Introduction to Soils and Soil Resources

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Introduction to Soils and Soil Resources

• 2001
• Lecture 7
• Soil Air and Soil Organic Matter

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Table 9.1. Relative concentrations of
atmospheric gases on Earth, Mars and Venus
Credit after Margulis and Hinkle, 1991
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Oxidation

• Oxidation A reaction in which atoms or
  molecules gain oxygen, or lose hydrogen or
  electrons Fe 2 Fe 3 e-

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Reduction

• Reduction A reaction in which atoms or
  molecules lose oxygen, or gain hydrogen or
  electrons N2 H2 NH3

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Oxidation State

• In a free state, oxidation state is zero. For
  example elemental sulfur (S0)
• Monoatomic ions have oxidation state equal to the
  ionic charge. For example, Ca2 has oxidation of
  2

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Oxidation State

• In a combined state, hydrogen has oxidation
  state of 1 oxygen has oxidation state of -2
• For example H2O
• For example H2SO4 (S 6)

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Examples

• Oxidation and reduction reactions occur in
  nature
• Responsible for energy transformations
• Examples are given in Table 9.2

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The atmosphere

• The atmosphere is a large, layer system
• Troposphere (80 air mass)
• Stratosphere (Ozone region)
• Mesosphere
• Thermosphere

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Fig. 9.1. The profile of the atmosphere
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Fig. 9.2. Radiation Budget
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Greenhouse gases

• Water vapor (H2O)
• Carbon dioxide (CO2) parts per million
• Methane (CH4) parts per million
• Ozone (O3) parts per million
• Nitrous oxide (N2O) parts per billion
• Halocarbons (CFCs) parts per trillion
• Refer to Table 9.3 (Section 9.3)

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Fig. 9.3. Active sites, mineral particles and
water films
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Atmospheric Air vs Soil Air ( volume)
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Soil Atmosphere

• The soil atmosphere is different from the
  atmosphere (Table 9.4 in Section 9.4)
• Soil is a biologically, porous medium
• Activities of plant roots and soil biota change
  soil atmosphere
• Metabolic pathways change if oxygen is limiting
  (Fig. 9.3 in Section 9.4)

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Aerobic Respiration

• C6H12O6 O2 ? 6 CO2 6 H2O energy

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Anaerobic Respiration

• NO3- ? NO2- ? NO ? N2O ? N2
• 5 3 2 1
  0
• Nitrate is terminal electron acceptor when oxygen
  is limiting

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Gas Conc. ( vol.) Corn field
Elliott and McCalla 1972. Soil Sci. Soc. Am.
Proc. 3668
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Gas Conc. ( vol.) Feedlot
Elliott and McCalla 1972. Soil Sci. Soc. Am.
Proc. 3668
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Gas Dynamics

• Normally O2 decreases and CO2 increases with
  depth
• Normally, CO2 lt0.5 in soil atmosphere while O2
  gt10

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Soil Organic Matter
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Soil Major Components
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Soil Organic Matter

• All organic substances, by definition, contain
  carbon
• The element carbon is the foundation of all life

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Soil Organic Matter

• SOM consists of living or dead plant material,
  living organisms, microbial and faunal products,
  and stabilized complex organic matter called
  humus
• Organic matter has a profound impact on soil
  physical, chemical and biological properties

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Credit U of A Extension Pedosphere.com
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Physical Properties

• The physical properties of soil horizons vary
  tremendously within a pedon
• Example is given in Table 11.2 in Section 11.2

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Soil C

• Present as organic matter
• Present as inorganic carbon in form of carbonates
  in some soils
• Example is given in Table 11.3 in Section 11.2

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OM Impacts

• Formation of organo-mineral complexes
• Aggregation
• Cation and anion exchange capacity
• Movement of pollutants
• Decomposition and nutrient cycling (next lecture)

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Fig. 1.9. Pores and particles in soil (Pawluk)
Credit Pedosphere.com
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Porosity/Structure
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Fig. 6.12. Structure of a model humic acid
(Schulten Schnitzer, 1997)
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Fig. 6.10. Impact of soil pH on net charge
oforganic acids
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Charge Characteristics
cmolc/kg
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Application

• Given a soil with 5 OM and 20 montmorillonite
  clay. Calculate total negative charge.
• Charge 0.05(200) 0.20(100) 10
  20 30 cmolc/kg

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Soils the Global C cycle

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How much organic C is present in this landscape?
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How much organic C is present in the Prairies?
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How Much C is present in Canadian Soils?
Credit Acton and Gregorich (1995)
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Credit Ecological Monitoring Assessment Network
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Fig. 5.14. A Catena
Credit Rennie and Ellis (1995)
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Global Carbon Cycle Units

