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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