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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? 2 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? e- 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 27 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 • minerals and other clays with interactions S tabilized tabilized SOM Formation: SOM Formation: Stabilization in by by _ _ _ _ _ (step 2) _ S _ Clay Organic matter Organic oil _ _ _ _ _ _ _ _ 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 • 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? 39 .
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