Abstract: Nitrification processes are highly sensitive to soil pH but the mechanisms through which pH affects soil nitrification are not fully understood. In the present study, three types of purple soils with different pH values were identified from the same parent material in southwest China (acid purple soil pH=5.7, neutral purple soil pH=7.3, calcareous purple soil pH=8.0). These soil types were used to investigate the long-term effects of soil pH on nitrification and ammonia oxidizers and the short-term effects of soil pH on nitrification and the activities and abundance of ammonia oxidizers. Tests were undertaken by artificially adding acid (H₂SO₄) or alkali (NaOH) to change soil pH. The results showed that short-term pH changes had no significant effect (P>0.05) on nitrification in acidic and calcareous purple soils whereas they had a significant effect (P<0.05) on nitrification in the neutral purple soil. The gene amoA of ammonia-oxidizing archaea (AOA) was expressed much more fully in the acid soil (3.04×10⁸/g dry soil, AOA/AOB=31.8), whereas ammonia-oxidizing bacteria (AOB) were more adapted to the calcareous purple soil environment (2.35×10⁸/g dry soil, AOA/AOB=0.07). These results indicate that soil pH can significantly affect nitrification in purple soils and that different pH levels have different mechanisms. The activities and community of nitrifying microorganisms were the dominant factors for nitrification in acidic and calcareous purple soils, while substrate concentration was the dominant factor in neutral purple soil. The key factor affecting the activity and community of nitrifying microorganisms is the long-term stable soil pH, while short-term pH changes mainly influence the substrate availability of nitrification.