Methane from urban river networks: distinct responses of methanogenic pathways to substrate stimulation

Urban rivers are critical hotspots for methane (CH₄) emissions due to anthropogenic activities. However, the understanding of methanogenic pathways and potential drivers across different urban functional zones remains unexplored. A combination of microcosm experiments and field evaluations was employed to investigate the substrate responses of methanogenic pathways in urban rivers influenced by industrial, residential, and agricultural activities within Guangzhou city, a developed metropolitan area in Southern China. Urban rivers in residential zones showed the highest dissolved CH₄ concentrations (3.2 ± 1.1 μmol L^-1), exceeding agricultural and industrial zones by 1.9-fold and 4.9-fold, respectively. Relatively higher methanogenic potential was observed in residential and agricultural zones, with significant activity for hydrogenotrophic (8.4 ± 1.5 μmol CH₄ g_DW^-1 h^-1) and methylotrophic methanogenesis (11.3 ± 8.8 μmol CH₄ g_DW^-1 h^-1) following H₂/CO₂ and trimethylamine amendment, respectively. Continuous amendments of H₂/CO₂ and trimethylamine markedly enhanced CH₄ production in river sediments across all zones, highlighting the effects of the constant supply of available organic substrates. The addition of glucose enhanced CH₄ production in a mixed sample composed of sediments from the three zones. This enhancement can be attributed to the stimulation of carbohydrate metabolism, which generated key substrates that supported both acetoclastic and hydrogenotrophic methanogenesis pathways. Overall, this study highlights the divergence of CH₄ production in urban rivers across functional zones due to differential organic pollution inputs. Optimizing urban functional zoning management, such as prioritizing advanced wastewater treatment in residential zones and implementing controlled fertilizer runoff in agricultural zones, emerges as a key actionable strategy to mitigate urban river CH₄ emissions amid ongoing urbanization.