JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, G04005, doi:10.1029/2009JG001202, 2010
Dissolved CO2 in small catchment streams of eastern Amazonia:
A minor pathway of terrestrial carbon loss
Eric A. Davidson,1 Ricardo O. Figueiredo,2 Daniel Markewitz,3
and Anthony K. Aufdenkampe4
Received 2 November 2009; revised 12 May 2010; accepted 18 May 2010; published 5 October 2010.
[1] Production of carbon dioxide (CO2) in soils can lead to supersaturation of dissolved
free CO2 (pCO2) in groundwater, which later evades to the atmosphere as groundwater
enters streams and rivers. This process could be a significant pathway for return of
terrestrially fixed C to the atmosphere. We measured pCO2 monthly over two years at
multiple stations along three streams from their headwaters in remnant mature forests
through multiple land covers in Pará, Brazil. The pCO2 averaged 19,000 matm in
headwaters and decreased to about 4,500 matm downstream. Similar values were measured
in headwaters of two small pristine mature forest catchments. Two approaches were
used to estimate groundwater pCO2 evasion: assuming that headwater pCO2 measurements
reflected incoming groundwater pCO2 or that all entering stream water was in
equilibrium with previously measured deep soil CO2. With these assumptions, losses from
the terrestrial environment through aquatic evasion of pCO2 would be 0.02–0.15 Mg C
ha−1 of land area yr−1, which is about 2–3 orders of magnitude lower than annual estimates
of soil respiration and net primary productivity. However, downstream pCO2 values that
appear to be in quasi‐steady state indicate contributions from other C sources, such as
aquatic primary production, soil erosion, dissolved organic matter, or litter inputs from
streamside vegetation. Hence, lateral pCO2 loss from groundwater to streams is minor for
most of the terrestrial ecosystems of this region, although C loss to streams could be
significant for net terrestrial budgets in riparian ecosystems or areas experiencing erosion.
Citation: Davidson, E. A., R. O. Figueiredo, D. Markewitz, and A. K. Aufdenkampe (2010), Dissolved CO2 in small
catchment streams of eastern Amazonia: A minor pathway of terrestrial carbon loss, J. Geophys. Res., 115, G04005,
doi:10.1029/2009JG001202.
1. Introduction
[2] Mature forest ecosystems of the Amazon Basin are
thought to be long‐term sinks of atmospheric C, in the range of
0.5–0.8 Pg C yr−1 [Phillips et al., 2008]. However, Richey et
al. [2002] estimated that the efflux of CO2 from rivers and
streams to the atmosphere in a 1.8 million km2 quadrant of the
central Amazon Basin is about 0.2 Pg C yr−1 and that a scaled‐
up estimate for the entire basin would be on the order of 0.5 Pg
C yr−1. These authors postulated that these fluxes were supported largely by terrestrial carbon entering streams and rivers,
either as groundwater CO2 or as terrestrial organic matter that
was subsequently mineralized by the aquatic ecosystem. They
further suggested that these could be significant pathways for
return of terrestrially fixed C to the atmosphere in the Amazon
1
Woods Hole Research Center, Falmouth, Massachusetts, USA.
Centro de Pesquisa Agroflorestal da Amazônia Oriental, Embrapa
Amazônia Oriental, Belém, Brazil.
3
Warnell School of Forestry and Natural Resources, University of
Georgia, Athens, Georgia, USA.
4
Stroud Water Research Center, Avondale, Pennsylvania, USA.
2
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JG001202
Basin, and one that is not measured or considered in most
terrestrial C balance studies [Richey et al., 2002]. Carbon
budgets for terrestrial ecosystems could be incomplete and net
C sequestration could be overestimated if hydrologic export of
evaded CO2, dissolved organic and inorganic carbon (DOC
and DIC) and particulate organic carbon are not considered
[Billett et al., 2004; Cole et al., 2007; Tranvik et al., 2009].
