NanoFe3O4 accelerates methanogenic straw degradation in paddy soil enrichments

Abstract

Methanogenesis is crucial in the carbon cycle, and conductive iron oxide minerals play an important role in methanogenesis in anoxic environments. The influence of iron oxides is ubiquitous, but the strength and nature of iron oxides effects are believed to vary in response to environmental conditions, especially the soil iron concentration. We evaluated the influence of conductive nanoFe3O4 on straw degradation in high-iron (HR) and low-iron (LR) paddy soil enrichments, as well as the corresponding responses of related microorganisms. Under conductive nanoFe3O4 amendment, methane production and straw degradation increased by 184%–285% and 7.8%–12.7%, respectively, in the two soil enrichments. The increase apparently coincided with increased interspecies interactions within the microorganismal community, especially enhanced syntrophic cooperation among fermenting bacteria, syntrophic bacteria and methanogenic archaea, and facilitated interspecies electron transfer, which improved carbon conversion and alleviated end-product inhibition. The methanogenic archaeal community also exhibited changes with nanoFe3O4 amendment, and the relative abundance of Methanosarcina increased. Pairwise comparisons revealed that although methanogenic straw degradation was higher in HR soil than in LR soil, the enhancement induced by the nanoFe3O4 amendment showed to a larger degree and the microbial responses were more distinct in LR soil than in HR soil. The abundances of Geobacter and Methanosarcina greatly increased, and the microbial community shifted towards a composition similar to that in HR soil. This work indicates that iron oxides facilitate the methanogenic degradation of rice straw and that the indigenous iron content significantly influences the microbial community composition. This study provides a technique for agricultural residue degradation and has clear implications for our understanding of soil C-Fe biogeochemical coupling.