Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole

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Carbon Dioxide Variation in a Hardwood Forest
Stream An Integrative Measure of Whole Catchment
Soil Respiration. Jeremy B. Jones, Jr. and
Patrick J. Mulholland. Ecosystems (1998) 1
183196
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There are 7 common ions in freshwater Cations
Ca2, Mg2, Na1, K1 Anions Cl-1, SO4-2,
HCO3-1
Sources of ions in freshwater From the
atmosphere. The principal atmospheric source of
ions is from sea spray. Accordingly, the ratios
of the common ions in rain and snow reflect the
ratios in seawater Na and Cl are often the most
abundant ions in rain. (The concentrations of
ions in rain are of course much lower than the
concentrations in seawater.) From rock
weathering. Watersheds contain soils and rock
which consist of a wide variety of different
minerals. Weathering of the soil and rock
minerals produces runoff containing the common
ions (and many other ions in lesser
concentration). Some sample reactions which
illustrate the general nature of
weathering Na-feldspar weathering 2 NaAlSi3O8
2 CO2 3 H2O lt----gt 4 SiO2 Al2Si2O5(OH)4 2
Na1 2 HCO3-1 Calcite weathering H2CO3 CaCO3
lt----gt Ca2 2 HCO3-1 There are of course a
host of other soil and rock minerals subject to
similar weathering reactions. In general,
weathering reactions can be characterized as a
weak acid (carbonic acid) slowly dissolving basic
minerals. Weathering reactions result in runoff
containing dissolved ions, and commonly,
undissolved particles of less soluble minerals
such as clays.
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Concentration patterns in surface water (the
Gibbs 1970 "boomerang" plot). Atmospheric
dominance Lakes which receive rainwater and
snowmelt (without much influence by rock
weathering) contain water with low
concentrations of ions. Because of the source of
atmospheric ions, from sea spray, chloride and
sodium are relatively more abundant than the
other ions. (The relative abundance of salts is
not precisely the same as sea water because
there is some fractionation in evaporation and
transport of sea salts.) In addition, rainwater
is in equilibrium with atmospheric CO2. In
effect, rainwater is (approximately) a dilute
solution of carbonic acid with an admixture of a
small amount of sea salt. Rock Dominance Water
more or less in equilibrium with the materials in
the drainage basin is characterized by higher
concentrations of ions and an increased
significance of Ca, Mg, and bicarbonate ions.
Many of the worlds great rivers discharge water
which can be characterized as rock-dominated. Eva
poration-Precipitation Evaporation and fractional
precipitation represent the third mechanism
identified by Gibbs. The world ocean is the end
of this process. Some rivers and lakes have a
similar chemistry if they occur in regions which
are arid. The overall concentration of ions
increases with evaporation until selected
minerals begin to precipitate because their
solubility product is exceeded. Because CaCO3 is
commonly the first mineral to precipitate, the
relative concentration of these ions begins to
decline with evaporation. With further
evap- oration, other minerals, such as gypsum
(CaSO4) will also precipitate. As a consequence,
the relative con- centrations of Na, K, and Cl-
increase. Comment Recent studies have produced
data sets which are not consistent with Gibbs
boomerang. For example, Kilham (1990, LO, vol
3580-83) presented data for African waters. For
his data set, the "rainwater" arm of Gibbs
boomerang is missing, possibly because of very
rapid rock weathering or because of terrigenous
dust contributing to precipitation chemistry.
Most of the African samples are above 100 ppm
total dissolved salts, and Ca and Cl are low. In
contrast, Eilers and Brakke (1992, LO, vol
371335-1337) report data for Oregon-Washington
and other locations which are generally of very
low overall ionic concentration, but with a range
of Na/Ca ratios which span most of Gibbs plot.
Eilers and Brakke argue that the Gibbs model is
misleading. Gibbs reply (1992, LO, vol
371338-1339) discusses Eilers comments and
defends the original model as still the most
suitable for "major" water bodies.
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The Carbonate System
The carbonate system is important for a number of
reasons. Natural waters are buffered with respect
to pH mostly because of the content of inorganic
carbon species. In turn, pH is an important
controlling variable for many important
geochemical reactions (e.g. solubility of
carbonates). Many important biochemical
reactions, such as photosynthesis and
respiration, interact with the pH and the
carbonate system. 1. The chemical species of
interest. The carbonate species CO2, H2CO3,
HCO3-, CO3-2 Water and its ionization products
H2O, H, OH- The other common ions Na, K, Ca,
Mg, Cl, SO4 2. Some definitions. Carbonate
alkalinity is defined as A HCO3- 2CO3-2
OH- - H (where species implies the
concentration, in moles/liter, of the species in
question.) By convention, H2CO3 H2CO3
CO2 Total inorganic carbon, CT H2CO3
HCO3- CO3-2
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The carbonate system may be completely described
by 6 equations (1) HHCO3-/H2CO3 K1
10-6.3 (1st dissociation of
carbonic acid) (2) HCO3-2/HCO3- K2
10-10.25 (2nd
dissociation of carbonic acid) (3) H2CO3
pCO2 x kCO2 (Henrys law
for CO2)
(pCO210
-3.5 atm, KH 10-1.5) (4) HOH- Kw
10-14
(Dissociation of water) (5) Ca2CO3-2 KSo
10-8.35 (Solubility
of calcite) (6) HNaK2Ca22Mg2
Cl-2SO4-2HCO3-2CO 3-2OH-

