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SOIL ORGANIC MATTER

SOIL ORGANIC MATTER. Organic Matter Decomposition: a cyclic view. population sizes, temperature, moisture. energy + CO 2. respiration. organic matter. synthesis. Biomass (more bugs). Microbe bodies become part of the organic matter pool. Symbol for control. SOM: Why it is important.

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SOIL ORGANIC MATTER

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  1. SOIL ORGANIC MATTER

  2. Organic Matter Decomposition: a cyclic view population sizes, temperature, moisture energy + CO2 respiration organic matter synthesis Biomass (more bugs) Microbe bodies become part of the organic matter pool Symbol for control

  3. SOM: Why it is important • Reservoir of plant nutrients • Food source for soil organisms • Provides cation exchange capacity • Increases water-holding capacity • Decreases Al toxicity at low pH • Improves soil structure • but doesn’t change soil texture

  4. SOIL ORGANIC MATTER Dead tissues and wastes: DETRITUS Living Organisms:BIOMASS Non-living, non-tissue: HUMUS SOM: What is it? Oa Oi

  5. Humus is not synonymous with Organic Matter • Relatively stable fraction of soil OM remaining after the major portion of plant and animal residues have decomposed • Can be chemically divided ad nauseum • Improves fertility  not through continued N release, but…? • High CEC • Anion retention

  6. Fate of Humus Rio Amazônas Rio Negro Rio Solimões

  7. Decomposition = transformations of SOM (remember, matter is neither created nor destroyed) CO2, nutrients, energy Microbes (biomass) Plants (biomass) Detritus (fresh) Humus (way dead) Symbol for control: by . . .?

  8. Rate of decomposition depends on: • Physical nature of the litter material • Chemical nature of the litter material (“quality”) • Temperature & moisture of the soil environment • Aeration (aerobic vs. anaerobic) • Respiration (with O2)  38 ATP (cellular “energy bus”) per glucose • Anaerobic  2 ATP per glucose • Leading toIncomplete Decomposition • The kinds & numbers of soil fauna

  9. 1. Physical nature of litter • Small stuff decomposes faster than big stuff • More Surface Area • Role for “Shredders”  Mechanical decomposition …

  10. fast slow 2. Chemical composition of plant matter  Chemical decomposition • Sugars, starches, simple soluble proteins

  11. Glucose structure • C6H12O6

  12. fast slow Chemical composition of plant matter • Sugars, starches, simple soluble proteins • Crude proteins • Hemicellulose • amorphous, short polysaccharide (200 units) • Cellulose • crystalline, long chains (7-15k glucose)

  13. Cellulose Structure • Simple, repeating structure • Polymer of Glucose units • “Easy” to decompose

  14. fast slow Chemical composition of plant matter • Sugars, starches, simple soluble proteins • Crude proteins • Hemicellulose • amorphous, short polysaccharide (200 units) • Cellulose • crystalline, long chains (7-15k glucose) • Fats, waxes • Lignin

  15. Lignin structure • Complex, non-repeating structure • Phenyl rings • Harder to decompose • Need lots of enzymes to do it

  16. fast slow Chemical composition of plant matter • Sugars, starches, simple soluble proteins • Crude proteins • Hemicellulose • amorphous, short polysaccharide (200 units) • Cellulose • crystalline, long chains (7-15k glucose) • Fats, waxes • Lignin  Humus – derived, slower yet

  17. Chemical nature of the litter Dry matter (25%) Water (75%) Types of Compounds Elemental Composition Lignin (20%) Hydrogen (8%) Hemi- cellulose (18%) Cellulose (45%) Ash (8%) Oxygen (42%) Carbon (42%) Sugars and starches (5%) Protein (8%) Other (4%)

  18. Remaining slides not shown in lecture, but concepts covered – Chemical “quality” of litter • Ratio of carbon to nitrogen (C:N) • Analogous to our nutrition: • carbohydrates (carbon) • protein (nitrogen) • Microbial uptake of N important: • microbes better scavengers than plants  If C:N high (N low), microbes can “rob” the soil “bank” of plant available N!

  19. C:N ratios • C always listed first, always in terms of certain number of grams C per gram N • e.g. 25:1, often written 25 • Lower litter C:N (high N content) means • microbes need less external (soil solution) N

  20. Critical Values of C:N • C:N < 24:1, N is released to plants  mineralization • C:N > 24:1, N locked up  immobilization • microbes out compete higher plants every time • microbes take N from soil “bank” • Microbial C:N (total organism makeup) = 8:1 • But: ⅔ of SOM carbon  lost on decomposition to CO2 • so microbes need to input 3 times as much C: (3*8):1 = 24:1 • Actual varies by microbe type & environmental conditions

  21. Organic material %C %N C:N soil organic matter 50 5.0 10 soil microbes 50 5.0 5-10 alfalfa 40 3.0 manure 41 2.1 corn stover 40 0.7 57 wheat straw 38 0.5 80 sawdust 50 0.05 600 “Quality” of different materials 13 20

  22. Effect of C:N on rate of decomposition

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