• Releases nutrients for plant uptake
• Influences ecosystem carbon storage and therefore climate
• Returns the bulk of carbon fixed by photosynthesis to atmosphere
• Is the 1st step in SOM formation which affects soil properties

1. Leaching by water (PHASE 1)
2. Fragmentation by soil animals (PHASE 2)
3. Chemical alteration by microbes  (PHASE 3)

Decomposition

Leaching by water (Phase 1)

• Transfers soluble materials
• Moves water-soluble compounds (sugars, amino acids) away from decomposing material
• Begins while leaves are still on plant
• Most important early in decomp

Decomposition

Fragmentation by soil animals (Phase 2)

• Increases surface area for microbial attack
• Fresh litter is protected from microbial attack
• Bark, epidermis, or skin on exterior
• plant cells protected by lignin in cell walls
• Time period where there is an increase in population of microbes

Decomposition

Chemical Alteration by Microbes (Phase 3)

• Changes chemical composition of detritus
• Breaks down OM into CO and nutrients₂
• Forms complex recalcitrant compounds
• Recalcitrant: "stubborn"

Comparison of Cell organics

Glucose

Comparison of cell organics

Cellulose

• Slightly more complex
• Specialized enzymes are necessary for breakdown

Comparison of Cell Organics

Lignin

• Very complicated structure
• Common component in bark and some leaves
• Very resistant to decomp

Decomposers

Who are they and Why do they do it?

• Decomposer organisms are subject to natural selection
• Decomp is result of their feeding activity and population dynamics
• C-cycling and nutrient mineralization are byproducts

Major Players in Decomp

Fungi

In Aerobic Environments

• Account for most decomp in aerobic environments
• 60-90% of microbial biomass in forests
• ~50% of microbial biomass in grasslands

Major Players in Decomp

Fungi

Broad enzymatic capability

• Cell walls: lignin, cellulose, hemicellulose
• Main lignin degraders
• cell contents: proteins, sugars, lipids

Major Players in Decomp

Fungi

Anatomy & Physiology

• Composed of long networks of hyphae
• Transport metabolites through hyphae
• Surface litter: import N from soil
• Wood degraders: import N from soil
• Mycorrhizae: plants trade carbohydrates for nutrients/minerals from fungi

Major Players in Decomp

Bacteria

Specialists

• Spatial specialists
• Rhizosphere, macropores, interior of soil aggregates
• Form biofilms on partical surfaces
• Chemical specialists
• Different bacteria produce different enzymes (consortia)

Major Players in Decomp

Bacteria

Dependence on substrates

• Dependent on substrates that diffuse to bacterium (not like fungi)
• Become inactive when substrate is exhausted
• 50-80% of soil bacteria inactive
• Activated by presence of substrate
• E.g., when root grows past

Major Players in Decomp

Bacteria

• Grow rapidly: live fast, die young
• Specialize on labile substrates
• Some function anaerobically

Major Players in Decomp

Microfauna

Protozoans

• Protozoans: Ciliates, amoebae
• Aquatic, mobile
• Bacterial predators (phagocytosis)
• Rhizospheres specialists

Major Players in Decomp

Microfauna

Nematodes & Mites

• Nematodes: many trophic roles
• Extremely abundant
• Often eat as much as above-ground grazers
• Mites: many trophic roles
• In Acari group, similar to spiders
• Feed on bacteria and fungi

Major Players in Decomp

Mesofauna

• Animals with greatest effect on decomp
• Fragment litter
• Ingest litter particles and digest the microbial jam

Major Players in Decomp

Mesofauna

Collembolans

• Aka Springtails
• Important mesofauna in Northern soils
• Mainly feed on fungi

Major Players in Decomp

Macrofauna

• Consists of: earthworms, termites, etc
• Fragment litter or ingest soil

Major Players in Decomp

Macrofauna

Ecosystem Engineers

• Mix soil, carry OM to depth
• Reduce compaction
• Create channels for water and roots

