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.