Carbon Dioxide In Soil

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Railsback's Some Fundamentals of Mineralogy and Geochemistry
Carbon Dioxide in Soil
Carbon dioxide (CO
) is much more abundant in
2
soil gas (the air in soil) than in the atmosphere. That's
Atmosphere = 0.03%
because plant roots respire and produce CO
, and
-3.5
Photosynthesis:
2
(P
= 10
)
CO2
because oxidative decay of organic matter produces
CO
+ H
O --> CH
O + O
2
2
2
2
CO
. As a result, concentrations of CO
in soil gas are
2
2
orders of magnitude greater than the atmospheric
concentration, as is shown at right.
That's significant to geochemistry because CO
2
combines with water to make carbonic acid:
P
(%)
CO
2
0
5
CO
+ H
O --> H
CO
2
2
2
3
(Carbonic Acid)
Carbonic acid in turn drives most chemical
Concentration of
weathering by providing the acidity that attacks
CO
decreases
2
minerals. For example:
upwards with
diffusion and
leakage to
Weathering of olivine:
atmos-
4H
CO
+ MgFeSiO
-->
phere.
2
3
4
-
2+
2+
Mg
+ Fe
+
4HCO
+ H
SiO
Warm wet
3
4
4(aq)
Plant respiration:
regions
Weathering of albite:
O
+ CH
O -->
CO
+ H
O
2H
CO
+ 2NaAlSi
O
+ 9H
O -->
2
2
2
2
2
3
3
8
2
Plant
-
Temperate
2Na
+
+
2HCO
+ Al
Si
O
(OH)
+ 4H
SiO
3
2
2
5
4(s)
4
4(aq)
carbohydrate
regions
Weathering of calcite:
Organic decay:
-
2+
H
CO
+ CaCO
--> Ca
+
2HCO
O
+ CH
O -->
CO
+ H
O
2
3
3
3
Cold
2
2
2
2
Organic
regions
Anyone who has taken physical geology might
matter
object that, although carbonate minerals react with
acid, silicate minerals do not. The answer is that,
although silicate minerals react slowly, they do react at
geologic time scales. Anyone familiar with chemistry
might object that carbonic acid is a weak acid, and that
Sources of concentration data:
its acidity would quickly be expended by the slightest
Brook, G.A., Folkoff, M.E., and Box, E.O., A world model of
reaction with a mineral. The answer is that CO
is
soil carbon dioxide: Earth Surface Processes and
2
Landforms, v. 8, p. 79-88.
Concen-
constantly replenished by plant respiration and decay,
Reardon, E.J., Allison, G.B, and Fritz, P., 1979, Seasonal
tration of
providing a long-term supply of weak acid.
chemical and isotopic variations of soil CO2 at Trout
CO
decreases
2
Creek, Ontario: Journal of Hydrology, v. 43, p. 355-371.
downwards with
The ultimate result is thus that the input of CO
Data from Russell, E.W., 1961, Soil Conditions and Plant
-
2
conversion to HCO
in
3
portrayed at right is what drives chemical weathering,
Growth, as presented in Holland, H.D., 1978, The
reactions like those at left.
3m
Chemistry of the Atmosphere and Oceans, p. 24.
-
and it's what causes HCO
(bicarbonate) to almost
3
10 ft
inevitably be the most abundant solute in groundwaters.
LBR 1121WeatheringCO207 3/2002 rev. 1/2009

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