Freshwater wetlands are critical drivers of global biogeochemical cycles, acting as significant carbon and nitrogen sinks, sources, and transformers. A novel linkage between the carbon and nitrogen cycles may be present by methanogens. Methanogens, known for methane production, potentially perform nitrogen fixation due to the harboring of the nifH gene. However, nitrogen fixation by methanogens remains largely unexplored as limited studies assess their functional activity. This study investigates the coupling of methanogenesis and nitrogen fixation across environmental gradients in freshwater wetlands by correlation analysis and the characterization of the active N-fixing microbial communities. Four northeastern Minnesota wetlands were selected based on their distinct geochemical characteristics of low phosphorus and high organic carbon, iron, and sulfate. Microcosms were incubated in the dark at 25℃ over 14 days with wetland sediments under atmospheric N and methanogenic substrate additions of hydrogen and carbon dioxide (4:1 v/v), acetate, trimethylamine, and nothing (control). Methane production and nitrogen fixation activities were assessed throughout the incubation. Microcosms with high nitrogen fixation rates in response to the substrate additions were further analyzed with nifH amplicon sequencing and 15N2-DNA-Stable isotope probing to identify the total and active N-fixing community, respectively. A significant positive correlation was observed between normalized methane production and nitrogen fixation rate constants at the organic carbon-rich wetland (p=0.05, rho=0.68, n=9). Conversely, methanogens with the nifH gene, such as Methanosarcina and Methanothrix, were present in low abundance regardless of substrate additions. Interestingly, the genus Sulfuricurvum, methane/sulfur-oxidizing bacteria, were identified as active N-fixers along with Azospirillum, a known N-fixing bacterium. These results suggest potential mixotrophic methanotroph nitrogen fixation in response to methanogenic substrates. Ultimately, this study provides a better understanding of the interconnected carbon, nitrogen, and sulfur cycles across these freshwater wetlands.