(Requirement for
electroneutrality)
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The ionization fractions (top panel of figure)
are defined algebraically as H2CO3 CT x a
0 HCO3- CT x a 1 CO3-2 CT x a 2 The
sum of the a s a 0 a 1 a 2 1 The
numerical value of the a s are a function of pH
alone. They are defined as a0 1 K1/H
K1K2/H2-1 a1 H/K1 1 K2/H-1 a2
H2/K 1K2 H/K2 1-1 Definitions of
equivalence points a. Endpoint for an alkalinity
titration point x This endpoint is defined by
the proton condition H HCO3 - 2 CO3-2
OH- (Or, a pure solution of CO2) b. Endpoint
for "phenolphthalein" alkalinity titration point
y H2CO3 H CO3-2 OH- (Or, a pure
solution of NaCO3) c. Point z 2H2CO3
HCO3- H OH- (Or, a pure solution of
Na2CO3) Notice that for all three of these points
(x, y, and z), the pH at which the endpoint
occurs is a function of CT. For example, the pH
for x decreases as CT increases.
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Alkalinity
Alkalinity is a measure of the ability of a water
system to resist changes in pH when acid is
added to water. A stream that has a high
alkalinity is well buffered so that large inputs
of acid (from acid rain for instance) can be
made with little affect on the stream pH. A
stream that has a low alkalinity is poorly
buffered and may undergo large, sudden drops in
pH in response to acid inputs.

• Alkalinity is roughly equivalent to the imbalance
  of cations and anions
• Alkalinity 2Ca2 2Mg2 Na K
  NH4 - 2SO42- NO3- Cl-
• The charge imbalance is corrected by changes in
  equilibrium in the DIC system.
• Thus Alkalinity HCO3- 2CO32-
  OH- - H
• Alkalinity increases by processes that consume
  SO42-, NO3- or other anions, or that release DIC.
• Alkalinity decreases by processes that consume
  cations or DIC.
• Acid rain decreases alkalinity due to additions
  of H and SO42-.

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An alternative
definition of alkalinity The current tendency is
to use "acid neutralizing capacity", or ANC,
instead of "alkalinity" to describe the acid
buffering capacity of a natural water. This usage
is justified because of the method used to
measure ANC an acid titration to a particular
pH endpoint (but note the use of the Gran plot)
An alternate definition of alkalinity (or ANC)
can be developed from the requirement for
electroneutrality. Beginning with the
electroneutrality equation the terms may be
rearranged to give HCO3-2CO3-2 OH--H
NaK2Ca22Mg2-Cl--2SO4
-2 Thus, Alkalinity (or ANC) can be defined
as Alk Na K 2 Ca2 2Mg2
-Cl- -2SO 4-2 This version of alkalinity
explains why it typically behaves as a
"conservative" quantity. Samples may be
collected, stored, and analyzed as much as 6
months later. The conceptual definition based on
carbon species would seem to imply that
alkalinity should be rather volatile. The
alternative definition based on common ions also
draws attention to conditions under which
alkalinity would not be conservative. Some
examples Redox processes which involve
important buffering species will alter
alkalinity. Examples are oxidation of ammonia
to nitrate (nitrification), oxidation of sulfide
or reduction of sulfate, oxidation or reduction
of iron. Precipitation or dissolution of minerals
involving participating species will alter
alkalinity. For example, the precipitation or
dissolution of calcium carbonate can alter
alkalinity. Other participating species, such as
organic acids, may influence ANC, and may not
behave conservatively. Note also that CT and pH
do not behave conservatively. They must be
measured as quickly as possible to obtain an
accurate estimate of the makeup of a water sample
at the time of collection.
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Long-term decline in carbon dioxide
supersaturation in rivers across the contiguous
United States
Jeremy B. Jones Jr., Emily H. Stanley, Patrick J.
Mulholland
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 10
2003
1. Introduction
The concentration of CO2 is an important property
of aquatic ecosystems, reflecting both internal
carbon dynamics and external biogeochemical
processes in the terrestrial ecosystem Cole et
al., 1994 Jones and Mulholland, 1998a Richey et
al., 2002. CO2 and dissolved inorganic
carbon (DIC) concentrations in rivers and streams
result from an interplay between inorganic carbon
fixation via aquatic primary production, organic
matter decomposition, import via ground waters,
and exchange with the atmosphere Hope et al.,
2001 Palmer et al., 2001.. (nice discussion)
2. Methods 2.1. Dataset 2.2. Chemical Analyses
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Table 1. Long-term Trends in the Partial
Pressure of CO2 (pCO2), Dissolved Inorganic
Carbon (DIC), and Associated Chemical and
Physical Parameters Across the Contiguous
United States Trend Trend
standard Variable Mean (y1) error
pCO2 (ppmv) 2109 -78.4
Alkalinity 2622 0.9 (mEq l1)

