Dissolved CO2 in Small Catchment Streams of Eastern Amazonia

Eric A. Davidson1, Ricardo de O. Figueiredo2,Daniel Markewitz3, and Patrício de S. Silva4

1. The Woods Hole Research Center;2. Embrapa Amazônia Oriental;

3. The University of Georgia; 4. Universidade do Estado do Pará / CNPq-CTHidro

Landsat 1999 sceneLandsat 1999 scene

Three watersheds near Paragominas

(Eastern Amazonia)

Igarapé 54 (Cq) 137 km2; 18% forest; 1.5 m3 s-1

Igarapé Sete (St) 161 km2; 34% forest; 2.9 m3 s-1

Igarapé Pajeú (Pj) 32 km2; 45% forest; 0.3 m3 s-1

Field campaigns April 2003 to July 2004, from headwaters in remnant mature forests, through pastures, secondary forests, and agricultural fields.

CropsPasture

IGARAPÉ DO SETE

IGARAPÉ PAJEÚ

A view of a farm area between two of the studied streamsA view of a farm area between two of the studied streams

• Water samples were collected from 20 cm below the stream surface using 60cc syringes

• Syringes were transported to a field laboratory

• 30 mL of water was injected into a serum bottle with 30 mL of CO2–free air and incubated in a water bath for gas and liquid phases to come into equilibrium

• Headspace CO2 concentration was determined using LiCor IRGA

• The pCO2 in the original streamwater sample was calculated, adjusting for temperature (Henry’s law), acidity (bicarbonate chemistry), and correcting for fugacity.

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

IG5

4-1

IG5

4-2

IG5

4-3

IG5

4-4

IG5

4-5

IG7

-1

IG7

-2

IG7

-3

IG7

-4

IG7

-5

IG7

-6

IG7

-7

IGP

-1

IGP

-2

IGP

-3

Sampling station

pH

0

5

10

15

20

25

Oct

-03

Dec

-03

Feb-

04A

pr-0

4Ju

n-04

Aug

-04

Oct

-04

Dec

-04

Feb-

05A

pr-0

5Ju

n-05

Aug

-05

mL

CO

2/L

wat

er

IGP-1

IGP-2

IGP-3

1 mL CO2/L 10,000 ppmv 40 mol CO2/L 1300 atm

0

5

10

15

20

25

Oct

-03

Dec

-03

Feb-

04A

pr-0

4Ju

n-04

Aug

-04

Oct

-04

Dec

-04

Feb-

05A

pr-0

5Ju

n-05

Aug

-05

mL

CO

2/L

wat

er

IG7-1

IG7-2

IG7-3

IG7-4

IG7-5

IG7-6

IG7-7

1 mL CO2/L 10,000 ppmv 40 mol CO2/L 1300 atm

0

5

10

15

20

Oct

-03

Dec

-03

Feb-

04A

pr-0

4Ju

n-04

Aug

-04

Oct

-04

Dec

-04

Feb-

05A

pr-0

5Ju

n-05

Aug

-05

mL

CO

2/L

wat

er IG54-1

IG54-2

IG54-3

IG54-4

IG54-5

1 mL CO2/L 10,000 ppmv 40 mol CO2/L 1300 atm

0

2

4

6

8

10

12

14

16

18

20

IG5

4-1

IG5

4-2

IG5

4-3

IG5

4-4

IG5

4-5

IG7

-1

IG7

-2

IG7

-3

IG7

-4

IG7

-5

IG7

-6

IG7

-7

IGP

-1

IGP

-2

IGP

-3

Sampling station

mL

CO

2 /

L w

ate

r

1 mL CO2/L 10,000 ppmv 40 mol CO2/L 1300 atm

Richey et al. 2002 – Amazon main stem and major tributaries

2,000 – 10,000 ppmv2,000 – 10,000 ppmv

Johnson et al. 2006 – small forested catchment, Mato Grosso

49,000 ppmv in emergent springwater49,000 ppmv in emergent springwater

Sousa, 2007 – Acre River, Acre

4,000 ppmv4,000 ppmv

Da Rosa, 2007 – small catchments Igarapé-Açu, Pará

77,500 – 24,000 ppmv near headwaters,500 – 24,000 ppmv near headwaters

This study – small catchments in largely deforested area of Pará

15,000 ppmv near headwaters15,000 ppmv near headwaters

 

