Soil carbon residence time regulates the age of dissolved organic matter in global rivers

Riverine dissolved organic carbon (DOC) plays a pivotal role in the Earth’s carbon cycle, yet little is known about the global patterns, sources, and factors governing lotic DOC. Here, we combine global data and employ machine-learning to generate a global atlas of riverine DOC concentration and radiocarbon (Δ14C) and stable-carbon (δ13C) isotopic signatures. Globally, riverine DOC has an average Δ14C value of –22.5 ± 144.0‰ (age of 221 years), with fossil carbon contributing only 6.7 ± 3.0% of riverine DOC. Terrestrial and river autochthonous production are the dominant DOC sources (>80%) at the global scale, with contemporary terrestrial DOC dominating in tropical rivers and within-river production being prominent in temperate and semi-arid regions. Rivers draining high-latitude regions and high-elevation sites have the lowest Δ14C values (–353‰ to –78‰; ages between 3400 and 600 years). River dissolved organic carbon Δ14C values correlate with soil organic carbon and riverine particulate organic carbon Δ14C values, but river DOC has much higher Δ14C values than subsurface soils indicating that riverine DOC originates from surface rather than subsurface soils. Soil carbon residence time regulates global variations in the age of DOC in rivers because warming mobilizes aged organic carbon from soils, glaciers, and permafrost into drainage networks. This mobilization will likely accelerate carbon cycling processes in downstream rivers, lakes, and coastal marine ecosystems under future warming climate.