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A long-term experiment (38 years) was conducted to elucidate the effects of long-term N addition on the net N mineralization in both paddy and upland soils, based on their initial soil N status, with and in connection with soil microbial biomass and N cycling extracellular enzyme activities. Two treatments without N addition CK (No fertilizer) and K (inorganic potassium fertilizer) and two treatments with N addition (inorganic nitrogen fertilizer) and NK (inorganic nitrogen and potassium fertilizer) were placed in incubation for 90 days. Results showed that the total N and soil organic carbon (SOC) contents were higher in the treatments with N application compared to the treatments without N in both paddy and upland soils. The SOC content of paddy soil was increased relative to upland soil by 56.2%, 45.7%, 61.1% and 62.2% without N (CK, K) and with N (N and NK) treatments, respectively. Site-wise, total N concentration in paddy soil was higher by 0.06, 0.10, 0.57 and 0.60 times under the CK, K, N and NK treatments, respectively, compared with upland soil. In paddy soil, soil microbial biomass nitrogen (SMBN) was higher by 39.6%, 2.77%, 29.5% and 31.4%, and microbial biomass carbon (SMBC) was higher by 11.8%, 11.9%, 10.1% and 12.3%, respectively, in CK, K, N and NK treatment, compared with upland soil. Overall, compared to upland soil, the activities of leucine-aminopeptidase (LAP) were increased by 31%, 18%, 20% and 11%, and those of N-acetyl-b-D-glucosaminidase (NAG) were increased by 70%, 21%, 13% and 18% by CK, K, N and NK treatments, respectively, in paddy soil. A significantly linear increase was found in the NO₃⁻-N and NH₄⁺-N concentrations during the 90 days of the incubation period in both soils. NK treatment showed the highest N mineralization potential (N_o) along with mineralization rate constant, <i>k</i> (NMR) at the end of the incubation. SMBC, SMBN, enzyme activities, NO₃⁻-N and NH₄⁺-N concentrations and the N_o showed a highly significant (<i>p</i> ≤ 0.05) positive correlation. We concluded that long-term N addition accelerated the net mineralization by increasing soil microbial activities under both soils.