fundamentals of soil science

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Fundamentals of Soil Science Soil Organic Matter

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Fundamentals of Soil Science. Soil Organic Matter. Lecture 6 SOM’s Influence on Soil Properties and Plants. 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 - PowerPoint PPT Presentation

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Page 1: Fundamentals of  Soil Science

Fundamentals of Soil ScienceSoil Organic Matter

Page 2: Fundamentals of  Soil Science

Lecture 6SOM’s Influence on Soil Properties and Plants

Page 3: Fundamentals of  Soil Science

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

Page 4: Fundamentals of  Soil Science

Lecture 6 - Topics• Factors controlling the level of soil organic

matter• Major soil C pools• Maintenance of soil organic matter• Summary and review

Page 5: Fundamentals of  Soil Science

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.

Page 6: Fundamentals of  Soil Science

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 cropsConservation tillageReturn of plant residuesLow temperatures and shadingControlled grazingHigh soil moistureSurface mulchesApplication of compost and manuresAppropriate nitrogen levelsHigh plant productivityHigh plant root:shoot ratio

ErosionIntensive tillageWhole plant removalHigh temperatures and exposure to sunOvergrazingLow soil moistureFireApplication of only inorganic materialsExcessive mineral nitrogenLow plant productivityLow plant root:shoot ratio

Page 7: Fundamentals of  Soil Science

Managing SOM• Management of soil organic matter leads to

reduction in greenhouse gas emission or to enhanced soil quality and plant production

Page 8: Fundamentals of  Soil Science

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.

Page 9: Fundamentals of  Soil Science

General Guidelines for Managing SOM

• Continuous supply of plant residues

Page 10: Fundamentals of  Soil Science

General Guidelines for Managing SOM

• Continuous supply of plant residues• Each system has its own “ideal” level of SOM

Page 11: Fundamentals of  Soil Science

General Guidelines for Managing SOM• Continuous

supply of plant residues

• Each system has its own “ideal” level of SOM

• Adequate N is requisite

Microbial activity,CO2 evolved

Microbial activity,CO2 evolved

Nitrate depression period

Soluble N level in soil

Soluble N level in soil

C/N ratio of residues

C/N ratio of residues

Residues added

Residues added Time

Time

C/N

ratio

C/N

ratio

60

40

20

0

80

60

40

20

0

(a)

(b)

Page 12: Fundamentals of  Soil Science

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

Page 13: Fundamentals of  Soil Science

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

Page 14: Fundamentals of  Soil Science

Pools of SOM

• Small % of residue is retained

• Offset by slow decomposition

• Often in equilibrium in mature ecosystems

• Disturbance can cause drastic change

Plant residuesStructural C

high lignin, low N2-4 years

C/N=100-200

Metabolic Clow lignin, high N

0.1-0.5 yearC/N=10-25

Slow SOM15-100 yearsC/N = 10-25

Active SOM1-2 years

C/N = 15-30

Passive SOM500-5000 years

C/N = 7-10

CO2

CO2

CO2

CO2

CO2

Page 15: Fundamentals of  Soil Science

SOM Active Pool• Active Pool - 10-20% 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.

Page 16: Fundamentals of  Soil Science

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.

Page 17: Fundamentals of  Soil Science

SOM Passive Pool• Passive Pool – 60-90 % 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

Page 18: Fundamentals of  Soil Science

Pools of SOM (cont.)

Page 19: Fundamentals of  Soil Science

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.

Page 20: Fundamentals of  Soil Science

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

Page 21: Fundamentals of  Soil Science

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

Page 22: Fundamentals of  Soil Science

GHG Emission from Soil

Page 23: Fundamentals of  Soil Science

Trace Gas Emission in CO2 Equivalents

Sugar cane

Napier grass

Page 24: Fundamentals of  Soil Science

Renewable Energy: Biofuels

Page 25: Fundamentals of  Soil Science

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