Aims <p>Mixed forests can generate interspecific complementarity through the regulation of root functional traits, thereby mitigating nitrogen (N) stress. However, the mechanisms governing the coexistence of Cunninghamia lanceolata and Schima superba under conditions of N deficiency and heterogeneous soil NH₄⁺-N/NO₃⁻-N distribution remain poorly understood.</p> Methods <p>To address this, one-year-old seedlings were cultivated in a pot experiment utilizing nutrient solutions with two N levels (deficiency vs. normal) and five NH₄⁺-N:NO₃⁻-N ratios (10:0, 8:2, 5:5, 2:8, and 0:10) under both monoculture and intercropping patterns. Root traits and seedling growth were quantified after 180 days.</p> Results <p>Under N deficiency, intercropped seedlings of C. lanceolata and S. superba exhibited increased absorptive traits (root length, surface area, volume, specific root length) and reduced conservative traits (tissue density, diameter) compared to normal N conditions. Notably, intercropping drove trait divergence: C. lanceolata adopted a "rapid absorption strategy," whereas S. superba employed a "conservative strategy" to adapt to N stress and varying N forms. Both species demonstrated improved growth and adaptation in intercropping systems. Specifically, C. lanceolata performed best at high NH₄⁺-N ratios, while S. superba thrived at high NO₃⁻-N ratios.</p> Conclusions <p>C. lanceolata and S. superba exhibit distinct, complementary root morphological strategies in response to N stress. These strategies are constrained by intraspecific competition in monocultures but enhanced by interspecific interactions in intercropping. Consequently, these root morphological adaptations facilitate species coexistence and enhance stand productivity under N-limited conditions.</p>