1. Fundamentals of Soil Science
Soil Organic Matter

2. Lecture 5 Creating SOM

3. Learning Objectives Lecture 5 –
Describe the importance of Carbon to Nitrogen ratio in litter decay
List the primary mechanisms contributing to soil C stabilization
Distinguish between factors that control decomposition of litter vs. decomposition of soil organic matter

4. Lecture 5 Topics
Factors that control litter decay, nutrient mineralization, and humus formation
Factors that control soil organic matter stabilization
Major soil carbon pools

5. Review - What is Soil Organic Matter?
Living biomass (plant tissues, animal tissues and microorganisms)
Dead roots and dead plant residues or litter
Mixture or organic substances no longer identifiable as tissues
Some carbon from decomposition process is converted to soil humus

6. polysaccharides, polyuronides, acids, etc.
Humus Creation
60-80 g
Organic C in residues
100 grams
3-8 g
3-8 g
10-30 g
Biomass
(soil organisms)
polysaccharides, polyuronides, acids, etc.
Complex compounds
Soil humus (15-35 g)
Incorporation year later

7. By-products of decomposition in aerobic soil
Basic reaction accounts for most of the organic matter decomposition in the soil, as well as oxygen consumption and CO2 release.
Aerobic: CH2O + O2 CO2 + H2O + energy (478 kJ mol-1 C)
In Aerobic soil activities of soil organisms create:
Carbon dioxide, water, energy and decomposer biomass
Release of essential nutrient elements such as nitrogen, phosphorus and sulfur and inorganic ions such as ammonium, nitrate an sulfate
Compounds resistant to microbial action
Mineralization – process that releases elements from organic compounds to produce inorganic forms

8. By-products of decomposition in anaerobic soil
Methanogenic bacteria and archaea reaction
Anaerobic: 4C2H5COOH + 2H2O 4CH3COOH + CO2 + CH4
In anaerobic soil decomposition activities are very slow
Wet, anaerobic soils accumulate large amounts of organic matter in partially decomposed condition.
Alcohols and methane gas contain energy
Foul odor and plant inhibitors

9. Controlling the Rate of Decomposition
Environmental conditions in the soil
Moisture
Air
Temperature
Residues as food source for soil organisms.
Physical location
Surface
Incorporated in soil by root deposition, faunal action, tillage
Particle size
Carbon/Nitrogen Ratio
Older plants higher proportion of slow decomposing lignin and cellulose

10. Carbon/Nitrogen Ratio
Soil organisms need carbon for building essential organic compounds and to obtain energy
They need nitrogen to synthesize nitrogen-containing cellular components such as amino acids, enzymes and DNA.
Microbes need 1 g of N for every 24 g of C in their food
Higher than 25:1 – not enough nitrogen so 1) microbes take from plant supply, 2) decay delayed because microbes can’t survive

11. Significance of C/N Ratio60 40 20
Adding readily decomposable organic material increases the consumption of microbial community which results in high CO2 yield. The microbes demand nitrogen which deprives plants of nitrogen. This in nitrate depression period.
Planting should be delayed until after nitrate depression period or additional sources of nitrogen applied.
Microbial activity,
CO2 evolved
C/N ratio
Soluble N level in soil
C/N ratio of residues
Nitrate depression period
Residues added
Time
(a) 80 60 40 20
Microbial activity,
CO2 evolved
Soluble N level in soil
C/N ratio
C/N ratio of residues
Residues added
Time
(b)

12. Mechanisms for SOM Stabilization
Protection within soil aggregates
Organo-mineral interaction (bound organic matter to mineral surfaces)
Recalcitrance (intrinsic chemical resistance to decay)
(Sollins et al. 1996, Geoderma)

13. Protection Root Microaggregates Plant and fungal debris
Silt sized microaggregate
Clay microstructure
Particulate OM
with hyphae
Hyphae
Macroaggregate
>250µm
Pore space
Interaggregate binding agents
(from Jastrow and Miller 1998)
Soil Processes and the Carbon Cycle, CRC Press.

14. Organo-mineral interaction
Organic Matter
Exchangeable
Hydrophilic
functional groups
Hydrophobic
structures
Direct bond with surface metal cation
Electrostatic Interaction
with soluble ions
Hydroxylated mineral surface
(Kleber et al. 2007, Biogeochemistry)

15. Organo-mineral interaction (cont.)
Hydrophobic Compounds
OCH3 OH
CH3O
COCH3
OCH3 OH
HC=CHCO2H
C/N means
lots of C,
little N
Phenolic groups = lignin O H C
Waxy, long chain fatty acids = cutin and suberin
Polar side chains for solubility, but will bind to minerals, other organic matter, each other preferentially
Very important role in ORGANO-MINERAL interactions

16. Recalcitrance Mean Residence Time (y)
Cellulose
Polyphenols
Complex proteins
Lipids
Lignin
Cuticular waxes
Black carbon
Hemicellulose
Starches
Simple sugars
Phospholipids
Peptides and AAs
Autofluorescence microsopy of pine wood
0.001
0.01
0.1 1 10
100
1000
10000
Mean Residence Time (y)
Free compounds

17. Recalcitrance (cont.) Stabilized in Soil Mean Residence Time (y)
LMW acids
Cellulose
Polyphenols
Complex proteins
Lipids
Lignin
Cuticular waxes
Black carbon
Hemicellulose
Starches
Simple sugars
Phospholipids
Peptides and AAs
Free compounds
STABILIZED in
the soil matrix
0.001
0.01
0.1 1 10
100
1000
10000
Mean Residence Time (y)

18. Pools of SOM Small % of residue is retained
Plant residues
Structural C
high lignin, low N
2-4 years
C/N=
Metabolic C
low lignin, high N
year
C/N=10-25
Small % of residue is retained
Offset by slow decomposition
Often in equilibrium in mature ecosystems
Disturbance can cause drastic change
CO2
CO2
Active SOM
1-2 years
C/N = 15-30
CO2
Slow SOM
years
C/N = 10-25
CO2
Passive SOM
years
C/N = 7-10
CO2

19. Pools of Soil Organic Matter (cont.)

20. SOM Active Pool
Active Pool % of SOM – labile materials with half-lives of only a few days to a few years.
Provides most of the accessible food for soil organisms and most of the readily mineralizable nitrogen.
Beneficial effects on structural stability that lead to enhanced infiltration of water, erosion resistance, ease of tillage.

21. SOM Passive Pool
Passive Pool – % of SOM – materials remaining in soil for hundreds or thousands of years.
Material physically protected in clay-humus complexes
Responsible for cation exchange and water-holding capacities contributed to soil by organic matter
Composed of humic substances

22. SOM Slow Pool Slow Pool – Between Active and Passive pools
Particulate matter high in lignin and other slowly decomposable and chemically resistant components. (Half-lives in decades)
Source of mineralizable N, P, and S
Important source of mineralized nitrogen and provides food source for k-strategist microbes.