To understand maize- and N-induced diel variations in CO2 emission, we examined hourly CO2 emissions during the three typical growth stages of maize in sandy loam soil. There was a distinct diel pattern in soil CO2 emissions, with the peak occurring between 14:00 and 18:00 and the trough occurring between 0:00 and 4:00. Maize presence delayed the time of the peak. The absolute amount and diel fluctuation of CO2 emissions tended to diminish with time in the bare soil fertilized with 150kgNha−1 (BS). In contrast, N-fertilized maize (N150) significantly enhanced the total amount of CO2 emissions and the peak-trough differences in CO2 emissions, which reached a maximum at the pollination stage and then decreased. Control soil (CK) containing maize but no N fertilizer had highest overall CO2 emissions but reduced diel fluctuation because rhizosphere respiration was elevated in the nighttime. Soil temperature accounted for 61–71% of diel variation in the BS treatment but for only 44–59% and 38–58% in the N150 and CK treatments, respectively. Photosynthesis rates affected diel variation at the seedling and pollination stages. Both temperature and photosynthesis rates together explained up to 67–84% of diel variation at the seedling and pollination stages in the N150 treatment, but only 61% at the seedling stage in the CK treatment due to more CO2 released in the nighttime. The increased nighttime CO2 release, in turn, decreased the effect of temperature and even reduced the influence of photosynthesis rate on diel variations in CO2 release. Based on the present results, the best time for obtaining a representative daily CO2 measurement was found to be approximately 8:00 at the seedling stage and 9:00–11:00 at the other growth stages. The current findings indicate that N addition reduces soil CO2 emissions and its diel fluctuation.