Chapter 7. Terrestrial Decomposition

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Chapter 7. Terrestrial Decomposition

• Principles of Ecosystem Ecology
• Chapin et al., 2002

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Decomposition

• Decomposition breaks down dead organic matter,
  releasing carbon to the atmosphere and nutrients
  in forms that can be used for plant and microbial
  production
• In the next two lectures we will discuss the key
  controls over decomposition and soil organic
  matter accumulation by ecosystems

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Decomposition

• Decomposition is the physical and chemical
  breakdown of detritus (i.e., dead plant, animal,
  and microbial material)
• Heterotrophic respiration is a sub-concept of the
  term decomposition Rh only concerns the
  production of CO2 during the decomposition of
  organic substrates used for growth and
  maintenance of the decomposers
• The balance between primary production and
  decomposition strongly influences C cycling at
  ecosystem and global scales

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Decomposition Processes

• Decomposition results from three types of
  processes with different controls and
  consequences
• (1) Leaching
• (2) Fragmentation
• (3) Chemical alteration
• Litter ? SOM ? humus
• Litter recognizable plant debris (e.g., Oi, Oe)
• Humus unrecognizable, microbially modified,
  amorphous, colloidal (e.g., Oa)
• SOM includes both litter and humus (sometimes
  living biomass in soil included as well)

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Leaching

• Leaching is the rate-determining step for mass
  loss of litter when it first falls to the ground
• Leaching is the physical process by which mineral
  ions and small water-soluble organic compounds
  dissolve in water and move through the soil
• Compounds leached from leaves include sugars,
  amino acids, and other compounds that are labile
  (readily broken down) or are absorbed intact by
  soil microbes

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Litter Fragmentation

• Fragmentation creates fresh surfaces for
  microbial colonization and increases the
  proportion of the litter mass that is accessible
  to microbial attack
• Fragmentation of litter greatly enhances
  microbial decomposition by piercing protective
  barriers and by increasing the ratio of litter
  surface area to mass.
• Soil fauna are the main agents of litter
  fragmentation
• More important generally in grasslands and
  deciduous forests than in coniferous forests
  also important in arid ecosystems

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Fig. 7.1 Representative types and sizes of soil
fauna.
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Soil Animals (Microfauna)

• Soil animals influence decomposition by
  fragmenting and transforming litter, grazing
  populations of bacteria and fungi, and altering
  soil structure
• Microfauna are the smallest animals (lt0.1 mm in
  width). They include nematodes, protozoans
  (ciliates and amoebae) and some mites
• Protozoans consist of a single cell and ingest
  their prey primarily by phagocytosis.
• Protozoans are usually mobile and are voracious
  predators of bacteria and other microfauna

Nematode (caught in a fungal web)
Amoeba protozoan
Ciliate protozoan
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Soil Animals (Microfauna)

• Nematodes are an abundant and trophically diverse
  group in which each species specializes on
  bacteria, fungi, roots or other soil animals
• Protozoans are particularly important predators
  in the rhizosphere
• Protozoans and nematodes are aquatic animals that
  move through water films on the surface of soil
  particles and are therefore sensitive to water
  stress

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Soil Animals (Mesofauna)

• The mesofauna are a taxonomically diverse group
  of soil animals 0.1 to 2 mm in width
• They fragment and ingest litter coated with
  microbial biomass, producing large amounts of
  fecal material with a greater surface area and
  moisture-holding capacity than the original
  litter
• Springtails (Collembola) are small insects that
  feed primarily on fungi
• Mites (Acari) are a more trophically diverse
  group of spider-like animals that consume
  decomposing litter or feed on bacteria and/or
  fungi

Collembolan
Mite
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Soil Animals (Macrofauna)

• Macrofauna (gt2 mm in width), such as earthworms
  and termites, alter resource availability by
  modifying the physical properties of soils and
  litter
• Some macrofauna fragment litter like the
  mesofauna others burrow or ingest soil, reducing
  soil bulk density, breaking up soil aggregates,
  and increasing soil aeration and the infiltration
  of water

Earthworm Millsonia anomala
Termites Macrotermes muelleri on fungus comb
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Soil Animals (Macrofauna)

• The passages created by earthworms create
  channels in the soil through which water and
  roots readily penetrate
• In temperate pastures earthworms may process 4 kg
  m-2 y-1 of soil, moving 3 to 4 mm of new soil to
  the ground surface each year this is a
  geomorphic force that is, on average, orders of
  magnitude larger than landslides or surface soil
  erosion

Casts of earthworm (Eudrilidae) scattered over of
the soil
Termites nest (Macrotermes bellicosus )
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Soil Animals (Macrofauna)

• Soil mixing by earthworms tends to disrupt the
  formation of distinct soil horizons
• Once the soil enters the digestive tract of an
  earthworm, mixing and secretions by the earthworm
  stimulate microbial activity, so soil microbes
  act as gut mutualists
• Earthworms are most abundant in the temperate
  zone, whereas termites are most abundant in
  tropical soils.
• Termites eat plant litter directly, digest the
  cellulose with the aid of mutualistic protozoans
  in their guts, and mix the organic matter into
  the soil

