Page 215 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

CBM1) that bind cellulose or hemicellulose chains, but lack catalytic activity. A further noticeable increase was observed in the lytic polysaccharide monooxygenases (mainly family AA10) and polysaccharide lyases (PLs), which are important enzymes involved in the deconstruction of cellulose and hemicellulose. Amongst the polysaccharide lyases (PLs) that cleave uronic acid-containing polysaccharides, a clear increase was observed in PL11. The increases in enzymes pectate lyase (EC 4.2.2.2), exo-polygalacturonosidase (EC 3.2.1.82), and cellulase (EC3.2.1.4) were very pronounced in TL and PF and less in AL. The CAZy-encoding microorganisms primarily belonged to the Bacteroidetes (Table S4) and Bacteroidetes- associated CAZy classes showed substantial increases in all the layers (Fig. S7).
Discussion
Greenhouse gas production rates showed an immediate response in CO2 release and a steadier response in CH4 release over the five-year incubation period. Thereby, initial CO2 production rates largely outweighed CH4 production rates. Rapid initial carbon degradation can result in high CO2/CH4 ratios (Gao et al. 2019), which coincides with our observations. High initial CO2 production was also observed in a 4-year incubation study on deep permafrost deposits from the Lena River Delta, Siberia, while CH4 production rates were much lower which is consistent to our study (Knoblauch et al. 2013). An excess of CO2 formation is indicative of fermentative processes as well as anaerobic respiration using alternative terminal electron acceptors, including NO2-/NO3-, Fe(III) and SO42- that are available in the sediment (Ewing et al. 2015). Indeed, Fe(III) and SO42- were detected in the soil pore water of all layers prior to long-term incubation (Fig. S2), while NO3- was not detected. Rapid initial activity is furthermore linked to the presence of a considerable labile carbon pool in permafrost soils (Rodionow et al. 2006; Mueller et al. 2015). It has been reported that this labile organic matter is rapidly turned over under suitable conditions (Rodionow et al. 2006; Strauss et al. 2013), while the less labile carbon was shown to fuel lower-rate CH4 production on the long-term (Douglas et al. 2020). A study on long-term carbon mineralization showed that under anoxic conditions only 25% of the aerobically released carbon was converted to CO2 and CH4 (Knoblauch et al. 2013). The calibrated carbon degradation model executed in this study predicted 15.1% (aerobic) and 1.8% (anaerobic) initial carbon release over 100 years, highlighting the slow carbon release in the
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