Download Weathering in Soil

NREM 301
Day 12
• Briefly review Tuesday’s lab – Agricultural
Ecosystems
• Discuss weathering, soil chemistry/fertility and
organic matter
• Reminder – Individual paper from Soil Bio- and
Toposequences Lab due next Thursday (Oct. 2)
Riparian Buffers Can:
1 ) Cut sediment in surface runoff as much as 90%
2 ) Cut nitrogen and phosphorus in runoff by 80%
3 ) Entice and support 5 times as many bird species as row
cropped or heavily grazed land
4 ) Allow water to infiltrate 5 times faster than row cropped
or heavily grazed land
5 ) Remove up to 90% of ground water nitrate
6 ) Cut streambank erosion by as much as 80% from row
cropped or heavily grazed land
7 ) Reach maximum efficiency for sediment removal in as
little as 5 years
8 ) Reach maximum nutrient removal efficiency in 10-15
years
9 ) Increase soil organic carbon up to 66%
10 ) Be most effective at upper reaches of a watershed
Soil Parent Materials – the raw mineral material
soils are developing in.
Rocks and
Minerals
Deposited in oceans -marine sediment
Deposited in lakes ----lacustrine sediment
Deposited in streams -alluvium – floodplain, delta
terrace, fan
ice
transport
Deposited by ice ----glacial till --- moraines
Deposited by water --outwash – alluvium, marine
lacustrine
Residual
Deposited by wind - eolian --- loess, eolian sand,
sediment
volcanic ash
parent material
(bedrock weathered
in place)
Deposited by gravity –colluvium – creep, landslides
types of
deposits
examples of
landforms or
deposits
Modified from Brady and Weil. 2002. The nature and properties of soils.
13th edition. Prentice Hall.
Physical Weathering
Weathering in
Soil
Definition: disintegration of rock material into smaller-sized
fragments. The general mechanism for physical weathering is the
establishment of sufficient stress for the rock material to break.
Common processes:
1. Expansion
Weathering refers
to the alteration of
rocks and
minerals at or
near the Earth’s
surface.
1) Crystallization: freezing water, salt crystal growth
2) Thermal expansion/contraction: differential
expansion/contraction of minerals, effects of fire.
3) Unloading
4) Plant and animal influences (e.g., roots, lichens)
2. Abrasion of material during transport by water, ice, wind, and
gravity.
Chemical Weathering
Definition: weathering of rock material that results in a change in
chemical composition.
Common processes:
1.
2.
3.
4.
Hydrolysis
Oxidation/Reduction
Hydration
Solution
Physical
weathering by
root action
Physical
weathering
due to
water and
wind action
Slot Canyon,
Zion National
Park, Utah
Physical weathering – wave abrasion of
limestone pebbles, Lake Michigan
Source: Sandor
Physical weathering – fire
cracked rock, Idaho
Source: Jim Gubbels
Weathering till boulder, Sierra Nevada Mountains
Source: Sandor
Chemical and physical weathering rind in diorite till
boulder, Des Moines Lobe
Source: Sandor
Deep (bio-geochemical)
weathering in
bedrock-Ultisol,
Georgia
Source: Sandor
Common
SilicateMinerals
Minerals
Common Primary
Primary Silicate
Primary minerals, the original minerals of the Earth, are formed at high temperature and/or
pressure in igneous and metamorphic rocks. They constitute the majority of sand and silt in soils.
Mineral
Quartz
Feldspar
K-feldspar (orthoclase)
Plagioclase
Composition
Si:O
Silicate Structure
Si02
1:2
Tectosilicate
(3-dimensional
framework)
Tectosilicate
KAlSi3O8
1:2
(Si or Al: O)
NaAlSi3O8
CaAl2Si2O8
Mafic Minerals:
Mica (biotite)
Fe, Mg
aluminosilicates
2:5
Phyllosilicate
(layer silicate)
Amphibole (hornblende)
Fe, Mg, Ca
aluminosilicates
4:11
Inosilicate
(double chain silicate)
Pyroxene (augite)
Fe, Mg, Ca
aluminosilicates
1:3
Inosilicate
(single chain silicate)
(Fe,Mg)2SiO4
1:4
Nesosilicate
(island silicate)
Olivine
Cation Exchange Capacity (CEC) – a key soil fertility property of
some clay minerals and humus
CEC definition: the amount of exchangeable cations a soil can adsorb. Adsorption
refers to the holding of these cations near clay surfaces by electrical attraction.
