image: Figure 1. Global distribution and latitudinal patterns of predicted riverine dissolved organic carbon (DOC) concentration, δ13C and Δ14C values.
Credit: ©Science China Press
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.
14C age and provenance of riverine DOC
To further disentangle DOC sources, the researchers applied a four-endmember isotope mixing model. Their results show that fossil or petrogenic carbon contributes only a small fraction (6.7%) to the global DOC pool, although this contribution can reach up to 40% in Arctic and alpine environments. In contrast, modern terrestrial organic carbon inputs and riverine autochthonous production dominate globally, contributing approximately 38% and 44%, respectively. Additionally, DOC derived from Holocene-aged sediments accounts for about 10.7%, particularly in high-latitude floodplains where permafrost degradation and sediment reworking release previously stored carbon. Together, these patterns illustrate that, although ancient carbon sources are locally important, modern carbon from terrestrial and autochthonous production, together with aged Holocene carbon, are predominant in the global riverine DOC pool, with distinct geographical controls governing their distribution.
Drivers of Δ14C in river DOC
The study highlights that climatic variables soil properties emerge as important controls of the spatial distribution of Δ14C-DOC. Climate variables, especially temperature and precipitation, regulate soil carbon turnover and leaching processes, thereby controlling the 14C signature of DOC exported to rivers. Hydrological pathways further influence DOC dynamics by transporting both recently fixed carbon and older carbon from deeper soil layers via surface and subsurface runoff. In addition to climate controls, the study finds a strong relationship between riverine Δ14C-DOC and soil organic carbon Δ14C, with riverine DOC closely resembling surface soil signatures. This indicates that riverine DOC is predominantly derived from surface soil rather than deeper soil horizons. Human activities introduce additional complexity. Reservoir impoundments can stimulate algal production, increasing the contribution of modern, 14C-enriched DOC, whereas agriculture and urbanization may enhance the export of older carbon through soil disturbance and fossil-derived inputs. These contrasting influences underscore the sensitivity of riverine carbon dynamics to land-use change.
By comparing Δ14C signatures across different carbon pools, the study shows that riverine DOC is closely correlated with soil organic carbon and more closely resembles surface soil signatures, indicating that it is primarily derived from topsoil. In contrast, particulate organic carbon (POC) is considerably older, reflecting differences in transport pathways and transformation processes. Overall, riverine DOC is dominated by relatively “young” carbon, making its age structure highly sensitive to climate change and human activities.
About the study
This work fills a critical gap in global-scale understanding of riverine carbon by linking terrestrial carbon storage, mobilization, and aquatic processing into a unified framework. By revealing how soil carbon residence time controls the age and origin of riverine DOC, the study provides a mechanistic basis for predicting how ongoing climate change may reshape carbon cycling across the land–water continuum. The study was carried out through close international collaboration. Co-first authors include Zhaohui Liu, a master student at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, and Professor Gerard Rocher-Ros from the Climate Impacts Research Centre at Umeå University. The research was led by Professor Yongqiang Zhou from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. Contributing authors include Professors Joshua F. Dean, Jack J. Middelburg, Pierre Regnier, Jan Karlsson, Wang Chenglong, R. Iestyn Woolway, Travis W. Drake, Robert G. M. Spencer, and Peter R. Leavitt, as well as researchers Liwei Zhang, Lei Zhou, Jianjun Wang, Yunlin Zhang, and a Ph.D. student Weipeng Lin. This work was supported by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the CAS President’s International Fellowship Initiative.