Carbon balance in an agriculture-disturbed forested headwater stream in South China: Variations and environmental controls.

Headwater streams are key contributors to global carbon emissions, yet the mechanisms regulating stream carbon transport and emissions in subtropical forested headwaters remain insufficiently understood. We conducted long-term monitoring campaign in a subtropical forested headwater watershed in South China to quantify stream carbon fluxes and to assess their seasonal patterns and environmental controls. The results show that gaseous carbon emissions dominated total carbon loss from the stream network, with annual CO2 and CH4 emissions (1843.56 t C·a-1 and 0.88 t C·a-1, respectively) substantially exceeding downstream exports of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) (203.72 t C·a-1 and 82.23 t C·a-1, respectively). Agricultural land cover was positively correlated with DIC, DOC, and CH4 across watershed, riparian, and sampling site scales, but negatively correlated with CO2, indicating that low-intensity agricultural disturbance can slightly alter carbon processing in forested headwater streams. In addition, environmental factors such as pH, dissolved oxygen, electrical conductivity, channel slope, and elevation also played key roles in regulating carbon fluxes, thereby highlighting the combined influences of land use and hydro-geomorphic controls. Overall, this study demonstrates that carbon fluxes in agriculture-disturbed forested headwater streams are governed by the interplay of watershed characteristics, water chemistry, and land cover patterns. These findings advance our understanding of stream carbon emission processes and impact factors that are useful for regional carbon budget assessment and environmental management.