Negatively charged clay
Cation Exchange
plant root
exchange
reactions
Cations in the soil water solution
Exchangeable cations
adsorbed to clay surface
Plant roots mainly take up nutrient
ions from the soil solution
Several plant nutrients occur in soil as cations (positively charged ions),
for example Ca2+, Mg2+, K+, Fe2+, Cu2+, Zn2+, and others. Negativelycharged clay and humus particles hold cations in soil, prevent leaching
loss of these cations, and make them available for uptake by plants.
Origin of Cation Exchange Capacity (CEC) in Clay Minerals
• CEC definition: the amount of exchangeable cations that a soil can adsorb.
Adsorption refers to the holding of these cations near clay surfaces by
electrical attraction.
Exchangeable cations
Ca2+
K+
tetrahedral sheet
octahedral sheet
tetrahedral sheet
• Mainly found in 2:1 clays
Ca2+
K+
• CEC mainly results from process of isomorphous substitution during clay
mineral formation. Isomorphous substitution involves the replacement of one
element for another inside the clay mineral at the time of formation.
Examples of isomorphous substitution:
Aluminum (Al3+) substitution for Silicon (Si4+) in tetrahedral sheet.
Magnesium (Mg2+) substitution for Aluminum (Al3+) in octahedral sheet.
Note in both cases a cation with lower valence replaces one of higher
valence, resulting in a net negative charge, or layer charge. This negative
charge is offset by the exchangeable cations.
Base saturation – a soil property related to Cation Exchange
Capacity and also essential to soil fertility
Base saturation % refers to the proportion of base-forming cations (calcium,
magnesium, potassium – all major plant nutrients, and sodium) on cation
exchange sites. The remaining cation exchange sites are occupied by
exchangeable acids (hydrogen and aluminum ions).
2+ + Mg2+ + K+ + Na+ x 100
Ca
Base Saturation % =
CEC
CEC Sites (%)
Base saturation is positively correlated with pH. Acidic, highly
weathered soils such as Ultisols and Oxisols have lower base
saturation (and lower CEC) than less weathered, less acidic
soils such as Mollisols and Alfisols.
Soil pH
Source: Birkeland 1999
Range of Soil pH
Relative
concentration
of H+ or OH
Acid
pH
3
acid sulfate soils
lemon juice
4
5
leached soils
Neutral
6
7
many fertile soils
pure water
Alkaline
8
9
10
calcareous soil sodic soil
soap
Source: Jenny. 1980. The Soil Resource
q
Effect of Parent Material on Soils and Ecosystems
Example from Sierra Nevada Mountains, California (Jenny, 1980)
Igneous Rock Parent Material
Soil Properties
Silicic Rocks
Mafic Rocks
Organic Carbon %
1.74
2.88
Total Nitrogen %
0.074
0.121
Clay %
12
21
Exchangeable
Bases (cmol+/kg)
5
11
Soils Contain Organic Matter
Living plant tissue
Soil organisms
Plant exudates
Litter
Coarse debris
Humus
Soil is dynamic & full of life!
Root Tip & Rhizosphere
Ectomycorrhizae
Special Soil Fungi
Mycorrhizal Fungi
Mycorrhizae = symbiosis between
fungi and root.
Fungi receives carbon from plant,
plant gets a 10-100X increase in
absorbing root surface area.
VA Mycorrhizae
Ectomycorrhizae
Basidiomycetes & Ascomycetes
* spores wind & water dispersed
* 2,100 species of fungi in NA
* most conifers, willow, aspen, oak,
hickory
Endomycorrhizae (VA)
Phycomycetes – spores below ground
* most widespread, associate with
* most plant families including crops
* most deciduous trees
(P 10 – Handout)
Group Activity
Identify 6 major kinds of OM that are
found in soil.
