Contrasting denitrifier communities relate to contrasting N2O emission patterns from acidic peat soils in arctic tundra

Cryoturbated peat circles (that is, bare surface soil mixed by frost action; pH 3–4) in the Russian discontinuous permafrost tundra are nitrate-rich ‘hotspots'' of nitrous oxide (N₂O) emissions in arctic ecosystems, whereas adjacent unturbated peat areas are not. N₂O was produced and subsequently consumed at pH 4 in unsupplemented anoxic microcosms with cryoturbated but not in those with unturbated peat soil. Nitrate, nitrite and acetylene stimulated net N₂O production of both soils in anoxic microcosms, indicating denitrification as the source of N₂O. Up to 500 and 10 μ nitrate stimulated denitrification in cryoturbated and unturbated peat soils, respectively. Apparent maximal reaction velocities of nitrite-dependent denitrification were 28 and 18 nmol N₂O g_DW⁻¹ h⁻¹, for cryoturbated and unturbated peat soils, respectively. Barcoded amplicon pyrosequencing of narG, nirK/nirS and nosZ (encoding nitrate, nitrite and N₂O reductases, respectively) yielded ≈49 000 quality-filtered sequences with an average sequence length of 444 bp. Up to 19 species-level operational taxonomic units were detected per soil and gene, many of which were distantly related to cultured denitrifiers or environmental sequences. Denitrification-associated gene diversity in cryoturbated and in unturbated peat soils differed. Quantitative PCR (inhibition-corrected per DNA extract) revealed higher copy numbers of narG in cryoturbated than in unturbated peat soil. Copy numbers of nirS were up to 1000 × higher than those of nirK in both soils, and nirS nirK⁻¹ copy number ratios in cryoturbated and unturbated peat soils differed. The collective data indicate that the contrasting N₂O emission patterns of cryoturbated and unturbated peat soils are associated with contrasting denitrifier communities.