Nitrification driven by bacteria and not archaea in nitrogen-rich grassland soils

The oxidation of ammonia to nitrate, nitrification, is a key process in the nitrogen cycle. Ammonia-oxidizing archaea are present in large numbers in the ocean and soils, suggesting a potential role for archaea, in addition to bacteria, in the global nitrogen cycle. However, the importance of archaea to nitrification in agricultural soils is not well understood. Here, we examine the contribution of ammonia-oxidizing archaea and bacteria to nitrification in six grassland soils in New Zealand using a quantitative polymerase chain reaction. We show that although ammonia-oxidizing archaea are present in large numbers in these soils, neither their abundance nor their activity increased with the application of an ammonia substrate, suggesting that their abundance was not related to the rate of nitrification. In contrast, the number of ammonia-oxidizing bacteria increased 3.2-10.4-fold and their activity increased 177-fold, in response to ammonia additions. Indeed, we find a significant relationship between the abundance of ammonia-oxidizing bacteria and the rate of nitrification. We suggest that nitrification is driven by bacteria rather than archaea in these nitrogen-rich grassland soils.