Watershed Max pCO2

(mLCO2/L) Mean

Discharge (m3/s)

Max CO2 evasion (MgC)

Area (ha)

Max efflux (MgC/ha/yr)

Pajeu

16 0.3 70 3250 0.02

Sete

15 2.9 690 16100 0.04

Cinqüenta e Quatro

8 1.5 360 13700 0.03

• Assume that all water entering the stream has the maximum observed mean pCO2

• Assume that all pCO2 is evaded to the atmosphere

• Realize that these assumptions yield maximum possible efflux rates, which is probably an overestimate

• Mean annual litterfall in the mature forest at Paragominas is 4-5 MgC/ha/yr

• Efflux of CO2 from pasture and forest soils is 10-20 MgC/ha/yr

• Therefore, <1% of the C fixed in the terrestrial system is lost via transport Therefore, <1% of the C fixed in the terrestrial system is lost via transport of pCO2 to streams and evasion from the streamof pCO2 to streams and evasion from the stream

Fluxo Vertical e Subsuperficial

Fluxo superficial (hortoniano e de retorno)

Fluxo superficial por saturação de área

Boa Drenagem

Drenagem impedida por camada de plintita

Sem drenagem, área saturada

Latossolo Amarelo Plintossolos Gleissolo

Lençol freático

plintita

(Schuler, 2004)Johnson et al. 2006. Biogeochem. 81:45–57

Where does the stream pCO2

come from?

Soil water in equilibrium with soil Soil water in equilibrium with soil atmosphere @ 8% COatmosphere @ 8% CO22 @ 25 @ 25°°C, C, would have pCOwould have pCO2 2 ~ 50 mL/L, which ~ 50 mL/L, which is about 3Xs what we measuredis about 3Xs what we measured

 

Watershed Max pCO2

(mLCO2/L) Mean

Discharge (m3/s)

Max CO2 evasion (MgC)

Area (ha)

Max efflux (MgC/ha/yr)

Pajeu

16 0.3 70 3250 0.02

Sete

15 2.9 690 16100 0.04

Cinqüenta e Quatro

8 1.5 360 13700 0.03

• Assume that all water entering the stream has the maximum observed mean pCO2

• Assume that all pCO2 is evaded to the atmosphere

• Realize that these assumptions yield maximum possible efflux rates, which is probably an overestimate

• Mean annual litterfall in the mature forest at Paragominas is 4-5 MgC/ha/yr

• Efflux of CO2 from pasture and forest soils is 10-20 MgC/ha/yr

• Therefore, ~1% of the C fixed in the terrestrial system is lost via transport Therefore, ~1% of the C fixed in the terrestrial system is lost via transport of pCO2 to streams and evasion from the streamof pCO2 to streams and evasion from the stream

0.06

0.12

0.09

0.004 Mg DOC-C ha-1 yr-1

Markewitz et al. 2004. Ecol. Appl. 14:S177–S199

Inputs of DOC and pCO2 to streams may be important for aquatic ecology, but are trivial relative to terrestrial C budgets

Conclusions

• Measured pCO2 concentrations in stream headwaters were consistent with previous reports in the literature for Amazonian streams and rivers

• If this C originates primarily from primary productivity in terrestrial ecosystems and soil respiration, it is a trivial pathway of CO2 loss from the terrestrial biosphere, representing no more than 1% of CO2 efflux from the soil surface on an area-weighted basis.

• 80-90% of the pCO2 in headwaters evaded to the atmosphere within 100m downstream