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4. Conclusions 16 The decline in pCO2, given
the lack of change in alkalinity, eqDIC, or eqH,
points to a reduction or alteration in the
quantity and/or quality of carbon import from
terrestrial ecosystems. These changes could be
caused by a myriad of factors but we suggest that
either a decline in soil respiration or
alteration in the nature of the hydrologic
connections between terrestrial and aquatic
ecosystems are the likely explanations. Baseflows
(i.e., groundwater fluxes to streams) have
declined in the southeastern U.S. over the past
30 years Lins and Slack, 1999, which could
reduce the import of terrestrial carbon to
streams Jones and Mulholland, 1998b. However,
baseflow increased in other parts of the country
where we also observed CO2 declines. Terrestrial
production appears to have increased given trends
in North America of increased plant growth
Myneni et al., 1997, afforestation Delcourt
and Harris, 1980 Dixon et al., 1994, and
nitrogen deposition Townsend et al., 1996,
which may lead to increased soil respiration and
CO2. More recent evidence, however, suggests that
increased nitrogen deposition may lead to reduced
soil CO2 results from experimental additions of
nitrogen to .
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forest soils have demonstrated a reduction in
the rate of organic matter decomposition Berg
and Matzner, 1997 Berg and Meentemeyer, 2002.
The reduction in riverine pCO2 may have also been
caused by a decline in riparian and wetland
habitat. Wetlands and near-stream environments
can affect in-river metabolism and pCO2 by
releasing organic matter and contributing water
supersaturated in CO2 from soil respiration.
Indeed, respiration rates in riparian soils can
exceed those in adjacent forest and cropland
ecosystems Tufekcioglu et al., 2001. High rates
of primary production and soil respiration of
wetlands and riparian zones, coupled with
dramatic losses of these habitats over the past
century Mitsch and Gosselink, 1993 should have
a strong and direct effect on stream pCO2.
Regardless of the cause, the trend in pCO2
indicates that gaseous carbon losses from
terrestrial ecosystems via aquatic pathways have
likely declined across much of the contiguous
U.S. and that ecosystem functioning significantly
shifted during the latter part of the 20th
century.
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Increase in the Export of Alkalinity from North
Americas Largest River Peter A. Raymond and
Jonathan J. Cole 4 JULY 2003 VOL 301 SCIENCE
In terrestrial systems, there are two
major processes that sequester atmospheric
CO2 Organic carbon produced during
photosynthesis can be stored on land or exported
in rivers, or terrestrial alkalinity (carbonate
and bicarbonate ions) produced during
chemical weathering in soils can be fluvially
exported. The fluvial export of terrestrial
alkalinity is also the major source of oceanic
alkalinity and is a key regulator of the CaCO3
saturation state of the oceans (1). During
chemical weathering, the atmosphere provides the
reservoir of CO2 either directly, or
indirectly through the respiration of
plant-derived organic matter. The lithosphere
converts some of this CO2 to dissolved
bicarbonate or carbonate through the weathering
of the parent rock material, and the hydrologic
cycle transports this dissolved inorganic carbon,
as alkalinity, to rivers and ultimately to the
ocean. Regional chemical weathering rates are
controlled..,
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Finally, these data demonstrate an increase in
the export of alkalinity from the Mississippi
watershed over the past five decades, which
demands an increase in the supply of protons to
mineral surfaces or an increase in the rate of
chemical weathering due to warming. The acidity
of rain has not increased in recent decades (38),
and therefore the increase in proton delivery
does not appear to be linked to acid rain.
Potential mechanisms for an increase in proton
delivery include an increase in atmospheric CO2,
an increase rainwater throughput, or an increase
in plant and microbial production of CO2 and
organic acids in soils due to biological
responses to increased rainfall and temperature.
Plant productivity responses to increased
temperature and rainfall are already documented
for the United States (39, 40). Determining the
relative contribution of these mechanisms, which
will vary in importance regionally, is a critical
component of future research. In particular,
because nitrogen loading has also increased in
the Mississippi watershed over the study period
and is linked to agricultural practices (41, 42),
studies must also determine the relationship
between nitrogen loading and alkalinity export.
Nitrogen loading could stimulate both soil
respiration (43) and nitrification (44), the
latter of which represents a source of acid to
weather minerals that does not consume
atmospheric CO2 and was found to be responsible
for 6 of the bicarbonate generation in a
carbonate-rich, extensively farmed watershed (45).
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