Long-term effects of four environment-related iron minerals on microbial anaerobic oxidation of methane in paddy soil: A previously overlooked role of widespread goethite

Due to a suitable ecological environment, the iron-dependent anaerobic oxidation of methane (Fe-AOM) occurs in paddy soil, reducing methane emissions to the atmosphere. Paddy soil contains a variety of iron minerals whose effects on Fe-AOM remain unclear. Here, paddy soil in the ferruginous zone was collected, and four environment-related iron minerals (ferrihydrite, goethite, hematite, and magnetite) were used as electron acceptors to cultivate Fe-AOM for 380 days. The Fe-AOM activity and microbial community structure were monitored using isotope activity assays and molecular biological methods, including quantitative polymerase chain reaction (qPCR) and high-throughput sequencing. The inoculum Fe-AOM rate was the highest with ferrihydrite, followed by goethite, hematite, and magnetite. However, it was ranked in the order goethite > ferrihydrite > hematite > magnetite after long-term cultivation. The specific activity of goethite-dependent AOM increased 14.2 times (from 0.9 +/- 0.2 to 12.9 +/- 0.4 nmol d(-1) g(-1) dry soil) during the 380 days, while that of ferrihydrite increased by only 1.2 times (from 7.9 +/- 2.4 to 9.1 +/- 0.3 nmol d(-1) g(-1) dry soil). The microbial community changed significantly after the emendation of ferrihydrite or goethite, while the other two did not. This implies that ferrihydrite and goethite had a significant impact on microbial processes, including Fe-AOM. Notably, although ferrihydrite promoted Fe-AOM, its reduction product of Fe(II) greatly stimulated methanogenesis and increased methane emission, whereas goethite was able to reduce methane emissions and did not stimulate methanogenesis. Thus, these findings indicate that goethite-dependent AOM occurs in paddy soils and, although previously overlooked, is a crucial sink of the global methane cycle.