Contents of fractions of organic nitrogen ( N ) in the soil profiles of fields that had been under different land uses (paddy field, upland and forestland) for 16 years were determined with the Bremner method. Results show that non-acidolysable N was the dominant fraction of organic nitrogen in all the three soils (0～60 cm in depth), and the contents of acidolysable total N and acidolysable soil organic nitrogen fractions and their proportions to total N were, on the whole, the highest in surface soil (0 ~ 20 cm）. The content of acidolysable total N in 0 ~ 60 cm soil layer and contents of various acidolysable organic N fractions in 0 ~ 40 cm soil layer decreased with soil depth, but the proportions of acidolysable total N and various acidolysable organic N fractions to total N in 0 ~ 60 cm soil layer did not show any obvious regulations. Within the same soil depth, the paddy soil and the upland soil showed a similar regularity in contents of acidolysable total N and acidolysable soil organic nitrogen fractions and their proportions to total N, that is, unknown N > ammonia N > amino acid N > amino sugar N, while the forest soil, did not show any obvious regularity in this aspect. Compared with paddy soil, upland soil and forest soil were more likely to increase the contents and their proportions to total N of acidolysable total N, acidolysable ammonia N, amino acid N and amino sugar N in the surface soil layer. In upland soil, the increase in acidolysable ammonia N was more significant, while in forest soil, the increase in acidolysable amino acid N and acidolysable amino sugar N was As a whole, when paddy field was turned into forest land, the increase in eadily decomposable acidolysable N was the most significant, thus indicating that its potential of increasing soil N supply is the highest.