Adoption of crop residue amendments has been increasingly recommended as an effective management practice for mitigating greenhouse gas emissions while enhancing soil fertility, thereby increasing crop production. However, the effect of biochar and straw on nitrous oxide (N₂O) and methane (CH₄) emissions in soils of differing pH remains poorly understood. Three treatments (control (i.e., no amendment), maize straw, and biochar derived from maize straw) were therefore established separately in soils with different pH levels, classified as follows: acidic, neutral, and alkaline. N₂O and CH₄ were investigated using a static chamber–gas chromatography system during 57 days of a mesocosm study. The results showed that cumulative N₂O emissions were significantly higher in acidic soils than in other experimental soils, with the values ranging from 7.48 to 11.3 kg N ha⁻¹, while CH₄ fluxes ranged from 0.060 to 0.089 kg C ha⁻¹, with inconclusive results. However, a weak negative correlation was observed between log N₂O and log NO₃-N in acidic soil with either biochar or straw, while the same parameters with CH₄ showed a moderate negative correlation, suggesting a likelihood that these amendments could mitigate GHGs as a result of the NO₃-N increase in acidic soils. It is also possible, given the alkaline nature of the biochar, that incorporation had a significant buffer effect on soil acidity, effectively increasing soil pH by >0.5 pH units. Our findings suggest that for the rates of application for biochar and straw used in this study, the magnitude of reductions in the emissions of N₂O and CH₄ are dependent in part on initial soil pH.