• Units1 ton 1 x 103 Kg 1 x 106 g
• 1 gigaton (Gt) 1 billion tons 1 x 109 tons 1
  Gt 1 x 109 tons 1 ton 1 x 106 g 1 Gt 1 x
  1015 g 1Pg (petagrams)

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Fig. 11.2. The Global Carbon Cycle
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Global Carbon Cycle Pools

• Atmosphere 750 Pg
• Vegetation 610 Pg
• Soil 1,580 Pg
• Fossil Fuels 5,000 Pg
• Oceans (organic) 1,020 Pg
• Oceans (inorganic) 38,100 Pg
• Carbonate Rocks 1 x 106 Pg

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Fig. 11.2. The Global Carbon Cycle
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Global Carbon Cycle Fluxes (Pg/yr)

• Atmosphere to vegetation 61.4
• Vegetation to atmosphere 60
• Deforestation (loss) 1.6
• Change in land use (gain) 0.5
• Fossil Fuel combustion 5.5

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Fig. 11.2. The Global Carbon Cycle
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Fig. 11.1. Atmospheric CO2 concentration trend
at Mauna Loa Station (1958-1996)
Credit Environment Canada
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Crops at the Breton Plots, University of Alberta
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Wheat Residue Composition (g/kg)

• Soluble components 288
• Cellulose 361
• Hemicellulose 184
• Lignin 141
• N-compounds (proteins) 9
• Ash 84

Credit Broder and Wagner, 1988
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Decomposition Rates

• Soluble Components Rapid
• Crude Proteins
• Cellulose
• Hemicellulose
• Fats, Waxes
• Lignins Slow

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Controls on Decomposition Rates

• Biochemical composition
• Chemical structure
• Amount present
• Microbial and faunal activity

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Processes

• Enzymatic solubilization of substrates through
  microbial activity
• Transformation of organic compounds
• Biophysical stabilization (reaction of organic
  compounds with mineral materials)
• Formation of organo-clay complexes

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Credit U of A Extension Pedosphere.com
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Decomposition Sequence
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Decomposition Kinetics

• First Order kinetics
• Amount decomposed rate constant
  pool size
• dc/dt kC

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Amount of Decomposition
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Simulation Models

• This model illustrates that substrates are
  decomposed at different rates
• Soil organisms grow and die
• Organic matter is recycled
• Part of carbon decomposed is used for energy
  production and is respired
• Model by Verberne et al. (1993)

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Linking the C Cycle to Nutrient Cycling

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Needs of Organisms

• Heterotrophic organisms need organic carbon for
  biosynthesis and energy production
• Need N for proteins, DNA, RNA
• Need P for DNA, RNA
• Need S for proteins
• Need trace elements and water

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Decomposition Mineralization of N
decomposition
mineralization
Nitrogen is excreted by microbesbecause
organisms have excess N
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Decomposition Immobilization of N
decomposition
immobilization
Nitrogen is taken up by microbesbecause
organisms need N for growth
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Carbon and Nutrient Cycling
Decomposition
Respiration (oxidation)
immobilization
Recycling
mineralization
Humification
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Tramin Model Demonstration

• N is mineralized in non-amended soil
• N is immobilized in glucose-amended soil
• N dynamics under field conditions is more complex
• The Tramin Model (Juma Paul, 1981)

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Ecosystem Processes
Photosynthesis
Respiration
Decomposition
Respiration (oxidation)
immobilization
Recycling
mineralization
Humification
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Humic Substances

• The less resistant, identifiable biomolecules are
  produced by microbial action are called nonhumic
  substances
• The ill-defined, complex, resistant, polymeric
  compounds are called humic substances

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Properties of Humic Substances

• Molecular weight range 2,000 to 300,000 daltons
  (mass of H atom is one dalton)
• Resistant to decomposition
• Form complexes with silicate clays
• Have pH dependent charge

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Fate of 100 g of Crop Residues
decomposition
synthesis
HumicCompounds
3 - 8
3 - 8
10 - 30
polymerization
60 - 80
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Factors Affecting SOM

• SOM increases as moisture content increases
  Example Brown vs. Black Chernozems

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Factors Affecting SOM

• Grasslands generally dominate in subhumid and
  semiarid areas (Chernozems)
• Forests dominate in humid regions (Luvisols)

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Factors Affecting SOM

• Texture All else being equal, soils high in
  clay and silt are generally higher in organic
  matter than are sandy soils

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Factors Affecting SOM

• In poorly drained soils, high moisture promotes
  plant dry matter production and relatively poor
  aeration inhibits organic matter decomposition.
  SOM accumulates.

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Cropping, Tillage Fertilization

• First cultivation results in a rapid loss of SOM
• In contrast, soils with low organic matter can be
  improved with crop rotations and increase in soil
  fertility.
• The University of Alberta Breton Plots

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Next Week

• Soil Ecology
• Review module 2