Nitrification activity and putative ammonia-oxidizing archaea in acidic red soils

Purpose Acidic red soils account for 21% of land area in China and contain low ammonia concentration due to ionization to ammonium. The unusual high affinity for ammonia of marine Nitrosopumilus maritimus and acidophilic soil Nitrosotalea devanaterra has suggested that ammonia-oxidizing archaea (AOA) may have greater selective advantage over ammonia-oxidizing bacteria (AOB) in ammonia-limited environment because ammonia rather than ammonium is thought to be the actual substrate for oxidation. The aim of this study was to assess whether nitrification activity can be attributed to AOA and/or AOB by relating community structures of AOA and AOB to nitrification activity in acidic red soils in southern China. Materials and methods In this study, the composition and abundance of AOA community were investigated in acidic red soils of coniferous Pinus forest, broad-leaf Cinnamomum forest, bush forest (BF), and a 30-year agricultural field converted from bush forest (BFA). The composition of AOA based on archaeal amoA genes were analyzed by denaturant gradient gel electrophoresis, and the abundances of AOA communities were deter-mined by real-time quantitative polymerase chain reaction, while soil nitrification activity was measured using N-15 pool enrichment technique. Results and discussion N-15 pool enrichment technique indicated nitrification activity in acidic red soils, but AOB were not detected. The absence of AOB in acidic red soils could be well explained by the low ammonia concentration ranging from 17.8 to 34.3 nM, which is far below the known threshold values required to support the growth of AOB in culture. Nitrification activity change coupled well with abundance and composition changes of archaeal amoA genes, particularly for acidic BF and BFA soils. Phylogenetic analysis demonstrated that the putatively active AOA were related to amoA transcripts in a hot spring within the soil Crenarchaeota group 1.1b lineage. Conclusions These results suggest that AOA play important roles in ammonia oxidation in acidic red soils tested in this study.