A 120-day indoor culture experiment was conducted by collecting soils with different carbon sources combined with nitrogen fertilizer in a field experiment set in Lijiabu Town, Dingxi City, Gansu Province since 2014. The Stanford batch elution culture method was used to study the effects of four fertilization methods, including no carbon and nitrogen addition (N0), only nitrogen application (N100), straw combined with nitrogen (SN100) and biochar combined with nitrogen (BN100), on soil nitrogen mineralization. The fertilization was applied to experimental fields in the dry farmland of the Loess Plateau in central Gansu. The results showed that compared with nitrogen application only, straw and biochar combined with nitrogen fertilizer significantly increased nitrogen mineralization in surface soil by 16.5% and 15.4% respectively. Soil nitrogen mineralization showed a rapid increase and then a rapid decrease, and the decreasing rate gradually leveled off. The nitrification rate and ammonification rate reached their maximum at 7 and 15 days, and stabilized at 45 and 30 days, respectively. The nitrogen mineralization rate of the upper soil was significantly increased by straw and biochar combined with nitrogen fertilizer. The nitrification rate and ammonification rate were in a relatively balanced state. The redundancy analysis between the changes of soil organic nitrogen fractions before and after culture and the amount of soil nitrogen mineralization showed that the largest contribution to soil mineralized nitrogen was amino nitrogen, followed by acid unknown nitrogen. Mineral nitrogen was negatively correlated with unacidolyzed nitrogen and positively correlated with all fractions of acidolyzed organic nitrogen. In addition, compared with no nitrogen application, adding straw and biochar under nitrogen application increased the variation of organic nitrogen fractions in the surface soil. These results indicated that the addition of straw and biochar could increase the content of easily mineralized nitrogen by changing the proportion of organic nitrogen fractions to total nitrogen, thus subsequently promote the mineralization of organic nitrogen fractions and improve the potential of soil nitrogen supply.