Sonya Remington 1 (e-mail: sunny9@u.washington), Vania Neu 2 , Jeffrey Richey 1

1. The role of sorption in retention of dissolved organic carbon in soils typical of the lowland Amazon basin For each grid cell, for each day: Hydrology Model + Soil DOC Model New DOC input minutes to hours Sorbed DOC DOC in solution To river via subsurface or groundwater years to decades Respired to CO2 Eroded to river To river via subsurface or groundwater Sonya Remington1 (e-mail: sunny9@u.washington.edu), Vania Neu2, Jeffrey Richey1 1University of Washington, Seattle, WA, USA 2CENA-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 CO2. Constraining this flux of CO2 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 CO2 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 Results Model Application To atmosphere 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. Figure 1. Example of isotherm used to derive partition coefficients.. Figure 2. Example of isotherm used to derive DOC sorption rates.. Conclusions • 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. Table 1. Sorption characteristics and soil properties for six soil types. Regression results: partition coefficient = 0.44 + 5.38(mineral-SA) – 4.4*%OC, r2=0.9266 sorption rate = -0.15 + 1.49(Al-oxide content) – 1.1(OC:Fe-oxide), r2=0.9623