Molecular Soil Biology 2026, Vol.17, No.1, 12-25 http://bioscipublisher.com/index.php/msb 19 6 Regulation Mechanism of Methane Production Pathways by Irrigation Modes 6.1 Changes in acetic acid-type and hydrogen-nutrient-type methane production pathways The methane production in paddy fields is generally classified into two main pathways: one is the acetic acid decomposition type, and the other is the hydrogen-nutrient type through CO2 reduction. However, these two pathways do not remain in a fixed proportion all the time. They often adjust constantly in response to changes in substrate supply, pH, water conditions, and available electron acceptors. Existing studies have found that the methane production pathway itself in paddy fields shows obvious temporal variations, and is often related to DOC, acetic acid concentration, water content, and NH4 +, NO3 -, etc., indicating that the pathway ratio is actually the result of the combined effect of "substrate - environment - community" (Conrad, 2020). For alternate wetting and drying irrigation, re-oxidation during the dry period usually reduces the available substrates under anaerobic conditions and makes some alternative electron acceptor processes more active, thus overall suppressing methane production. However, as for which pathway is more dominant and which is less, the results often vary depending on the context, and need to be determined by combining stable isotopes, key enzyme gene typing, and process measurements (Figure 4) (Angle et al., 2017). Based on this, a hypothesis that requires further verification can be proposed: AWD may cause systematic changes in the ratio of methane production pathways. Therefore, conducting quantitative studies on different growth stages and soil layers in subtropical paddy fields would be more appropriate, and it can also avoid using single-season or single-point data to infer the entire process. Figure 4 Methane concentrations and production rates across soil depths. a Porewater dialysis peepers provide 2.8 cm resolved depth methane measurements. b, d Monthly in situ porewater dissolved methane concentrations in mud and water-covered soils with data collected from June (blue), July (yellow), August (red), and September (purple). Black dashed lines depict the 95% confidence interval for location of the oxic to anoxic transition. c, e. The calculated net methane volumetric fluxes in soils columns from mud and water ecosites show seasonal methane production (orange and red) in oxic soils (Adopted from Angle et al., 2017)
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