Here we examine the potential magnitude of pCO2 moving
through groundwater from terrestrial Amazonian ecosystems
into several small streams relative to other terrestrial C fluxes
in the eastern Amazon region.
[3] Production of carbon dioxide (CO2) in soils can lead to
supersaturation of dissolved CO2 (pCO2) in groundwater
seeps [Johnson et al., 2006]. Concentrations of CO2 within
deep soil air profiles in Amazonian forests and pastures often
exceed 50,000 ppmv [Davidson and Trumbore, 1995;
Davidson et al., 2004; Johnson et al., 2008], indicating that
supersaturated groundwater may be common. As groundwater
enters streams and rivers, much of the pCO2 rapidly evades to
the atmosphere, thus transferring C that was fixed in terrestrial
ecosystems to the atmosphere via an aquatic pathway.
[4] We measured pCO2 along three streams from their
headwaters in mature evergreen forest remnants, through
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Table 1. Stream and Catchment Characteristics and Estimates of Possible CO2 Evasion Based on Two Methods of Estimating
Groundwater pCO2 and Subsequent Evasion, as Described in the Discussion Section
Watershed
Groundwater CO2
Evasion Per Hectare
Maximum Groundwater
Land Based on Headwater
Area Upstream
Groundwater CO2
CO2 Evasion Per
Evasion Based on
Hectare Land Based on
Concentrations
of Gauging
Mean Discharge Headwater pCO2
(mmole CO2 L−1) Headwater (Mg C yr−1)
Soil CO2 (Mg C ha−1 yr−1)
(Mg C ha−1 yr−1)
Station (ha)
(m3 s−1)
Igarape‐7
Igarape‐Pajeú
Igarape‐54
Capitão Poço
16,100
3,250
13,700
∼20
2.9
0.3
1.5
no data
650
600
360
610
690
70
360
landscapes of mixed land use, including pastures, secondary
forests, and agriculture, in the municipality of Paragominas,
Pará, Brazil. Our objective was to measure pCO2 in headwaters and downstream stations in these small streams and to
estimate the upper limit of plausible evasion of CO2 that could
have been derived from the surrounding terrestrial ecosystems
of these small catchments. We also present pCO2 measurements from two small watersheds within an intact mature
forest in the neighboring municipality of Capitão Poço.
2. Methods
2.1. Research Site
[5] The region around Paragominas supported high‐
statured evergreen forest before it became a regional center
for logging and ranching following the construction of the
Belém‐Brasilia highway [Nepstad et al., 1991]. More
recently, intensive agricultural production for upland soybean,
rice, and corn has also increased. These trends are similar to
those for much of the Amazon region, particularly along the
eastern and southern arc of deforestation [Fearnside, 2001].
[6] As a result of this extensive land‐use change, it is
impossible to find large areas of pristine forest that encompass an entire watershed basin of a perennial stream in the
Paragominas area. The forested headwater areas of the three
streams selected for this research (Table 1) were established
as references relative to downstream samples in a larger study
of soil water quality (see Figueiredo et al. [2010] for delineation of watershed boundaries superimposed on Landsat
images). Even these headwater areas, however, have been
selectively logged, and have suffered some entry of pastoral
and agricultural activities. As will be discussed later, the
data demonstrate that one of these three headwater streams
behaves much differently than the others, despite no obvious
apparent difference during our site selection. More complete
descriptions of land cover within these watersheds are given
in a companion paper by Figueiredo et al. [2010].
[7] In order to include streams that flow through fully forested catchments, two streams in small catchments, each about
20 ha, were sampled in the Capitão Poço municipality, which
is about 100 km from Paragominas. These streams, including
their headwaters, are located entirely within a 3,700 ha segment of unharvested mature forest within a single property.
The watershed areas for these forested streams are much
smaller than in the Paragominas study (Table 1), but are
characterized by similar geologic and pedogenic history.
