【Objective】 This paper was oriented to explore effects of cultivation, relative to duration, on gross and net N transformation rates in black soil in Northeast China. 【Method】 Two tracts of upland farmlands of typical black soils, different in cultivation history, 2 and 30 years, were selected in Northeast China for the study. Soil samples were collected for in-lab incubation using the ¹⁵N pair-labeled technique. Gross N transformation rates in the soils were calculated using the numerical algorithm model (FLUAZ), and net mineralization rates and net nitrification rates were by duration of the incubation and variation of the samples in inorganic nitrogen content or nitrate nitrogen content with the incubation. 【Result】 The soil of the farmland, 30 years in cultivation, was 20.8% and 16.0% lower, respectively, than the one 2 years in cultivation in organic C and water soluble organic C content, and only 62.6% and 42.1% of the latter, respectively, in gross mineralization and immobilization rate. However, the former was significantly higher than or 1.77, 2.00, and 7.29 times as high as the latter, respectively, in gross nitrification rate, net nitrification rate, and net mineralization rate. The gross nitrification rate was 1.15 and 1.02 times as high as the net nitrification rate, respectively, in the latter and former, and the immobilized ammonium nitrogen accounted for 60%~97% of the total inorganic nitrogen during the whole incubation period, indicating that little nitrate nitrogen was immobilized by soil microorganisms in these two soils. The values of m/i (the ratio of gross N mineralization rate to immobilization rate) and n/ia (the ratio of gross nitrification rate to ammonium immobilization rate) of the former were both significantly higher than 1, while those of the latter were approximate to 1. 【Conclusion】 The processes of mineralization and immobilization of nitrogen in the soil, 2 years in cultivation, were coupled tightly, posing little risk of nitrogen loss to environment, whereas the amount of mineralized nitrogen was much higher than that of immobilized nitrogen in the soil 30 years in cultivation, thus providing substrate for nitrification and increasing denitrification and leaching risk of nitrate.