Download Lecture 9. Decomposition -we will examine this briefly

Lecture 9. Decomposition
-we will examine this briefly
-not because it is unimportant:
**decomposition is as vital to ecosystem functioning as photosynthesis
-but I cover the details of decomposition in exhaustive detail in Environmental Biology of Soil
-I invite anyone interested to take that course for all details (also well explained in textbook)
Ecological Significance
(1) decomposition is the primary pathway of energy flow from primary production
in terrestrial ecosystems
-roughly 70-90% of all NPP on land is recycled directly through decomposers
-decomposition supports an amazing array of soil bacteria, fungi, protozoans, invertebrates,
and above-ground organisms.
-quantitatively less in aquatic ecosystems, where more NPP is consumed, but still essential
-fundamentally, terrestrial ecosystems are based on photosynthesis and decomposition
-the so-called grazing food chain, including aphids, songbirds, foxes and tigers, is secondary
(2) decomposition is an essential part of the carbon cycle
-degrades and removes all kinds of organic detritus and recycles carbon as humus and CO2
-decomposition is the dominant source of energy for soils and streams
-an important energy source in all ecosystems
(3) critical for nutrient cycling and therefore supply of nutrients for GPP
-decomposition recycles organic nutrients (N, P, K, Ca, Mg, etc.) in simple chemical forms
that can be absorbed by plant roots.
-controls productivity and development of terrestrial and aquatic ecosystems
-by controlling the supply of limiting nutrients.
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(Aside: note the biassed perspective here. Decomposers are seen as important because they
provide a service to plants. No one says that the function of plants is to provide a source of
carbon and nutrients for soil organisms, although that statement is equally accurate.)
(4) responsible for long-term storage of C as stable residues
-stable soil organic matter (humus) is an end product of decomposition
-may be an enormous reservoir of organic C in northern climates
-especially where complete decay is inhibited, such as in bogs and wetlands generally
(5) essential to soil formation
-SOM modifies pH, aeration, water retention, density and microstructure
-also soil’s ability to retain nutrients and ease of penetration by roots
What is Decomposition?
-a simple definition of decomposition:
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A biological process in which organic matter is systematically disassembled into
its component parts
-note (1) it is a biological process: organisms are essential all the way
(2) it is a systematic disassembly of organic matter, not simply things falling apart
-in fact, a better definition of decomposition would be:
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The transformation of dead organic matter into live microbial and animal tissue
-decomposers are organisms that derive their energy and nutrients from dead things,
-exactly as we do, except that most decomposers digest organic matter externally
-do not forget that decomposition is not about waste removal; it is organisms making a living
-all the other effects of decomposition on the ecosystem are secondary and incidental
What decomposes?
-all plant and animal matter eventually becomes litter or detritus
-most litter is produced by plants, especially on land
-animals of all kinds, microbes, produce relatively little litter
-because their standing crops are small compared with plants
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-plant parts producing litter include stems, boles, branches, roots, leaves, flowers and fruits
-of these, leaves are perhaps the most important as a nutrient source
-while slow-decaying structures such as wood and roots are important for physical structure,
habitat and carbon storage
Who does it?
-if decomposition is a biological process, there must be some organisms involved
-central players are decomposer micro-organisms, especially fungi, less so bacteria
-in addition, a variety of soil animals, invertebrates like earthworms, are very important
-physical forces, especially leaching, may also be important.
-also abrasion, burial, freezing-thawing may accelerate or retard decomposition
What are the mechanisms?
-there are three: biochemical, mechanical and physical
-central and most important process is degradation of detritus by microbial enzymes
-these enzymes break down the large structural molecules such as cellulose into smaller units
-that the fungus or bacterium can absorb to sustain itself
-decomposers work systematically:
-first degrading small, light, high-energy compounds such as sugars, proteins,
and other cell cytoplasm components
-then moving to more resistant substances, when labile substances are exhausted
-mostly cellulose and lignin from the cell wall
mechanical effects:
-soil detritivores (earthworms, mites, insects) feed on decaying litter
-they may not digest very much of it: instead they digest the microbes growing on the litter
-or use internal symbionts, gut bacteria, to help (as do mammals and most other organisms)
-animals contribute to decomposition through shredding and masticating of large particles
-breaks fibres, softens tissues and exposes much greater surface area for microbial colonization
-animals also help spread fungi and bacteria by transporting cells or spores
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physical effects:
-some fraction of most organic matter is soluble in water
-this fraction corresponds to the residual cell cytoplasm left behind when the tissue died
-soluble fraction varies from <5% in large wood and roots to 40% in some deciduous leaves
-rainfall and snowmelt may quickly remove soluble material from fallen leaves and debris
-this material is transported as dissolved organic carbon (DOC) to the soil
-because fresh DOC is so labile, it is decomposed before it gets very far, usually by bacteria
-some other physical processes such as abrasion or freezing and thawing, may also matter
What are the end products?
-there are three: respired carbon dioxide, microbial biomass and stable organic matter
-this last is better known as humus
-humus is the stable, recalcitrant material left over when decomposition is finished
-it may remain in the soil for years, decades or centuries
-it eventually decomposes if conditions are favourable
-in northern peat bogs, humus has been accumulating for thousands of years
-Figure 1 on the following page shows the general pattern of decomposition
(“Comminution” means shredding by litter-feeding animals)
Controls on Decomposition
-at the proximate scale, decomposition rates are controlled by three factors:
Climate: all other things being equal, decomposition proceeds faster at higher temperatures
-because of temperature-dependence of microbial metabolism
-but in nature, the warmest places and the warmest seasons are often dry
-so lack of water may limit decomposition rate as much as temperature
-parallel with what we see with water and temperature limitation on primary production
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Figure 1. Schematic diagram showing the general process of litter decomposition.
