Carbon dioxide and nitrogenous gases in the soil atmosphere

Carbon dioxide and nitrogenous trace gases (N₂O, NO) in the soil atmosphere are mainly the products of microbially mediated processes. Once produced, these gases pass to the overlying atmosphere primarily via molecular diffusion, a process which is described by Fick's law of diffusion.In a diffusion-dominated soil, the partial pressure, or concentration, of CO₂ in the soil atmosphere varies as a function of soil depth and is dependent on the production rate and diffusivities. Since these parameters are highly variable, CO₂ concentrations vary widely both between, and within, differing ecosystems. In a compilation of data from around the world, arranged according to an ecosystem classification, soil CO₂ concentrations varied from 0.04 to 13.0% by volume in the upper several meters of soil. These data also highlight the importance of organic substrate (soil organic matter, roots, root exudates), temperature, and (to some extent) moisture on CO₂ production and the resulting concentration in soil profiles. The δ¹³C of the soil CO₂ also varies as a function of depth due to differences in the δ¹³C of the organic substrate undergoing decomposition and the mixing with CO₂ of the overlying atmosphere. Recent work suggests that the δ¹⁸O of the soil CO₂ may hold some promise in estimating the δ¹⁸O of soil water.Biological production and consumption of N₂O and NO results primarily from activity of nitrifying and denitrifying bacteria. Ammonium limitation of nitrification and organic carbon limitation of denitrification usually restricts these processes to surface soil horizons, although denitrification may be an important process for reducing NO₃⁻ in groundwater. These microbial processes and the relative proportions of their gaseous end products are strongly influenced by redox conditions. Microsite variation in sources of electron donors and acceptors is critical to understanding rates and distributions of N trace gas production. Several abiological oxidation and reduction reactions are also important, and interaction of biological and abiological processes deserves more research attention.