Balanced riverine export of millennial and decadal carbon from the Amazon Basin

The riverine mobilization and export of OC from different compartments of the terrestrial biosphere such as standing biomass, organic-rich surface soils and deep mineral soils have contrasting impacts on the carbon cycle. Heretofore, the distribution of these OC sources in river sediments remains poorly constrained, hampering attempts to close the carbon budget of river catchments. Here, we use a time series of radiocarbon measurements that spans four decades to show that particulate OC in the Amazon River is composed of millennial-age soil OC, mixed with up to 60% plant and soil litter contributions, characterized by a turnover of ca. 20 years. This reflects concomitant organic matter stabilization over millennial timescales in soil horizons via mineral protection and mobilization of abundant primary production from the Amazon rainforest. The Amazon Basin thus holds large reservoirs of millennial and decadal carbon that are susceptible to destabilization upon increasing temperature and deforestation, respectively.