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The integrated effects of environmental factors on soil nitrification are largely unknown. Here, we performed a microcosm experiment to investigate the interactive effects of pH and NH 4 + on nitrification activity in two acidic soils with different land use patterns (Anhui soil, a forest soil; Jiangxi soil, a brush land soil). Both soils were incubated under native pH and CaCO 3 -manipulated pH in the presence or absence of added ammonium for 60 days. The addition of CaCO 3 alone did not change the nitrification activity of either soil. Ammonium addition stimulated nitrification in Anhui soil, but not in Jiangxi soil, and this stimulation was more pronounced with increased CaCO 3 . The ammonia monooxygenase ( amoA ) gene copy number of both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) was much higher in Anhui soil than in Jiangxi soil. The amoA gene copy number of AOB in Anhui soil was more highly upregulated under CaCO 3 + NH 4 + than NH 4 + during incubation. In Anhui soil, changes in the denaturing gradient gel electrophoresis (DGGE) fingerprint patterns of bacterial amoA genes were parallel to changes in the amoA gene copy number of AOB. In Jiangxi soil, DGGE could not be performed because the PCR for bacterial DGGE did not yield any products, while quantitative PCR revealed that the amoA gene copy number of AOB changed during incubation. These results suggest that AOB plays an important role in CaCO 3 -enhanced nitrification of Anhui soil with ammonium addition. The low nitrification rates of Jiangxi soil regardless of CaCO 3 with or without NH 4 + supply may be ascribed to the lower activity of both AOB and AOA, especially AOA. [ABSTRACT FROM AUTHOR]