Soil Carbon Residence Time Regulates the Age of Dissolved Organic Matter in Global Rivers

It has long been recognized that rivers transport large amounts of carbon across the Earth system, yet the factors controlling the “age” of this carbon have remained poorly understood. Now, writing in National Science Review, a research team led by Professor Yongqiang Zhou from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences offers a new explanation. By integrating an unprecedented global dataset with machine learning approaches, the team constructed the first high-resolution global atlas of riverine dissolved organic carbon (DOC), including its radiocarbon (Δ¹⁴C) and stable carbon isotope (δ¹³C) signatures. Their results suggest that the age of riverine DOC is fundamentally governed by how long carbon resides in soils before entering aquatic systems, providing a new framework for understanding how climate, hydrology, and soil processes interact to regulate carbon cycling in rivers.

Global patterns of riverine DOC concentration and its δ¹³C and Δ¹⁴C signatures

Rivers, once viewed as passive conduits transporting terrestrial carbon to the ocean, are increasingly recognized as active processors of carbon. Globally, riverine DOC concentrations span three orders of magnitude, with a mean of 6.6 mg C L^-1. The highest concentrations occur in permafrost-influenced and forested catchments, whereas glacial rivers exhibit extremely low DOC levels. Model predictions further indicate that more than half of global rivers contain less than 5 mg C L^-1, with pronounced peaks in Arctic and boreal regions and relatively lower concentrations in the tropics. The study shows that δ¹³C-DOC values vary widely (–43.8‰ to –12.1‰), reflecting diverse sources and biogeochemical processing pathways. Tropical rivers are largely dominated by inputs from C3 vegetation, while temperate rivers exhibit a mixture of terrestrial plant material and in-stream primary production. In contrast, Δ¹⁴C-DOC reveals a strikingly broad age spectrum, ranging from modern carbon to material exceeding 29,000 years in age. On average, riverine DOC has a Δ¹⁴C value of –22.5‰, corresponding to a mean radiocarbon age of approximately 221 years, with nearly 60% of DOC has ¹⁴C ages < 100 years. Nevertheless, distinctly aged carbon persists in high-latitude and high-elevation regions, particularly where permafrost thaw and glacial processes mobilize long-stored carbon pools.