Gas molecules in soil are in continuous thermal motion according to the kinetic theory of gases, and there is also collision between molecules – a random walk process. In soil, a concentration gradient causes net movement of molecules from high concentration to low concentration, which gives the movement of gas by diffusion. Numerically, it is explained by the Fick's law of diffusion. Soil gas migration, specifically that of hydrocarbon species with one to five carbons, can also be caused by microseepage.
The soil atmosphere's variable composition and constant motion can be attributed to chemical processes such as diffusion, decomposition, and, in some regions of the world, thawing, among other processes. Diffusion of soil air with the atmosphere causes the preferential replacement of soil gases with atmospheric air. More significantly, moreover, variation in soil gas composition due to seasonal, or even daily, temperature and/or moisture change can influence the rate of soil respiration.
According to the USDA, soil respiration refers to the quantity of carbon dioxide released from soil. This excess carbon dioxide is created by the decomposition of organic material by microbial organisms, in the presence of oxygen. Given the importance of both soil gases to soil life, significant fluctuation of carbon dioxide and oxygen can result in changes in rate of decay, while changes in microbial abundance can inversely influence soil gas composition.
In regions of the world where freezing of soils or drought is common, soil thawing and rewetting due to seasonal or meteorological changes influences soil gas flux. Both processes hydrate the soil and increase nutrient availability leading to an increase in microbial activity. This results in greater soil respiration and influences the composition of soil gases.