Ammonia oxidizing archaea and bacteria respond dynamically to drought in rewetted fen peatlands

The impact of drought on ammonia oxidizing microbes (AOM) in peatlands remains unclear, despite their role in facilitating a rate-limiting step in nitrification and the increasing prevalence of drought. This study aims to identify trends in ammonia oxidizing archaea (AOA) and bacteria (AOB) responses to summer drought in a rewetted percolation (PW) and coastal fen (CW) in northeastern Germany. AOA and AOB abundances were evaluated using 16S rRNA amplicon sequences and SSU RNA metatranscriptomes. We further quantified amoA gene copies with qPCR and transcript copies with reverse-transcription (RT-)qPCR from in situ peat soil sampled bi-monthly between April 2018 and February 2019. The magnitude of AOM drought response was unexpectedly more pronounced in PW than CW, despite PW exhibiting higher hydrological stability. RT-qPCR showed that PW had an increase in bacterial and archaeal amoA transcript abundances during drought. Additionally, there was evidence in the PW mRNA metatranscriptome for shifts in soil nitrogen cycling, first from a decrease in nitrogen fixation after drought onset, then due to a late-drought increase in assimilatory nitrate reduction to ammonium. In contrast, CW had no significant shifts in RT-qPCR amoA transcripts or the nitrogen cycling functional metatranscriptome during the drought. These results suggest that AOM react significantly to drought, responding to changes in soil hydrology and amplifying shifts in nitrogen cycling gene transcription. More frequent droughts will increase the role of AOM in rewetted fens; however, this response may vary between peatland types based on whether the ecosystem is frequently exposed to hydrological changes (such as in coastal fens) or is accustomed to hydrological stability (percolation fens). • Transcripts of bacterial and archaeal amoA increased during summer drought. • Ammonia oxidizers were more abundant in the percolation fen than in the coastal fen. • Ammonia oxidizing bacteria outnumbered ammonia oxidizing archaea in both fens. • Nitrogen cycling microbiomes were more impacted by drought in the percolation fen. • Hydrologically stable fens are more vulnerable to drought impact.