Highly abundant acidophilic ammonia-oxidizing archaea causes high rates of nitrification and nitrate leaching in nitrogen-saturated forest soils

In southern China, high levels of atmospheric nitrogen (N) are being deposited in forests. Soil acidification and high rates of soil nitrification and subsequent NO3− leaching have been observed in the N-saturated forests. We previously did not detect NH3-oxidizing bacteria in non-N-saturated or N-saturated forest soils. However, NH3-oxidizing archaea were present in both soils, more in the saturated ones. The purpose of this study was to investigate the roles of autotrophic NH3-oxidizing archaea and heterotrophic nitrifiers in soil N transformations in N-saturated and non-N-saturated forests in southern China. We investigated the contribution of heterotrophic nitrifiers in the soils by determining the gross nitrification rates with and without an inhibitor of autotrophic nitrification, acetylene (C2H2). We also reevaluated nitrification by NH3-oxidizing archaea by correlating the C2H2-inhibited gross nitrification rates with the abundance of the amoA transcripts of NH3-oxidizing archaea. We further measured the gross NH4+ production rates and analyzed the community composition of the NH3-oxidizing archaea. The results suggest that NH3-oxidizing archaea, rather than heterotrophic nitrifiers and NH3-oxidizing bacteria, are responsible for the nitrification in the N-saturated forest soils. NH3-oxidizing archaea in the soils could be acidophilic, having low amoA diversity, indicating their strong adaptation to the highly acidified soils. The gross NH4+ production rate did not differ between N-saturated and non-N-saturated forests; however, the gross nitrification rate was higher in N-saturated forests. Consequently, the high abundance and NH3 oxidation activity of NH3-oxidizing archaea caused the high rates of nitrification and subsequent leaching of NO3− in the N-saturated forest. This study suggests that acidophilic NH3-oxidizing archaea have a great impact on soil N cycling in N-saturated forests.