The role of sorption in control of riverine dissolved organic carbon concentrations by riparian zone soils in the Amazon basin

Abstract

Terrestrially derived dissolved organic carbon (DOC) is an important component of biogeochemical cycling in river channels. Despite this, the processes controlling its export from terrestrial ecosystems to river channels are not well known. Sorption is thought to be an important process in controlling riverine DOC concentrations. We describe the sorption of litter-derived DOC by soils of the Barreiras sediment formation in the Amazon basin. Soils were collected along a single transect of a soil toposequence. Clay-rich soils dominate on plateaus and slopes, whereas sandy soils dominate in valleys that compose riparian zones of the region. Soils from each topographic position were subjected to sorption experiments, and soil properties were analyzed. Based on our results, the toposequence was divided into two sorption regions. Plateau and slope soils sorbed 60 ± 5% of initial DOC, whereas valley soils sorbed 34 ± 4%. Plateau and slope soils sorbed DOC twice as quickly (t1/2 ≤ 1440 min) as valley soils (t1/2 ≤ 2880 min). A regression of sorption experiment results and soil properties showed that sorption correlates with both soil organic C content and mineral surface area. Our results suggest that control of riverine DOC concentrations by riparian zones is the result of the sorption mechanism operating in soils of this region of the Amazon River basin. In conjunction with hydrologic models and more detailed soil data, it may be possible to apply results from similar replicated studies to the landscapes of the Amazon basin in an effort to better understand C dynamics in tropical river basins. Copyright © 2007 by Lippincott Williams & Wilkins, Inc.