Community structure, ecophysiology, and regulation of new, acid-tolerant denitrifiers as cause of high N2O emissions from cryoturbated peat circles of acidic tundra soils

Abstract

Denitrification is the main source of N2O emissions from cryoturbated peat circles in the Eastern European Arctic tundra. These permafrost-affected bare peat circles lack vegetation, have a high amount of available nitrate, and a low carbon to nitrogen ratio. Together with a soil moisture of ~ 70%, these are optimum conditions for denitrification and associated microorganisms. An acidic in situ pH of these peat circles is hypothesized to be a major controlling factor influencing denitrification and triggering the release of N2O as emitted end product of denitrification, as the final enzymatic step of denitrification is impaired by a low pH. This thesis aimed to test if the ecophysiology of new and acid-tolerant denitrifiers affects the emissions of N2O from cryoturbated peat circles. Microcosm experiments with soil from bare peat circles and surrounding vegetated peat plateau were conducted to test denitrification potentials of both soils. Additionally, the influence of pH on denitrification was tested. Nitrate reduction and associated production of gaseous N-products by denitrification were detected in both soils. In peat circle soil microcosms, independent of pH, the released end product was N2. Though nitrate reduction was slower at pH 4. In peat plateau microcosms N 2 was as well the emitted main end product at pH 6, whereas at pH 4 N2O was emitted. Therefore, data indicate the potential for denitrification in both soil types, as well as complete denitrification potential at acidic pH in peat circles. Microcosms experiments supplemented with labeled acetate were conducted to reveal the active, organic acid utilizing archaeal and bacterial community of denitrifiers based on 16S rRNA stable isotope probing coupled to Illumina MiSeq amplicon sequencing. Results suggest Burkholderiaceae as key nitrate reducers and acetate assimilators. This is supported by a new species within the Burkholderiaceae (Caballeronia) isolated from peat circle soil that encodes diverse nitrate and nitrite reductases. Potential key denitrifiers responsible for the reduction of N2O to N2, the detected end product and product of complete denitrification, are Sphingobacteriacea. Altogether, collected data suggest an electron donor limitation in peat circle soil leading to high in situ emission of N2O, since peat circle soil demonstrated the potential for complete denitrification despite an acidic pH.