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

Wei Tang,Yang Shen,Yibin Yuan,Hong Liu,Xingzu Wang

Published 2025 in Water Research

ABSTRACT

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.

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