With the increasing of anthropogenic nitrogen (N) deposition, understanding the underlying mechanisms of external N effect on residue decomposition is critical to improving our prediction of the ecosystem carbon (C) sequestration. An experiment of decay subjected to N addition was conducted by 360 days incubation under laboratory condition, using leaf residues collected from Chinese fir (Cuninghamia lanceolata) (Cl) and eucalyptus (Eucalyptus urophylla) (Eu). Decay of needle leaf (Cl) was slower and more limited by N than broadleaf (Eu). Added N significantly accelerated initial decay, and then the N effect tended to be negative. The switched time point coincided with the C concentration peak, implying C quality might affect the external N impacts. More importantly, our results, together with previous decay studies using multiple residues, illuminated that both the positive and negative N effects became evident as residue quality tended to be low. We supposed that this phenomenon might be attributed to the fact that the specific microbe (fungi, which possessed higher N and C use efficiency than bacteria) gradually exerted a more dominant role in decay with the residue quality varying from high to low, and this shape of microbe would mediate the N impacts. For the isotope signatures, the supplied N caused elevated residue ¹⁵N in the early stage, and the increased magnitude was much greater for the hardly-degradable residue (Cl). These ¹⁵N dynamics patterns revealed enhanced initial microorganism activity by added N, and also probably confirmed the microbe-mediated effect we presumed. Overall, our research delivered very helpful implications for nutrient management and ecosystem restoration under heightened N scenario in the future.