Afforestation can make a substantial contribution to soil characteristics and nutrient cycling. This study aims to determine the effects of two tree species (Robinia pseudoacacia L. and Pinus nigra Arnold.) on some soil characteristics and nitrogen mineralization four decades after the afforestation. Soil sampling was carried out in four seasons of a year at 0-10 and 10-20 cm soil depths. Soil samples were aerobically incubated under both field and laboratory conditions for 30 days to determine net nitrogen mineralization rate. Four decades after the afforestation, soil organic C and total N concentrations were considerably higher in planted areas in comparison to open field. Both species significantly increased net N mineralization under laboratory conditions whereas only black locust had a higher rate of mineralization in the field. Furthermore, nitrate was the dominant form of the mineral N, accounting for 60 to 100 percent of the mineral nitrogen for all types of cover. These results indicate that forest cover, especially the one with black locust, has a pronounced effect on soil C and N storage and cycling.