<p>Climate change challenges forestry in temperate regions, with drought and insect infestations affecting major tree species. Introducing drought-tolerant non-native species into managed forests is a widely discussed adaptive strategy. However, the impact of such interventions on forest nitrogen (N) cycling are still unknown. Here, we investigate the influence of species identity and competition on sapling N form preferences and root morphological traits of non-native Douglas fir (<i>Pseudotsuga menziesii (</i>Mirbel<i>) Franco</i>) and native European beech (<i>Fagus sylvatica</i> L.).</p><p>We conducted an in-situ experiment in a close-to-nature beech forest in Germany, growing beech and Douglas fir saplings for two years in bottomless pots under intra- and interspecific competition. Using highly ¹⁵N-enriched ammonium nitrate, we tracked N uptake in plant compartments to compare the species’ inorganic N uptake form preference and allocation.</p><p>Under intraspecific competition, Douglas fir and beech showed higher uptake of ammonium than nitrate. Under interspecific competition, N form uptake was complementary, with beech exhibiting a preference for nitrate. Fine root morphology did not significantly affect N form preference. Roots displayed species-specific traits; Douglas fir’s traits point to a slow resource acquisition strategy (e.g., large fine root diameter) and traits of beech to a fast strategy (e.g., high branching frequency), yet fine root morphology was unaffected by species mixing.</p><p>Our findings suggest that Douglas fir and European beech saplings can coexist without hampering each other’s N nutrition. The study highlights the necessity of field-based investigations to elucidate the complex interactions governing N cycling in mixed-species forests, providing insights for climate change-adapted forest management.</p>