Diverse Altitudinal Patterns and Drivers of Greenhouse Gas Dynamics in Southwest China Alpine Streams and Rivers

Streams and rivers are globally significant sources of greenhouse gases (GHGs) to the atmosphere. However, GHG evasion from mountain streams remains poorly constrained due to scarce data. In this study, we measured concentrations and estimated fluxes of riverine carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) across three alpine catchments in Southwest China. GHG concentrations in these turbulent streams and rivers are slightly oversaturated but much lower than the global average, likely due to high gas transfer velocities that rapidly deplete GHGs. Headwater streams (first-order) exhibited higher gas evasion rates than large rivers (fourth-order), despite having lower CO₂ and N₂O concentrations. The partial pressure of CO₂ and dissolved N₂O concentrations decreased linearly with elevation, likely linked to the altitudinal patterns of forest cover and groundwater table depth. Dissolved CH₄ concentrations and the three GHGs fluxes showed weak relationships with elevation. We observed significant seasonal differences in GHG fluxes, with higher evasion rates during the wet season. The seasonal and spatial heterogeneity in stream GHG concentrations and fluxes was primarily controlled by hydrology, climate, and geomorphology. Our analyses also revealed that GHG fluxes were positively correlated with stream water temperature, velocity, and channel slope. This study demonstrates that these alpine streams are underestimated net sources of GHGs, particularly CO₂ and N₂O, highlighting the importance of mountain headwater systems in regional and global GHG budgets. The diverse altitudinal patterns of GHG dynamics also suggest complex controls of GHG in alpine streams and rivers.