[8] According to the Geological Survey of Brazil (www.
cprm.gov.br), both Paragominas and Capitão Poço watersheds
overlay the Ipixuna and Barreiras formations, where the pre-
0.04
0.02
0.03
0.15
0.08
0.09
dominant clay mineral is kaolinite. Paragominas soils are
classified as deeply weathered Oxisols (Haplustox, in the U.S.
soil taxonomy, or Yellow Latosol in the Brazilian classification) in upper landscape positions, while in lower landscape
positions some soils are classified as a clay‐rich (40–60%)
Plintosolos Haplicos (Plinthustults in the U.S. soil taxonomy),
developed from both clay‐rich colluvium from upslope and
from the sandier Barreiras Formation [Markewitz et al., 2004;
Moraes et al., 2006]. Soil studies in Capitão Poço municipality [Embrapa‐SNLCS, 1990] also identified similar soils in
this region (i.e., Yellow Latosols and Plinthosols), and clay
contents are similar to Paragominas soils [Davidson et al.,
2007]. Annual average rainfall for this region is 1800mm,
most of which falls within a 6‐month rainy season [Jipp et al.,
1998]. Deep roots maintain an evergreen forest canopy,
despite the long dry season [Nepstad et al., 1994].
2.2. Stream Water Sampling and Analyses
[9] Sampling along each stream was largely driven by
issues of access. Land in this region is mostly privately held,
and roads are typically unimproved dirt roads on private
ranches. As such, sample points were identified at five locations on Igarape‐54 (IG54), seven locations on Igarape‐7
(IG7), and three locations on Igarape‐Pajeú (IGP) where we
were able to obtain permission for access (see Figueiredo et al.
[2010] for exact locations of sampling stations). For this reason, it was impossible to use consistent distances between
sampling stations. For the Paragominas streams, we adopt a
convention of calling the headwater sampling station within
each watershed S1, and successive downstream sampling
stations S2, S3, etc (e.g., IGP‐S1 is the headwater sampling
station in Igarape‐Pajeú and IG54‐S5 is the fifth downstream
sampling station along Igarape‐54). For each of the two
Capitão Poço pristine watersheds (CP1 and CP2), we were
able to find small groundwater seeps for the first sampling
stations (S1), which were located uphill of the organized
headwater streams. The organized headwater streams were
used as the second sampling stations (S2). In all three Paragominas streams, sampling for pCO2 was initiated in
November 2003 and continued on a nearly monthly basis until
August 2005. In the Capitão Poço streams sampling was
begun in February 2004 and continued on a nearly monthly
basis until June 2005.
[10] Several measurements of stream water quality,
including turbidity, pH, temperature, conductivity, alkalinity, dissolved oxygen and anion and cation concentrations
are described in a companion manuscript by Figueiredo et al.
[2010]. Here we take advantage of these data on stream
temperature, pH and alkalinity in order to convert pCO2
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values measured under laboratory conditions to pCO2 values
under in situ stream conditions (described below). Temperature and pH were measured in situ using a field multimeter (WTW, model Multiline P3 pH/Oxygen, Gold River,
CA). Stream water grab samples were collected in previously
acid washed 250‐ml polypropylene bottles. Bottles were filled to capacity to minimize headspace and were placed in cold
storage (∼4°C) within a few hours of collection. Samples
were returned to a field laboratory in Paragominas, where
they were again analyzed for pH and for alkalinity by endpoint titrations with 1 mM HCl to pH 4.5 [Clesceri et al.,
1998]. If possible, samples were analyzed for alkalinity on
the same day of collection, but many were retained cold for
up to 4–5 days until analysis.
