Biochar and magnetite promote methanogenesis during anaerobic decomposition of rice straw

Conductive materials like magnetite nanoparticles (nanoFe3O4) are known to stimulate methanogenesis from syntrophic oxidation of butyrate and propionate. Whether conductive materials increase the decomposition of complex organic substrates like plant residues is unknown. Here, we investigated the effects of biochar and nanoFe3O4 on anaerobic decomposition of rice straw and methanogenesis in a rice paddy soil. The straw decomposition and CH4 production were greatly stimulated by both treatments. Major bacterial populations were Bacteroidetes, Clostridia, Deltaproteobacteria, Actinobacteria and Anaerolineae, while methanogens were predominately Methanosarcina, Methanosaeta and Methanobacterium. Bacteroides were relatively enriched in the biochar treatment, while the Deltaproteobacteria and Anaerolineae were favored by nanoFe3O4. The relative abundance of typical syntrophs including Syntrophomonas, Syntrophus and Smithella were positively correlated with the degradation of intermediate short-chain fatty acids. Notably, the relative abundance of Geobacter, which increased during the incubation, was approximately tenfold greater than all other syntrophs. The co-occurrence of Geobacter together with Methanosarcina and Methanosaeta suggests that the enhanced decomposition of rice straw and methanogenesis by biochar and magnetite may be related to direct interspecies electron transfer involved in the syntrophic oxidation of intermediate products.