Denitrification as a potential source of N2O in palsa peat soils

Palsa peats are characterized by elevated, circular frost heaves (peat soil on top of a permanently frozen ice lense), emit the greenhouse gas N2O, and are predicted to react sensitively to global warming. The acidic palsa peat Skalluvaara (app. pH 4.4) is located in the sporadic permafrost zone in northwestern Finnish Lapland (69°49'13"N, 27°9'47"E). Fertilization experiments with nitrate stimulated in situ N2O emissions without apparent delay. N2O was likewise produced in unsupplemented anoxic microcosms with palsa peat soil from 0-30 cm depth (active layer depth), and stimulated by supplemental nitrate and nitrite. Initially produced N2O was subsequently consumed. Maximal reaction velocities (vmax) of nitrate dependent denitrification were 2.75 ± 0.3 and 1.4 ± 0.03 nmol N2O h-1 gDW-1 in soil from 0-20 cm and soil from greater depths, respectively. vmax of nitrite dependent denitrification was 2-5 x higher than of nitrate dependent denitrification. Up to 12 species-level operational taxonomic units (OTUs) were retrieved for structural denitrifier gene markers (narG, nirK/S, nosZ). Detected denitrifier diversity was similar in 0-20 cm and soil from greater depths. OTUs were indicative of diverse uncultured soil denitrifiers, new species, Actinobacteria, Alpha- and Betaproteobacteria. Structural gene abundances were essentially similar in soil from 0-20 cm and greater depths. The collective data indicate that (i) denitrification is a source of N2O in palsa peat soil, (ii) denitrification potentials are higher in upper soil layers, and (iii) palsa peats harbor novel and diverse denitrifiers associated with N2O fluxes that might be prone to react to global warming.