[11] For pCO2 measurements, stream samples were collected in 60 mL syringes from 20 cm below the water surface, talking care to avoid air bubbles in the syringe before
closing the syringe valve and transporting the samples to the
field laboratory in Paragominas. The syringe samples were
allowed to equilibrate to room temperature, and then 30 mL
of water were injected into 60 mL serum bottles that had
previously been flushed with CO2‐free air. An extra needle
in the serum stopper allowed excess air to vent while injecting the water sample. The half‐filled serum bottles were
shaken and allowed to sit at room temperature for one hour
so that the headspace gas would come into equilibrium with
the gases in solution. A 3 mL sample of the headspace gas
was then injected through a 1 mL sample loop, and the gas
in the sample loop was then injected into a stream of CO2‐
free air leading to a LiCor infrared gas analyzer [Davidson
and Trumbore, 1995]. The analyzer integrated the area
under the absorption peak as the sample passed through the
analyzer. The area was converted to CO2 concentration
using a standard curve generated from injecting standards of
known concentrations (2,000 and 80,000 matm CO2 in N2)
through the sample loop in the same manner as the unknown
samples.
[12] The concentrations of CO2 in the liquid phase in the
serum bottles after equilibration with the headspace was
calculated as follows:
mol CO2 L1 water ¼ L CO2 L1 air kH F
ð1Þ
where kH is Henry’s Law constant and F is fugacity
[Dickson and Goyet, 1994]. For the conditions of these
measurements, we assumed that fugacity was 0.997. The
temperature dependence of Henry’s Law Coefficient
[Plummer and Busenberg, 1982; Szaran, 1998] was calculated as:
kH ¼ 10^ ðð108:3865 0:01985076T þ 6919:53=T 669365
=T2 þ 40:451*LOGðTÞÞÞ
ð2Þ
where T is the equilibration temperature (K) in the lab. The
dissolved free CO2 present in the original syringe sample
was then calculated based on the sum of CO2 in air and
water phases in the serum bottle, divided by the volume of
the water sample injected into the bottle. This approach does
not consider the redistribution of other carbonate species.
Specifically, as dissolved free CO2 concentrations decrease
during degassing, bicarbonate is converted to additional
dissolved free CO2. However, this conversion is self limit-
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ing because of increasing pH with CO2 degassing. At the
low pH values observed in the study streams (typically <5),
most dissolved inorganic‐C occurs as CO2 rather than
bicarbonate and carbonate, so the bias in our estimations is
minor. The in situ stream pCO2 was then calculated by reapplying Henry’s Law (equation (1)) but using in situ stream
temperature.
2.3. Stream Discharge
[13] Stream discharge was estimated monthly on a subset
of points in the three Paragominas streams from July 2004 to
June 2005 by measuring cross‐sectional area and flow with
a current meter (General Oceanic, model 2030W, Miami,
FL). This measurement was done at the most downstream
sampling stations in each stream for which a good cross
section could be obtained (IG54‐S5, IG7‐S6 and IGP‐S2)
and followed the methods of Rantz [1982]. Due to the
shallow depth of the small stream channel at the Capitäo
Poço sites, it was not possible to measure stream discharge.
3. Results
[14] For two of the Paragominas streams, IG7 and IGP,
the pCO2 ranged from about 11,000 to 25,000 matm at the
headwater, and declined to around 4,000 to 5,000 matm at
downstream positions (Figures 1a and 1b). This decline is
likely due to rapid evasion in shallow streams as supersaturated stream water is exposed to the atmosphere, combined
with decreasing downstream proportions of newly emergent
groundwater to total streamflow. The headwater pCO2
concentrations at the CP streams in undisturbed mature
forest catchments were within a similar range. There was
some tendency for the seeps (CP1‐S1 and CP2‐S1) to have
higher pCO2 than the headwater streams (CP1‐S2 and CP2‐
S2), but not consistently so (Figure 1d). In contrast, the
IG54 stream in the Paragominas region gave unusual results
at the headwater sampling station (IG54‐S1) and the following two stations (IG54‐S2 and IG54‐S3; Figure 1c).
While the average pH was about 4.5 in all other streams, the
headwaters of IG54 had a mean pH of 6.2 and, also unlike
all other streams, pCO2 concentrations were very low
(averaging 1100 and 1600 matm at IG54‐S1 and IG54‐S2).
