Editing Soil (section)

== Soil gas ==
{{main|Soil gas}}
The atmosphere of soil, or [[soil gas]], is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO<sub>2</sub> concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.<ref>{{cite journal |last1=Qi |first1=Jingen |last2=Marshall |first2=John D. |last3=Mattson |first3=Kim G. |journal=[[New Phytologist]] |volume=128 |issue=3 |title=High soil carbon dioxide concentrations inhibit root respiration of Douglas fir |year=1994 |pages=435–442 |doi=10.1111/j.1469-8137.1994.tb02989.x |pmid=33874575 |doi-access=free |bibcode=1994NewPh.128..435Q }}</ref> Calcareous soils regulate CO<sub>2</sub> concentration by [[carbonate]] [[Buffering agent|buffering]], contrary to acid soils in which all CO<sub>2</sub> respired accumulates in the soil pore system.<ref>{{cite journal |last1=Karberg |first1=Noah J. |last2=Pregitzer |first2=Kurt S. |last3=King |first3=John S. |last4=Friend |first4=Aaron L. |last5=Wood |first5=James R. |journal=[[Oecologia]] |volume=142 |issue=2 |title=Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone |url=https://www.researchgate.net/publication/8337234 |year=2005 |pages=296–306 |doi=10.1007/s00442-004-1665-5 |pmid=15378342 |access-date=26 January 2025 |bibcode=2005Oecol.142..296K |s2cid=6161016 }}</ref> At extreme levels, CO<sub>2</sub> is toxic.<ref>{{cite journal |last1=Chang |first1=H.T. |last2=Loomis |first2=Walter E. |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=20 |issue=2 |title=Effect of carbon dioxide on absorption of water and nutrients by roots |year=1945 |pages=221–32 |doi=10.1104/pp.20.2.221 |pmid=16653979 |pmc=437214 |doi-access=free }}</ref> This suggests a possible [[negative feedback]] control of soil CO<sub>2</sub> concentration through its inhibitory effects on root and microbial respiration (also called [[soil respiration]]).<ref>{{cite journal |last1=McDowell |first1=Nate J. |last2=Marshall |first2=John D. |last3=Qi |first3=Jingen |last4=Mattson |first4=Kim |journal=Tree Physiology |volume=19 |issue=9 |title=Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations |year=1999 |pages=599–605 |doi=10.1093/treephys/19.9.599 |pmid=12651534 |doi-access=free }}</ref> In addition, the soil voids are saturated with water vapour, at least until the point of maximal [[hygroscopic]]ity, beyond which a [[vapour-pressure deficit]] occurs in the soil pore space.<ref name="Vannier1987"/> Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by [[diffusion]] from high concentrations to lower, the [[diffusion coefficient]] decreasing with [[Soil compaction (agriculture)|soil compaction]].<ref>{{cite journal |last1=Xu |first1=Xia |last2=Nieber |first2=John L. |last3=Gupta |first3=Satish C. |journal=[[Soil Science Society of America Journal]] |volume=56 |issue=6 |title=Compaction effect on the gas diffusion coefficient in soils |url=https://www.academia.edu/6547475 |year=1992 |pages=1743–1750 |doi=10.2136/sssaj1992.03615995005600060014x |access-date=26 January 2025 |bibcode=1992SSASJ..56.1743X }}</ref> Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including [[greenhouse gases]]) as well as water.<ref name="Smith2003">{{cite journal |last1=Smith |first1=Keith A. |last2=Ball |first2=Tom |last3=Conen |first3=Franz |last4=Dobbie |first4=Karen E. |last5=Massheder |first5=Jonathan |last6=Rey |first6=Ana |journal=European Journal of Soil Science |volume=54 |issue=4 |title=Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes |url=https://www.academia.edu/14433607 |year=2003 |pages=779–91 |doi=10.1046/j.1351-0754.2003.0567.x |bibcode=2003EuJSS..54..779S |s2cid=18442559 |access-date=26 January 2025 }}</ref> [[Soil texture]] and [[soil structure|structure]] strongly affect soil porosity and gas diffusion. It is the total pore space ([[porosity]]) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air [[turbulence]] and temperature, that determine the rate of diffusion of gases into and out of soil.