Denitrification Activity of a Remarkably Diverse Fen Denitrifier Community in Finnish Lapland Is N-Oxide Limited

Peatlands cover more than 30% of the Finnish land area and impact N(2)O fluxes. Denitrifiers release N(2)O as an intermediate or end product. In situ N(2)O emissions of a near pH neutral pristine fen soil in Finnish Lapland were marginal during gas chamber measurements. However, nitrate and ammonium fertilization significantly stimulated in situ N(2)O emissions. Stimulation with nitrate was stronger than with ammonium. N(2)O was produced and subsequently consumed in gas chambers. In unsupplemented anoxic microcosms, fen soil produced N(2)O only when acetylene was added to block nitrous oxide reductase, suggesting complete denitrification. Nitrate and nitrite stimulated denitrification in fen soil, and maximal reaction velocities (v(max)) of nitrate or nitrite dependent denitrification where 18 and 52 nmol N(2)O h(-1) g(DW) (-1), respectively. N(2)O was below 30% of total produced N gases in fen soil when concentrations of nitrate and nitrite were <500 μM. v(max) for N(2)O consumption was up to 36 nmol N(2)O h(-1) g(DW) (-1). Denitrifier diversity was assessed by analyses of narG, nirK/nirS, and nosZ (encoding nitrate-, nitrite-, and nitrous oxide reductases, respectively) by barcoded amplicon pyrosequencing. Analyses of ~14,000 quality filtered sequences indicated up to 25 species-level operational taxonomic units (OTUs), and up to 359 OTUs at 97% sequence similarity, suggesting diverse denitrifiers. Phylogenetic analyses revealed clusters distantly related to publicly available sequences, suggesting hitherto unknown denitrifiers. Representatives of species-level OTUs were affiliated with sequences of unknown soil bacteria and Actinobacterial, Alpha-, Beta-, Gamma-, and Delta-Proteobacterial sequences. Comparison of the 4 gene markers at 97% similarity indicated a higher diversity of narG than for the other gene markers based on Shannon indices and observed number of OTUs. The collective data indicate (i) a high denitrification and N(2)O consumption potential, and (ii) a highly diverse, nitrate limited denitrifier community associated with potential N(2)O fluxes in a pH-neutral fen soil.