Major Players in Decomp

Macrofauna

Soil Animals

• Collectively account for only 5-10% of soil respiration
• Major impacts on decomp are indirect:
• Alter soil environment
• Graze on bacteria and fungi
• Excrete N and P

Rhizosphere is the

MAJOR ZONE OF DECOMPOSTION

• High inputs of labile C "prime decomposition
• Microbes break down SOM for N

Rhizosphere is the

MAJOR ZONE OF DECOMPOSTION

Microbial Processes

• Bacterial starvation
• Predation by protozoans
• Rapid bacterial growth

Rhizosphere is the

MAJOR ZONE OF DECOMPOSTION

Root Processes

• Nitrogen uptake
• Root exudation
• Sloughing of root cap

• Declines almost exponentially with time
• Rates differ among different substrates (branch, leaf, needle)
• L_t = L₀e^-kt

LITTER MASS

Meaning of variables

• L₀: mass at time zero
• L_t: mass at time t
• k: decomposition constant
• Litterfall/litterpool (at a steady state)
• 1/k: mean residence time
• kt: negative because litter mass is expected to decline over time

LITTER MASS

Litterfall vs. Litterpool

• Litterpool: steady amount of litter throughout the year
• Litterfall: altered throughout the year
• Use litter bags to measure
• bages filled with typical amounts of litter

LITTER MASS

K is not constant over time:

• Due to composition changes
• Phase 1: leaching dominates
• Phase 2: high value of K, labile substrates broken down by fragmentation plus chemical alteration
• Phase 3: low value of K, recalcitrant substrates predominate
• Time scale depeds on environment (tropics vs. arctic)

LITTER MASS

Olsen (1963)

• Decomposition rates:
• Highest: warm and moist
• Lowest: cool and/or dry    OR    cool and/or wet

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

Direct Temperature effect on microbial activity

• Temp optimum much higher than ambient temp
• Maintenance respiration: increasing proportion of total at high temp (high temp not necessarily optimal for microbes)
• Growth respiration dominates at optimal temp, but maintenance respiration increases with temp
• Temp Fluctuations: freeze-thaw lyses microbes, increases substrate supply

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

Indirect Temperature effects

• Effects of evaporation and soil moisture
• Effects on quantity and quality of litter inputs

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

Moisture Effects

• Response to decomp to moisture is similar to that of photosynthesis: declines at extremely low/high moisture
• Less sensitive to low moisture:  No litter accumulation in deserts
• More sensitive to high moisture:  SOM accumulation in waterlogged soils

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

pH

• Bacteria predominate at high pH
• Fungi predominate at low pH
• Lower rates at lower pH

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

Soil Texture

• Protection of SOM by the large surface area of clay layers
• Aggregate structure (anaerobic microsites)
• Some chemical groups unavailable to enzymes when OM binds to clay particles

CONTROLS OVER DECOMPOSITION

1. PHYSICAL ENVIRONMENT

Soil Disturbance

• Reduces SOM protection by clays
• Breaks up soil aggregates
• Increases aeration

CONTROLS OVER DECOMPOSITION

1. Substrate Quantity & Quality

THE major controls over decomposition

NPP: Net Primary Productivity

CONTROLS OVER DECOMPOSITION

2. Substrate Quantity & Quality

Substrate Quality

• Susceptibility to decomp
• THE predominant control over decomp
• Climate exerts large effect on substrate quality through effects of vegetation

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on:

1. Size of a molecule
2. Types of chemical bonds
3. Regularity of structure
4. Toxicity
5. Nutrient concentrations

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on: Size of a Molecule

• Large molecules (cellulose, proteins) must be broken down outside cells
• Limits metabolic control that microbes can exert over breakdown processes
• Requires production of exoenzymes
• Comparison of cell organics
• Glucose → Cellulose → Lignin

