Riverine dissolved organic carbon (DOC) is a key player in Earth's carbon cycle, but the global patterns, sources, and controls of lotic DOC have remained largely unknown. However, a recent study published in National Science Review revealed that the age of DOC in global rivers is governed by how long carbon resides in soils before entering aquatic systems.

Led by Prof. ZHOU Yongqiang from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, the research team has constructed the first high-resolution global maps of riverine DOC concentration, along with its radiocarbon (Δ¹⁴C) and stable carbon isotope (δ¹³C) signatures.

To achieve this, the team combined a comprehensive global database with machine learning approaches. The study uncovers the sources, spatial distribution, and age characteristics of riverine DOC, quantifying contributions from different endmembers.

The results show that riverine DOC spans an age range from modern carbon to material over 29,000 years old, with a mean radiocarbon age of roughly 221 years. Nearly 60% of DOC has a ¹⁴C age of less than 100 years. Yet aged carbon persists in high-latitude and high-elevation regions, especially where permafrost thaw and glacial processes mobilize long‑stored carbon pools.

Using a four‑endmember isotope mixing model, the researchers found that fossil or petrogenic carbon contributes only 6.7% to the global DOC pool. In contrast, modern terrestrial organic carbon and riverine autochthonous production dominate, accounting for about 38% and 44%, respectively. DOC derived from Holocene sediments makes up about 10.7%, particularly in high‑latitude floodplains.

The researchers highlight that soil carbon residence time—shaped by climate and hydrological processes—is a key regulator of riverine DOC age. Notably, warming‑induced permafrost thaw is accelerating the release of ancient "old carbon" into river systems. Once mobilized, this carbon can be transported downstream and participate in aquatic biogeochemical processes, potentially intensifying carbon‑cycle feedbacks to the climate system.

These findings fill a gap in the global‑scale understanding of riverine carbon cycling, linking terrestrial carbon storage, mobilization, and aquatic processing into a unified framework, the researchers said. They also provide a mechanistic basis for predicting how ongoing climate change may reshape carbon cycling across the land–water continuum, according to the team.