anatomical traits were largely independent of morphological and chemical traits and were significantly associated with soil phosphorus and pH. Cortex thickening increased faster than stele development.</p> Conclusions <p>Long-term N addition reshapes both functional and anatomical dimensions of fine roots, altering acquisitive traits. These findings highlight the independence of anatomical traits within fine-root trait spectrum and provide trait-based indicators to support the management and resilience of temperate forests under continuing global nitrogen enrichment.</p>