Biochar has been attracting increasing attention for its potentials of C sequestration and soil amendment. This study aimed to understand the effects of combining biochar with additional external N on soil C mineralization. A typical red soil (Plinthudults) was treated with two biochars made from two types of plantation-tree trunks (soil-biochar treatments), and was also treated with external N (soil-biochar-N treatments). All treatments were incubated for 42 d. The CO2-C released from the treatments was detected periodically. After the incubation, soil properties such as pH, microbial biomass C (MBC), and microbial biomass N (MBN) were measured. The addition of biochar with external N increased the soil pH (4.31-4.33) compared to the soil treated with external N only (4.21). This was not observed in the comparison of soil-biochar treatments (4.75-4.80) to soil only (4.74). Biochar additions (whether or not they were associated with external N) increased soil MBC and MBN, but decreased CO2-C value per unit total C (added biochar C + soil C) according to the model fitting. The total CO2-C released in soil-biochar treatments were enhanced compared to soil only (i.e., 3.15 vs. 2.57 mg and 3.23 vs. 2.45 mg), which was attributed to the labile C fractions in the biochars and through soil microorganism enhancement. However, there were few changes in soil C mineralization in soil-biochar-N treatments. Additionally, the potentially available C per unit total C in soil-biochar-N treatments was lower than that observed in the soil-biochar treatments. Therefore, we believe in the short term, that C mineralization in the soil can be enhanced by biochar addition, but not by adding external N concomitantly.