Soil Biological Processes

Lecture 15

• Soil Biological Processes

1 Questions

• What are oxidation-reduction reactions? Give an example • How do metabolisms differ between microbes? Give examples • How does microbial respiration change as soils become flooded? • What affects organic matter decomposition in soil? • What does C/N ration mean?

Oxidation-Reduction Reactions

Reduction Electron Oxidized Donor Donor e- e-

Electron Reduced Acceptor Acceptor Oxidation Aerobic respiration ------>

C6H12O6 + O2 => CO2 + H2O

3 Classifying Microbial Metabolism

Where does carbon come from? Where does energy come from?

What is the electron donor for What is the electron acceptor for reactions? reactions?

4 Classifying Microbial Metabolism

Where does carbon come from? Where does energy come from?

Organic molecules – heterotrophs Light - phototrophs Carbon dioxide - autotrophs Chemical reactions – chemotrophs

What is the electron donor for What is the electron acceptor for reactions? reactions?

Organic molecules – organotrophs Oxygen: aerobes Other than oxygen: anaerobes Inorganic (e.g. H2S, NH4) – lithotrophs Both: facultative anaerobes

5 Metabolism of Microbes

Yes Fixes No Carbon?

Yes Yes Energy Photoautotroph Energy Photoheterotroph from light? from light?

No No

Yes Energy from Yes Energy from Chemoheterotroph Chemoautotroph chemical chemical reactions? reactions?

No No

Other Autotroph Other Heterotroph 6 • Photoheterotroph • Photoheterotroph

Electron • Chemoheterotroph • Chemoheterotroph Electron Donor • Photoautotroph • Photoautotroph Acceptor • Chemoautotroph • Chemoautotroph

Uses Yes Yes Organic Aerobe only O2? organotroph electron donor? No

No Other Yes Anaerobes than O2? Yes Inorganic lithotroph electron No donor? Can use Yes Facultative either? Anaerobe

7 Examples of Microbial Metabolism

Protozoa, fungi, bacteria

O H H H H R C O + O O C + O + H O

Organic matter Oxygen Carbon dioxide Water Energy

• Source of Carbon: • Source of Energy: – Modified organic molecules – Oxidation of organic originally from plants molecules – Sugars, amino acids, fatty • Electron Donor: acids, nucleotides, etc. – Organic molecules – Organic pollutants (petroleum, • Electron Acceptor: pesticides, solvents) – Oxygen

Aerobic Chemoorganoheterophy 8

Examples of Microbial Metabolism

Cyanobacteria

O H H H C + O + C O + O O H O Formaldehyde Oxygen Carbon dioxide Water Light

• Source of Carbon: • Electron Donor: – Carbon dioxide – Water • Source of Energy: • Electron Acceptor: – Light – NADP (coenzyme)

Aerobic Photolithoautotropy 9

Examples of Microbial Metabolism

Nitrosomonas

O H + _ _ H H + N O O N H C + H H + O O + O + + H O Oxygen Nitrite Water Hydrogen Ammonium Energy Carbon dioxide ion

• Source of Carbon: • Electron Donor: – Carbon dioxide – Ammonium • Source of Energy: • Electron Acceptor: – Oxidation of ammonia – Oxygen

Aerobic Chemolithoautotropy

10 Examples of Microbial Metabolism

Thiobacillus denitrificans

O H H H H _ _ O R O N N N + C + + C + O O H Nitrogen O Water Energy Nitrite gas Organic matter Carbon dioxide

• Source of Carbon: • Electron Donor: – Organic molecules – Organic molecules • Source of Energy: • Electron Acceptor: – Oxidation of organic – Nitrate molecules

Anaerobic Chemoorganoheterotrophy 11

Examples of Microbial Metabolism

Carbon Source Energy Source Electron Donor Inorganic Organic Light Chemical Inorganic Organic

Organisms Autotroph Heterotroph Phototroph Chemotroph Lithotroph Organotroph Most bacteria x x x

Fungi and Protozoa x x x Nitrifying + + bacteria CO2 NH4 NH4 Sulfur oxidizing CO2 H2S H2S bacteria Purple sulfur bacteria CO2 x H2S Cyanobacteria CO2 x H2O

