A laboratory incubation experiment was established to examine the impacts of nitrate and ammonium nitrogen additions on soil microbial attributes of a subtropical Pinus elliottii forest ecosystem in southern China. Soils were subjected to three different treatments: the control with no nitrogen addition (CK), the ammonium nitrogen addition (-N), and the nitrate nitrogen addition (-N). Samples from bulk and two different size fractions (macroaggregate (>250 µm) and microaggregate (53–250 µm)) were analyzed for soil properties, enzyme activities and microbial communities on day 7 and 15 of the incubation. Our study demonstrated that -N had a greater influence on soil microbial activities than -N. -N additions resulted in significant increases in β-1,4-glucosidase (βG) and β-1,4-N-acetyl glucosaminidase (NAG) enzyme activities in bulk, macroaggregate and microaggregate soils after 7 and 15 days incubation. -N additions only significantly increased in βG and NAG enzyme activities in bulk, macroaggregate soils after 7 and 15 days incubation, but not in microaggregate. All -N and -N additions resulted in significant increases in gram-positive bacterial PLFAs in microaggregates. Only a significant correlation between soil nutrient contents and enzyme activities in macroaggregates was founded, which suggests that the soil aggregation structure played an important role in the determining enzyme activities.