Only the downstream sampling stations, IG54‐S4 and IG54‐
S5 had pCO2 and pH values in the range of the other
streams. This result suggests that the headwater of IG54
either was not fed by groundwater, or that the groundwater
was unlike any other sampled in the other Paragominas and
Capitão Poço streams. Figueiredo et al. [2010] also report
unusual stream solute concentrations measured at IG54‐S1,
and they also were forced mostly to abandon attempts to use
it as a reference point for downstream samples. This stream’s
headwater is located near a major highway and an agricultural field where there had been significant burning. The
stream then passes through an urban area. We do not know
the source of this variation and conclude that the headwater
of this stream is uncharacteristic of most streams of the
region that drain forested areas. Enough CO2‐saturated
groundwater apparently entered this stream by the time it
reached our fourth and fifth sampling station (IG54‐S4 and
IG54‐S5) for the pCO2 to be elevated there. The fifth sampling station is within a large cattle ranch, with a remnant
forest in upslope positions [Davidson et al., 2000].
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[15] Although the pCO2 varied temporally in the present
study, no clear seasonal pattern is apparent. The annual
mean of pCO2 for each of the three Paragominas streams is
20,000, 19,000, and 11,000 matm, respectively, for IG7‐S1
and IGP‐S1 headwater stations and station IG54‐S4 (which
we substitute here for the IG54‐S1 headwater station as a
minimum indicator of headwater pCO2 of this watershed,
since the headwater station had unexplained undersaturation
of CO2).
4. Discussion
Figure 1. Concentrations of pCO2 by sampling date and
sampling station for the (a) Igarape‐7, (b) Igarape‐Pajeú,
(c) Igarape‐54, and (d) Capitão Poço streams.
[16] The pCO2 values reported for the streams in this study
are in the range of other reported values for Amazonian
streams and rivers, including, 52,000 matm in a groundwater
seep in a small forest catchment of Mato Grosso [Johnson
et al., 2008], 10,000 to 31,000 matm near headwaters of
agricultural catchments near Igarapé‐Açu, Pará [da Rosa,
2007], 5,000 matm in the Acre River [Sousa, 2007], and
2,500–13,000 matm in the main stem and major tributaries of
the Amazon River [Richey et al., 2002]. Although the number
of studies is limited, there appears to be a trend of higher
pCO2 values reported for seeps and headwaters than for large
rivers, indicating expected within‐stream outgassing.
[17] Deep soil CO2 concentrations were previously measured in the IG54 watershed, increasing from 10,000 to
30,000 ppmv at 1 m depth to 60,000 to 70,000 ppmv at 8 m
depth [Davidson and Trumbore, 1995]. These values are
comparable to deep soil CO2 concentrations ranging from
30,000 to 90,000 ppmv measured elsewhere in the Amazon
Basin [Davidson et al., 2004; Johnson et al., 2008].
Groundwaters supporting base flow in these watersheds
should be in near equilibrium with these deep soil atmosphere CO2 concentrations [Johnson et al., 2008].
[18] One approach to estimating outgassing of pCO2 is to
assume that all of the groundwater entering the stream
throughout the watershed had the same pCO2 as was measured in the headwaters, and that all of the pCO2 in
groundwater is eventually evaded to the atmosphere, either
in the stream or river or in the estuary once the freshwater
reaches the ocean. Gas evasion studies for small streams
ranging in depth from 10 to 50 cm indicate gas exchange
velocities (k600, normalized to the Schmidt number of 600
for CO2 in freshwater at 20°C) in the range of 10–25 cm h−1
[Bott et al., 2006; Melching and Flores, 1999; Raymond and
Cole, 2001] and turnover times for dissolved CO2 of 0.4 to
5 h, with likely turnover distances of 150 to 3000 m. Hence,
most groundwater pCO2 is likely evaded within a few kilometers of its entry into the stream, but the location of evasion
need not be known for this analysis. Multiplying the mean
headwater pCO2 concentration by the measured mean annual
discharge rate (based on monthly measurements), and
dividing this flux by the area of the watershed upstream of the
discharge measurement point, the amount of terrestrial C lost
via groundwater CO2 to stream evasion of pCO2 would be on
the order of 0.02–0.04 Mg C ha−1 of land area yr−1 (Table 1).
