Editing Humus (section)

==Stability==
Much of the humus in most soils has persisted for more than 100 years, rather than having been decomposed into CO<sub>2</sub>, and can be regarded as stable; this organic matter has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles, or tightly [[Sorption|sorbed]] or [[Complex (chemistry)|complexed]] to [[clay]]s.<ref name=Dungait2012>{{cite journal |last1=Dungait |first1=J. A. |last2=Hopkins |first2=D. W. |last3=Gregory |first3=A. S. |last4=Whitmore |first4=A. P. |title=Soil organic matter turnover is governed by accessibility not recalcitrance |journal=[[Global Change Biology]] |date=14 February 2012 |volume=18 |issue=6 |pages=1781–96 |doi=10.1111/j.1365-2486.2012.02665.x |url=https://www.desmog.com/wp-content/uploads/files/Dungait%20SOM%20article.pdf |access-date=22 September 2024 }}</ref> Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or more [[Lability#Soils|labile]].<ref>{{cite journal |last1=Baldock |first1=Jeffrey A. |last2=Skjemstad |first2=Jan Otto |date=July 2000 |title=Role of the soil matrix and minerals in protecting natural organic materials against biological attack |journal=[[Organic Geochemistry]] |volume=31 |issue=7 |pages=697–710 |doi=10.1016/S0146-6380(00)00049-8 |url=https://www.academia.edu/78009563 |access-date=22 September 2024 }}</ref> The mixing activity of soil-consuming invertebrates (e.g. [[earthworm]]s, [[termite]]s, some [[millipede]]s) contribute to the stability of humus by favouring the formation of organo-mineral complexes with clay at the inside of their [[Gastrointestinal tract|guts]],<ref>{{cite journal |last1=Angst |first1=Šárka |last2=Mueller |first2=Carsten W. |last3=Cajthaml |first3=Tomáš |last4=Angst |first4=Gerrit |last5=Lhotáková |first5=Zuzana |last6=Bartuška |first6=Martin |last7=Špaldoňová |first7=Alexandra |last8=Frouz |first8=Jan |date=1 March 2017 |title=Stabilization of soil organic matter by earthworms is connected with physical protection rather than with chemical changes of organic matter |journal=Geoderma |volume=289 |pages=29–35 |doi=10.1016/j.geoderma.2016.11.017 |url=https://www.academia.edu/80259832 |access-date=6 October 2024 }}</ref><ref>{{cite journal |last=Brauman |first=Alain |date=July 2000 |title=Effect of gut transit and mound deposit on soil organic matter transformations in the soil feeding termite: a review |journal=European Journal of Soil Biology |volume=36 |issue=3–4 |pages=117–25 |doi=10.1016/S1164-5563(00)01058-X |url=https://fr.articles.sk/book/18221151/100de8 |access-date=6 October 2024 }}</ref> hence more [[carbon sequestration]] in [[humus form]]s such as mull and amphi, with well-developed mineral-organic [[Soil horizon|horizons]], when compared with [[Moder humus|moder]] where most organic matter accumulates at the soil surface.<ref>{{cite journal |last1=Andreetta |first1=Anna |last2=Ciampalini |first2=Rossano |last3=Moretti |first3=Pierpaolo |last4=Vingiani |first4=Simona |last5=Poggio |first5=Giorgio |last6=Matteucci |first6=Giorgio |last7=Tescari |first7=Francesca |last8=Carnicelli |first8=Stefano |date=2011 |title=Forest humus forms as potential indicators of soil carbon storage in Mediterranean environments |journal=Biology and Fertility of Soils  |volume=47 |pages=31–40 |doi=10.1007/s00374-010-0499-z |url=https://www.researchgate.net/publication/226417489 |access-date=6 October 2024 }}</ref>

Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure.<ref name="Oades1984">{{cite journal |last1=Oades |first1=J. Malcolm |title=Soil organic matter and structural stability: mechanisms and implications for management |journal=[[Plant and Soil]] |date=February 1984 |volume=76 |issue=1–3 |pages=319–337 |doi=10.1007/BF02205590|bibcode=1984PlSoi..76..319O |s2cid=7195036 |url=https://edisciplinas.usp.br/pluginfile.php/5168021/mod_resource/content/1/Grupo%206_Oades%2C%201984.%20Soil%20organic%20matter%20and%20structural%20stability%20mechanisms%20and%20implications%20for%20management.pdf |access-date=13 October 2024 }}</ref> A very stable form of humus is formed from the slow oxidation ([[redox]]) of [[soil carbon]] after the incorporation of finely powdered [[Biochar|charcoal]] into the [[topsoil]], suggested to result from the grinding and mixing activity of a tropical earthworm.<ref>{{cite journal |last1=Ponge |first1=Jean-François |last2=Topoliantz |first2=Stéphanie |last3=Ballof |first3=Sylvain |last4=Rossi |first4=Jean-Pierre |last5=Lavelle |first5=Patrick |last6=Betsch |first6=Jean-Marie |last7=Gaucher |first7=Philippe |date=July 2006 |title=Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: a potential for tropical soil fertility |journal=[[Soil Biology and Biochemistry]] |volume=38 |issue=7 |pages=2008–9 |doi=10.1016/j.soilbio.2005.12.024 |url=https://www.researchgate.net/publication/44735820 |access-date=13 October 2024 }}</ref> This process is speculated to have been important in the formation of the unusually fertile Amazonian {{lang|es|[[Terra preta|terra preta do Indio]]}}.<ref>{{cite book |date=July 2017 |title=Archaeological soil and sediment micromorphology |editor-last1=Nicosia |editor-first1=Cristiano |editor-last2=Stoops |editor-first2=Georges |pages=345–57 |chapter=Amazonian Dark Earths |last=Arroyo-Kalin |first=Manuel |doi=10.1002/9781118941065.ch33 |isbn=9781118941065 |publisher=[[Wiley (publisher)|Wiley]] |location=Hoboken, New Jersey |url=https://www.researchgate.net/publication/319444794 |access-date=13 October 2024 }}</ref> However, some authors<ref name="Lehmann2015"/> suggest that complex soil organic molecules may be much less stable than previously thought: "the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds.″