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, USA2CENA, Piracicaba, Brazil

From hydrology model:60% of rain = subsurface flow 30% of rain = groundwater flow10% 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 inSoil Solution

To RiverPermanently

Sorbed(becomes SOM)

Respired to CO2

To Atmosphere

Eroded intoRiver

To River

(via subsurface or groundwater flow)

(via surface runoff)

(via subsurface or groundwater flow)

Minutes to hours

Years to decades

Decades to centuriesErosion

Partition Coefficient

(Devol and Hedges, 2001)

t

cv

xD

t

cR

2

2c ?

Plateau

Oxisols(Ferralsols=FAO)

(Latossolos = Brazil) Slope

Ultisols(Acrisols = FAO)

(Argisols = Brazil) Valley

(Spodosols) (Bravard and Righi 1989)

Batch sorption experimentsfor 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

CO2 Evasion

Subsurface:

Entrainment: Litterfall

DOC DIC

25%

25%15%

35%

Sources of carbon fueling evasion: CO2 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):CO2 evasion: 470 TgC/yrRiverine transport: 70 TgC/yr (Amazon)

800 TgC/yr (global flux)

Implications: Role of tropical systems as net source or sink of CO2

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

SiteLocation:

Asu Catchment

Batch Sorption Experiments

DOC

DOC Stock Solution

Distilled water

Natural DOC stock solution

0.7um (GF/F) filter

DilutionsmL of DOC stock mL of artificial

stock solution inorganic solutionConc 1 18 2Conc 2 15 5Conc 3 10 10Conc 4 5 15Conc 5 2 18Conc 6 0 20

(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 andanalyze for DOCInitial Solution

Dry and weigh soilPoison filtratewith HgCl2 andanalyze for DOCFinal Solution

Equilibrium = 24 hoursKinetic = 1min to 48 hours

Plateau B Horizon

0

5

10

15

20

25

30

35

0 500 1000 1500 2000Time (min)

DO

C S

orb

ed

(

mg

DO

C/g

so

il)

Kinetic batch experiments(1 min – 48 hrs)

24 hr batch experiments

Plateau, B Horizon

R2 = 0.9976, s(y) = 0.02

-0.1

0.1

0.3

0.5

0.7

0.0 0.5 1.0

Initial DOC (mg DOC/g soil)

DO

C S

orb

ed

(m

g D

OC

/g s

oil)

0.58 0.57 0.68 0.57 0.30 0.37

Plateau = OxisolSlope = UltisolValley = Spodosol

PlateauB Horizon

PlateauA Horizon

SlopeB Horizon

SlopeA Horizon

ValleyB Horizon

ValleyA Horizon

Multiple linear regression

sorption partition coefficient = 0.44 + 5.38*mineral surface area – 4.7*%OCr2 = 0.93

Sample size, n = 5

partitioncoefficient

= f (mineral-SA, %OC, …..)

DOC loss: respiration versus sorption

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

DO

C l

ost

(mg

)

C1r

C2r

C3r

C4r

C5r

C1s

C2s

C3s

C4s

C5sRespiration Sorption

Experimental results = Maximum DOC sorption

Applying results in a real-world model

Film diffusion?

Flow conditions

SometimesYesMatrix interaction?

No flow Both

No Yes

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/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)

Test catchment: Reserva Ducke, Annual DOC retention1.5 km2, Oxisols, Riparian Zone Width = 20m

(McClain et al 1997)

•••

1

2

3

25

•••

Hillslope

Annual retention = 99.9 %Annual retention = 92.2 %

1

2

4

3

5

Riparian

(McClain et al 1997 = 99.8%)(Riparian zone as main DOC source to river)

80 g C/m2 yr input as DOC(10% of C input solubilized to DOC)

Flow predominatelyvertical.

(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

PanamaPeruvian AmazonQueenslandRondônia (Rancho Grande)

PanamaRondônia

PeninsularMalaysia

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