Soil Organic Matter and Decomposition

Basic Decomposition Equation Organic compound + O2 (or other electron acceptor) CO2 + H2O + energy + inorganic nutrients a form of respiration. an oxidation reaction aided by microbial enzymes.

Review of food chain concept: 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)

Oxidation Loss of electrons Fe+2 Fe+3 e- -26 -25 +28 +28 Fe+2 Fe+3

Reduction Gain of electrons Fe+3 Fe+2 e- -26 -25 +28 +28 Fe+2 Fe+3

Photosynthesis (brought to you by autotrophs) CO2 + H2O + energy O2 + C6H12O6

Respiration (required by all others) C6H12O6 + O2 CO2 + H2O + energy Energy-rich energy-poor Reduced carbon oxidized carbon (no energy available for further reactions)

Decomposition revisited… Organic compound + O2(or other electron acceptor) CO2 + H2O + energy + inorganic nutrients 1) Organic substrate (dead plants and animals) 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 soil atmosphere. 4) Carbon cycles through decomposition and photosynthesis, serving as vehicle of energy flow among hetero and autotrophic organisms.

Most soil bacteria are heterotrophic and aerobic Get carbon from organic compounds Get energy from aerobic respiration Use oxygen as electron acceptor in decomposition

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

Oxygen harmful to anaerobes: 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

What do microbes need? 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 (NH4+, SO4-2, HPO4-2)

organic compounds mineralization inorganic 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

Organic matter cycle

Composition of plant residues 1 rapid to 6 slow 4 5 1 2 3 6 6

Humus “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

Humus is colloidal 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

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

Notice: 1.CO2 levels 2.Feeding frenzy 3.Priming effect 4.Arrows: C transfers 5.Humus levels Microbial biomass

Each type of plant residue has a C:N ratio 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

Amount of N is more critical than amount of C : Carbon usually makes up 45 – 55% of dry weight of tissue Nitrogen can vary from < 0.5% - >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)

C:N ratio in organic residue determines rate at which residue will decay and whether it will release (mineralize) or immobilize N after incorporation into soil.

What is cutoff for high and low C:N? 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

Organic residues with WIDE C:N ratios: 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 CO2 Higher proportion of C in resistant compounds (cellulose, lignin) slower decomposition

Wide C:N examples: Sawdust Newspaper Wood chips Straw

Organic residues with NARROW C:N ratios: 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

Narrow C:N examples: Manure Cover crop Household compost (composted)

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

Calculating C:N of SAP compost pile 5(100:1) + 7 (45:1) + 28 (80:1) = 500 + 315 + 2240 = 3055 = 76 : 1 5 + 7 + 28 40