Editing Ecosystem (section)

===Decomposition===
{{See also|Decomposition}}
[[File:Decomposition stages.jpg|thumb|upright=3.0|Sequence of a decomposing pig carcass over time]]
The carbon and nutrients in [[soil organic matter|dead organic matter]] are broken down by a group of processes known as decomposition. This releases nutrients that can then be re-used for plant and microbial production and returns carbon dioxide to the atmosphere (or water) where it can be used for photosynthesis. In the absence of decomposition, the dead organic matter would accumulate in an ecosystem, and nutrients and atmospheric carbon dioxide would be depleted.<ref name="Chapin-2011f" />{{rp|183}}

Decomposition processes can be separated into three categories—[[leaching (agriculture)|leaching]], fragmentation and chemical alteration of dead material. As water moves through dead organic matter, it dissolves and carries with it the water-soluble components. These are then taken up by organisms in the soil, react with mineral soil, or are transported beyond the confines of the ecosystem (and are considered lost to it).<ref name="Chapin-2011h">{{Cite book|last=Chapin|first=F. Stuart III|title=Principles of terrestrial ecosystem ecology|date=2011|publisher=Springer|others=P. A. Matson, Peter Morrison Vitousek, Melissa C. Chapin|isbn=978-1-4419-9504-9|edition=2nd|location=New York|chapter=Chapter 9: Nutrient cycling|oclc=755081405}}</ref>{{rp|271–280}} Newly shed leaves and newly dead animals have high concentrations of water-soluble components and include [[sugar]]s, [[amino acid]]s and mineral nutrients. Leaching is more important in wet environments and less important in dry ones.<ref name="Chapin-2011c" />{{rp|69–77}}

Fragmentation processes break organic material into smaller pieces, exposing new surfaces for colonization by microbes. Freshly shed [[leaf litter]] may be inaccessible due to an outer layer of [[plant cuticle|cuticle]] or [[Bark (botany)|bark]], and [[protoplasm|cell contents]] are protected by a [[cell wall]]. Newly dead animals may be covered by an [[exoskeleton]]. Fragmentation processes, which break through these protective layers, accelerate the rate of microbial decomposition.<ref name="Chapin-2011f">{{Cite book|last=Chapin|first=F. Stuart III|title=Principles of terrestrial ecosystem ecology|date=2011|publisher=Springer|others=P. A. Matson, Peter Morrison Vitousek, Melissa C. Chapin|isbn=978-1-4419-9504-9|edition=2nd|location=New York|chapter=Chapter 7: Decomposition and Ecosystem Carbon Budgets|oclc=755081405}}</ref>{{rp|184}} Animals fragment detritus as they hunt for food, as does passage through the gut. [[Freeze-thaw cycle]]s and cycles of wetting and drying also fragment dead material.<ref name="Chapin-2011f" />{{rp|186}}

The chemical alteration of the dead organic matter is primarily achieved through bacterial and fungal action. Fungal [[hypha]]e produce enzymes that can break through the tough outer structures surrounding dead plant material. They also produce enzymes that break down [[lignin]], which allows them access to both cell contents and the nitrogen in the lignin. Fungi can transfer carbon and nitrogen through their hyphal networks and thus, unlike bacteria, are not dependent solely on locally available resources.<ref name="Chapin-2011f" />{{rp|186}}

==== Decomposition rates ====
Decomposition rates vary among ecosystems.<ref name="Ochoa-Hueso-2019">{{cite journal |last1=Ochoa-Hueso |first1=R |last2=Delgado-Baquerizo |first2=M |last3=King |first3=PTA |last4=Benham |first4=M |last5=Arca |first5=V |last6=Power |first6=SA |title=Ecosystem type and resource quality are more important than global change drivers in regulating early stages of litter decomposition |journal=Soil Biology and Biochemistry |date=February 2019 |volume=129 |pages=144–152 |doi=10.1016/j.soilbio.2018.11.009 |bibcode=2019SBiBi.129..144O |s2cid=92606851 |hdl=10261/336676 |hdl-access=free }}</ref> The rate of decomposition is governed by three sets of factors—the physical environment (temperature, moisture, and soil properties), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself.<ref name="Chapin-2011f" />{{rp|194}} Temperature controls the rate of microbial respiration; the higher the temperature, the faster the microbial decomposition occurs. Temperature also affects soil moisture, which affects decomposition. Freeze-thaw cycles also affect decomposition—freezing temperatures kill soil microorganisms, which allows leaching to play a more important role in moving nutrients around. This can be especially important as the soil thaws in the spring, creating a pulse of nutrients that become available.<ref name="Chapin-2011h" />{{rp|280}}

Decomposition rates are low under very wet or very dry conditions. Decomposition rates are highest in wet, moist conditions with adequate levels of oxygen. Wet soils tend to become deficient in oxygen (this is especially true in [[wetland]]s), which slows microbial growth. In dry soils, decomposition slows as well, but bacteria continue to grow (albeit at a slower rate) even after soils become too dry to support plant growth.<ref name="Chapin-2011f" />{{rp|200}}