Freeze-thaw seasonal variations and environmental controls of CO2 and CH4 diffusive emissions from reservoirs in the upper Yellow River

Reservoirs are significant sources of atmospheric carbon greenhouse gas (carbon dioxide [CO₂] and methane [CH₄]) emissions. This study analysed the seasonal emission patterns of CO₂ and CH₄ concentrations and diffusive fluxes from water bodies, along with the associated environmental controls, in the three large reservoirs of the upper Yellow River on the Tibetan Plateau. The results indicated that the soil thawing period represented a critical window for carbon greenhouse gas emissions from reservoirs (CO₂: 239.3 ± 94.4 mmol m^-2 d^-1 and CH₄: 201.1 ± 366.6 mmol m^-2 d^-1). Emissions of these gases from reservoirs were predominantly driven by photodegradation of organic carbon during the soil freezing period, whereas anaerobic respiration by microorganisms was the primary process during the soil thawing period, supplemented by photodegradation. CO₂ emissions from reservoirs were driven primarily by natural environmental pressures, particularly the alpine climate and high altitude of the plateau region. Precipitation and altitude were the main factors influencing the carbon input from terrestrial sources, whereas dissolved oxygen and chlorophyll-a predominantly governed the metabolism of carbon from endogenous sources. CH₄ emissions from reservoirs were influenced primarily by anthropogenic environmental pressures, including reservoir siltation and the confluence of tributaries characterized by high pollutant loads. Ammonia nitrogen served as a critical limiting factor for CH₄ emissions, which, along with dissolved oxygen, pH, and oxidation‒reduction potential, collectively affect CH₄ releases. These findings enhance the understanding of the balance of greenhouse gas carbon emissions in reservoirs on the Tibetan Plateau and provide a scientific basis and theoretical support for greenhouse gas emission reduction in this region.