|Palmer, K; Biasi, C; Horn, MA: Contrasting denitrifier communities relate to contrasting N2O emission patterns from acidic peat soils in arctic tundra, ISME Journal, 6, 1058-1077 (2012)|
Cryoturbated peat circles (i.e., bare surface soil mixed by frost action; pH 3-4) in the Russian discontinuous permafrost tundra are nitrate-rich ‘hotspots’ of N2O-emissions in arctic ecosystems, while 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 µM nitrate stimulated denitrification in cryoturbated and unturbated peat soil, respectively. Apparent maximal reaction velocities of nitrite-dependent denitrification were 28 and 18 nmol N2O gDW-1h-1, for cryoturbated and unturbated peat soil, 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 soil differed. Quantitative PCR (inhibition-corrected per DNA extract) revealed higher abundances of narG in cryoturbated than in unturbated peat soil. Copy numbers of nirS were up to 1000x higher than those of nirK in both soils, and nirS nirK-1 copy number ratios in cryo- and unturbated peat soil differed. The collective data indicate that the contrasting N2O emission patterns of cryoturbated and unturbated peat soils are associated with contrasting denitrifier communities.