【Objective】This study aims to investigate the impacts of varying nitrogen (N) addition levels on the decomposition dynamics of fine roots in poplar plantations and to elucidate the key regulatory factors influencing this process. The findings are expected to provide critical insights into the role of N deposition in ecosystem carbon (C) cycling and nutrient turnover.【Method】Fine roots of Populus deltoides were collected in 2018 from experimental plots subjected to five different N addition treatments: N0 (0 g/(m²·a)), N1 (5 g/(m²·a)), N2 (10 g/(m²·a)), N3 (15 g/(m²·a)), and N4 (30 g/(m²·a)). After measuring their initial chemical properties, all root samples were buried in the control (N0) plot for decomposition experiments. Samples were collected at seven time points over a three-year period to track residual mass and assess the decomposition patterns and influencing factors associated with different N treatments.【Result】Initial chemical properties of poplar fine roots differed significantly among different N treatments. N concentration in the roots increased with higher N input, while the C/N declined. Decomposition rates decreased with increasing N addition. Although root mass continuously declined over three years, higher N levels consistently resulted in greater residual mass at each time point, indicating that N addition inhibited fine root decomposition. Initial C/N and N concentrations were the primary factors influencing decomposition rate. A higher C/N significantly increased the decomposition rate (R² = 0.86, P < 0.001), while higher N concentration significantly decreased it (R² = 0.80, P < 0.001), suggesting that the initial chemical composition of fine roots strongly regulates their decomposition.【Conclusion】In summary, this study demonstrates that nitrogen addition inhibits fine root decomposition by altering root initial chemistry-specifically by increasing N concentration and reducing the C/N which may in turn affect soil organic carbon accumulation in forest ecosystems.