Unravelling substrate availability and redox interactions on methane production in peat soils of China

The availability of electron acceptors (EAs) in peatlands determines the potential of methane (CH₄) formation under waterlogged conditions. Previous studies suggested that EAs can suppress CH₄ production based on Gibbs free energy under the Redox Ladder Theory. However, growing evidence challenges this theory, raising the question of how the coupling of soil substrates with EAs influences CH₄ emissions. To answer this key question, peat soils were collected across different climatic zones with different degrees of soil degradation. Anoxic incubation experiments were set up, and continuous addition of SO₄²⁻, Fe³⁺ and humic acid (HA) at different concentrations was followed by characterization of dissolved organic matter using fluorescence spectroscopy. Results suggest that low concentrations of SO₄²⁻ (1000 μmol L⁻¹), Fe³⁺ (100 μmol L⁻¹) and HA (30 mgC L⁻¹) promoted CH₄ production in most of the peat soils. With the addition of SO₄²⁻ and HA, increased CH₄ emissions were attributed to the facilitation of dissolved organic carbon and increased quinone-like component C1, which increased the substrate availability for methanogenesis. Furthermore, strengthened microbial activity as indicated by fluorescence component C2 led to higher CH₄ production under Fe³⁺ treatments. On the other hand, at high concentrations of SO₄²⁻ (5000 μmol L⁻¹), Fe³⁺ (500 μmol L⁻¹) and HA (50 mgC L⁻¹), CH₄ emissions rapidly decreased by 70.65 ± 1.57% to 96.25 ± 0.45% compared to control group without EAs addition, accompanied by increased δ¹³C-CH₄ signatures indicating the outweighed CH₄ production under anaerobic oxidation of methane (AOM) when coupling with reduced EAs. The effect of EAs on CH₄ emissions in peat soils could also be related to lability and characteristics of natural organic matter. Our results suggest that the CH₄ production in waterlogged peatlands could be facilitated by regulating organic substrates at low EAs concentrations, but excessive EAs will reduce net CH₄ emissions through AOM. The valuable discovery of CH₄ production and oxidation processes provides insights for mitigating methane emissions from peatlands and regulating global climate change.