1. SOIL ORGANIC MATTER

2. Organic Matter Decomposition: a cyclic view organic matter population sizes, temperature, moisture energy + CO 2 Biomass (more bugs) respiration synthesis 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 Living Organisms: BIOMASS Dead tissues and wastes: DETRITUS Non-living, non-tissue: HUMUS SOM: What is it? Oi Oa

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 Solimões Rio Negro Rio Amazônas

7. Decomposition = transformations of SOM (remember, matter is neither created nor destroyed) Symbol for control: by...? Detritus (fresh) Humus (way dead) Microbes (biomass) Plants (biomass) CO 2, nutrients, energy

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 O 2 )  38 ATP (cellular “energy bus”) per glucose –Anaerobic  2 ATP per glucose Leading to Incomplete 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. 2. Chemical composition of plant matter Sugars, starches, simple soluble proteins slow fast  Chemical decomposition

11. Glucose structure C 6 H 12 O 6

12. 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) slow fast

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

14. 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 slow fast

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

16. 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 slow fast

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

18. 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! Remaining slides not shown in lecture, but concepts covered –

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 CO 2 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%NC:N soil organic matter505.010 soil microbes505.05-10 alfalfa403.0 manure412.1 corn stover400.757 wheat straw380.580 sawdust500.05600 “Quality” of different materials 20 13

22. Effect of C:N on rate of decomposition