ESPM 121: Homework Set 1: Due Feb. 21 @ 5pm (return to my mailbox in 145
Hilgard or slide under my door in 317 Hilgard).
Soil processes on the western slope of the Sierra Nevada, Fresno County, CA
References and resources for calculations:
Dahlgren, R.A., J.L Boettiner, G.L. Huntington, and R.G. Amundson. 1997. Soil
development along an elevational transect in the western Sierra Nevada, California.
Geoderma 78:207-236.
For all calculations, you are encouraged to use Excel to perform the calculations,
and:
• Please provide the approach to the problem on this sheet.
• Attach the Excel sheet of results for all questions.
• Attach plots where for all problems where they are required.
Residence time vs. elevation/climate
1. In lecture 2, we covered the concept of residence time as a guide to soil age on
sloping landscapes.
a. For the 7 soils examined by Dahlgren et al., calculate their residence time.
Assume all sites have an average erosion rate of 4 cm/1000 yrs (a value
both probably too high and not applicable to all sites, but one calculated
from reservoir sedimentation by Janada, 1967).
See the plot below.
Problem 1
5 10 4
residence time
4.5 10
4
Residence Time (yr)
4 10 4
3.5 10 4
3 10 4
2.5 10 4
2 10 4
1.5 10 4
1 10 4
0
500
1000
1500
Elevation (m)
2000
2500
3000
b. Discuss why the trends occur as they do with MAP/MAT. In this
discussion, remember that soil thickness (which essentially drives the
residence time calculations) is the balance of erosion rates and soil
production (rock conversion to soil) rates.
See plot below of residence time vs. the ratio of MAP/MAT. This ratio increases with
elevation due to declining temps and increasing precip. It seems that there is a critical
ratio which promotes the establishment of thick soils and long residence times. So, the
thickest soils are found at mid elevation with a favorable combination of both
temperature and moisture which leads to high rates of chemical weathering and
conversion of rock to soil. Given our assumption of constant erosion rates regardless of
elevation, the longest residence times are found with the thickest soils.
Problem 1
5 10 4
residence time
4.5 10 4
Residence Time (yr)
4 10 4
3.5 10 4
3 10 4
2.5 10 4
2 10 4
1.5 10 4
1 10 4
0
5
10
15
20
25
30
35
MAP/MAT
Clay production vs. elevation
2. a. Calculate the rate of clay formation (kg clay/cm2x1000 yrs) for all seven soils.
You can assume all soils have a bulk density of 1.4 g cm-3. Use clay data from
Table 3 in paper, and data from #1 above.
See plot below. I apologize for making you convert to cm2 scale! It was a typo on my part
– my data are on a m2 scale, 10,000x the values I accidentally asked for.
Problem 2
8
clay prod rate
Clay Prod Rate (kg/m2 1000 yrs)
7
6
5
4
3
2
1
0
500
1000
1500
2000
2500
3000
Elevation (m)
b. Discuss why the weathering rates (i.e. clay production rates) vary as they do
with MAT and MAP.
As mentioned above to some degree, the combination of water vs. temperature is
critical for weathering rates. Water is needed as the solvent in which silicate
weathering occurs, and temperature (heat) drives the reactions. The mid
elevations in the Sierra Nevada appear to be the most ideal combination in that
region.
Use of CEC to evaluate soil clay mineralogy
3. a. Calculate the CEC of the clay fraction (meq/100g of clay) as a function of
both soil depth and elevation for the 7 soils. Obtain CEC and clay from Table
3.
Please see the Excel sheet for the numerical results.
b. Given the clay mineralogical data in Figs. 7 and 9, do the CEC values seem
consistent with these data. If not, what might be contributing to the CEC of the
soils beyond the clay minerals?
In general, the results (CEC/100 g clay) are not consistent with the
mineralogical measurements, which show increasing kaolinite/gibbsite with
elevation (and loss of mica/vermiculite). The former are low in CEC, the later
higher.
The relatively high apparent clay CEC is most likely due to OM contributions.
To support this, CEC decreases with depth (as does OM). If I had the
numerical values for OM vs depth for all soils, we could have easily corrected
the CEC values and gotten a better handle on the actual clay mineral
contributions (but I didn’t have that information).
Soil color and soil mineralogy
4. In the lecture, we discussed that soil Munsell values are related to soil
organic C contents. We hypothesized that Munsell chroma should be related to
degree of weathering and the amount of secondary Fe oxides the soil contains.
To test this hypothesis, plot the soil chroma (for moist soils) vs. the Fe(d) of
the B horizons only (neglect any A horizons or any Cr horizons) (citratedithionite extractable Fe)(Fe(d) is a chemical extractant that removes most
important secondary Fe oxides, including hematite, goethite, and ferrihydrite).
a. Discuss the results of the analysis. Does it conform to the hypothesis?
The results of this analysis very nicely supported our hypothesis that soil color reflects
degree of weathering and the amount of secondary Fe oxides present. See plot below.
western Sierra Nevada: Dahlgren et al., 1997
Citrate-dithionite extractable Fe (g/kg)
2
4
6
8
10
12
14
16
1
2
3
4
5
6
7
8
9
Moist Chroma
Soil morphology and climate
5. a. Plot O and Bt horizon thicknesses vs. elevation. Discuss reasons why these
trends occur as they do. For the O horizons, considered the vegetation (Table 1) and the
effect of climate.
See plot below. As we would expect from the clay analysis above, the thickest Bt horizons
are at the middle elevation zones. The O horizons loosely follow this pattern as well. In a
few days we will go into the soil C cycle in more detail, but at this stage, the O horizon
thickness corresponds to forest/plant density. The mid elevation is the premier
commercial forest of California. Sites above and below it (in elevation) have lower
productivity due to climatic limitations (water or temp). Also, the persistence of organic
matter once added to the soil surface is related to temperature: high temps promote rapid
decomposition. We will go into this more in the future.
western slope of Sierra Nevada
150
Horizon Thickness (cm)
O horizon
A horizon
Bt horizon
100
50
0
0
500
1000
1500
Elevation (m)
2000
2500
3000