Soil nitrogen (N) pools are important sources and sinks of N for ecosystems. A Pinus massoniana plantation in the Three Gorges reservoir area was used as the research object. We analyzed the patterns of responses of soil nutrients and enzyme activities to the addition of N from the perspective of soil aggregates and the corresponding changes in the mineralization of N. These factors were analyzed to provide a reference to predict the dynamics of soil N in the region against the background of a continuous increase in the deposition of atmospheric N. Four levels of the accumulation of N from its addition were established. The amounts added included N₀:0 kg N; N₃₀:30 kg N hm^-2 a^-1; N₆₀:60 kg N hm^-2 a^-1; and N₉₀:90 kg N hm^-2 a^-1. The soil was divided into three component aggregates of >2000 μm (large macroaggregates), 250-2000 μm (small macroaggregates) and <250 μm (microaggregates) by particle size to observe the characteristics of nitrogen mineralization by the aggregates. The results showed the following:(1) Compared with the control, the N₃₀ and N₆₀ treatments increased the levels of soil organic matter (SOM), but the soil SOM and levels of total N began to decrease under the addition of N₉₀. This addition decreased the levels of available phosphorus (P) in the soil, which was most significant in the small macroaggregates. The activities of all three enzymes were increased under N addition treatment except POD in small macroaggregates as well as AP and NAG in microaggregates. (2) The overall average net nitrification rate in the soil was higher than the average net ammonification rate of the soil. The net ammonification rate in large and small macroaggregates was significantly lower after the addition of N, and the net nitrification rate of the macroaggregates was lower than that of the other two sizes of aggregates. The rate of conversion of net N in the soil was highest under the N₉₀ treatment. (3) Soil nutrient and inorganic levels of N significantly correlated with the activities of soil acid phosphatase, N-acetyl-β-D-glucosidase (NAG), peroxidase (POD), nitrate reductase, and urease (UE), and the changes in enzyme activities were the result of a combination of multiple factors. Α redundancy analysis showed that there was a significant positive correlation between UE and net soil ammonification. NAG and POD were the key soil enzymes that significantly positively and negatively correlated with the net conversion of N, respectively. In summary, nitrification is the primary contributor to the net transformation of N in the soil. Microaggregates play a major role in the mineralization of soil N, and NAG and POD are the primary biological enzymes that alter the transformation of net soil N. In addition, the addition of N causes the loss of soil N and limitation of P and significantly affects the cycling of soil nutrients.