CH₄ and CO₂ emissions and dissolved carbon exporting in rivers on the upper Lanzhou section of the Yellow River, China

Abstract

The Yellow River (YR), China's second-longest river, remains understudied regarding its greenhouse gases (GHGs) emissions, particularly the impacts of urban drainage ditches and wastewater treatment facilities on regional GHGs dynamics. This study investigated methane (CH₄) and carbon dioxide (CO₂) concentrations, fluxes and stable carbon isotopes (δ¹³C-CH₄ and δ¹³C-CO₂) across six main stream, three ditches, and one wastewater treatment site along the upper Lanzhou section of the YR, spanning from the urban entrance (36.176 °N, 103.449 °E) to the exit of Lanzhou city (36.056 °N, 104.020 °E). Measured CH₄ diffusion fluxes in mainstem sites ranged from 0.01 to 2.58 mmol ·m^-2 ·d^-1 (mean: 0.36 mmol ·m^-2 ·d^-1), while ebullitive fluxes (gas bubbles) ranged from 0.01 to 18.89 mmol ·m^-2 ·d^-1 (mean: 0.90 mmol ·m^-2 ·d^-1). CO₂ diffusion fluxes varied between 9.16-92.80 mmol ·m^-2 ·d^-1 (averaged: 39.11 mmol ·m^-2 ·d^-1) at these locations. Ebullition (bubble) fluxes accounted for 53.1% ± 22.4% (range: 9.0% to 98.4%) to total CH₄ emissions (diffusion plus ebullition), with peak fluxes occurring during summer, indicating its significance as a CH₄ transport mechanism. Notably, both diffusion CH₄ and CO₂ fluxes and ebullitive CH₄ rates at ditch sites substantially exceeded those in mainstream reaches. The lowest CH₄ and highest CO₂ concentrations were observed at a wastewater treatment site, likely resulting from the removal of high organic loads. Acetoclastic methanogenesis €"the process converting acetate-derived methyl groups to CH₄^- was identified as the dominant production pathway in both mainstream and ditch environments. CH₄ and CO₂ flux magnitudes in the upper YR (Lanzhou section) were comparable to those observed in subtropical Yangtze River tributaries. These results demonstrate that anthropogenic influences significantly enhance CO₂/CH₄ emissions, and the lateral exports of dissolved carbon (DIC and DOC) in the main stream site was quantified., which cannot be overlooked. The findings emphasize the critical need to account for pronounced spatiotemporal variations in arid-region GHG fluxes to improve basin-scale estimates for the YR.

Keywords

Yellow River / CH₄ and CO₂ fluxes / δ¹³C isotope / Ditch and Wastewater treatment water / Acetoclastic methanogenesis

CRediT authorship contribution statement

Zhiheng Du: Writing - original draft, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. Hao Cui: Data curation,Investigation, Writing - original draft. Fangping Yan: Inves-tigation, Conceptualization, Writing - original draft. Lei Wang: Inves-tigation, Conceptualization, Writing - original draft. Zhiqiang Wei: Writing - review & editing, Investigation, Data curation, Funding acquisition. Wenhan Hu: Investigation, Writing - original draft. Simin Xie: Investigation, Writing - original draft. Changlian Tao: Investigation, Writing - original draft. Qian Xu: Investigation, Writing - original draft. Qiangqiang Xu: Writing - original draft. Yicheng Wang: Data curation, Writing - original draft. Jingfeng Liu: Writing -original draft. Xiaoxiang Wang: Data curation, Writing - original draft. Minzhu He: Writing - original draft.

Data availability

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was funded by the Strategic Priority Research Pro-gram of the Chinese Academy of Sciences (Grant No. XDB0950000), the NSFC (Grant No. 42201155; 42401147; 42201137) and the State Key Laboratory of Cryospheric Science and Frozen Soil Engineering (CSFSE-ZQ-2410). We especially acknowledge Kai Wang for logistics, and samples collecting in field.

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