In order to find out the change characteristics of soil total nitrogen (TN), organic carbon (SOC) and its components (dissolved organic carbon (DOC), microbial biomass carbon (MBC), light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC)) after four years of carbon and nitrogen addition, local field experiment of nitrogen application with different carbon sources was conducted at the Rainfed Agricultural Experimental Station of Gansu Agricultural University in Lijiabao Town, Anding District, Dingxi City, central Gansu Province. Three factors including straw, biochar and nitrogen were set up in the experiment. The straw was set at two levels, including no straw and straw application, and the experiment involved two biochar levels, which were no biochar and biochar application, and the nitrogen application was at three levels of no application, 50 kg/hm² and 100 kg/hm², totally nine treatments. The results showed that the contents of TN, organic carbon and their components decreased with the deepening of soil layer under different treatments. Application of biochar increased the concentrations of SOC, TN, DOC, MBC, LFOC and HFOC. Application of straw significantly increased the contents of SOC, TN, DOC, MBC and LFOC, but only significantly increased HFOC content in 0-5 cm soil layer. Application of nitrogen significantly increased the concentrations of SOC, TN, DOC, MBC and LFOC. Among the treatments, application of biochar showed the best improve effect on the concentrations of SOC, TN and HFOC, and adding straw represented the best enhancement effects on the concentrations of DOC, MBC and LFOC. From the perspective of improving soil quality, straw combined with nitrogen was the recommended application mode, which had a higher soil carbon availability that facilitated the carbon utilization of the microorganisms, and benefited the growth of crops. Considering the improvement of soil carbon sequestration, biochar combined with nitrogen was the recommended application mode, which was beneficial to the carbon sequestration.