Influences and mechanisms of iron input for methane productions in peatlands

Atmospheric deposition provides a stable iron source for peatlands. The influences of Fe input on methane (CH₄) productions and the underlying mechanisms remain unclear. We conducted a microcosm experiment with peat sediments collected from the Qinghai-Tibet Plateau of China to explore the effects of ferrihydrite reductionfor CH₄ productions in peatlands by using geochemical analyses including ⁵⁷Fe Mössbauer spectroscopy and three-dimensional fluorescence spectroscopy (3D-EEM) in combination with high-throughput sequencing of 16S rRNA and real-time fluorescence quantitative PCR (qPCR). Results showed that ferrihydrite reduction significantly increased CH₄ production, being 30 times of that under the control. Selective extractions for iron oxides and ⁵⁷Fe Mössbauer spectroscopy measurements revealed that no crystalline secondary iron minerals were formed during the ferrihydrite reduction process. The addition of ferrihydrite enhanced the degradation of dissolved organic matter (DOM) in peat soil, resulting in a reduction in the concentration of dissolved organic carbon (DOC). Furthermore, the relative abundance of typical fermentative microorganisms in peat sediments, including Acidobacteriota and Bacteroidota, significantly increased. Such a result indicated that reduction of ferrihydrite accelerated organic matter decomposition and increased substrate concentration required for methanogenesis. Furthermore, a co-increase in relative abundance of Geobacter, Geothrix, and Methanobacterium in the ferrihydrite-amended group suggested a potential synergistic interaction that may promote the CH₄ production. Our results demonstrated that ferrihydrite reduction could significantly enhance CH₄ production and play a vital role in regulating CH₄ emissions in peatlands.