A highly agricultural river network in Jurong Reservoir watershed as significant CO₂ and CH₄ sources

Freshwaters are receiving growing concerns on atmospheric carbon dioxide (CO₂) and methane (CH₄) budget; however, little is known about the anthropogenic sources of CO₂ and CH₄ from river network in agricultural-dominated watersheds. Here, we chose such a typical watershed and measured surface dissolved CO₂ and CH₄ concentrations over 2 years (2015-2017) in Jurong Reservoir watershed for different freshwater types (river network, ponds, reservoir, and ditches), which located in Eastern China and were impacted by agriculture with high fertilizer N application. Results showed that significantly higher gas concentrations occurred in river network (CO₂: 112 ± 36 μmol L^-1; CH₄: 509 ± 341 nmol L^-1) with high nutrient concentrations. Dissolved CO₂ and CH₄ concentrations were supersaturated in all of the freshwater types with peak saturation ratios generally occurring in river network. Temporal variations in the gas saturations were positively correlated with water temperature. The saturations of CO₂ and CH₄ were positively correlated with each other in river network, and both of these saturations were also positively correlated with nutrient loadings, and negatively correlated with dissolved oxygen concentration. The highly agricultural river network acted as significant CO₂ and CH₄ sources with estimated emission fluxes of 409 ± 369 mmol m^-2 d^-1 for CO₂ and 1.6 ± 1.2 mmol m^-2 d^-1 for CH₄, and made a disproportionately large, relative to the area, contribution to the total aquatic carbon emission of the watershed. Our results suggested the aquatic carbon emissions accounted for 6% of the watershed carbon budget, and fertilizer N and watersheds land use played a large role in the aquatic carbon emission.