{{sfn|Russell|1957|pp=35–36}}<ref name="Smith2003"/> [[Ped#Platy|Platy]] soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO<sub>3</sub> to the gases N<sub>2</sub>, N<sub>2</sub>O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called [[denitrification]].<ref>{{cite journal |last1=Ruser |first1=Reiner |last2=Flessa |first2=Heiner |last3=Russow |first3=Rolf |last4=Schmidt |first4=G. |last5=Buegger |first5=Franz |last6=Munch |first6=J.C. |journal=[[Soil Biology and Biochemistry]] |volume=38 |issue=2 |title=Emission of N<sub>2</sub>O, N<sub>2</sub> and CO<sub>2</sub> from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting |url=https://downloads.regulations.gov/APHIS-2011-0023-0105/attachment_4.pdf |year=2006 |pages=263–74 |doi=10.1016/j.soilbio.2005.05.005 |access-date=26 January 2025 }}</ref> Aerated soil is also a net sink of [[methane]] (CH<sub>4</sub>)<ref>{{cite journal |last1=Hartmann |first1=Adrian A. |last2=Buchmann |first2=Nina |last3=Niklaus |first3=Pascal A. |journal=[[Plant and Soil]] |volume=342 |issue=1–2 |title=A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization |year=2011 |pages=265–275 |doi=10.1007/s11104-010-0690-x |bibcode=2011PlSoi.342..265H |hdl=20.500.11850/34759 |s2cid=25691034 |url=https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/34759/2/11104_2010_Article_690.pdf |access-date=26 January 2025 }}</ref> but a net producer of methane (a strong heat-absorbing [[greenhouse gas]]) when soils are depleted of oxygen and subject to elevated temperatures.<ref>{{cite journal |last1=Moore |first1=Tim R. |last2=Dalva |first2=Moshe |journal=Journal of Soil Science |volume=44 |issue=4 |title=The influence of temperature and water table position on carbon dioxide and methane emissions from laboratory columns of peatland soils |url=https://www.researchgate.net/publication/229878721 |year=1993 |pages=651–64 |doi=10.1111/j.1365-2389.1993.tb02330.x |access-date=26 January 2025 }}</ref>

Soil atmosphere is also the seat of emissions of [[Volatile (astrogeology)|volatiles]] other than carbon and nitrogen oxides from various soil organisms, e.g. roots,<ref>{{cite journal |last1=Hiltpold |first1=Ivan |last2=Toepfer |first2=Stefan |last3=Kuhlmann |first3=Ulrich |last4=Turlings |first4=Ted C.J. |journal=Chemoecology |volume=20 |issue=2 |title=How maize root volatiles affect the efficacy of entomopathogenic nematodes in controlling the western corn rootworm? |url=https://www.researchgate.net/publication/215470509 |year=2010 |pages=155–62 |doi=10.1007/s00049-009-0034-6 |bibcode=2010Chmec..20..155H |s2cid=30214059 |access-date=2 February 2025 }}</ref> bacteria,<ref>{{cite journal |last1=Ryu |first1=Choong-Min |last2=Farag |first2=Mohamed A. |last3=Hu |first3=Chia-Hui |last4=Reddy |first4=Munagala S. |last5= Wei |first5= Han-Xun |last6= Paré |first6=Paul W. |last7= Kloepper |first7= Joseph W. |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=100 |issue=8 |title=Bacterial volatiles promote growth in Arabidopsis |year=2003 |pages=4927–32 |doi=10.1073/pnas.0730845100 |pmid=12684534 |pmc=153657 |bibcode=2003PNAS..100.4927R |doi-access=free }}</ref> fungi,<ref>{{cite journal |last1=Hung |first1=Richard |last2=Lee |first2=Samantha |last3=Bennett |first3=Joan W. |journal=[[Applied Microbiology and Biotechnology]] |volume=99 |issue=8 |title=Fungal volatile organic compounds and their role in ecosystems |url=https://www.researchgate.net/publication/273638498 |year=2015 |pages=3395–405 |doi=10.1007/s00253-015-6494-4 |pmid=25773975 |s2cid=14509047 |access-date=2 February 2025 }}</ref> animals.