Editing Ecosystem (section)

===Nutrient cycling===
{{See also|Nutrient cycle|Biogeochemical cycle|Nitrogen cycle}}
[[File:Nitrogen Cycle.jpg|thumb|right|upright=1.8|Biological nitrogen cycling]]
Ecosystems continually exchange energy and carbon with the wider [[environment (systems)|environment]]. Mineral nutrients, on the other hand, are mostly cycled back and forth between plants, animals, microbes and the soil. Most nitrogen enters ecosystems through biological [[nitrogen fixation]], is deposited through precipitation, dust, gases or is applied as [[fertilizer]].<ref name="Chapin-2011h" />{{rp|266}} Most [[terrestrial ecosystems]] are nitrogen-limited in the short term making [[Nitrogen cycle|nitrogen cycling]] an important control on ecosystem production.<ref name="Chapin-2011h" />{{rp|289}} Over the long term, phosphorus availability can also be critical.<ref>{{Cite journal|last1=Vitousek|first1=P.|last2=Porder|first2=S.|date=2010|title=Terrestrial phosphorus limitation: mechanisms, implications, and nitrogen–phosphorus interactions|journal=Ecological Applications|volume=20|issue=1|pages=5–15|doi=10.1890/08-0127.1|pmid=20349827|bibcode=2010EcoAp..20....5V |doi-access=free}}</ref>

Macronutrients which are required by all plants in large quantities include the primary nutrients (which are most limiting as they are used in largest amounts): Nitrogen, phosphorus, potassium.<ref name="Chapin-2011g">{{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 8: Plant Nutrient Use|oclc=755081405}}</ref>{{rp|231}} Secondary major nutrients (less often limiting) include: Calcium, magnesium, sulfur. [[Micronutrient]]s required by all plants in small quantities include boron, chloride, copper, iron, manganese, molybdenum, zinc. Finally, there are also beneficial nutrients which may be required by certain plants or by plants under specific environmental conditions: aluminum, cobalt, iodine, nickel, selenium, silicon, sodium, vanadium.<ref name="Chapin-2011g" />{{rp|231}}

Until modern times, nitrogen fixation was the major source of nitrogen for ecosystems. Nitrogen-fixing bacteria either live [[symbiosis|symbiotically]] with plants or live freely in the soil. The energetic cost is high for plants that support nitrogen-fixing symbionts—as much as 25% of gross primary production when measured in controlled conditions. Many members of the [[legume]] plant family support nitrogen-fixing symbionts. Some [[cyanobacteria]] are also capable of nitrogen fixation. These are [[phototroph]]s, which carry out photosynthesis. Like other nitrogen-fixing bacteria, they can either be free-living or have symbiotic relationships with plants.<ref name="Chapin-2011k" />{{rp|360}} Other sources of nitrogen include [[acid deposition]] produced through the combustion of fossil fuels, [[ammonia]] gas which evaporates from agricultural fields which have had fertilizers applied to them, and dust.<ref name="Chapin-2011h" />{{rp|270}} Anthropogenic nitrogen inputs account for about 80% of all nitrogen fluxes in ecosystems.<ref name="Chapin-2011h" />{{rp|270}}

When plant tissues are shed or are eaten, the nitrogen in those tissues becomes available to animals and microbes. Microbial decomposition releases nitrogen compounds from dead organic matter in the soil, where plants, fungi, and bacteria compete for it. Some soil bacteria use organic nitrogen-containing compounds as a source of carbon, and release [[ammonium]] ions into the soil. This process is known as [[ammonification|nitrogen mineralization]]. Others convert ammonium to [[nitrite]] and [[nitrate]] ions, a process known as [[nitrification]]. [[Nitric oxide]] and [[nitrous oxide]] are also produced during nitrification.<ref name="Chapin-2011h" />{{rp|277}} Under nitrogen-rich and oxygen-poor conditions, nitrates and nitrites are converted to [[nitrogen|nitrogen gas]], a process known as [[denitrification]].<ref name="Chapin-2011h" />{{rp|281}}

Mycorrhizal fungi which are symbiotic with plant roots, use carbohydrates supplied by the plants and in return transfer phosphorus and nitrogen compounds back to the plant roots.<ref>{{Cite journal|last=Bolan|first=N.S.|date=1991|title=A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants|journal=Plant and Soil|volume=134|issue=2|pages=189–207|doi=10.1007/BF00012037|bibcode=1991PlSoi.134..189B |s2cid=44215263}}</ref><ref name="Hestrin-2019" /> This is an important pathway of organic nitrogen transfer from dead organic matter to plants. This mechanism may contribute to more than 70 Tg of annually assimilated plant nitrogen, thereby playing a critical role in global nutrient cycling and ecosystem function.<ref name="Hestrin-2019">{{Cite journal|last1=Hestrin|first1=R.|last2=Hammer|first2=E.C.|last3=Mueller|first3=C.W.|date=2019|title=Synergies between mycorrhizal fungi and soil microbial communities increase plant nitrogen acquisition|journal=Commun Biol|volume=2|page=233|doi=10.1038/s42003-019-0481-8|pmid=31263777|pmc=6588552}}</ref>

Phosphorus enters ecosystems through [[weathering]]. As ecosystems age this supply diminishes, making phosphorus-limitation more common in older landscapes (especially in the tropics).<ref name="Chapin-2011h" />{{rp|287–290}} Calcium and sulfur are also produced by weathering, but acid deposition is an important source of sulfur in many ecosystems. Although magnesium and manganese are produced by weathering, exchanges between soil organic matter and living cells account for a significant portion of ecosystem fluxes. Potassium is primarily cycled between living cells and soil organic matter.<ref name="Chapin-2011h" />{{rp|291}}