Role of Sorption in Retention of Dissolved Organic Carbon in Soils of the Lowland Amazon Basin Sonya Remington 1, Jeff Richey 1, Vania Neu 2 1 University of Washington, Seattle, USA 2 CENA, Piracicaba, Brazil

From hydrology model: 60% of rain = subsurface flow 30% of rain = groundwater flow 10% of rain = surface runoff For each grid cell, for each time step: Sorption Mineralization Hydrology New DOC enters soil from various sources DOC Sorbed DOC Remaining in Soil Solution To River Permanently Sorbed (becomes SOM) Respired to CO 2 To Atmosphere Eroded into River To River (via subsurface or groundwater flow) (via surface runoff) (via subsurface or groundwater flow) Minutes to hours Years to decades Decades to centuries Erosion Partition Coefficient

(Devol and Hedges, 2001) ? Plateau Oxisols (Ferralsols=FAO) (Latossolos = Brazil) Slope Ultisols (Acrisols = FAO) (Argisols = Brazil) Valley (Spodosols) (Bravard and Righi 1989) Batch sorption experiments for soils of Tertiary Barreiras formation Soils highly variable in space Grain-size not uniform Flow not always saturated Sandy soils of relatively uniform grain-size, saturated flow

Why develop a large-scale biogeochemical model for river basins? Why focus on DOC in soils?

Estimated % contributions from Richey et al (Nature 2002). Macrophytes CO 2 Evasion Subsurface: Entrainment: Litterfall DOC DIC 25% 15% 35% Sources of carbon fueling evasion: CO 2 and organic carbon Dissolved carbon from soils = 40% DOC is about ½ Export of carbon from the Amazon River system (Richey et al, Nature 2002): CO 2 evasion: 470 TgC/yr Riverine transport: 70 TgC/yr (Amazon) 800 TgC/yr (global flux) Implications: Role of tropical systems as net source or sink of CO 2 Role of rivers in global carbon cycle

A Horizon sample: 0-15cm depth B Horizon samples: at ~ 1 m depth Sample collection: 2mm sieve Sample processing: Dry soil sample Sorption experiments Site Location: Asu Catchment

Batch Sorption Experiments DOC DOC Stock Solution Distilled water Natural DOC stock solution 0.7um (GF/F) filter Dilutions (tree leaves as major source of OM to rivers, Hedges et al 1994) 20mL + ~ 2 grams soil Sorption Experiments (soil:solution ratio = 1:10) 40mL DOC solution Mix and filter through 0.7um (GF/F) filter Poison 20mL with HgCl2 and analyze for DOC Initial Solution Dry and weigh soil Poison filtrate with HgCl2 and analyze for DOC Final Solution Equilibrium = 24 hours Kinetic = 1min to 48 hours

Kinetic batch experiments (1 min – 48 hrs) 24 hr batch experiments Plateau = Oxisol Slope = Ultisol Valley = Spodosol Plateau B Horizon Plateau A Horizon Slope B Horizon Slope A Horizon Valley B Horizon Valley A Horizon

Multiple linear regression sorption partition coefficient = *mineral surface area – 4.7*%OC r 2 = 0.93 Sample size, n = 5 partition coefficient = f (mineral-SA, %OC, …..) RespirationSorption

Experimental results = Maximum DOC sorption Applying results in a real-world model Film diffusion? Flow conditions SometimesYesMatrix interaction? No flowBoth NoYes Experiment conditions In-situ soil conditionsFactors affecting sorption Soil layer depth = f (soil:solution ratio = 1:10) Oxisol partition coefficient = 0.60 Average bulk density = 1.2 g/cm 3 (riparian) 1.4 g/cm 3 (hillslope) (Nortcliff and Thornes 1989) Depth to groundwater = 50 cm (riparian zone) 250 cm (hillslope) (McClain et al 1997) Test catchment: Reserva Ducke, Annual DOC retention 1.5 km 2, Oxisols, Riparian Zone Width = 20m (McClain et al 1997) Hillslope Annual retention = 99.9 %Annual retention = 92.2 % Riparian (McClain et al 1997 = 99.8%) (Riparian zone as main DOC source to river) 80 g C/m 2 yr input as DOC (10% of C input solubilized to DOC) Flow predominately vertical. (Nortcliff and Thornes 1989, Elsenbeer et al)

Conclusions Soil toposequence of Tertiary Barreiras formation divided into two “sorption regions” based on partition coefficient: plateaus and slopes = sorb ~60% valleys = sorb ~ 35% Experimental results represent maximum sorption in the field Model results support conclusions of McClain et al (1997) that most DOC is generated in riparian zone/valley bottoms in this region of the Amazon basin

Future Plans Lateral Hydrological Flowpaths in Rainforest Ecosystems (Elsenbeer et al) Central Amazônia Panama Peruvian Amazon Queensland Rondônia (Rancho Grande) Panama Rondônia Peninsular Malaysia Central Amazônia (Reserva Ducke) Paragominas More detailed DOC analyses (DOC size fractions, LMWOAs) in different hydrological regimes Scaling up DOC dynamics from small streams to large rivers

Acknowledgements Napoleao, Antonio Nobre, Martin Hodnett, Javier Tomasela, Regina Luizao and others at INPA and the ZF-2 site. Vania Neu, Alex Krusche, Luiz Martinelli, Reynaldo Victoria and many others at CENA. Anthony Aufdenkampe and Bonnie Dickson, Fieldwork in fall 2002 Jeff Richey, Kellie Balster, Simone Alin, Erin Ellis and the rest of the CAMREX group at UW NSF, NASA and LBA