Effect of biochar on anaerobic co-digestion performance and microbial communities of hulless barley straw and pig manure

Biochar has been reported as an effective additive to enhance methane production during anaerobic digestion due to its large specific surface area, ability to enrich microorganisms, electrical conductivity, and low cost. However, it is not clear whether these effects are applicable to the anaerobic co-digestion (co-AD) of hulless barley straw and pig manure. Under medium temperature batch anaerobic digestion, the enhancing effects of different concentrations of biochar on co-AD of hulless barley straw and pig manure were investigated. The results show the addition of biochar improved the methane production performance of the co-AD system, and a maximum cumulative methane production of 322.94 mL/g VS was obtained when 4 g/L biochar was added to the system. This value was 17.73% higher than that of the control (CK), without biochar supplementation. The addition of biochar produced certain economic benefits. The addition of biochar increased the relative abundance of bacterial groups, such as unclassified Anaerolineaceae, Clostridium sensu stricto 1, and Syner-01, and acetate trophic methanogens, such as Methanothrix and Methanosarcina, but it did not promote the growth of hydrogenotrophic methanogens. Notably, the addition of biochar increased the electron transport system (ETS) activity of the co-AD and enhanced the enrichment of microorganisms involved in direct interspecies electron transfer (DIET), such as unclassified Anaerolineaceae, Clostridium sensu stricto 1, Methanothrix, and Methanosarcina. This may represent one of the key reasons behind the observed increase in methane production.