Soil gas

Soil gases (soil atmosphere) are the gases found in the air space between soil components. The spaces between the solid soil particles, if they do not contain water, are filled with air. The primary soil gases are nitrogen, carbon dioxide and oxygen. Oxygen is critical because it allows for respiration of both plant roots and soil organisms. Other natural soil gases include nitric oxide, nitrous oxide, methane, and ammonia. Some environmental contaminants below ground produce gas which diffuses through the soil such as from landfill wastes, mining activities, and contamination by petroleum hydrocarbons which produce volatile organic compounds. Gases fill soil pores in the soil 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. 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. The composition of gases present in the soil's pores, referred to commonly as the soil atmosphere or atmosphere of the soil, is similar to that of the Earth's atmosphere. Unlike the atmosphere, moreover, soil gas composition is less stagnant due to the various chemical and biological processes taking place in the soil. 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 of carbon dioxide and water vapor in comparison to the atmosphere. Furthermore, concentration of other gases, such as methane and nitrous oxide, are relatively minor yet significant in determining greenhouse gas flux and anthropogenic impact on soils. 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.