Global Riverine Export of Dissolved Lignin Constrained by Hydrology, Geomorphology, and Land-Cover

The land-to-ocean export of terrestrial dissolved organic matter (tDOM) links the two largest organic carbon (OC) pools on Earth and is changing due to anthropogenic activity and climate change. Lignin phenols are often utilized as diagnostic markers for tDOM in aquatic systems, but information on the global dissolved lignin flux is still uncertain as linkages between future changes in export and global changes are poorly constrained. Thirty-two new measurements from 15 Chinese rivers and 548 measurements collected from 97 global rivers were combined to analyze its distributions, fluxes, and potential controls. Dissolved lignin concentrations (in μg L⁻¹) varied by more than two orders of magnitude (0.7–138 μg L⁻¹) and were positively correlated with dissolved OC (DOC) concentrations globally, suggesting coupled export of dissolved lignin and DOC. The carbon-normalized lignin yields (in mg g OC⁻¹) varied by an order of magnitude (<1–30 mg (g OC)⁻¹) and was significantly lower in the Arctic than in temperate and tropical rivers. Correlation analyses indicated that the dissolved lignin flux (F_Lignin) was mainly affected by water discharge and mean basin slope, while the area-normalized yield of dissolved lignin as well as discharge-weighted lignin concentration strongly and positively correlated with forest coverage. We, for the first time, estimated global F_Lignin ranges from 0.58 to 1.2 Tg yr⁻¹ by using different approaches. Our synthesis reveals that future changes in land-cover that is driven by climate change as well as other human activities would impose a significant influence on the land-to-ocean export of tDOM.