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Pedological Aspects of Soil Science
Soil Science and Pedology
• Coined in mid 19th Century by French scientist
• Derived from Greek: pedon=ground, logia = discourse
• “The study, in situ, of the biogeochemical processes that form
and distribute soils”
• An observational, vs. an experimental, science - nature is the
laboratory
• Origins attributed to two centers: Russia (Dokuchaev) and
Berkeley (Hilgard)
Role of Pedology in Scientific and
Societal Problems
•Carbon and nitrogen cycles
•Are soils part of an unidentified sink for CO2?
•What is the effect of agricultural on soil C (and atm CO2)?
•Will soils store excess N from human activity?
•Chemistry of natural waters
•How do soils release elements with time and space?
•Earth history
•‘Paleosols’ and evolution of land plants, atmospheric CO2 records,
human evolution
•Soils and archaeology
•Biodiversity
•Is soil diversity analogous to, and complementary to, biodiversity
•Microorganisms in soil represent unknown biodiversity resources
Soils as a Physical System
• System is open to surroundings (exchange
energy and matter)
• System Properties = f (initial state, external
surrounding, time)
• “Soil is those portion’s of the earth’s crust
whose properties vary with soil forming
factors”
rain, OM
runoff
Four processes:
• Additions
• Losses
• Translocations
OM,
clay,
ions
OM  humus
1º minerals  clays
Transformations
•Transformation
capillary
leaching rise
Key Processes of Soil Formation
CO2 flux
•
•
•
•
Additions
OM additions, OM
– Organic C
transformations,
– Dust
weathering
Removals
– CO2
– Weathering products
Transfers
– Clay
– Organic matter
– Carbonate
Transformations
– Plants to SOM
– Primary silicates to secondary silicates, carbonates
Leaching
Clay transfers
Clay and
carbonate
transfers
Pedogenic Processes
Key processes involve interactions
among chemical, physical and
biological components of ecosystems
Factors of Soil Formation
State Factor Model of Soil and
Ecosystem Formation
•Idea that soils form predictably in
response to environmental factors
attributable to Dokuchaev in ~ 1880
•Hans Jenny (1920’s to 1930’s) transformed
conceptual model to a more quantitative
theory following tenets of physical
chemistry
Conditioning Variables
• Jenny (1941, 1980) “Factors of Soil Formation”
S=f(cl,o,r,p,t,h…)
•
•
•
•
•
•
Climate
Vegetation
Relief
Time
Parent Material
Humans
State Factor Model Concepts
•Earth surface a continuum of objects
–Soils and ecosystems are human constructs
•Continuum broken into systems for study
–Size is arbitrary
–System has the following properties
•Open to surroundings
•Can exchange matter and energy
•Properties at any time depend on surroundings
•Sytem properties depend on
–1. Initial state of system
–2. External conditions
–3. Age of system
State Factor Equation
Soil = f (initial conditions, external
conditions, time)
or, based on field observation
Soil = f(cl,o,r,p,t,h…)
Definitions of State Factors
1. Climate (cl)
• regional climate
• climate inside ecosystem is a dependent variable (on all state
factors in addition to climate0
2. Biota ()
• Potential biota
• gene flux that enters an ecosystem over time t
• Actual (existing) biota reflects  dependent on other state
factors.
3. Topography
• configuration of land at t=0
• slope, curvature, aspect, depth to water, etc.
4. Parent Material
• initial state at t=0 (t=0 can be pre-existing soil)
5. Time
•elapsed time during present state factor configuration
6. Humans
• Culture is a human variable that dictates land use, etc.
Constituent Mass Balance
• Is used to determine elemental gains
and losses during pedogenesis and is
calculated according to the method
described by Chadwick et al. (1990) and
revised by Egli and Fitze (2000).
Physical & Chemical Changes in a Pedon
V
leached
horizon
vp
water
Horizon
vs
2
enriched
horizon
unaltered
PARENT
MATERIAL
SOIL
STRAIN FOR SOIL:
 p Ci,p
1
horizon1  sCi,s
z
i,s= V  
V
p

ELEMENTAL GAIN/LOSS FOR SOIL
 j,s 
z

horizon1
sC j,s (i,horizon(x) 1)   p C j,p
100
1
3
Constituent Mass Balance involves the
calculation of four parameters
• Strain (ε), a measure (%) of the volume change
of a mobile element, such as Ca, relative to an
immobile element, usually Ti or Zr;
• Transport (τ), a relative measure (no units) of
elemental movement between soil horizons;
• Mass flux, a measure (g cm-2)of the quantity of
elemental gain or loss from the soil profile;
• Enrichment, a measure of mobile element
enrichment or depletion in the soil profile
relative to the immobile element
Profile Strain
• Profile strain is calculated using the following
equation:
εi,w = (rpCi,p/rwCi,w) -1
• where, ρp is the bulk density of the parent material
(g/cm3), Ci,p is the immobile element i in the parent
material (weight %), ρw is the bulk density of the
weathered layer, or horizon (g/cm3), and Ci,w is the
immobile element i in the weathered layer (weight
%). Strain values > 0 indicate soil dilation; values < 0
indicate soil collapse.
Transport
The transport function of a mobile element j (τj,w)
is defined by:
τj,w = (Cj,w Ci,p/Cj,p Ci,w) -1
where Cj,w is the mobile element j in the weathered
layer (weight %), Ci,p is the immobile element i in
the parent material (weight %), Cj,p is the mobile
element j in the parent material (weight %), and Ci,w
is the immobile element i in the weathered layer
(weight %). Transport values > 0 indicate elemental
addition to a horizon; values < 0 indicate elemental
removal from a horizon.
