Coupling Mechanism of Methane Production and Carbon Fixation Regulated by Artificial Humic Acid in Paddy Soils: Insights into Microbial Communities and Metabolic Pathways

The impact of artificial humic acid (HA) under varying water management conditions on the interactions between carbon metabolic pathways in rice soils remains unclear. Therefore, this study employed incubation experiments using HA-treated soil columns under continuous flooding (CF) and alternating wet–dry (AWD) to explore the relationship between CH 4 production and CO 2 fixation. The results demonstrate that HA significantly increased CH 4 emissions, with a 102.87% rise under CF conditions and a 135.13% increase under AWD. In contrast, CO 2 emissions remained unaffected under both conditions. Key microbial groups, such as Methanosarcina, were enriched by HA, with a 15.01% increase in methanogen abundance under CF and a 6.36% increase under AWD. Additionally, the upregulation of key intermediates, such as acetyl-CoA and tetrahydrofolate, was significantly higher in HA treatments. A Mantel test demonstrated the coupling between carbon fixation and methanogenesis pathways. In conclusion, HA promoted the Wood–Ljungdahl pathway in Methanobacterium for CO 2 conversion into acetyl-CoA and facilitated acetoclastic methanogenesis in Methanothrix through acetate utilization. These findings advocate prioritizing AWD-HA integration in water-secure regions to balance soil carbon retention and emission reduction. This mechanistic insight advances strategies for optimizing HA use in climate-smart rice cultivation.