[19] Another approach to estimating an upper limit of
outgassing of pCO2 is to assume that all of the groundwater
entering the stream throughout the watershed had a pCO2
equivalent to deep soil CO2, and that all of the pCO2 is evaded
to the atmosphere [Johnson et al., 2008]. This approach assumes that our headwater measurements of pCO2 underesti-
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mate groundwater inputs, because of outgassing that may
have occurred upstream of our headwater sampling locations.
This is unlikely in the case at Capitão Poço, where we sampled directly in groundwater seeps, but is likely true for the
Paragominas watersheds, where limited access on private
property required sampling that was sometimes as much as
0.1 to 1.0 km from headwater seeps (which varied by season).
Multiplying the reported average deep forest soil CO2 concentration (70,000 ppmv) [Davidson and Trumbore, 1995] by
the measured mean annual discharge rate, and dividing this
flux by the area of the watershed upstream of the discharge
measurement point, the maximum amount of terrestrial C lost
via groundwater CO2 to streams would be on the order of
0.08–0.15 Mg C ha−1 of land area yr−1 (Table 1). This is likely
an overestimate, because it does not account for lower concentrations of pCO2 in groundwater from degraded lands
[Davidson and Trumbore, 1995]. Also, quickflow into streams
following precipitation events initially has lower pCO2, due to
its lower residence time in the soil and lower soil CO2 concentrations near the soil surface, followed by high pCO2 water
[Johnson et al., 2006, 2007], but we do not have temporal
resolution to consider these dynamics or their associated errors.
[20] Depending on which of the two approaches described
above is used, the flux of C via stream outgassing of
groundwater pCO2 is about 2–3 orders of magnitude lower
than annual estimates of soil CO2 efflux from pastures and
forests of this region, which are in the range of 10–20 Mg C
ha−1 yr−1 [Davidson et al., 2000] and estimates of aboveground net primary productivity of 4–5 Mg C ha−1 yr−1
[Nepstad et al., 1994]. Hence, while the groundwater is
generally supersaturated in pCO2, this is a minor pathway of
C loss from the terrestrial ecosystem in these small Eastern
Amazon watersheds. Although root and microbial respiration in the soil generate impressively high concentrations of
soil CO2 and groundwater pCO2, the mass of these gases
transported in groundwater in this region is small relative to
rates of photosynthesis by terrestrial plants and efflux of
CO2 as soil respiration. In contrast, Johnson et al. [2008]
estimated groundwater‐derived CO2 evasion of 0.4 Mg C
ha−1 yr−1 in undisturbed small forested catchments near
Juruena, Mato Grosso, Brazil, which could be in the range of
2–10% of soil respiration and NPP and could be more important
relative to net ecosystem exchange of C. These higher losses of
groundwater‐derived CO2 estimated by Johnson et al. [2008]
compared to the present study could be due to either higher
rates of runoff and/or the presence of productive, undisturbed,
mature forests throughout the small Juruena catchments.
[21] Reported rates of CO2 evasion per unit area of water
surface in the Ji‐Paraná River network of Rondônia, Brazil,
ranged from 0.7 to 12.7 mmoles CO2 m−2 s−1 [Rasera et al.,
2008], which is similar to reported soil surface efflux rates
in Amazonia [Davidson et al., 2000, 2004]. However,
because stream surface usually covers on the order of only
0.5–2% of the area of the watershed, when the stream
evasion of CO2 is normalized to the area of land that the
stream drains and that may be the source of the stream
pCO2, it becomes a minor amount of C per unit land area
compared to the stocks and fluxes of C in those terrestrial
ecosystems. Outgassing of CO2 in white water streams has
been reported to be 2–40 times larger than hydrologic export
of DOC [Johnson et al., 2006; Rasera et al., 2008], but this
result does not contradict the conclusions reached here.
