Role of Sorption in Retention of Dissolved Organic Carbon in Soils of the Lowland Amazon Basin

1. Role of Sorption in Retention of Dissolved Organic Carbon in Soils of the Lowland Amazon Basin Sonya Remington1, Jeff Richey1, Vania Neu2 1University of Washington, Seattle, USA 2CENA, Piracicaba, Brazil

2. For each grid cell, for each time step: Hydrology From hydrology model: 60% of rain = subsurface flow 30% of rain = groundwater flow 10% of rain = surface runoff New DOC enters soil from various sources Partition Coefficient Minutes to hours Sorption DOC Sorbed DOC Remaining in Soil Solution To Atmosphere To River Permanently Sorbed (becomes SOM) Decades to centuries Years to decades (via subsurface or groundwater flow) Eroded into River Mineralization Respired to CO2 Erosion (via surface runoff) (via subsurface or groundwater flow) To River

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

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

5. Export of carbon from the Amazon River system (Richey et al, Nature 2002): CO2 evasion: 470 TgC/yr Riverine transport: 70 TgC/yr (Amazon) 800 TgC/yr (global flux) Sources of carbon fueling evasion: CO2 and organic carbon CO2 Evasion Dissolved carbon from soils = 40% DOC is about ½ Entrainment: Litterfall Macrophytes Subsurface: DOC DIC 25% 35% 15% 25% Estimated % contributions from Richey et al (Nature 2002). Implications: Role of tropical systems as net source or sink of CO2 Role of rivers in global carbon cycle

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

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

8. 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 Kinetic batch experiments (1 min – 48 hrs)

9. Sample size, n = 5 Multiple linear regression sorption partition coefficient = 0.44 + 5.38*mineral surface area– 4.7*%OC r2 = 0.93 partition coefficient = f (mineral-SA, %OC, …..) Respiration Sorption

10. Test catchment: Reserva Ducke, Annual DOC retention 1.5 km2, Oxisols, Riparian Zone Width = 20m (McClain etal 1997) Riparian Hillslope 1 1 2 2 3 3 4 • • • • • • Annual retention = 92.2 % Annual retention = 99.9 % 5 (Riparian zone as main DOC source to river) 25 (McClain et al 1997 = 99.8%) Applying results in a real-world model Matrix interaction? Yes Sometimes Factors affecting sorption Experiment conditions In-situ soil conditions Flow conditions No flow Both Film diffusion? No Yes Flow predominately vertical. (Nortcliff and Thornes 1989, Elsenbeer et al) 80 g C/m2 yr input as DOC (10% of C input solubilized to DOC) Soil layer depth = f (soil:solution ratio = 1:10) Oxisol partition coefficient = 0.60 Average bulk density = 1.2 g/cm3 (riparian) 1.4 g/cm3 (hillslope) (Nortcliff and Thornes 1989) Depth to groundwater = 50 cm (riparian zone) 250 cm (hillslope) (McClain et al 1997) Experimental results = Maximum DOC sorption

11. 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

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

13. 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