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

Transcription

1 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

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

3 c R t = D x 2 c? 2 c v t Sandy soils of relatively uniform grain-size, saturated flow Soils highly variable in space Grain-size not uniform Flow not always saturated Plateau Oxisols (Ferralsols=FAO) (Latossolos = Brazil) Slope Batch sorption experiments for soils of Tertiary Barreiras formation Ultisols (Acrisols = FAO) (Argisols = Brazil) Valley (Devol and Hedges, 2001) (Bravard and Righi 1989) (Spodosols)

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): 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 Sources of carbon fueling evasion: CO 2 and organic carbon Dissolved carbon from soils = 40% DOC is about ½ CO 2 Evasion Entrainment: Litterfall 35% Macrophytes 25% Subsurface: DOC DIC 15% 25% Estimated % contributions from Richey et al (Nature 2002).

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

8 Plateau Plateau B Horizon A Horizon Slope Slope B Horizon A Horizon Plateau = Oxisol Slope = Ultisol Valley = Spodosol Valley Valley B Horizon A Horizon DOC Sorbed (mg DOC/g soil) hr batch experiments Plateau, B Horizon R 2 = , s(y) = Initial DOC (mg DOC/g soil) Kinetic batch experiments (1 min 48 hrs) DOC Sorbed (mg DOC/g soil) Plateau B Horizon Time (min)

9 Multiple linear regression sorption partition coefficient = *mineral surface area r 2 = *%OC Sample size, n = 5 partition coefficient = f (mineral-sa, %OC,..) DOC lost (mg) DOC loss: respiration versus sorption Respiration Sorption C1r C2r C3r C4r C5r C1s C2s C3s C4s C5s

10 Flow predominately vertical. (Nortcliff and Thornes 1989, Elsenbeer et al) Test catchment: Reserva Ducke, Annual DOC retention 1.5 km 2, Oxisols, Riparian Zone Width = 20m (McClain et al 1997) Riparian 80 g C/m 2 yr input as DOC (10% of C input solubilized to DOC) Hillslope Annual retention = 92.2 % (Riparian zone as main DOC source to river) Factors affecting sorption Matrix interaction? 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) Applying results in a real-world model Annual retention = 99.9 % Experiment conditions In-situ soil conditions Yes Sometimes (McClain et al 1997 = 99.8%) Flow conditions No flow Both Film diffusion? No Yes 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 More detailed DOC analyses (DOC size fractions, LMWOAs) in different hydrological regimes Lateral Hydrological Flowpaths in Rainforest Ecosystems (Elsenbeer et al) Central Amazônia (Reserva Ducke) Central Amazônia Peninsular Malaysia Panama Rondônia Paragominas Panama Peruvian Amazon Queensland Rondônia (Rancho Grande) 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

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