Abstract: A comparative study was carried out in Mayombe, between the soil of natural forest and the soil under four Terminalia superba plantations of 7, 12, 32 and 48 year old. In each plantation type and natural forest composite soil samples were taken in 0-10, 10-20, 20-30, 30-40 and 40-50 cm layers. The goal was to assess the impact of reforestation on soil organic matter and its biological characteristics. Statistically differences between the sites were tested using the Analysis of Variance (ANOVA). The results showed that there was a general decrease of soil organic matter in the profile of all sites except in the 48 year old plantation for total nitrogen. In the surface soil the carbon content and total nitrogen were respectively, 22.2 mg g-1 and 1.56 μg g-1 in the forest. The carbon content was between 14.9 and 23.5 mg g-1 while total nitrogen was between 1.31 and 2.24 μg g-1 in the plantations. The microbial biomass carbon (Cmic) was 732 μg g-1 soil in the forest, it varied between 461 and 740 μg g-1 in the plantations. The metabolic quotient (qCO2) was between 3.6 mg C-CO2 mg-1 Cmic day-1 and 4.8 mg C-CO2 mg-1 Cmic day-1 in plantations. The qCO2 in forest was 3.6 mg C-CO2 mg-1 Cmic day-1.