Current study is based on the data collected in January 2010 from field under authorization of Çatalca-Karacaköy the Chief Forest Enterprise located in the northern Istanbul. Net nitrogen mineralization rates were estimated at soils taken from mixed and thicket age (1.30 m Ø ≈ 20 cm) Oak sp. (Quercus frainetto Ten., Q. cerris L., Q. robur L. and Q. petraea (Mattuschka) Lieblein)) forest (OFL), shrub land (dominated with Cistus spp.) (SL), recently abandoned agricultural land (RAAL ≈5 y), old abandoned agricultural land (OAAL ≈20 y) by standard laboratory incubation method. Net N mineralization was by laboratory incubation method whereas soil humidity were suited to %60 in terms of weight and incubated at 21oC. According to result we achieved: at pre-incubation stage the ammoniumN amounts at OAAL, shrub land and forest ecosystems were significantly higher than RAAL lands according to Tukey HSD p<0.05, while at post-incubation three to four fold higher net nitrogen mineralization was recorded according to Dunnett T3 p<0.05. Except RAAL lands, OAAL, SL and OFL were expected to deduce similarity, however, the actual results should be accounted for possibly higher microbial activity.