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Soil Animals (Macrofauna)

• Soil food webs are complex, so many of the
  effects of soil animals on decomposition are
  indirect
• Loss or exclusion of soil invertebrates can
  reduce decomposition rate (and therefore nutrient
  cycling) substantially, indicating the important
  role of animals in the decomposition process

A GREATLY simplified soil food web.
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Soil Animals

• The soil fauna is critical to the carbon and
  nutrient dynamics of soils. Microbes contain 70
  to 80 of the labile C and N in soils, so
  variations in predation rates of microbes by
  fauna dramatically alter C and N turnover in
  soils
• Soil animals account for only about 5 of soil
  respiration, so their major effect on
  decomposition is their enhancement of microbial
  activity through fragmentation, rather than their
  own processing of energy derived from detritus

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Chemical Alteration by Fungi

• Fungi are the main initial decomposers of
  terrestrial dead plant material and, together
  with bacteria, account for 80-90 of the total
  decomposer biomass and respiration
• Fungi have networks of hyphae (i.e., filaments
  that enable them to grow into new substrates and
  transport materials through the soil over
  distances of cm to m)
• Hyphal networks enable fungi to acquire their
  carbon in one place and their N in another
• White-rot fungi decompose lignin to get at N

A network of fungal hyphae binding soil particles
on to wheat stubble
The white-rot fungus, Bjerkandera adusta, growing
on a beech stump
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Chemical Alteration by Fungi

• Fungi account for 60-90 of the microbial biomass
  in forest soils, where litter frequently has a
  high lignin and low N concentration
• They have a competitive advantage at low pH,
  which is also common in forest soils
• Fungi make up about half the microbial biomass in
  grassland soils where pH is higher, and wood is
  absent
• Most fungi lack a capacity for anaerobic
  metabolism and are therefore absent from or
  dormant in anaerobic soils and aquatic sediments

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Chemical Alteration by Bacteria

• The small size and large surfacevolume ratio of
  bacteria enable them to rapidly absorb soluble
  substrates and to grow and divide quickly in
  substrate-rich zones
• The major functional limitation resulting from
  their small size is that each bacterium is
  completely dependent on the substrates that move
  to the bacterium
• There is a wide range of bacterial types in
  soils. Rapidly growing gram-negative bacteria
  specialize on labile substrates secreted by roots

Mixed culture of large Gram positive rod and
small Gram negative rods.
Gram Stain The Gram technique involves the
application of two dyes, crystal violet and
safranin. If the cell wall is composed of
peptidoglycan, crystal violet would adhere to the
molecules and will appear violet purple in color.
If the cell wall contains little or no
peptidoglycan, the crystal violet will decolorize
and a counterstain of safranin is used.
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Chemical Alteration by Bacteria

• Actinomycetes are slow-growing gram-positive
  bacteria that have a filamentous structure
  similar to that of fungal hyphae
• Like fungi, actinomycetes produce
  lignin-degrading enzymes and can break down
  relatively recalcitrant substrates
• They often produce fungicides to reduce
  competition from fungi
• Much of the fungal and bacterial biomass is
  metabolically inactive at any given point in time

Myxococcus xanthus G- bacterium
Nocardia colony G bacterium
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Temporal Pattern of Decomposition

• The predominant controls over decomposition
  change with time

Mean Residence Time (MRT)
Units time
Units time-1
Lt is mass at time t, Lo is initial mass, k
decomposition constant
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Fig. 7.2 Time course of litter
decomposition Phase 1 Leaching Phase 2
Relatively constant fractional mass loss Phase
3 Stabilization
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Temporal Pattern of Decomposition

• Methods for studying
• Tethered litter
• Litterbags
• Isotopically labeled

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Litterbag 3 x 4 mm mesh, bridal veil material,
containing ponderosa pine needles
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• Problems with litterbags
• May exclude soil fauna
• Alter microenvironment
• Utility decreases with time
• Use of uniformly labeled litter
• Has none of the disadvantages above
• Can track the movement of labeled elements
  out of the bag
• Example
• Hart (1990)
• 14C and 15N labeled Bromus mollis litter

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Factors Controlling Decomposition

• Decomposition is controlled by three types of
  factors
• (1) the physical environment
• (2) the quantity and quality of substrate
  available to decomposers
• (3) the characteristics of the microbial community

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Physical Environment

• Temperature direct Q10 effects and indirect
  effects

Relationship between soil respiration and soil
temperature across ecosystems (standardized so
that soil respiration at all sites is equal at
10C)
Temperature response curve for a single soil
(Arrhenius plot linear relation between log of
rate vs. 1 / temperature)
Fig. 7.4
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Fig. 7.5 - Forest Floor biomass and aboveground
litter inputs for selected evergreen forests
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Physical Environment

