Unveiling the regulation of biochar and ferrihydrite on organic carbon stabilization, CH4 emission and microbial community in paddy soil.

Biochar (BC), an accessible and economic soil improvement, has influenced the biogeochemical cycling of iron and organic carbon (OC) in paddy soil. However, little is known about the effect of BC on iron-organic carbon (Fe-OC) associations, as well as how they regulate the associated microbial community in paddy soil. This study employed anoxic soil microcosms with distinct treatments including BC alone, ferrihydrite (Fh) alone, and BC combined with Fh. Results demonstrated that Fh alone significantly reduced CH₄ emission by suppressing key methanogens and stabilizing humic-like dissolved organic matter. BC alone slightly inhibited CH₄ while promoted CO₂ emission, consistent with its function as an electron shuttle promoting dissimilatory iron reduction and labile OC mineralization. Interestingly, the combined application of BC and Fh significantly suppressed the CH₄ emission. It might be due to that BC accelerated microbial iron reduction to divert electrons from methanogenesis and Fh preferentially sequestered humic-like organic components into mineral-associated complexes, which reduced substrate availability for methanogens (e.g., Methanosarcina) while enriched iron-reducing bacteria like Thermoleophilia and Desulfuromonadia. This study revealed that the BC and Fh together had significant effects on the OC stabilization, CH4 emission and microbial community, which provides fundamental information for future synergistic application of BC and Fh in rice paddy ecosystems.