Mass Flux
The mass flux of a mobile element j (mj,flux) is defined by:
mj,flux = (rp * Cj,p * Dzw * tj,w * (1/( εi,w + 1)))/100
where ρp is the bulk density of the parent material
(g/cm3), Cj,p is the mobile element j in the parent material
(weight %), Δzw is the depth of the weathered layer (cm),
τj,w is the transport factor of a mobile element; and εi,w is
the strain of an immobile element. A positive mass flux
indicates elemental gain in the soil profile; a negative value
indicates loss.
Enrichment
The enrichment of a mobile element j (Cj,w/Cj,p) is
defined by:
Cj,w /Cj,p = rp/rw * 1/( εi,w + 1) *(1 + τj,w)
where ρp is the bulk density of the parent
material (g/cm3), ρw is the bulk density of the
weathered layer (g/cm3), εi,w is the strain of an
immobile element, and τj,w is the transport
factor of a mobile element. Enrichment values >
1 indicate enrichment of an element; values < 1
indicate depletion.
Soil Based View of Weathering:
Chemical Mass Balance
•Give soil profile based perspectives
–Depth variations
–Climate variations
–Age variations
•Gives us view of both physical and chemical changes
in soils during development
–Soils commonly undergo initial expansion
followed by collapse
–Soils, given enough time, can become chemically
depleted and must rely on atmospheric inputs
Mass Balance Theory
Increasing Soil Age (Ky)
8.0
0.3
Dilation & Gain
Averaged over the top 1 m
20
660
350
Collapse & Gain
Loss/Gain (%)
0
-20
Collapse & Loss
Dilation & Loss
-40
% Na
-60
% Ca
% Mg
-80
-100
-100
% O.C.
-50
0
50
Volume Change (%)
100
150
Pedological Rules
• There is a pressing need to identify the extent to
which we can extrapolate beyond individual study
sites and model systems.
• Pedological rules are “general principles that
underpin and create patterns” within and among
ecosystems and strengthen our ability to
generalize biogeochemistry more broadly
(regionally and globally).
• Our investigations should allow us to probe the
limits of these rules and potentially identify key
contingent factors that may alter their
manifestation
Elemental Distribution of Soil Versus
the Earths Crust (Amundson, 2004)
Elemental Distribution of Vegetation
Versus the Earth’s Crust (Amundson, 2004)
Scientific Approach
• Utilize environmental gradients to establish the
range and variability in soil properties, processes
and behavior that constrain regional and global
biogeochemical models.
• State Factor Analyses (Jenny, 1941, 1980;
Vitousek, 2004)
• Integrate geochemical, biochemical and mass
balance approaches with traditional pedological
measurements.
Carbon Storage and Variability
in Grassland Systems
Environmental Gradients
– Toposequences (Aguilar et al, 1988;Kelly et al,
1988)
– Lithosequences (Aguilar et al, 1988)
– Climosequences (Honeycutt et al, 1987; Kelly,
1989)
summit
shoulder
backslope
footslope
native
cultivated
toeslope
Topographic Gradient
nd sis
nd ss
nd shale
chey wells
goodland
oberlin
hockley
2.0
1.8
OC (g cm-2 m-1)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Summit
Shoulder
Backslope
Footslope
Parent Material Gradient
OC (g cm-2 m-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Control Section Clay (%)
10
20
30
40
50
60
sandstone
siltstone
shale
Topographic Gradient
2.0
1.8
1.6
Parent Material Gradient
nd sis
nd ss
nd shale
1.4
1.2
1.0
OC (g cm-2 m-1)
0.8
0.6
0.4
0.2
1.6
1.4
1.2
Moisture Gradient
chey wells
goodland
oberlin
1.0
0.8
0.6
0.4
0.2
0.0
Summit
Shoulder
Backslope
Footslope
Climosequence
OC (g cm-2 m-1)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Mean Annual Precipitation (cm)
30
40
50
60
70
80
90
34.4 cm MAP
46.2 cm MAP
50.2 cm MAP
57.5 cm MAP
65 cm MAP
88.4 cm MAP
Pedological Rules
• Grassland ecosystems vary systematically
in C storage as a function of landscape
position, bioclimatic and Edaphic conditions
• The relationship between conditioning
variables and soil properties provide a
potential avenue for extrapolating beyond
site level and constraining regional studies.
Mass Balance Theory
Increasing Soil Age (Ky)
20
0.3
Dilation & Gain
Averaged over the top 1 m
20
4100
170
Collapse & Gain
Loss/Gain (%)
0
-20
Collapse & Loss
Dilation & Loss
-40
% Na
-60
% Ca
% Mg
-80
-100
-100
% O.C.
-50
0
50
Volume Change (%)
100
150
Note large
difference in C13 content in
surface
All sites reach same value
deep within profile ?
% C4 vegetation
C-13 content
Past vs Present
Wet-
Mostly C3
Plants
IntermediateC3 and C4
Plants
Dry Mostly
C4 plants
Location of Ancient
Agricultural Fields
Soil Organic
C follows
productivity
gradients
Soils and
Parent
materials
rich in Al !
No losses of Al
but major
transformations
Fire and
erosion
losses
Ca losses
at dry and
wet end
are highest
Leaching
losses
Fire and
erosion
losses
P losses similar
to Ca but
retention
highest in
surface soils of
intermediate
zone
Some
Leaching
losses
Pedology & Biogeochemcial
Research
• Earth Sciences are now at the forefront of research
addressing biogeochemical questions at regional and global
scales.
• Identify pedological processes that operate consistently, or
at least change predictability, across similar ecosystems
within and between regions ?
• There is a pressing need to identify those pedological
processes that are predictive (quantifiable) rather than
descriptive.
• Once established and tested the “Pedological Rules” can be
utilized to help quantify the range and variability of
biogeochemical responses to key drivers (climatic extremes
and land use).