|Palmer, K; Horn, MA: Actinobacterial nitrate reducers and Proteobacterial denitrifiers are abundant in N2O metabolizing palsa peat, Applied and Environmental Microbiology, 78, 5584–5596 (2012)|
Palsa peats are characterized by elevated, circular frost heaves (peat soil on top of a permanently frozen ice lense), and are strong to moderate sources or even temporary sinks for the greenhouse gas nitrous oxide (N2O). Palsa peats are predicted to react sensitively to global warming. The acidic palsa peat Skalluvaara (app. pH 4.4) is located in the discontinuous permafrost zone in northwestern Finnish Lapland. In situ N2O fluxes were spatially variable, ranging from 0.01 to -0.02 µmol N2O m-2 h-1. Fertilization with nitrate stimulated in situ N2O emissions and N2O production in anoxic microcosms without apparent delay. N2O was subsequently consumed in microcosms. Maximal reaction velocities (vmax) of nitrate dependent denitrification approximated three and one nmol N2O h-1gDW-1 in soil from 0 to 20 cm and below 20 cm of depth, respectively. vmax of nitrite dependent denitrification were two to five fold higher than of nitrate dependent denitrification, and vmax of N2O consumption were one to six fold higher than of nitrite dependent denitrification, highlighting a high N2O consumption potential. Up to 12 species-level operational taxonomic units (OTUs) of narG, nirK/S, and nosZ were retrieved. Detected OTUs suggested the presence of diverse uncultured soil denitrifiers and dissimilatory nitrate reducers, hitherto undetected species, as well as Actino-, Alpha- and Betaproteobacteria. Copy numbers of nirS always outnumbered those of nirK by 2 orders of magnitude. Copy numbers of nirS tended to be higher, while copy numbers of narG and nosZ tended to be lower in 0 to 20 cm soil compared to soil below 20 cm. The collective data suggest that (i) the source and sink functions of palsa peat soils for N2O are associated with denitrification, (ii) Actinobacterial nitrate reducers, and nirS-type and nosZ harboring Proteobacterial denitrifiers are important players, and (iii) acidic soils like palsa peats represent reservoirs of diverse acid-tolerant denitrifiers associated with N2O fluxes.