12 Winogradsky Column

13 More or less

14 Putting it all together: N biogeochemical cycle

Nitrogen in Atmosphere

Volatilization

- (NO3 )

Plants

Assimilation Denitrifying (Uptake) bacteria

Nitrogen-fixing bacteria Nitrates Decomposers (aerobic and anaerobic bacteria and fungi) Nitrifying Ammonification Nitrification bacteria Ammonium Nitrites Nitrogen-fixing Nitrifying soil bacteria bacteria

15 Global Carbon Cycle

18 Flooded Soil

19 Anaerobic Respiration Free Energy Change (kJ/mol) O 2 Aerobes -119 H2O Oxic - -113 NO3 Denitrifiers N2 MnO2 Manganese reducers -97 Mn2+ Post-Oxic Fe(OH)3 Iron reducers -47 Fe2+ SO 2- 4 Sulfate reducers -21 H2S Sulfidic CO 2 Methanogens -18 CH4 H O Methanic 2 -1

H2 20 Controls on Microbial Activity • Soil conditions – Extreme pH (> 8 or < 5) – Salinity – Nutrient deficiencies (ex. C, N, P, Mo, Cu) – Sufficient soil moisture (fastest -10 to -50 kPa) – Good aeration – Warm temperatures (doubling rate every 10°C between 5-25 °C) – Heavy metals and agrochemicals – Microbial community composition

• Quality of added residues – Physical condition and accessibility – C/N ratio – Content of lignins and polyphenols

24 Organic Matter Accessibility

• Incorporated?

• Residue particle size

25 Organic Mater Quality

Sugars and starches Rapid

Proteins

Hemicellulose

Cellulose

Lipids

Lignins and polyphenols (tannins) Slow

26 C/N ratio of residue affects N availability to plant

• If C/N ratio < 20 – Excess N released to soil (ammonification)

• If C/N ratio > 20 – Mineral N is taken from soil reserve reducing plant access to N

Examples of N Release During Decay

Microbial activity, 60

CO2 evolved

40 C/N ratio

of residues C/N C/N Ratio

Soluble N level 20 Soluble Nitrogen Soluble 0 0 Time

Residues added

Microbial activity,

CO2 evolved 60

Soluble N level 40 C/N C/N Ratio

C/N ratio of residues 20 Soluble Nitrogen Soluble

0 0 Time 28 Residues added Nitrogen Liberation

• After ingestion by protozoa, excess N is released for plant uptake O C O

C C C C C CN Bacterium C C Protozoa C C C C C C CN CN C C C C

%N C:N Bacteria 8-12 5:1

Protozoa 15 >10:1 29 C/N Ratio of Various Organic Material

Organic Material %C %N C/N Sawdust 46 0.1 400 Wheat straw 38 0.5 80 Corn stover 40 0.7 57 Mature alfalfa hay 40 1.8 25 Barnyard manure 41 2.1 20 Compost 30 2 15 Vetch cover crop 40 3.5 11 Soil OM (Mollisol) 56 4.9 11 Fungi 50 5 10 Bacteria 50 10 5

31 SOM Formation:(step 1) Preparation of Components

Plant and animal residues

Sugar, starch, proteins, Lignins, hemicellulose, tannins, etc. cellulose, lipids, etc.

Fungi on petri plate have H O, CO , Microbial H2O, CO2, cell wall components that 2 2 Microbial NH3, H2S, etc. decomposition NH3, etc. are precursors to SOM decomposition and modification

Microbial biomass and Modified lignin metabolic byproducts and aromatics

Aromatic Ammino acids, structures proteins, etc.

Mineralization Mineralization Humus Benzene: aromatic molecule34 SOM Formation: (step 2) Stabilization in Soil

• Stabilized by

interactions with

_ _

Clay

_ _

_ _ _

_ _

clays and other _

_ _ _ minerals _

Organic matter

36 Changes in OM During Conversion to Humus

These reactions give humus three related properties: • Increases pH buffer capacity • pH dependent charge • Ability to chelate certain cations

37 Soil organisms are involved in many aspects of soil quality

Structure/ Aggregation

Nutrient Organic Cycling Matter

Soil Community

Decomposition Nitrate Leaching Humification

38 Questions