1. Fundamentals of Soil Science
Soil Organic Matter

2. Lecture 6 SOM’s Influence on Soil Properties and Plants

3. Learning Objectives Lecture 6 –
Identify factors that lead to a loss or gain of organic matter in soils
Explain the conundrum of soil organic matter management
List five guidelines for managing soil organic matter
Discuss changes in active and passive pools of organic matter as a result of management
Name the greenhouse gases of importance to soil processes and the relative warming potential of each

4. Lecture 6 - Topics
Factors controlling the level of soil organic matter
Major soil C pools
Maintenance of soil organic matter
Summary and review

5. Carbon Inputs – Outputs = Storage
Plants Litter Soil organic matter
Gains in carbon come from plant residues and applied organic materials
Losses in carbon are due to respiration (CO2 losses), plant removals, and erosion.

6. Factors promoting gains Factors promoting losses
Balance of Carbon
Factors Affecting the Balance Between Gains and Losses or Organic Matter in Soils
Factors promoting gains
Factors promoting losses
Green manures or cover crops
Conservation tillage
Return of plant residues
Low temperatures and shading
Controlled grazing
High soil moisture
Surface mulches
Application of compost and manures
Appropriate nitrogen levels
High plant productivity
High plant root:shoot ratio
Erosion
Intensive tillage
Whole plant removal
High temperatures and exposure to sun
Overgrazing
Low soil moisture
Fire
Application of only inorganic materials
Excessive mineral nitrogen
Low plant productivity
Low plant root:shoot ratio

7. Managing SOM
Management of soil organic matter leads to reduction in greenhouse gas emission or to enhanced soil quality and plant production

8. Conundrum – SOM must simultaneously decompose and accumulate.
SOM must decompose to become a source of nutrients for plants and organic compounds that promote biological diversity, disease suppression, aggregate stability and metal chelation.
SOM must accumulate for these same functions as well as for sequestering of C, enhancement of soil water-holding, adsorption of exchangeable cations, immobilization of pesticides and detoxification of metals.

9. General Guidelines for Managing SOM
Continuous supply of plant residues

10. General Guidelines for Managing SOM
Continuous supply of plant residues
Each system has its own “ideal” level of SOM

11. General Guidelines for Managing SOM
Microbial activity,
CO2 evolved
Nitrate depression period
Soluble N level in soil
C/N ratio of residues
Residues added
Time
C/N ratio 60 40 20 80
(a)
(b)
Continuous supply of plant residues
Each system has its own “ideal” level of SOM
Adequate N is requisite

12. General Guidelines for Managing SOM
Continuous supply of plant residues
Each system has its own “ideal” level of SOM
Adequate N is requisite
Tillage should be reduced or eliminated

13. General Guidelines for Managing SOM
Continuous supply of plant residues
Each system has its own “ideal” level of SOM
Adequate N is requisite
Tillage should be reduced or eliminated
Encourage perennial vegetation and natural ecosystems

14. 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

15. 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.

16. 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.

17. 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

18. Pools of SOM (cont.)

19. Changes in Active and Passive Pools with Soil Management
Monitoring the Active C Pool can serve as an early warning of soil quality changes
The Active Pool reflects the greatest change in organic matter, either loss through cultivation or gain through addition of organic material.

20. Global Climate Change
Levels of certain gases in Earth’s atmosphere cause concern
Carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs)
Greenhouse gases (GHG) trap much of the outgoing long-wavelength radiation
GHG produced by biological processes, such as those occurring in soil, account for ½ of the rising greenhouse effect.
Root respiration, decomposition of exudates and SOM produce CO2
Methanogenesis produces CH4
Nitrification and denitrification produce N2O

21. Global Warming Potential
N2O and CH4 are present in lower concentrations than CO2
Their potential to trap infrared radiation is greater
GWP of N2O is 298 x and CH4 is 25 x CO2 over 100 years
Small increases in the production of these trace gases impacts net emissions of an ecosystem or production system

22. GHG Emission from Soil

23. Trace Gas Emission in CO2 Equivalents
Sugar cane
Napier grass

24. Renewable Energy: Biofuels

25. Summary
SOM is beneficial to soil biological, physical, and chemical properties
To realize this potential you must build SOM up, but also have mineralization, in balance
Management can have enormous impact particularly on the active soil C pools
Trace GHG that originate from soils, such as CH4 and N2O have disproportionate effects on climate change compared to CO2