DOC =dissolved organic carbon. Soluble material lost from the litter follows the lefthand
pathway, dominated by bacterial metabolism; insoluble material follows the righthand pathway,
dominated by fungal metabolism and animal feeding (comminution).
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Litter Quality: refers to the chemical and physical characteristics of litter
-wood decomposes much more slowly than leaves, for example
-two features are important: the nutrient content and the proportion of available carbon
-nutrients, especially N, tend to be in short supply in litter, so they frequently limit decomposers,
-which need N and P and other elements to build tissue
-C/N ratio is a crude but widely used quick indicator of litter quality
-wood decomposes very slowly partly because there is very little N in wood (high C/N ratio)
-available carbon refers to carbon compounds that are susceptible to microbial attack
-and which provide a positive energy return to the decomposer
-some part of all plant material is long-chain polymers
-especially cellulose and lignin found in cell walls
-lignin, and cellulose bound up in lignin, is difficult to break down enzymatically
-and provides a poor (sometimes negative) energy return
-again, wood is only about 5% available carbon, while a typical leaf may be 60-70%
-recent evidence suggests fungi only degrade lignin to get at the small amount of N it contains
Biology: refers to the differences among decomposer organisms
-fungi decompose initially, bacteria later (and dissolved material);
decomposer growth rates determine decay rates
-decay may be accelerated when litter-feeding animals such as earthworms are abundant
Aquatic Decomposition
-how does decomposition in water differ from that on land?
-first, distinguish between aquatic litter and terrestrial litter decomposing in water
-terrestrial litter in water would be, for example, leaves falling into a stream
-decomposition in water is the same as on land except
(1) it is much faster: leaf that takes 3 years to decay on land may take 3 month in a stream
(2) feeding by insect shredders may be more important
(3) water limitation is replaced by oxygen limitation
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-a consequence of the relatively slow diffusion of oxygen into water
-leaves on the bottom of a lake may decompose slowly because of lack of oxygen
-benthic sediments are anoxic below the first few centimetres
-rapid decomposition of large masses of terrestrial litter may cause anoxia, leading to fish kills
-what about decomposition of phytoplankton in the open water?
-the same sequence as shown on Figure 1 applies
-but the controls on decomposition are very different
1. Litter quality
-material decomposing in the open ocean or a lake is very different from on land
-because of buoyancy providing support, there is no need for structural tissue
-therefore most algal cells are entirely composed of labile material (available carbon)
-these cells decompose very rapidly when the cell dies
-a large part of the cell is composed of soluble substances
-this material is quickly degraded by bacteria in the water column
-cell contents may be completely mineralized within hours of cell death
-and nutrients re-assimilated into growing algal cells
2. Zooplankton
-animal feeding is at least as important in water as on land
-but because most algae are unicellular, the entire organism is consumed
-because cells settle relatively slowly, there is plenty of time for zooplankton to find them
-therefore, most cells are consumed long before they reach the bottom of the lake or ocean
-zooplankton excrete neat packages of waste material, that continues settling
-but is often consumed again before it gets to the bottom
** most algal decomposition occurs in the water column, not on the bottom
-this is especially true in the deep oceans, less so in lakes
** bacteria are far more important, fungi less, for aquatic decomposition
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3. Physical Environment
-the aquatic equivalent of climate
-this is a distant third in controls on decomposition rate
-have already mentioned impeded decomposition in low dissolved oxygen waters
-in severely polluted lakes, undecayed material may accumulate on the bottom
-temperature probably controls metabolic rates as it does on land, but a minor factor generally
Ecosystem Perspective on Decomposition
-now scale up from particles of litter to the level of a whole ecosystem
** aerobic decomposition is the major route of carbon loss from ecosystems
and its return to the atmosphere
-this fits with decomposers being the main consumers of NPP
-Figure 6.1, p. 158 in Textbook shows the relative magnitude of the fluxes
-Rplant is plant respiration, the difference between GPP and NPP
**once C is fixed in plant tissue, decomposition is the major route of release
-contribution of animals is not insignificant, but is small relative to decomposition
** stand-level controls on decomposition are generally the same as on plant production
-where climates are moist and warm, plants produce much litter,
-which supports abundant decomposers
-high nutrient concentrations in soil leads to more NPP,
-from which the litter is more nutritious and decomposes faster
-nutrient poor sites produce less litter, supporting fewer decomposers,
-and litter they drop is also nutrient poor, therefore slowing decomposition.
-sites that have rapid plant growth also support rapid decomposition
-hence, NPP and decomposition are both higher in the tropics than in the arctic
-and higher in rain forests than in grasslands
-on a global scale, we can see the correspondence between NPP and soil respiration rate
(which includes root respiration) in this OVERHEAD (Figure 7.14, p. 206 in Text)
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-most decomposition activity by soil microbes is based on relatively recent litter
-therefore quantity of freshly fallen litter is a much better predictor of microbial activity
than total SOM
-most soil OM is humus and other resistant end-products that decompose very slowly, if at all
** ironically, greatest soil storage of organic C is in cool northern climates
-where production is considerable but decomposition is impeded
-in tropics, massive NPP is matched by very rapid decomposition
-in far north, there is little NPP to decompose or store
-soil storage of organic carbon is a large and significant reservoir in the global C cycle
-so much that disturbances such as forestry and replanting can alter global climate change
-northern forests of Canada, especially deep bogs and wetlands, are a critical global store of C
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