Underestimated Carbon Losses as Dissolved Organic Carbon in Forested Headwater Streams

Abstract Large amounts of dissolved organic carbon (DOC) are transported laterally from uplands and wetlands to headwater streams; however, the global magnitude of this flux and its role in the carbon (C) budget remain unclear. By compiling 20,403 DOC concentration observations and applying machine learning models, we estimated the annual DOC flux from forested headwater streams north of 30°S to be 116.2 Tg C yr −1 (90% CI: 86.5–145.2). Higher yields were observed in tropical regions (3.8 g C m −2 yr −1 ), where vegetation productivity is high, and in boreal regions (3.1 g C m −2 yr −1 ), where peatland cover and soil organic C stocks are substantial. When comparing our results with global terrestrial C flux estimates upscaled from eddy covariance data, we found that the fraction of net ecosystem production (NEP) lost annually as DOC ranged from negligible (<0.1%) to 20.7%, with discharge explaining 34% of the variation. Our study suggests that neglecting the lateral export of DOC could lead to an overestimation of NEP in forested headwater catchments, a bias that is further amplified by increased discharge.