Terrestrially-derived dissolved organic matter (DOM) is an important component of biogeochemical cycling in river channels. Despite this, the processes regulating its export from terrestrial ecosystems to river channels and its subsequent processing within river channels remains unresolved. During transport from land to river channels, soil type is thought to be the primary controlling factor. However, the specific mechanism operating in soils that regulates DOM transport is uncertain. In the Amazon basin, soils are dominated by clay-rich and sandy varieties. In this dissertation, I estimate the capacity of these two soil types to retain DOM by sorption in terrestrial ecosystems and the kinetics of the sorption mechanism. Results of these experiments revealed that clay-rich soils retained 60 ± 5% of litter-derived dissolved organic carbon (DOC), whereas sandy soils retained 34 ± 4%. The clay-rich soils sorbed DOC twice as rapidly as the sandy soils. A regression of sorption experiment results and soil properties showed that sorption correlates with both soil organic C content and mineral surface area.