Gases fillsoil pores in thesoil structure as water drains or is removed from a soil pore by evaporation or root absorption. The network of pores within the soil aerates, or ventilates, the soil.[11] This aeration network becomes blocked when water enters soil pores. Not only are both soil air and soil water very dynamic parts of soil, but both are often inversely related.[12]
The composition ofgases present in thesoil's pores, referred to commonly as the soil atmosphere or atmosphere of the soil, is similar to that of the Earth'satmosphere.[13] Unlike the atmosphere, however, soil gas composition is less stagnant due to the various chemical andbiological processes taking place in thesoil.[13] The resulting changes in composition from these processes can be defined by their variation time (i.e. daily vs. seasonal). Despite this spatial- and temporal-dependent fluctuation, soil gases typically boast greater concentrations ofcarbon dioxide andwater vapor in comparison to the atmosphere.[13] Furthermore, concentration of other gases, such asmethane andnitrous oxide, are relatively minor yet significant in determininggreenhouse gasflux and anthropogenic impact onsoils.[3]
Automated CO2 exchange system measuring soil respiration
Gas molecules in soil are in continuous thermal motion according to thekinetic theory of gases, and there is also collision between molecules – arandom walk process.[14] In soil, aconcentration gradient causes net movement of molecules from high concentration to low concentration, which gives the movement of gas bydiffusion.[15] Numerically, it is explained by theFick's law of diffusion. Soil gas migration, specifically that ofhydrocarbon species with one to five carbons, can also be caused by microseepage.[16]
The soil atmosphere's variable composition and constant motion can be attributed to chemical processes such asdiffusion,decomposition, and, in some regions of the world,thawing, among other processes. Diffusion of soil air with theatmosphere causes the preferential replacement of soil gases withatmospheric air.[13] More significantly, moreover, variation in soil gas composition due to seasonal, or even daily, temperature and/or moisture change can influence the rate ofsoil respiration.[17]
According to theUSDA,soil respiration refers to the quantity ofcarbon dioxide released from soil.[18] This excess carbon dioxide is created by the decomposition of organic material bymicrobial organisms, in the presence ofoxygen.[17] Given the importance of both soil gases tosoil life, significant fluctuation of carbon dioxide and oxygen can result in changes in rate of decay,[17] while changes in microbial abundance can inversely influence soil gas composition.[19]
In regions of the world wherefreezing of soils ordrought is common, soil thawing and rewetting due toseasonal ormeteorological changes influence soil gasflux.[3] Both processes hydrate the soil and increasenutrient availability leading to an increase in microbial activity.[3] This results in greatersoil respiration and influences the composition of soil gases.[17][3]
Soil gases have been used for multiple scientific studies to explore topics such as microseepage,[16]earthquakes,[20] and gaseous exchange between thesoil and theatmosphere.[21][3] Microseepage refers to the limited release ofhydrocarbons on the soil surface and can be used to look forpetroleum deposits based on the assumption thathydrocarbons vertically migrate to the soil surface in small quantities.[16] Migration of soil gases, specificallyradon, can also be examined asearthquake precursors.[20] Furthermore, for processes such as soilthawing and rewetting, for example, large sudden changes insoil respiration can cause increasedflux of soil gases such ascarbon dioxide andmethane, which aregreenhouse gases.[3] These fluxes and interactions between soil gases andatmospheric air can further be analyzed by distance from the soil surface.[21]
^Pierzynski, Gary M.; Sims, J. Thomas; Vance, George F., eds. (2005).Soils and environmental quality (3rd ed.). Boca Raton, Florida:CRC Press. Retrieved8 July 2025.
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