Utilizing conductive materials for reducing methane emissions in postharvest paddy rice soil microcosms

Paddy fields are a major anthropogenic source of global methane (CH₄) emissions, a powerful greenhouse gas (GHG). This study aimed at gaining insights of different organic and inorganic conductive materials (CMs) - biochar, fungal melanin, and magnetite - to mitigate CH₄ emissions, and on their influence on key microbial populations, mimicking the postharvest season throughout the degradation of rice straw in microcosms under anaerobic conditions encompassing postharvest paddy rice soils from the Ebro Delta, Spain. Results showed that fungal melanin was the most effective CM, significantly reducing CH₄ emissions by 29 %, while biochar amendment also reduced emissions by 10 %. Magnetite slightly increased CH₄ production (3 %), but this result was non-significant compared to unamended control microcosms. All treatments (with and without CM) displayed the acetoclastic methanogenesis pathway according to isotopic signature of δ¹³C-CH₄, δ¹³C-CO₂ and δ₂H-CH₄. In the presence of CMs, the archaeal populations showed a major abundance of Methanobacteria, Methanosarcina, and Bathyarchaeia. Furthermore, linear discriminant analysis effect size (LefSe) revealed specific positive linkages between fungal melanin and electroactive bacteria like Geobacter, biochar with Clostridia, and magnetite with Thiobacillus, and specifically related with archaea, particularly Bathyarchaeia. Biochar may diversify volatile fatty acids (VFA) utilization leading to a final mitigation of cumulative CH₄ emissions through complex microbial interactions in the later stages of incubation. In contrast, fungal melanin increased VFA production, while delaying CH₄ production, and may have diverted the electron flow towards melanin quinone reduction, suppressing methanogenesis by oxidizing organic compounds. These results suggest that CMs might facilitate specific potential direct interspecies electron transfer (DIET) between syntrophic electroactive bacteria (i.e. Geobacter, Clostridia) and electroactive methanogens such as Methanosarcina and Methanobacteria, but also with alternative microbial populations with the potential for hampering methanogenesis in a certain extent.