Betaproteobacterial clade II nosZ activated under high N ₂ O concentrations in paddy soil microcosms

Aims: Microbial communities in paddy soils act as potential sinks of nitrous oxide (N ₂ O), a notorious greenhouse gas, but their potential to reduce external N ₂ O is unclear. The direct observation of N ₂ O reduction in submerged field soils is technically difficult. Here, we aimed to identify soil microbial clades that underpin the strong N ₂ O mitigation capacity. Methods and Results: We constructed paddy soil microcosms with external N ₂ O amendment that enabled the simultaneous evaluation of N ₂ O reductase gene ( nosZ ) transcripts and N ₂ O consumption. Although the amount of N ₂ O amended was large, it was mostly consumed after 6–8 days of microcosm incubation. Metatranscriptomic sequencing revealed that betaproteobacterial nosZ , especially those classified as clade II nosZ belonging to the orders Rhodocyclales or Nitrosomonadales , occupied >50% of the nosZ transcripts in three of the five paddy soils used. On the other hand, publicly available shotgun metagenomic sequences of 46 paddy soils were not dominated by betaproteobacterial clade II nosZ sequences, although they were ubiquitous. The same applied to the 16S rRNA sequences of Rhodocyclales or Nitrosomonadales . Conclusions: The results indicated that betaproteobacterial N ₂ O reducers potentially serve as powerful N ₂ O sinks. Betaproteobacteria holding clade II nosZ can be targets of biostimulation, although further studies are required to understand their ecophysiology.