Use the pictures to help identify the
different kinds.
Types of Soil Organic Matter
1. Living tissues of plants & symbionts
2. Soil biomass – living organisms other than plants (microbes,
invertebrates – ants, beetles, earthworms, etc.)
3. Exudates and leachates – biochemicals from living & dead
tissues – provide energy
4. Litter – fresh or partly composed – above & below ground
light fraction – easily decomposed (L & F)
heavy fraction - humic material slow to decompose
5. Coarse woody debris – larger pieces of wood - > 1 in diam
water storage & habitat for meso & micro-fauna
6. Humus – mostly synthesized OM released by microbes &
invertebrates; 80-90% of SOM; major CO2 storage.
O Horizons – Forest Floor
L – litter (Oi)
F – fermentation (Oe)
H – humus (Oa)
Kinds of FF’s
Mor – Oi (L)
Oe (F)
Oa (H)
Conifer
Acid
Low C/N
Mull – Oi (L)
Oe (F)? Deciduous
Basic
Moder – Oi (L)
Oe (F)
Oa (H)?
Mor Forest Floor
Conifer/Acid
Slow Decomposition
Clear Break Between FF & Soil
Oi (L)
Oe (F)
Oa (H)
Mineral soil
Mull Forest Floor
Deciduous
Basic/Low C/N
Oi (L)
No Oe & Oa
Incorporated
Into A
No Break Between FF & Mineral Soil
Bogs
1. Northern or high elevation
climates
2. Water source - precipitation
3. Anaerobic water
4. Low pH < 5
5. Slows decomposition
6. Sphagnum moss
Histisol
Fens
1. Similar locations
2. Water source is
groundwater or stream
3. Higher pH
4. More nutrients
5. More plant diversity
Organic Soil
Fall is here and the leaves are falling off of the trees
Group Activity
These leaves play an important role in maintaining the organic matter
in the soil. How do you think they become part of the OM?
Please use this figure to expand your answer
Group Activity
A) What % of the aspen leaves
is water?
B) What % of the aspen leaves
are C, H, O and ash?
Lignins & phenolic compounds
C) Rank the compounds from Cellulose
Sugars, starches & simple proteins
fastest decomposition to
slowest.
Hemicellulose
Fats, waxes, etc.
Crude proteins
Page 31 Soil & Plant Handout
Composition of typical
green plant material
Sugars, starches & simple proteins
Crude proteins
Hemicellulose
Cellulose
Fats, waxes, etc.
Lignins & phenolic compounds
Rapid decomposition
Slow decomposition
Group Activity
A) What is meant by the term
detrital food web?
B) How is this food web
similar, different from the
traditional terrestrial food
web?
C) What are the three major
steps of decomposition?
Soil Food Web
Decomposition
1. Oxidized to water & CO2
2. Heat energy
3. Nutrients released
(mineralized)
4. Resistant compounds
produced by organisms
C/N
Dry Plant OM
42% = C
1-2% = N
1. Microbes need
8 C for 1N to
build cells
2. Need 16 C for Rs
3. Need C/N = 24/1
C/N = 30:1
4. If OM > 24:1
microbes take
C/N = 120:1
N from NO3
in soil
C/N = 300:1
Temperature & Moisture
Influences on SOM
What does this figure show?
Higher temperature decreases
SOM
• faster decomposition
• higher respiration
Higher moisture increases SOM
• more production of OM
• possibly slower decomposition
Page 29 Soil & Plant Handout
Soil pH
Microbial activity
&
Nutrient availability
What is the effect of
pH on the decomposition?
Check the impact on fungi
and bacteria
Thus ends our in-depth
discussion of soils –
Questions??
But don’t forget these processes as you
continue to discuss forest ecology
Soil – The Basic Natural Resource (CliPROT)
Molly McGovern, INHF