Soil Organic Matter and Decomposition

Organic compound + O 2 (or other electron acceptor) CO 2 + H 2 O + energy + inorganic nutrients  a form of respiration.  an oxidation reaction  aided by microbial enzymes.

 Trophic levels  Autotrophs: ( get C from CO2)  Photoautotrophs ▪ (Get energy from sun)  Chemoautotrophs ▪ (Get energy from inorganic chemical reactions): ▪ Oxidation of N,S, Fe  Heterotrophs  (get C from organic compounds)

 Loss of electrons  Fe +2 Fe Fe Fe +2 e-e-

 Gain of electrons  Fe +3 Fe Fe Fe +2 e-e-

CO 2 + H 2 O + energy O 2 + C 6 H 12 O 6

C 6 H 12 O 6 + O 2 CO 2 + H 2 O + energy Energy-richenergy-poor Reduced carbonoxidized carbon (no energy available for further reactions)

Organic compound + O 2 (or other electron acceptor) CO 2 + H 2 O + energy + inorganic nutrients 1) Organic substrate is oxidized by inorganic oxidizing agent (O, N, S, etc). 2) Nutrient elements are contained in organic substrate too. These are mineralized in respiration. Decomposition frees nutrients (N,P,S,etc). 3) CO2 escapes to atmosphere. 4) Carbon cycles through decomposition and photosynthesis, serving as vehicle of energy flow among hetero and autotrophic organisms.

 Get carbon from organic compounds  Get energy from aerobic respiration  Use oxygen as electron acceptor in decomposition

1. Anaerobic respiration use nitrate, sulfate (or others) as electron acceptor 2. Fermentation  use organic substrate as electron acceptor (instead of oxygen)  reduced to by-product, such as alcohol or organic acid

 In aerobes, when oxygen accepts electrons, and is reduced, toxic compounds (e.g., hydrogen peroxide) are produced.  Aerobic organisms have adapted mechanisms (2 enzymes) to counteract toxins  ANAEROBES LACK THESE ENZYMES

 Nutrients, Carbon, energy. ▪ Up to 50% of C in decomposed compounds is retained as microbial tissue ▪ Some N,P,S also ▪ If amount of nutrients exceeds amount needed by microbes, they released as inorganic ions ▪ (NH 4 +, SO 4 -2, HPO 4 -2 )

organic compounds mineralization inorganic compounds immobilization

 In mineralization, nutrients formerly stored in organic form are released for use by living organisms ORGANIC INORGANIC  In immobilization, these nutrients are reabsorbed and assimilated by living organisms INORGANIC ORGANIC

1 rapid to 6 slow

 “Amorphous, colloidal mixture of complex organic substances, not identifiable as tissue”.  C:N:P:S = 100:10:1:1  Composed of humic substances  Resistant, complex polymers ▪ 10s to 100s of years  and nonhumic substances  Less resistant, less complex

 Large surface area per unit volume  Greater than clay  Negatively charged  OH - and COOH - groups  High nutrient holding capacity (high CEC)  High water-holding capacity

 Zymogenous: opportunists; eat “easy” food; reproduce rapidly (r-strategists)  Autochthonous: eat very resistant organic compounds; slowly reproducing  (K-strategists)

Notice: 1.CO 2 levels 2.Feeding frenzy 3.Priming effect 4.Arrows: C transfers 5.Humus levels Microbial biomass

 Decomposing residue is not only a source of energy, but also a source of nutrients for microbial growth.  N is the element most often lacking in soil/residue to point of limiting microbial population growth ▪ Limiting factor

 Carbon usually makes up 45 – 55% of dry weight of tissue  Nitrogen can vary from 6.0% For a residue with: 50% carbon and 0.5% N, C:N ratio would be ? 100:1 (wide/high C:N) 50% carbon and 3.0% N, C:N ratio would be ? 16:1 (narrow/low C:N)

 determines rate at which residue will decay and whether it will release (mineralize) or immobilize N after incorporation into soil.

Soil microbe cells need 8 parts C for 1 part N (C:N = 8:1) only 1/3 of C from food is incorporated into cells therefore, they need food with a C:N of ? 24:1

 If C:N ratio > 24:1, intense competition among microbes for soil N  If ratio is too wide, N will be used (immobilized) by microbes and plants may suffer N deficiency.  Compost those materials before adding to soil

 Comparatively low N  Microbes suffer a shortage as they begin decomposing, so have to get N from soil at a cost in energy expenditure and decomposition rate  Greater energy expense and release of CO 2  Higher proportion of C in resistant compounds (cellulose, lignin)  slower decomposition

 Sawdust  Newspaper  Wood chips  Straw

 Comparatively high N content  Mineralized N will be released soon after decay starts  So microbes won’t suffer a shortage as they begin decomposing  More C from residue can be diverted to microbial growth  Higher proportion of total C in easily decomposable compounds  Faster decomposition

 Manure  Cover crop  Household compost (composted)

1.Add high/wide C:N residue: microbial activity, CO 2 long nitrate depression final N level 2.low/narrow C:N: microbial activity, CO2 no nitrate depression final N level