fundamentals of soil science soil organic matter
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Fundamentals of Soil ScienceSoil Organic Matter
Lecture 6SOM’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– 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
Lecture 6 - Topics• Factors controlling the level of soil organic
matter• Major soil C pools• Maintenance of soil organic matter• Summary and review
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.
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
Managing SOM• Management of soil organic matter leads to
reduction in greenhouse gas emission or to enhanced soil quality and plant production
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.
General Guidelines for Managing SOM
• Continuous supply of plant residues
General Guidelines for Managing SOM
• Continuous supply of plant residues• Each system has its own “ideal” level of SOM
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)
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
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
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
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.
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.
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
Pools of SOM (cont.)
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.
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
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
GHG Emission from Soil
Trace Gas Emission in CO2 Equivalents
Sugar cane
Napier grass
Renewable Energy: Biofuels
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
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