This study aimed to compare in situ net nitrogen mineralization and nitrification amounts in different land use types in northern Turkey (Karacakoy-Catalca-Istanbul). The soils in the study area are loamy textured with generally slightly acidic (pH(H2O) between 4.29 -6.02) soil reaction. The vegetation types studied were pasture in abandoned agricultural lands, Cystus and Rubus sp. dominated shrubs at shrub land, and mixture of Quercus petrea, Q. cerris, Q. frainetto and Q. robur in the oak forest which was clear-cut. Thirteen sample plots were used to measure net mineralization and nitrification amounts during a month consisting from 3 recently abandoned agricultural lands (RAAL) (no application for 5 years), 3 old abandoned agricultural lands (OAAL) (no application for 20 years), 3 shrub lands (SL) and 4 oak forest lands (OFL). Sampling took place on April 24th and May 22nd in 2009. The 29 d net nitrogen mineralization amounts showed no differences among RAAL, OAAL and SL (0.223, 2.398 and 3.951 mg NH4+-N kg(-1) for RAAL, OAAL and SL respectively), while OFL site produced more amount of net-nitrogen mineralization (10.976 mg NH4+-N kg(-1)) than the others (Tukey, p<0.05). The nitrification amounts also showed similar results with mineralization such as 0.604, 0.434, 0.499 and 2.602 mg NO3--N kg(-1) for RAAL, OAAL, SL and OFL respectively (Tukey, p<0.05). The initial ammonium-N and nitrate-N concentration in the soils showed that for all land use types nitrate was mobile whereas ammonium was more stable mineral nitrogen form. In this study, the initial ammonium and nitrate amounts had a minor effect on the mineralization and nitrification. Amounts of organic carbon amounts were more important than total carbon and mineral nitrogen concentrations in affecting rates of N transformations. In conclusion, microclimatic conditions and differences in organic carbon are likely related to the differences in nitrogen transformation rates we measured.