Biochar facilitates methanogens evolution by enhancing extracellular electron transfer to boost anaerobic digestion of swine manure under ammonia stress

This study explored the mechanisms by which biochar mitigates ammonia inhibition in anaerobic digestion (AD) of swine manure. Findings show 2-8 g/L exogenous ammonia dosages gradually inhibited AD, leading to decreases in the efficiencies of hydrolysis, acidogenesis and methanogenesis by 3.4-70.8%, 6.0-82.0%, and 4.9-93.8%, respectively. However, biochar addition mitigated this inhibition and facilitated methane production. Biochar enhanced microbial activities related to electron transport and extracellular electron transfer. Moreover, biochar primarily enriched Methanosarcina, which, consequently, upregulated the genes encoding formylmethanofuran dehydrogenase and methenyltetrahydromethanopterin cyclohydrolase for the CO2reducing methanogenesis pathway by 26.9-40.8%. It is believed that biochar mediated direct interspecies electron transfer between syntrophic partners, thereby enhancing methane production under ammonia stress.