SOIL ORGANIC MATTER

Organic Matter Decomposition: a cyclic view

organic matter

population sizes, temperature, moisture

energy + CO2

Biomass (more bugs)

respiration

synthesisMicrobe bodies become part of the organic matter pool

Symbol for control

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

SOIL ORGANICMATTER

LivingOrganisms:BIOMASS

Dead tissues

and wastes:DETRITUS

Non-living, non-tissue:

HUMUS

SOM: What is it?

Oi Oa

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

Fate of Humus

Rio Solimões

Rio Negro

Rio Amazônas

Decomposition = transformations of SOM (remember, matter is neither created nor destroyed)

Symbol for control: by . . .?

Detritus (fresh)

Humus (way dead)

Microbes (biomass)

Plants (biomass)

CO2, nutrients, energy

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 to Incomplete Decomposition

• The kinds & numbers of soil fauna

1. Physical nature of litter

• Small stuff decomposes faster than big stuff– More Surface Area– Role for “Shredders”

Mechanical decomposition …

2. Chemical composition of plant matter

• Sugars, starches, simple soluble proteins

slow

fast Chemical decomposition

Glucose structure

• C6H12O6

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

Cellulose Structure• Simple, repeating structure

– Polymer of Glucose units– “Easy” to decompose

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

Lignin structure

• Complex, non-repeating structure– Phenyl rings– Harder to

decompose– Need lots of

enzymes to do it

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

Chemical nature of the litterChemical 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%)

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 –

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

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

Organic material %C %N C:N

soil organic matter 50 5.0 10soil microbes 50 5.0 5-10alfalfa 40 3.0manure 41 2.1corn stover 40 0.7 57wheat straw 38 0.5 80sawdust 50 0.05 600

“Quality” of different materials

2013

                                                                                                                                                                        

                                                                                                                                                                                                                                                                 

Effect of C:N on rate of decomposition