Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams

We analyzed the impacts of land use/land cover types on carbon dioxide (CO2) and methane (CH4) concentration and diffusion in 1st to 4th Strahler order tributaries of the Longchuan River to the upper Yangtze River in China by using headspace equilibration method and CO2SYS program. Field sampling and measurements were conducted during the dry and wet seasons from 2017 to 2019. The average of calculated CO2 partial pressure (pCO2, mean ± SD: 2389 ± 3220 μatm) by CO2SYS program was 1.9-fold higher than the value (mean ± SD: 1230 ± 1440 μatm) 10 years ago in the Longchuan River basin, where the urban land area increased by a factor of 7 times. Further analysis showed that corrected pCO2 by headspace method and dissolved CH4 (dCH4) decrease as the stream order and flow velocity increase. The pCO2 and dCH4 in the wet season was lower than that in the dry season. The explanatory ability of land use types on the variation of corrected pCO2 and dCH4 was stronger at the reach scale than at the riparian and catchment scales in two seasons. Urban land at reach scale further showed much higher explanation on corrected pCO2 and dCH4 than cropland, grassland and forest land in the wet season. The Longchuan River emits approximately 112.5 kt CO2-C and 1.0 kt CH4-C per year, being 1.7-fold of the total lateral export of dissolved inorganic and dissolved organic carbon (68.3 kt C y-1). The findings highlight the scale effects of land use on the observed seasonality in dissolved carbon gases in low-order streams.