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Rather, both DIC and DOC export from these upland
Amazonian terrestrial ecosystems, although potentially
important for in‐stream C processes, are small relative to
other terrestrial C stocks and flows [Markewitz et al., 2004].
In other soils and ecosystems of other regions, aquatic
export, processing, and burial can be a larger percentage of
terrestrially fixed C than observed in the present study
[Billett et al., 2004; Cole et al., 2007; Johnson et al., 2008,
Tranvik et al., 2009].
[22] The persistence of 4000–5000 matm pCO2 values at
most downstream sites of the present study (Figure 1) suggests a downstream CO2 source that is in quasi‐equilibrium
with stream outgassing rates. To properly estimate and
integrate total stream CO2 outgassing fluxes would require
data on the gas exchange velocities (k600) for these streams
at each reach and on the total stream surface area, neither of
which we have. However, using literature values in the
typical range of 10–25 cm h−1 for k600 [Bott et al., 2006;
Melching and Flores, 1999; Raymond and Cole, 2001], and
using the downstream average pCO2 for IG7 of 4200 matm,
the average respiration‐driven outgassing from the stream is
likely in the range of 3 to 9 mmoles CO2 m−2 of water
surface s−1, which is in the range reported for other Amazonian rivers and streams [Rasera et al., 2008]. Assuming
that 0.5% to 1.5% of the watershed area is covered by
streams and reservoirs, total CO2 outgassing from streams
could be equivalent to 0.06–0.5 Mg C ha−1 of land area yr−1.
The lower end of this range includes the estimates based on
groundwater inputs of pCO2 only (Table 1), but the upper‐
end of this range is several times larger than any of the
estimates of groundwater inputs of pCO2. Not all of this
additional C would be derived from land, however, as algae
and macrophytes in the numerous reservoirs respire CO2
from plant parts below the water surface, and their growth
creates organic matter that eventually decomposes, contributing an unknown fraction of the stream respiration. The
few carbon budgets that have been derived for Amazonian
floodplain ecosystems indicate that net primary productivity
within the floodplain forest and by aquatic macrophytes is
sufficient to account for the majority of within‐stream respiration [Melack and Engle, 2009; Melack et al., 2009].
Other allocthonous C sources include DOC in surface water
and groundwater, soil carbon eroded from stream banks or
from degraded pastures and croplands, and litter inputs from
stream‐side vegetation [Hamilton et al., 1995, Mayorga et al.,
2005]. Hence, the C budgets of watersheds drained by
DOC‐rich blackwater rivers, of upland areas experiencing
significant soil erosion, and of riparian forests may need to
account for export of C via an aquatic pathway.
[23] If all Amazonian land (7 million km2) loses C via
hydrologic export of CO2 in groundwater at similar areal
rates as reported in the present study, the basin‐wide flux
would be on the order of 0.01 to 0.10 Pg C yr−1. Scaling up
with our second approach, if all of the water discharged by
the Amazon River (long‐term average of 209,000 m3 s−1;
estimated for the mouth of the Amazon [Molinier et al.,
1996]) originated from groundwater that was supersaturated at about 70,000 matm, the flux would be about 0.18 Pg
C yr−1. However, it is unrealistic to assume that all water
entering the Amazon River is supersaturated groundwater.
Higher published estimates of total CO2 evasion from the
Amazon River system indicate a potentially important role
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for autotrophic inputs and for other allocthonous sources of
carbon, including streamside vegetation, soil erosion, and
DOC export from organic soils.
[24] Acknowledgments. We thank Patrício de S. Silva and Ewerton
Cunha for assistance in the field. This research was supported by grants
NCC5‐686 and NNG06GE88A of NASA’s Terrestrial Ecology Program
as part of the Large‐scale Biosphere‐Atmosphere (LBA) project.
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