Page 220 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 9. Long-term warming effects on permafrost soil microbial communities
within these layers upon long-term thaw exposure is lacking. In our study, concentrations of nitrite and nitrate in the initial samples were negligible, and ammonium concentrations were highest in AL and lowest in PF. This data suggests nitrogen supply is potentially limiting microbial growth and nitrogen supply is largely controlled by organic matter degradation and nitrogen fixation. After long-term incubation, gene abundances linked to general nitrogen cycle processes decreased while nitrogen fixation genes increased in TL and PF. Nitrogen fixation is an expensive process, which supports a general decrease in nitrogen fixation-associated genes when energy is limiting or nitrogen supply is sufficient (Gutschick 1978). Our findings thus indicate that for TL and PF nitrogen availability can be a controlling factor and nitrogen demand may have increased during the incubations. Overall, nitrate reduction and denitrification gene abundances decreased, which support a decrease in nitrate availability. Denitrification is mainly carried out by heterotrophic bacteria and requires organic carbon compounds as electron donor.
The majority of canonical denitrifiers are facultative anaerobes (Zumft 1997). Long-term anoxic incubation is therefore expected to result in a decrease of denitrifiers due to depletion of alternative terminal electron acceptors, as was observed in this study. Within all layers an increase was observed in norB, which encodes for nitric oxide (NO) reductase subunit B and is involved in N2O production (Suzuki et al. 2006). Unlike a previous observation for polygonal tundra soils (Taş et al. 2018), this suggests the potential of N2O formation associated with continuous permafrost thaw. It is further supported through initially high gene abundances of nitrate reduction and denitrification genes. Growth rates with N2O and nitrite are comparable to growth on nitrate (Strohm et al. 2007). Potential N2O production is highly relevant in the context of climate feedbacks of thawing permafrost, as N2O has a global warming potential (GWP) of 298 over a time span of 100 years when including climate-carbon feedbacks (Etminan et al. 2016). Although there is no direct measurement of N2O production in this study, N2O production was shown to be enhanced in poorly drained thawing permafrost by previous studies (Elberling, Christiansen and Hansen 2010; Marushchak et al. 2011; Abbott and Jones 2015; Voigt et al. 2017a). Therefore, it is necessary to experimentally confirm whether N2O emissions will increase under long-term anoxic thaw.
Little is known about microbial sulfur cycling in permafrost (Mackelprang et al. 2016). 16S rRNA gene and metagenomic sequences from sulfate-reducing microbes have been detected in
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