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

Nitrous oxide (N 2 O), a greenhouse gas with ozone destruction potential, is mitigated by the microbial reduction to dinitrogen catalyzed by N 2 O reductase (NosZ). Bacteria with NosZ activity have been studied at circumneutral pH but the microbiology of low pH N 2 O 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) N 2 O assessed N 2 O reduction at pH 4.5 and 7.3. All microcosms consumed N 2 O, but long lag times of up to 7 months were observed in microcosms with 2 mM N 2 O. Comparative metagenome analysis revealed that Rhodocyclaceae dominated in circumneutral microcosms under both N 2 O feeding regimes. In acidic microcosms, Peptococcaceae dominated in high-N 2 O, and Hyphomicrobiaceae in low-N 2 O microcosms. Seventeen metagenome-assembled genomes (MAGs) recovered from these microcosms harbored nos operons, with all eight MAGs derived from acidic microcosms carrying the clade II type nosZ , lacking nitrite reductase genes ( nirS , nirK ). Five of these MAGs represented novel taxa indicating an unexplored N 2 O-reducing diversity exists in acidic tropical soils. A survey of pH 3.5-5.7 soil metagenome datasets revealed that nosZ genes commonly occur, suggesting broad distribution of N 2 O reduction potential in acidic soils.