Active metabolic pathways of anaerobic methane oxidation in paddy soils

Anaerobic oxidation of methane (AOM) is a globally important CH4 sink. However, the AOM pathways in paddy soils, the largest agricultural source of methane emissions (31 Mio tons per year) are not yet well described. Here, a combination of C-13 isotope tracer, phospholipid fatty acids (PLFA) analyses, and microbial community analysis was used to identify AOM pathways in fertilized (pig manure, biochar, NPK, and the control) paddy soils amended with alternative electron acceptors (AEAs) (NO3-, Fe3+, SO42-, humic acids, and the reference without AEAs addition). After 84 days of anaerobic incubation, the microbial co-occurrence network got tightened and became more complex relative to unincubated samples. Fertilization and AEAs addition led to a strong divergence of the microbial community structure as indicated by abundances of AOM-related microbiota and C-13 incorporation into microbial PLFA, thus suggesting an environmental niche differentiation of AOM-involved microorganisms. Comparative analyses revealed a set of major and minor AOM pathways with synergistic relations to complementary anaerobic microbial groups. NO3--driven AOM, performed by members of the candidate group ANME-2d, was the major AOM pathway. Minor AOM pathways involved NO2- reduction by NC10, reduction of humic acids and Fe3+ by Geobacter species, and SO42- reduction by sulfate-reducing bacteria linked with anaerobic methanotrophs. As identified by the network analysis, these active AOM pathways compensated a fraction of CH4 produced during ongoing methanogenesis. From a broader ecological perspective, nitrogendriven AOM will become a more important methane sink in the future with the increases of nitrogen fertilization and deposition.

Automated summary

Through the use of various methods such as isotope tracing and microbial community analysis, different pathways of anaerobic oxidation of methane (AOM) in paddy soils were identified, highlighting the impact of fertilization and alternative electron acceptors on the microbial community structure. These findings suggest a niche differentiation for AOM-involved microorganisms and the potential for nitrogen-driven AOM to become a more significant methane sink in the future.