The role of sorption in retention of dissolved organic carbon in soils typical of the lowland Amazon basin Sonya Remington 1 ( Vania Neu 2, Jeffrey Richey 1 1 University of Washington, Seattle, WA, USA 2 CENA-Universidade de S o Paulo, Piracicaba, SP, Brazil Introduction Soils play an important role in regulating carbon transport from terrestrial ecosystems to river channels, where carbon may be evaded from river surfaces as CO 2. Constraining this flux of CO 2 from river surfaces to the atmosphere is important in balancing the carbon budget of tropical terrestrial ecosystems. Sorption is a major soil process that retains DOC in terrestrial ecosystems and was studied here in order to better understand sources of carbon fueling CO 2 evasion from river surfaces. Question: How much dissolved organic carbon (DOC) is initially sorbed by soils of the lowland Amazon basin? Hypothesis: The quantity of DOC initially sorbed to soils of the lowland Amazon basin is a function of the Fe- and Al-oxide content, the mineral surface area, and the percent organic carbon content of soils. Methods Study site: ZF-2 forest site, N of Manaus Collected six soil samples from A and B horizons (0-15cm and 1m, respectively) from plateau-slope-valley sequence: plateau = Oxisols, slope = Ultisols, valley = Entisols Derived partition coefficients (fraction of initial DOC sorbed after 24 hours) and DOC sorption rates (mg DOC sorbed/g soil per minute) for six soils types from batch sorption experiments Measured Fe- and Al-oxide content, mineral-SA, and %OC of soils Multiple linear regressions dependent variables: partition coefficient, DOC sorption rate independent variables: measured soil properties Regression results: partition coefficient = (mineral-SA) – 4.4*%OC, r 2 = sorption rate = (Al-oxide content) – 1.1(OC:Fe-oxide), r 2 = ResultsModel Application Figure 1. Example of isotherm used to derive partition coefficients.. Figure 2. Example of isotherm used to derive DOC sorption rates.. Table 1. Sorption characteristics and soil properties for six soil types. Conclusions To atmosphere Sorbed DOC minutes to hours Soil DOC Model For each grid cell, for each day: Hydrology Model + New DOC input DOC in solution Respired to CO 2 years to decades To river via subsurface or groundwater Eroded to river To river via subsurface or groundwater Figure 3. Example of a soil map for the Amazon basin that may be used in a coupled biogeochemical- hydrology model. Different colored squares represent different soil types. Both A and B horizons for plateau and slope samples sorbed about 60% of new DOC, while the Valley samples sorbed significantly less (about 30%). The sorption rate for all soils is about 0.2 mg DOC/g soil per minute, except for the Slope B horizon sample (which is about half or 0.1 mg DOC/g soil per minute) Given the experimental conditions during the batch experiments of constant contact of DOC with soil particles and no flow, these results represent maximum DOC sorption possible in the field. All results need to be tested with field data for validation.