Plastic film mulching and nitrogen (N) fertilization are two important field management practices used to increase crop yields in rain-fed agriculture in arid and semi-arid areas. These two practices, however, can have opposing effects on soil organic carbon (SOC) stock and turnover. To clarify and analyze the combined effects of these practices, we conducted a 7-year continuous maize cultivation experiment with four treatments: (1) no plastic film mulching without N fertilization (Control), (2) plastic film mulching without N fertilization (PFM), (3) N fertilization without plastic film mulching (N), and (4) plastic film mulching with N fertilization (PFM + N). The ¹³C natural abundance of in the light and heavy fractions of organic carbon (LFOC and HFOC) was used to differentiate “old” (>7 years) and “new” (<7 years) C in the soil after C₃ to C₄ vegetation change. PFM increased soil temperature and moisture content over 7 years and led to a 150 % and 108 % increase in the above-ground and root biomass of maize, respectively. Hot and wet conditions under PFM accelerated the decomposition of both labile C (i.e., LFOC) and persistent C (i.e., HFOC), as measured by CO₂ efflux from the soil. PFM decreased the LFOC and HFOC pools due to the fast decomposition of “old” C and inadequate stabilization of “new” C. Furthermore, PFM accelerated HFOC decomposition to a greater extent than that of LFOC. The Q₁₀ values of SOC decomposition remained similar both in the presence and absence of PFM. Nitrogen fertilization increased the aboveground and root biomass of maize by 54 % and 40 %, respectively. In the control soil, microorganisms decomposed organic N owing to limited availability of mineral N and in the absence of any from replenishment by fertilizer N. Thus, the increase in LFOC content under N fertilization, was primarily attributed to the reduced decomposition of “old” C. The use of N fertilization produced a similar HFOC content as that of the control soil, while also accelerating the decomposition of “old” C and promoting the stabilization of “new” C, resulting in a faster turnover rate. Consequently, the impact of N fertilization on SOC turnover was negligible owing to the contrasting effects on LFOC and HFOC. The application of N fertilization resulted in a reduction of the Q₁₀ value as compared to control. The combination of PFM and N fertilization accelerated the decomposition of HFOC and SOC, while having absent effect on SOC, labile C and persistent C contents. This indicates that 7 years weren’t long enough to trigger a response to the combination of PFM and N fertilization from SOC and HFOC contents. In summary, PFM led to a decline in the SOC content by accelerating the decomposition of both LFOC and HFOC. Conversely, N fertilization maintained SOC content by inhibiting LFOC decomposition and enhancing HFOC decomposition.