• Moisture organic matter accumulation is greatest
  in wet soils because decomposition is more
  restricted by high soil moisture than is NPP, but
  is less restricted by low soil moisture than is
  NPP (note deserts have low SOM)
• Oxygen diffusion is 10,000 slower through water
  than through air
• Generally, microbial activity optimal near 0.01
  MPa (field capacity about 0.033 MPa)

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Soil moisture-temperature interactive effects on
microbial respiration
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Physical Environment

• Soil properties
• Decomposition highest at circum-neutral pH
• Disturbances increases decomposition by promoting
  aeration and new surfaces for microbial attack

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The influence of environment on the decomposition
of ponderosa pine needles. Partial restoration
treatment involved thinning of small diameter
trees. Complete restoration treatment included
this thinning and forest floor biomass reduction
and reintroduction of prescribed fire. Higher
moisture (more canopy throughfall) and warmer
soils (more isolation) likely contributed to the
faster rates of litter decomposition in the
restored stands (Hart, unpublished data).
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Substrate Quality

• Carbon quality of substrates may be the
  predominant chemical control over decomposition
  (five- to ten-fold range in decomposition rate of
  litter in a given climate)
• Substrate quality susceptibility of a substrate
  to decomposition measured under standardized
  conditions
• (1) labile, metabolic compounds, such as sugars
  and amino acids
• (2) moderately labile structural compounds such
  as cellulose and hemicellulose
• (3) recalcitrant structural material such as
  lignin and defensive compounds such as condensed
  polyphenols

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Fig. 7.8 Comparison of decomposition dynamics
of substrates of varying qualities.
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Substrate Quality Predictors

• CN ratio (or N) Why does it work?
• Litter CN 1001, microbe 101
• If respire 50 of C, CN 501 still too much C
• Need to import N, slows decomposition
• ButN fertilization doesnt always lead to higher
  decomposition rates Why?
• Limitation of available C
• N interaction with organic matter

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Leaf Litter Decomposition Rates in Ecosystems of
the Colorado Plateau
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Fig. 7.10 -Relationship for hardwood leaf litters
in NE USA other factors like climate would
change overall values
The decomposition constants shown here should NOT
have a negative sign attached to them!
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Substrate Quality Predictors

• LigninN Why does it work?
• Same principle as with CN
• Lignin better measure of C quality than total C

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Nutrient Dynamics During Decomposition
Inverse linear relationship denotes net N
immobilization
Nearly vertical lines denote mass loss in
proportion to N release
Initial N lt0.5 Initial CN gt100
Initial N gt1.25 Initial CN lt 42
N in Remaining Material
N in Remaining Material
The LIDET data set 27 species of leaf and root
litter decomposing across over 28 contrasting
sites in North and Central America (LIDET
unpublished).
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Nutrient Dynamics During Decomposition
Net N immobilization in ponderosa pine needles
during decomposition, suggesting N-limitation of
microbial growth of decomposers.
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Nutrient Dynamics During Decomposition
Net P release in ponderosa pine needles during
decomposition, suggesting P is in excess of
decomposer demand (Hart, unpublished data).
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Controls on SOM Decomposition

• Controls on humus decomposition different than
  litter
• Temperature may not be as strong a driver
• Why? Interactions with surfaces (clays) become
  important

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Physical Environment
Fig. 7.7 - Clay interacts with SOM impeding its
decomposition
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Microbial Community Composition

• Believed to be important to decomposition, but
  not well understood
• Many enzymes involved in decomposition are
  ubiquitous (e.g., proteases, peptidases) others
  are not (e.g., phenol oxidases produced by
  white-rot fungi and some actinomycetes)

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Long-term Storage of SOM

• In climates that are favorable for decomposition,
  humus is the major long-term reservoir of soil
  carbon
• Humification several important biotic and
  abiotic processes and several theories
• Lignin Theory
• Polyphenol Theory
• Sugar-Amine Theory (browning reactions)

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Fig. 7.12 - This diagram is a mixture of all
three major theories of humus formation
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Fig. 7.13 - Many important processes in
humification
Note the quinones should have O double bonded to
C, NOT OH groups
A hypothetical humic acid
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Decomposition at the Ecosystem Scale

• Aerobic heterotrophic respiration is the major
  avenue of carbon loss from ecosystems
• The controls over stand-level decomposition are
  similar to the controls over GPP and NPP
• Most of the annual heterotrophic decomposition
  (respiration) occurs from recent litter (not
  humus)
• Currently, we cannot measure heterotrophic
  respiration directly (surrogates soil
  respiration, ecosystem respiration during night
  both of these include root respiration)

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Fig. 7.15 - Relationship between mean annual soil
respiration rate and mean annual NPP for Earth's
major biomes
KEY Agricultural lands (A), boreal forest and
woodland (B), desert scrub (D), temperate forest
(F), temperate grassland (G), moist tropical
forest (M), tropical savanna and dry forest (S),
tundra (T), and Mediterranean woodland and heath
(W)
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Root respiration probably accounts for the 25
greater soil respiration than NPP at any point
along this regression line