[Objective] Soil acidification intensifies extensively in cropland due to anthropogenic activities, thus restraining sustainable development of agriculture on the soil and affecting environmental safety. Long-term excessive application of ammonium nitrogen fertilizer results in strong nitrification, and intensive leaching of nitrate accompanied with base cations, which are the main mechanisms of soil acidification in cropland. However, so far little has been reported in the literature on effective methods to exert source control over soil acidification triggered by nitrogen fertilizer application. It has been reported that amendment of readily utilizable carbon sources, such as glucose and sucrose, can promote microbial assimilation of soil inorganic nitrogen and reduce N mineralization and nitrification in forest soils. Based on the relationship of nitrogen transformation and proton flux, this type of organic-carbon-promoted microbial assimilation of inorganic nitrogen could also retard N-fertilization-induced soil acidification. Does the amendment of this type of readily utilizable carbon sources also have the effect of controlling or retarding nitrogen transformation and soil acidification in cropland? This question deserves further investigation.[Method] For this study, an in-door incubation experiment was carried out to explore effects of extraneous carbon on N transformation and soil acidification. Glucose was amended as model carbon source at a rate of 8 g·kg^-1, and (NH₄) ₂SO₄ or KNO₃ applied as model nitrogen fertilizer at a rate of 100 mg·kg^-1.[Result] Results show that in the treatments of CK, application of (NH₄) ₂SO₄ and application of KNO₃ only, net mineralization of organic N and nitrification of ammonia N dominated the process of N transformation in the soil, which is attributed mainly to the lack of usable carbon sources in Ultisol. Amendment of adequate glucose as extraneous carbon source promoted rapid microbial assimilation of almost all the NH₄⁺-N and NO₃^--N native in the soil and amended in the form of fertilizer within 2 days. Within 30 days of incubation, N microbial assimilation quickly transformed 28%~50% of fertilizer N into solid organic N. However, application of glucose might trigger the risk of N loss through denitrification. Change in soil pH was closely related to chemical reaction of chemical fertilizer on the surface of the soil and transformation of C and N in the cropland. Application of (NH₄) ₂SO₄ or KNO₃alone lowered soil pH through nitrification and salt effect. Decomposition of glucose generated some mid products like organic acids, such as lactic acid, pyruvic acid, citric acid, etc., thus lowering soil pH. However, these organic acids consumed protons when decarboxylating, thus neutralizing soil acidity. Addition of glucose induced microbial assimilation or fixation of NH₄⁺-N and NO₃^- , thus inhibiting NH₄⁺-N-nitrification-triggered soil acidification, which could raise soil pH by up to 0.78.[Conclusion] Consequently, amendment of extraneous readily utilizable carbon sources, such as glucose, can promote microbial assimilation of NH₄⁺-N and NO₃^--N in the soil and mitigate the soil acidification induced by N transformation in the cropland. All the findings in this study are of important significance for building up soil N retention capacity, improving N fertilizer use efficiency, and inhibiting soil acidification in croplands of acid soil.