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on: Types of Chemical Bonds

• Some bonds are easier to break than others:
• Peptide bonds compared to aromatic rings
• Most litter N (80%) in proteins
• Most N is in old SOM

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on: Regularity of Structure

• Lignin and humus have irregular structure

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on: Toxicity

• Phenolics evolved to protect plants from herbivores and pathogens
• May affect decomposers
• Importance of this effect is uncertain

CONTROLS OVER DECOMPOSITION

 2. Substrate Quantity & Quality

 Substrate Quality  

Depends on: Nutrient Concentrations

• Nutrients are essential to support microbial growth

CONTROLS OVER DECOMPOSITION

1. Physical Environment
2. Substrate Quantity and Quality
3. Properties of Microbial Community

MINERALIZATION & IMMOBILIZATION

• Mineralization: decomp of SOM by soil microbes releasing mineral N in the process
• Immobilization: Conversion of mineral N to organic N by microbes
• Organisms that decom OM as an energy source require N

MINERALIZATION & IMMOBILIZATION

Graph Information

• CO₂ released as byproduct
• C:N ratio is high in beginning
• Becomes smaller, leaving most mineral N behind (decreases exponentially)
• Microbial biomass reflects C:N ratio

PREDICTORS OF DECOMP

C:N ratio

• Index of ratio of cytoplasm to cell walls
• Measure of N concentration
• Directly affects decomp mainly in the presence of labile C

PREDICTORS OF DECOMP

Lignin:N ratio

• Integrated measure of N concentration and substrate size/complexity
• As the ratio in hardwood leaf litter increases, decomp rates decrease

Plant species differ predictably in litter quality

• High-resource-adapted leaves decomp quickly due to higher concentrations of labile C and N
• High-resource-adapted plants grow quickly when resources are available and die quickly with resource exhaustion
• Not invested in making compounds, making them resistant to decomp

Substrate Quality of SOM

• Much of SOM is old & recalcitrant
• Consists of "leftovers" and microbial products
• Density and particle size fractionations can distinguish pools with different turnover rates
• Ligh, particulate SOM (>53mm) decomposes faster than dense, silt+clay SOM

Long-term Storage of SOM:

Humification

• Formation of SOM that doesn't decompose easily
• Critical determinant of soil properties

MAJOR CONTROLS OVER DECOMP

• Quantity & Quality of litter input
• Environmental conditions that favor biological activity
• Microbial activity is more important than microbial mass

Soil respiration correlates closely with rates of Photosynthesis

• Some is decomposition: depends on litter quantity & quality
• Some is root respiration: correlates with photosynthesis

Decomposition in Aquatic Environments

• Depends on stability of environment (intertidal)
• Flowing water
• Shredders: aquatic anthropods, fragment organic particles, eat bacteria/fungi on litter surface
• Collectors: clams, mussels, filter water
• Grazers/Scrapers: feed on algae, bacteria, fungi, and OM collected on rocks and debris

AQUATIC ORGANIC MATTER

Particulate Organic Matter (POM)

• Poorly available, Large organic particles
• Humic compounds in substrate
• Low O at bottom of still water: very low decomp
• Ratio of anaerobic respiration to aerobic respiration is high

AQUATIC ORGANIC MATTER

Dissolved Organic Matter (DOM)

• Readily Available
• Small suspended/dissolved organic compounds
• Excretia of phytoplankton, macroalgae, zooplankton
• Bodies dissolve rapidly upon death
• Substrate for bacterial growth
• Reconcentration of OM

Benthic Zone vs. Photic Zone

• Photic zone: top of water system
• Where decomp occurs
• Benthic zone: bottom of water system
• Very low O
• Little to no decomp

DECOMPOSITION SUMMARY

• Major avenue of carbon loss from ecosystems
• Determined primarily by factors regulating photosynthesis
• Sensitive to global change
• Has potentially large feedbacks to climate