In order to investigate the effect of adding plant residues on total and microbial biomass nitrogen, an incubation experiment was carried out by adding ¹⁵N-labeled maize (Zea mays L.) leaves, stems, or roots to brown earths with different fertility levels. The soil samples were collected from a long-term fertilization experiment site (27 years) that had both low fertility (LF) and high fertility soil (HF). The ¹⁵N stable isotope labeling technique has been used to monitor the accumulation dynamics of maize-derived N in different fertility soils. Our results showed that (1) the addition of roots, leaves, and stems increased the total N content of LF soils by 5.75, 4.77, and 3.75%, respectively, when compared to that of corresponding HF soils. Moreover, the proportion of the total N derived from maize was significantly higher in the LF soils supplemented with roots (3.54%), stems (3.28%), and leaves (2.49%) than in the HF soils. These results suggested that low fertility soil was more sensitive to organic input and had a greater ability to sequester extraneous N. (2) During the 56-day incubation period, the microbial biomass N of the LF soils increased by 0.83-0.98, and that of the HF soils increased by 0.87-1.56 times, which indicated that the addition of maize residues stimulated microbial biomass, especially in HF soils. In addition, there was no significant difference in the microbial biomass of the LF soils supplemented with different maize parts. However, in the HF soils, the addition of stems and leaves had a more pronounced effect on microbial biomass compared than the addition of roots. We conclude that the contribution of exogenous organic N to the total pool of soil N is dependent on both the initial soil fertility and the biochemical composition of the plant residues.