Responses of nitrification and ammonia oxidizers to a range of background and adjusted pH in purple soils

Soil pH is often changed by anthropogenic activities such as agronomic management, land use or acidifying pollution, and is widely considered to be a dominant factor affecting soil nitrogen cycling. In this study, three purple soils originating from similar parent materials but varying in pH (5.7, 7.3, and 8.0), representing acidic, neutral and alkaline soils, were selected to determine the effect of background pH on net nitrification rate (NNR) and ammonia oxidizers (AOA and AOB). The background pH of each soil was modified to the pH of the other soils to investigate the effect of adjusted pH on NNR and ammonia oxidizers. Net nitrification rates varied significantly with adjusted pH in neutral soils but did not change in acidic and alkaline soils, suggesting that soil at neutral pH is more sensitive to changes in nitrification. The AOB abundance increased in neutral soils adjusted to high pH, whereas AOA decreased with increased pH in acidic and neutral soils, which indicated that the activity and abundance of AOA and AOB is the more important factor affecting nitrification in neutral soils. The ratios of AOA to AOB in the unmodified acidic, neutral and alkaline soils were 120, 1.55 and 0.07, respectively. The highest AOA and AOB abundances occurred in unmodified pH neutral soil. However, the highest NNR was found in alkaline soils (7.04 mg N kg(-1) dry soil d(-1)), which was significantly higher than that in neutral and acidic soils (2.31 and - 0.23 mg N kg(-1) dry soil d(-1), respectively). These results indicate that substrate competition between AOA and AOB exists in neutral soils, which can provide insight and improve our understanding of microbial regulation of N cycling in terrestrial ecosystems.