Urban development elevates methane and carbon dioxide emission in headwater streams

Urban development has been a significant force impacting terrestrial and aquatic ecosystems. Our current knowledge on the exact effects of the magnitude and extent of urbanization on the magnitude of riverine methane (CH₄) and carbon dioxide (CO₂) concentration and outgassing is still limited. This study investigated the spatiotemporal variations of CH₄ and CO₂ dynamics in three headwater streams that drain rapidly urbanizing areas in subtropical China. Field measurements and sampling were conducted during three cool-dry months and three warm-rainy months in 2024 at 19 sites that represented varied urban development. The findings from the study showed that carbon emission and nutrient load were significantly higher in the stream reaches draining more urbanized areas than those draining less urbanized areas. There were 2.85-21.76 fold higher riverine CH₄ and CO₂ outgassing rates observed at the sites with high urban area coverage (Group U1: 21.05±31.79 CH₄ mmol m^-2 d^-1 and 985.95±454.21 CO₂ mmol m^-2 d^-1), when compared to those with intermediate urban coverage (Group U2: 2.7 ± 3.1 CH₄ mmol m^-2 d^-1 and 345.4 ± 446.7 CO₂ mmol m^-2 d^-1) and low urban area coverage (Group U3: 1.1 ± 0.7 CH₄ mmol m^-2 d^-1 and 45.3 ± 86.4 CO₂ mmol m^-2 d^-1). Significant seasonal difference in CH₄ and CO₂ outgassing was respectively found in groups U1 and U3, with fluxes being higher during the cool-dry season (26.89±35.92 CH₄ mmol m^-2 d^-1 and 79.75±87.60 CO₂ mmol m^-2 d^-1). River water parameters such as NH₄⁺-N and protein-like organic matter can be used as an indicator proxy to the change caused by urbanization. These findings underscore the need of sustainable land-use planning and nutrient mitigation to reduce anthropogenically driven carbon emissions in river ecosystems.