Biochar, a solid coproduct from the thermochemical production of bioenergy, has been reported to increase nutrient availability in soils through increased cation retention and decreased phosphate adsorption. The objectives of this study were to determine biochar effects on N and P availability in the presence and absence of external nutrient inputs. Biochar was obtained from hydrothermal pyrolysis of corn cobs at 305°C with 20 min of retention time. Biochar was added to two soils at three biochar rates (0, 2, and 20 g/kg) in combination with either two N rates (0 and 100 mg/kg) or two P rates (0 and 20 mg/kg) and incubated for 56 days. Soils were extracted for Mehlich-3 P and KCl-extractable NO3-N and NH4-N at 0, 3, 10, 17, 28, and 56 days after soil amendment. Biochar application at 20 g/kg increased NH4-N concentrations by 1.1 to 4.8 mg kg−1 during the first 10 days and consistently decreased NO3-N recovery by 5 to 10 mg kg−1 for the duration of the study. Biochar decreased Mehlich-3 P concentrations in soil by 0.9 mg kg−1 in the absence of P additions and increased Mehlich-3 P concentrations by 3.3 mg kg−1 when added with a P source. Furthermore, biochar increased Mehlich-3-extractable P by 5.4 mg kg−1 in the soil when applied with N fertilizer. Results indicate that biochar-amended soils may need additional N after biochar addition to maximize crop production.