Urban streams as hotspots of carbon dioxide and methane diffusive emissions compared to small ponds

Streams and ponds significantly contribute to the global freshwater carbon gases budget. To understand the role of urbanization in affecting aquatic carbon emissions, here we examined the carbon dioxide (CO₂) and methane (CH₄) concentrations and diffusive emissions from streams and small ponds across three headwater catchments with varied urbanization intensities in the upper Yangtze River. We found that riverine partial pressure of CO₂ (pCO₂) and dissolved CH₄ concentration (dCH₄) increased, while pond pCO₂ and dCH₄ decreased as urban land proportion of catchments increased from 1.0% to 14.1%. In addition, catchments with a lower urban land proportion (1.0% or 6.5%) showed significantly higher pCO₂ and dCH₄ in ponds than in streams, while catchment with a high urban land proportion (14.1%) showed comparable values between stream and ponds. Total dissolved nitrogen (TDN) proved a crucial differentiating factor among streams experiencing varying urbanization intensities and explained 56% of riverine pCO₂ variability. By contrast, dissolved organic carbon (DOC) emerged as a key variable in distinguishing ponds located in catchments with differing proportions of urban land and a significant predictor of pond dCH₄. At the catchment scale, the diffusive emissions of carbon gases (i.e., CO₂ and CH₄) from streams and ponds increased from 2.59 CO₂-eq kt y^-1 in the catchment with 1.0% urban land to 3.93 CO₂-eq kt y^-1 in the catchment with 6.5% urban land, peaking at 6.66 CO₂-eq kt y^-1 in the catchment with 14.1% urban land. Unexpectedly, the majority of these carbon gases diffusive emissions were contributed by streams, and their contribution rate ranged from 65% to 83%. Our results highlight urban streams as primary sources of atmospheric CO₂ and CH₄ sources in headwater catchments.