Decomposition
In a detritus-based trophic system, bacteria and fungi breakdown dead organic plant, animal and microbial material. Primary detritivores absorb the breakdown products for their own growth and maintenance. Decomposition is a consequence of the feeding activity of soil animals (fragmentation) and heterotrophic microbes, chemical alteration and the interacting physical and chemical processes occurring inside and outside of living soil microbes and animals. Dead plant material, leaf, stem and root litter and animal residues are gradually decomposed until their original identity is no longer recognizable, at which point they are considered soil organic matter (SOM).
Decomposition occurs to meet the energetic and nutritional demands of decomposer organisms. Decomposition ultimately releases carbon to the atmosphere and nutrients in forms that can be used for plant and microbial production.
Decomposition results from three types of processes:
- leaching
- fragmentation and
- chemical alteration
Leaching
Leaching is the physical process by which mineral ions and small water-soluble organic compounds dissolve in water and flow out of the detritus and transfers soluble material away from decomposing organic matter into the environment. These soluble materials are either absorbed by organisms, react with the mineral phase of soil or sediments or are lost from the system in solution. Leaching loss from litter is greater for soluble compounds and nutrients than for carbon. Leaching occurs most rapidly in environments with high rainfall, and is negligible under dry conditions.
Fragmentation
Animals are the main agents of litter fragmentation, although freeze-thaw and wetting-drying cycles can also disrupt the cellular structure of litter. Animals influence decomposition by fragmentation, and by transforming and transporting litter. Bears, voles and other mammals tear apart wood or mix the soil as they search for insects, plant roots and other food. Fragmentation occurs when soil animals break large pieces or organic matter into smaller ones which they eat. Soil invertebrates, earthworms, nematodes termites and collembola, fragment the litter. In the process they create fresh surfaces for microbial colonization.
Soil animals also mix the decomposing organic mater into the soil and return organic matter to the soil or sediments as fecal pellets which have a higher surface-to-volume ratio and provide a more favourable environment for soil microbes than does the original material consumed. Fragmentation creates fresh surfaces for microbial colonization and increases the proportion of the litter mass that is accessible to microbial attack.
Chemical Alteration
Chemical alteration of dead organic matter results primarily from the activity of soil microbes, although some chemical reactions also occur spontaneously in the soil without microbial mediation. Microbes (microscopic and single cell organisms, protozoans and mesofauna) and animals feed on live and dead organic matter to support their energetic and nutritional demands.
Seventy five percent of terrestrial organic carbon is dead organic material in soils. Cellulose is the most abundant chemical constituent of plant litter.
Mycorrhizae are a symbiotic association between plant roots and fungi in which the plant gains nutrients from the fungus in return for carbohydrates. Fungi with networks of hyphae are able to acquire their carbon in one place and their nitrogen in another. Mycorrhizal fungi play a role in decomposition by breaking down proteins into amino acids which support fungal growth and are also transferred to their host plants. Fungi secrete enzymes. Cellulose breakdown requires three separate enzyme systems. Mycorrhizosphere around mycorrhizal fungal hyphae move plant carbon into the bulk soil through a combination of hyphal turnover and exudation.
Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Lignin is degraded slowly because only some organisms (fungi) produce the necessary enzymes.
When microbes die, their bodies become part of the organic substrate available for decomposition.
Factors Controlling Decomposition
Most decomposition occurs near the soil surface where litter inputs are concentrated. Decomposition in ecosystems is controlled by three major factors:
- substrate quality (soil and rock minerals)
- the physical environment (moisture and temperature)
- characteristics of the animal and microbial community
Substrate Quality
Decomposition of soil organic matter is strongly influences by its reaction with soil and rock minerals. Clay reduces the decomposition rate. Soil organic matter decomposes more slowly in acidic soil. Soil disturbance increases decomposition by promoting aeration and exposing new surfaces.
Physical Environment
Decomposition increases with increasing moisture until soils become so waterlogged that anaerobic conditions inhibit decomposition. Microbial respiration and decomposition increase with temperature in the short term, indirect effects constrain their temperature sensitivity over annual to decadal time scales.
Animal Community
Sometimes soil animals inhibit decomposition through direct consumption of microbial biomass.
In climates that are favourable for decomposition, substantial quantities of carbon persist in mineral soils for thousands of years.
Impact of Decomposition
Decomposition causes a decrease of detrital mass, as materials are fragmented and converted to other organic compounds and as organic matter transforms into complex organic compounds that are recalcitrant (resistant to further microbial breakdown). Decomposition causes the mineralization of organic matter to inorganic components, CO2, mineral nutrients and water.
Terrestrial Release of Carbon
Soil fauna is critical to the carbon and nutrient dynamics of soils. Microbes constitute 70-80% of the labile carbon and nitrogen in soils. Microbes significantly alter carbon and nitrogen turnovers in soils.
In addition to the release of carbon through plant respiration, carbon leaches from ecosystems in dissolved and particulate forms and moves laterally through erosion and deposition of soil and movement of animals.
Carbon is transferred back to the atmosphere through the production of carbon-containing trace gases such as methane. Decomposer microbes and their predators account for most of this respiration. Large methane emissions from wetlands cause the region to exert a positive greenhouse-gas warming effect. Heterotrophic respiration (by organisms that produce CO2 without photosynthesizing), respiration by soil microbes and animals is one of the largest avenues of carbon loss from ecosystems.
When decomposition did not keep pace with primary production in ages past, vast stores of carbon accumulated in coal, oil, and gas.
Aquatic Carbon Budgets
Lateral fluxes of carbon from terrestrial ecosystems are critical energy subsidies to aquatic ecosystems and constitute a significant component of the carbon budgets of many ecosystems. There is a continuum in stream metabolism from headwater to ocean. Flood events dislodge primary producers and transport sediments, woody debris and other organic matter downstream.
In large unregulated rivers, flooding converts much of the floodplain from a terrestrial to an aquatic habitat. Decomposition in lakes is faster than in streams or on land because of the high litter quality of algae.
Eutrophication of rivers, enriched with minerals and nutrients, greatly stimulates the productivity of many estuaries and increases the rain of dead organic matter. Too many nutrients, to the point where oxygen depletion occurs, creates dead zones where aquatic animals can’t obtain oxygen from the water.
Patterns of ocean decomposition are qualitatively similar to those in lakes. Most of the planktonic carbon acquired through photosynthesis returns to the environment. The turnover of carbon and nutrients in phytoplankton in the euphotic zone (the top most layer of lakes and oceans) occurs rapidly, about weekly. The net effect is to move carbon from the atmosphere to the deep waters and to ocean sediments. Over decades to centuries some of the carbon in deep waters recirculates to the surface through upwelling and mixing
