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Abstract
Although soil-borne methanogens are known to be highly diverse and adapted to extreme environments, their application as potential (anaerobic) inocula to improve anaerobic digestion has not been investigated until now. The present study aimed at evaluating if soil-derived communities can be beneficial for biogas (methane, CH4) production and endure unfavorable conditions commonly associated with digestion failure. Nine study sites were chosen and tested for suitability as inoculation sources to improve biogas production via in situ measurements (CH4 fluxes, physical and chemical soil properties, and abundance of methanogens) and during a series of anaerobic digestions with (a) combinations of both sterile or unsterile soil and diluted fermenter sludge, and (b) pH-, acetate-, propionate-, and ammonium-induced disturbance. Amplicon sequencing was performed to assess key microbial communities pivotal for successful biogas production. Four out of nine tested soil inocula exerted sufficient methanogenic activity and repeatedly allowed satisfactory CH4/biogas production even under deteriorated conditions. Remarkably, the significantly highest CH4 production was observed using unsterile soil combined with sterile sludge, which coincided with both a higher relative abundance of methanogens and predicted genes involved in CH4 metabolism in these variants. Different bacterial and archaeal community patterns depending on the soil/sludge combinations and disturbance variations were established and these patterns significantly impacted CH4 production. Methanosarcina spp. seemed to play a key role in CH4 formation and prevailed even under stressed conditions. Overall, the results provided evidence that soil-borne methanogens can be effective in enhancing digestion performance and stability and, thus, harbor vast potential for further exploitation.
Highlights
Regardless of being produced in biogas reactors or natural habitats, the formation of methane (CH4) is driven by a delicate balance between functionally distinct microorganisms mainly from the domains of Bacteria and Archaea that are kinetically, physiologically, and thermo-dynamically linked and dependent on mutual and syntrophic interactions (Gerardi, 2003; Akuzawa et al, 2011; Soil Inoculation for Anaerobic DigestionSchink and Stams, 2013)
Soils harbor diverse and functionally dynamic microbial communities that are able to thrive under acidic as well as nitrogen-rich conditions and occupy natural niches in which various volatile fatty acids (VFA) are major intermediates in the carbon flow to methane (Gan et al, 2012; Liu et al, 2017). This contradicts with the specified and narrow range suitable for anaerobic digestion in biogas plants. Considering their natural tolerance toward parameters commonly associated with digestion failure, we investigated whether soil-borne communities have the potential to improve CH4 production in biogas plants under-disturbed conditions
The positive in situ fluxes in the grassland (AG) and pastureland (AP) can most probably be attributed to inputs of methanogenic Archaea due to fermentation residue applications and grazing cattle excretions, while high water saturation creating ideal conditions allow the settlement of methanogenic Archaea in fens (Chasar et al, 2000; Blodau, 2002). Quantitative PCR (qPCR) conducted with the native soil samples of the four chosen soils provided further evidence for the abundance of methanogens (Figure 3B) that further coincided with cumulative CH4 production in these soils (Figures 3A,B)
Summary
Regardless of being produced in biogas reactors or natural habitats, the formation of methane (CH4) is driven by a delicate balance between functionally distinct microorganisms mainly from the domains of Bacteria and Archaea that are kinetically, physiologically, and thermo-dynamically linked and dependent on mutual and syntrophic interactions (Gerardi, 2003; Akuzawa et al, 2011; Soil Inoculation for Anaerobic DigestionSchink and Stams, 2013). As input materials with a high content of fat can lead to increased production of acetate and propionate, high concentrations of VFA without an increase in alkalinity might result in adverse operational conditions and are associated with digestion imbalance or stress (Gerardi, 2003; Li et al, 2012) In this context, a key factor that directly influences on CH4 production and efficacy of the entire AD process is the choice of inoculum (Wojcieszak et al, 2017). To the best of our knowledge, the application of soil-derived communities as potential inocula to improve biogas production has not been investigated until now but harbors significant benefits as they are fairly ubiquitous in nature
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