Divergent response of ammonia oxidizers to various amino acids

Amino acids (AAs) have an important role on the biogeochemical cycle of N in terrestrial environments. However, the effect of AAs on chemolithotrophic ammonia-oxidizing bacteria (AOB) and archaea (AOA) and nitrification rate has not been elucidated. Supplying soil microcosms with three AAs (asparagine, glutamate, and phenylalanine) with distinct intrinsic properties and equivalent amount of ammonium (NH4+), we investigated their effect on AOB and AOB amoA gene abundances and on nitrification rate. A parallel experiment was also performed in which nitrification was inhibited, using dicyandiamide (DCD), to get insights on amoA-dependent nitrification activity and the mineralization of AAs. The source of N strongly affected nitrification with the highest rate to be observed in NH4+-treated soils followed by asparagine, glutamate, and phenylalanine. Nitrate accumulation was associated with an increase in the abundance of AOB amoA genes suggesting a stronger contribution of AOB in the regulation of nitrification compared to AOA. Differences in N-mineralization rate of AAs appear as the main driver of the divergent nitrification rates is soils treated with various sources of N, which probably was driven by their different properties. Moreover in the case of phenylalanine, inhibitory effects on nitrification were documented. Nitrification was completely inhibited by DCD, indicating amoA-dependent nitrification activity. The present study demonstrates the crucial role of AAs on performance of ammonia oxidizers and nitrification, particularly in neutral or slightly alkaline soils with low organic matter content.