Excessive amounts of nitrate have accumulated in many soils on the North China Plain due to the large amounts of chemical N fertilizers or manures used in combination with low carbon inputs. We investigated the potential of different carbon substrates added to transform soil nitrate into soil organic N (SON). A 56-d laboratory incubation experiment using the ¹⁵N tracer (K¹⁵NO₃) technique was carried out to elucidate the proportion of SON derived from accumulated soil nitrate following amendment with glucose or maize straw at controlled soil temperature and moisture. The dynamics and isotopic abundance of mineral N (NO^-₃ and NH⁺₄) and SON and greenhouse gas (N₂O and CO₂) emissions during the incubation were investigated. Although carbon amendments markedly stimulated transformation of nitrate to newly formed SON, this was only a substitution effect of the newly formed SON with native SON because SON at the end of the incubation period was not significantly different (P > 0.05) from that in control soil without added C. At the end of the incubation period, amendment with glucose, a readily available C source, increased nitrate immobilization by 2.65 times and total N₂O-N emission by 33.7 times, as compared with maize straw amendment. Moreover, the differences in SON and total N₂O-N emission between the treatments with glucose and maize straw were significant (P < 0.05). However, the total N₂O-N emission in the straw treatment was not significantly (P > 0.05) greater than that in the control. Straw amendment may be a potential option in agricultural practice for transformation of nitrate N to SON and minimization of N₂O emitted as well as restriction of NO₃-N leaching.