Context: Nitrogen (N) deficiency is common in forest soils, and N addition is increasingly applied to the cultivation of plantations. Knowledge of the potential effect of high-efficiency fertilization techniques based on successive field experiments on fine root morphology and growth in trees with respect to nutrient acquisition strategy is insufficient. Aim: We aimed to quantitatively investigate the responses of fine roots in morphology and growth to N addition through surface drip fertigation (DF) over multiple years in a Populus × euramericana “Guariento” plantation. Methods: A field experiment that included four DF treatments with N addition levels (0, 60, 120, and 180 kg N ha-1 yr-1) was conducted for three successive years. A coring method was used to sample soils and quantify root morphological traits, foraging precision, and soil N content along 0-60 cm profiles. Results: The distribution pattern of fine roots along a vertical profile was not affected by the DF treatment. Root biomass, length, surface area, and specific length were significantly higher in N addition treatments than that in the control after the first year, but the positive effect decreased in the second year. Root biomass even showed a 11-39% reduction in the N addition treatments compared with that in the control in the third year. N addition affected fine root growth positively in the early period and negatively in later years. There was a positive correlation between root growth and soil N content in the first two years during which the soil N availability was low, whereas the relationship became negative in the third year when the soil N availability increased greatly. Conclusion: The responses of fine roots in the poplar clone to N addition include a comprehensive adjustment in morphology and growth. The species has a high degree of plasticity of root adaptation to soil N conditions and a tradeoff between energy cost and root production.