Identification of the carbon (C) sources of methane (CH₄) and methanogenic community structures after organic fertilization may provide a better understanding of the mechanism that regulate CH₄ emissions from paddy soils. Based on our previous field study, a pot experiment with isotopic 13C labelling was designed in this study. The objective was to investigate the main C sources for CH₄ emissions and the key environmental factor with the application of organic fertilizer in paddies. Results indicated that 28.6%, 64.5%, 0.4%, and 6.5% of 13C was respectively distributed in CO₂, the plants, soil, and CH₄ at the rice tillering stage. In total, organically fertilized paddy soil emitted 3.51 kg·CH₄ ha-1 vs. 2.00 kg·CH₄ ha-1 for the no fertilizer treatment. Maximum CH₄ fluxes from organically fertilized (0.46 mg·m-2·h-1) and non-fertilized (0.16 mg·m-2·h-1) soils occurred on day 30 (tillering stage). The total percentage of CH₄ emissions derived from rice photosynthesis C was 49%, organic fertilizer C < 0.34%, and native soil C > 51%. Therefore, the increased CH₄ emissions from paddy soil after organic fertilization were mainly derived from native soil and photosynthesis. The 16S rRNA sequencing showed Methanosarcina (64%) was the dominant methanogen in paddy soil. Organic fertilization increased the relative abundance of Methanosarcina, especially in rhizosphere. Additionally, Methanosarcina sp. 795 and Methanosarcina sp. 1H1 co-occurred with Methanobrevibacter sp. AbM23, Methanoculleus sp. 25XMc2, Methanosaeta sp. HA, and Methanobacterium sp. MB1. The increased CH₄ fluxes and labile methanogenic community structure in organically fertilized rice soil were primarily due to the increased soil C, nitrogen, potassium, phosphate, and acetate. These results highlight the contributions of native soil- and photosynthesis-derived C in paddy soil CH₄ emissions, and provide basis for more complex investigations of the pathways involved in ecosystem CH₄ processes.
Keywords: 13C; methane; methanogen; organic fertilizer; paddy.