Slash-and-burn tropical forests had been cited as the main cause that contributes to the global warming, specifically because of the large amount of CO₂ that is emitted to the atmosphere. In the Brazilian Amazon, this practice occurs in a large proportion and it has been done to give place for grasslands and agriculture, mainly soybean and maize rotation.  Although, these changes on land uses may result on lose in soil C and N stocks. The aim of this study was to quantify and compare total soil C and N stocks and to assess the origin of soil organic matter through δ¹³C and δ¹⁵N isotopic signals, in grassland and agriculture areas with 10 years, and we used native forest areas as a reference. Soil samples were collected in 0-10, 10-20 and 20-30 cm layer, and were analyzed for total soil C and N contents, bulk density and with these values we calculated the total soil C and N stocks. For the same samples, we determined δ ¹³C and δ ¹⁵N natural abundance. The results showed that after 10 years of grassland implementation soil C and N stocks had increased compared with native forest. On the other hand, agriculture had contributed to decrease those attributes. The total soil C stocks for grassland areas was 59 Mg ha^-1, for agriculture was 50 Mg ha^-1 while for the forest areas it was 58 Mg ha^-1, in the 0-30 cm layer. In relation to isotopic signal, it did not showed much differences, but we can note that the higher δ ¹³C value was found for the grassland areas (-24‰) and the lowest was found for the native forest areas (-27‰) indicating that it is occurring an isotopic dilution.