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

Abstract

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.