Greenhouse Gas Emissions from a Main Tributary of the Yangtze River, Eastern China

Abstract

Rivers and streams are recognized as potential greenhouse gas (GHGs: CO2, CH4, and N2O) sources, contributing to global warming. However, GHG emissions from rivers and streams have received insufficient attention compared to other ecosystems (forests, grasslands, wetlands, etc.). In this study, dissolved GHG concentrations were measured in the Qingyijiang River, the longest tributary in the lower reaches of the Yangtze River, during two campaigns in September 2020 and April 2021. Our results showed that the Qingyijiang River was oversaturated with dissolved GHGs. The dissolved GHG concentration in the surface river water ranged from 8.70 to 67.38 μM CO2, 0.03 to 2.06 μM CH4, and 12.30 to 32.22 nM N2O. The average diffusive GHG emission rates were 31.89 ± 22.23 mmol CO2 m−2 d−1, 697.22 ± 939.82 μmol CH4 m−2 d−1, and 18.12 ± 7.73 μmol N2O m−2 d−1. The total emissions (CO2-e) were CO2 (58%) dominated, while CH4 (38%) played a moderate role in total emissions. Temporally, average GHG concentrations and fluxes from the studied river in April were higher than those in September. The concentration and flux of CH4 exhibited high spatial variability, similar to those in most rivers. In contrast, we found that there was no obvious spatial variability in CO2 and N2O concentrations but a significant difference among reaches in N2O fluxes. We found that water temperature and flow velocity were the potential drivers for the regulating spatial variability in GHGs. However, no other observed limnological parameters were found in governing the spatial patterns of GHGs, suggesting a complex combination of factors governing GHG fluxes; thus, these inconspicuous mechanisms underscore the need for further research. Overall, our study suggests that this river acts as a minor source of GHGs relative to other rivers, and CH4 cannot be ignored when considering aquatic carbon emissions.