Divergent Effects of Agriculture on River Methane and Carbon Dioxide Concentrations and Emissions: Insights from Multiple Techniques

This study examined the impacts of agriculture on methane (CH₄) and carbon dioxide (CO₂) dynamics in an agricultural river in eastern China. The dissolved CH₄ concentrations (dCH₄; 422 ± 188 nmol L^–1 in summer and 193 ± 190 nmol L^–1 in winter) and CH₄ emissions (6430 ± 11,650 μmol m^–2 d^–1 in summer and 396 ± 1190 μmol m^–2 d^–1 in winter) were closely associated with agriculture-enhanced allochthonous CH₄ inputs and methanogenesis by methanogens and algae. The shallow water depth and low flow velocity of the plain river were favorable for bubble release. The pressure of CO₂ (pCO₂) values were substantially higher than the atmospheric level, and the CO₂ emission rates were 96 ± 159 mmol m^–2 d^–1 in summer and 27 ± 64 mmol m^–2 d^–1 in winter. The pCO₂ and CO₂ emissions were largely regulated by the pH, which was not particularly relevant to agricultural activities. Notably, methanotrophs contributed significantly to the pCO₂ in the winter. Therefore, agriculture could generate more impacts on fluvial CH₄ than on CO₂, which is crucial for understanding the anthropogenic impacts on fluvial carbon emissions and the simulation of carbon budgets in global biogeochemical models.