ESPM 120
Homework 5
Carbon Dioxide Transport in Soils and Impact on Soil Chemistry
Due: Monday November 14, 2005 @ 5 pm
Friday, we examined CO2 concentrations vs. depth in a grass covered and a bare area
outside of Mulford Hall. We will use these data to calculate the rates of CO2 fluxes from
the soil to the atmosphere. We also directly measured the flux of CO2 from the bare soil
surface into a chamber. Jon converted the raw data into real concentrations (both ppm
and umol/m3). He also calculated the flux rate in the chamber. He also calculated the
diffusion coefficient that allows us to calculate the flux from the concentration profiles.
Finally, I quickly derived an equation to calculate the pH of the soil water in equilibrium
with the CO2 at any given depth assuming no other compound is available to buffer the
pH.
1. Calculate the fluxes (using data) for the bare and grass soils (and include the correct
units).
2. Compare the calculated to actual measurement on the bare soil - does it differ greatly
and if so, do you have any suggestions why?
3. How does the CO2 flux in the grass and bare soils differ, and why?
4. Calculate the pH of soil water at the 3 depths in the 2 soils. How does potential
weathering vary with depth and plant cover?
5. What other compounds might control the pH in the soil in addition to CO2 and how?
(look at your soil acidity chapter if you wish for help).
EXTRA CREDIT : 50 POINTS
Jon did much of the work for the data provided above. If you want an additional
50 points, take the raw data (vs. depth and in the chamber vs. time) and calculate the
fluxes (you need to use some expressions Jon put on the board, calculate concentration
gradients, calculate the diffusion coefficient).
Gas Well CO2 Flux Calculation
F = -Ds x dC/dz
Ds = t x Da
where t = tortuosity and Da = diffusion constant in air
t = A^(10/3) / P^2 where A = volumetric air content and P = porosity
P = A + W = 1 - BD/PD where W = volumetric water content, BD = bulk density and PD = particle density
(2.65 g/cm3)
A=P-W
Units: Ds = 10^-6 m2/s; dC/dz = umol/m3/m; t = unitless
BD =
1.4
g/cm3
W=
0.2
cm3/cm3
P=
0.47
A=
0.27
t=
0.058
Da
14.7
10^-6 m2/s
Ds = t*Da =
0.86
10^-6 m2/s
CO2 concentration data:
Depth (cm)
grass
0
-5
-10
-20
bare ground
ppm
0
-5
-10
-20
377
3181
5354
9104
umol/m3
16844
142015
239023
406440
377
1884
3006
5950
16844
84123
134192
265622
*Unit conversion from ppm (or umol/mol) to umol/m3 = (umol/mol) x (1 mol/22.4 L) x (1000 L/m3)
- we made the simplifying assumption that we are at standard pressure and temperature here
Flux gradient (dC/dz):
grass
bare ground
(umol/m3/m)
-1916000
-1231200
*Calculated as a linear regression of CO2 concentration (umol/m3) v. depth (m)
Chamber measurements - CO2 build up over time (dC/dt) into a known volume across a known area
CO2 concentration at 3 points in time
minutes
sec ppm (or umol/mol)
4.5
270
581.8
8.8
528
711.1
13
780
891.8
dC/ds (umol/mol/s)
0.6074
volume (cm3)
2500
vol (m3)
calculated as a linear regression of CO2 concentration
(umol/mol) v. time (s)
0.0025
diameter = 21cm
area (m2)
0.03462
F = (dC/dt x Vol)/area x (p/RT)
R
Pressure
8.21E-05
1
Temp (5cm)
288
F (umol/m2/s)
1.86
m3 atm mol-1 K-1
atm
K