Land-surface models are important tools for simulation of the past, present, and future capacity of terrestrialecosystems to absorb anthropogenic CO2emissions. However, fluvial carbon (C) transfers are presently ne-glected in these models. Using the Amazon basin as a case study, we show that this negligence leads to sig-nificant underestimation of the net uptake of atmospheric C while terrestrial C storage changes are overesti-mated. These biases arise from the fact that C—in reality, leached from soils and exported through the rivernetwork—is instead represented as partly being respired and partly being stored in soils. Moreover, thesebiases scale mainly to the fluvial C export to the coast, despite aquatic CO2emission to the atmosphere beingthe major pathway of riverine C exports. We further show that fluvial C transfers may change significantly inresponse to changes in either hydrology or in atmospheric C uptake by vegetation.