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Terrestrially derived dissolved organic carbon (DOC) is an importantcomponent of biogeochemical cycling in river channels. Despite this, theprocesses controlling its export from terrestrial ecosystems to riverchannels are not well known. Sorption is thought to be an importantprocess in controlling riverine DOC concentrations. We describe thesorption of litter-derived DOC by soils of the Barreiras sedimentformation in the Amazon basin. Soils were collected along a singletransect of a soil toposequence. Clay-rich soils dominate on plateaus andslopes, whereas sandy soils dominate in valleys that compose riparianzones of the region. Soils from each topographic position were subjectedto sorption experiments, and soil properties were analyzed. Based on ourresults, the toposequence was divided into two sorption regions. Plateauand slope soils sorbed 60 T 5% of initial DOC, whereas valley soils sorbed34 T 4%. Plateau and slope soils sorbed DOC twice as quickly (t1/2 e 1440min) as valley soils (t1/2 = 2880 min). A regression of sorption experimentresults and soil properties showed that sorption correlates withboth soil organic C content and mineral surface area. Our results suggestthat control of riverine DOC concentrations by riparian zones is theresult of the sorption mechanism operating in soils of this region of theAmazon River basin. In conjunction with hydrologic models and moredetailed soil data, it may be possible to apply results from similarreplicated studies to the landscapes of the Amazon basin in an effort tobetter understand C dynamics in tropical river basins.

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