pH selects for distinct N₂O-reducing microbiomes in tropical soil microcosms

Abstract

Nitrous oxide (N2O), a greenhouse gas with ozone destruction potential, is mitigated by the microbial reduction to dinitrogen catalyzed by N2O reductase (NosZ). Bacteria with NosZ activity have been studied at circumneutral pH but the microbiology of low pH N2O reduction has remained elusive. Acidic (pH<5) tropical forest soils were collected in the Luquillo Experimental Forest in Puerto Rico, and microcosms maintained with low (0.02mM) and high (2mM) N2O assessed N2O reduction at pH 4.5 and 7.3. All microcosms consumed N2O, but long lag times of up to 7 months were observed in microcosms with 2 mM N2O. Comparative metagenome analysis revealed thatRhodocyclaceaedominated in circumneutral microcosms under both N2O feeding regimes. In acidic microcosms,Peptococcaceaedominated in high-N2O, andHyphomicrobiaceaein low-N2O microcosms. Seventeen metagenome-assembled genomes (MAGs) recovered from these microcosms harborednosoperons, with all eight MAGs derived from acidic microcosms carrying the clade II typenosZ, lacking nitrite reductase genes (nirS,nirK). Five of these MAGs represented novel taxa indicating an unexplored N2O-reducing diversity exists in acidic tropical soils. A survey of pH 3.5-5.7 soil metagenome datasets revealed thatnosZgenes commonly occur, suggesting broad distribution of N2O reduction potential in acidic soils.