<ref>{{cite journal |last1=Purrington |first1=Foster Forbes |last2=Kendall |first2=Paricia A. |last3=Bater |first3=John E. |last4=Stinner |first4=Benjamin R. |journal=Great Lakes Entomologist |volume=24 |issue=2 |title=Alarm pheromone in a gregarious poduromorph collembolan (Collembola: Hypogastruridae) |url=https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1732&context=tgle |year=1991 |pages=75–8 |access-date=2 February 2025 }}</ref> These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks<ref>{{cite journal |last1=Badri |first1=Dayakar V. |last2=Weir |first2=Tiffany L. |last3=Van der Lelie |first3=Daniel |last4=Vivanco |first4=Jorge M |journal=[[Current Opinion in Biotechnology]] |volume=20 |issue=6 |title=Rhizosphere chemical dialogues: plant–microbe interactions |url=https://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |doi=10.1016/j.copbio.2009.09.014 |pmid=19875278 |year=2009 |pages=642–50 |access-date=2 February 2025 |archive-date=21 September 2022 |archive-url=https://web.archive.org/web/20220921224421/http://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Salmon |first1=Sandrine |last2=Ponge |first2=Jean-François |journal=[[Soil Biology and Biochemistry]] |volume=33 |issue=14 |title=Earthworm excreta attract soil springtails: laboratory experiments on Heteromurus nitidus (Collembola: Entomobryidae) |url=https://www.academia.edu/20508985 |doi=10.1016/S0038-0717(01)00129-8 |year=2001 |pages=1959–69 |bibcode=2001SBiBi..33.1959S |s2cid=26647480 |access-date=2 February 2025 }}</ref> playing a decisive role in the stability, dynamics and evolution of soil ecosystems.<ref>{{cite journal |last1=Lambers |first1=Hans |last2=Mougel |first2=Christophe |last3=Jaillard |first3=Benoît |last4=Hinsinger |first4=Philipe |journal=[[Plant and Soil]] |volume=321 |issue=1–2 |title=Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective |url=https://www.academia.edu/25517742 |doi=10.1007/s11104-009-0042-x |year=2009 |pages=83–115 |bibcode=2009PlSoi.321...83L |s2cid=6840457 |access-date=2 February 2025 }}</ref> [[Biogenic substance|Biogenic]] soil [[volatile organic compound]]s are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.<ref>{{cite journal |last1=Peñuelas |first1=Josep |last2=Asensio |first2=Dolores |last3=Tholl |first3=Dorothea |last4=Wenke |first4=Katrin |last5=Rosenkranz |first5=Maaria |last6=Piechulla |first6=Birgit |last7=Schnitzler |first7=Jörg-Petter |journal=[[Plant, Cell and Environment]] |volume=37 |issue=8 |title=Biogenic volatile emissions from the soil |year=2014 |pages=1866–91 |doi=10.1111/pce.12340 |pmid=24689847 |doi-access=free |bibcode=2014PCEnv..37.1866P }}</ref>

Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,<ref>{{cite journal |last1=Buzuleciu |first1=Samuel A. |last2=Crane |first2=Derek P. |last3=Parker |first3=Scott L. |journal=[[Herpetological Conservation and Biology]] |volume=11 |issue=3 |title=Scent of disinterred soil as an olfactory cue used by raccoons to locate nests of diamond-backed terrapins (Malaclemys terrapin) |url=https://www.herpconbio.org/Volume_11/Issue_3/Buzuleciu_etal_2016.pdf |year=2016 |pages=539–51 |access-date=2 February 2025 }}</ref> a bulk property attributed in a [[reductionist]] manner to particular biochemical compounds such as [[petrichor]] or [[geosmin]].