To investigate the decomposition dynamics of maize straw, the transformation of straw-derived organic carbon functional groups, and their effects on soil organic matter and humic acid in fluvo-aquic soil, we conducted an incubation experiment on soils from the North China Plain. Four treatments were established: 1) a soil-to-straw ratio of 200:1 without N fertilizer (L); 2) 200:1 with 450 kg/ hm² of N fertilizer (LN); 3) 50:1 without N fertilizer (H); and 4) 50:1 with 450 kg/ hm² of N fertilizer (HN). The results showed that: 1) The amount of maize straw degraded increased progressively over the incubation period. By day 180, the H and HN treatments showed the highest degradation rate, reaching 76.87%. 2) During straw decomposition, the relative proportions of carbonyl C, aromatic C, and alkyl C increased, whereas the relative proportion of O-alkyl C decreased. 3) Soil organic matter content increased with both straw application rate and incubation duration. The greatest increase occurred under the HN treatment after 180 days, with an increment of 4.34 g/kg. Under the same N level, compared with low straw addition, increasing straw input enhanced soil organic matter content by 13.88% (without N) and 12.19% (with N). The absolute content of humic acid in the fluvo-aquic soil was strongly influenced by straw application rate. Compared with the initial soil, humic acid increased by 3.33% and 5.58% under the HN and H treatments, respectively, whereas the LN and L treatments showed decreases of 12.32% and 13.79%.