The effects of combined application of biochar (a new type of soil conditioner) and nitrogen fertilizers on soil nutrient conditioning were investigated. The incubation experiment based on stoichiometric method was carried out to analyze the additive effects of the combined application of corn straw biochar (SBC), chicken manure biochar (MBC), and nitrogen fertilizer (urea). Results show that biochar improved the contents of soil nutrients, but its conditioning effect was dose-dependent and heterogeneous among species. The nutrient content of the soil increased in parallel with the dosage of biochar. Treatment with SBC increased the contents of soil organic carbon (SOC) and total phosphorous (TP) by 101.754% and 34.592% respectively, while treatment with MBC increased the contents of SOC and TP by 23.684% and 84.396% respectively. SBC increased C/N (ratio of SOC to total nitrogen (TN)) and C/P (ratio of SOC to TP) by promoting SOC, while the MBC treatment decreased C/P and N/P (ratio of TN to TP) by the dose-diluting effect of phosphorus. There was not a single additive effect between the biochar and nitrogen fertilizer, and the interactive effect of nitrogen fertilizer and biochar should be fully considered in actual application.