SUMMARY
Production and consumption processes in soils contribute to the global cycles of many trace gases (CH4, CO, OCS, H2, N2O, and NO) that are relevant for atmospheric chemistry and climate. Soil microbial processes contribute substantially to the budgets of atmospheric trace gases. The flux of trace gases between soil and atmosphere is usually the result of simultaneously operating production and consumption processes in soil: The relevant processes are not yet proven with absolute certainty, but the following are likely for trace gas consumption: H2 oxidation by abiontic soil enzymes; CO cooxidation by the ammonium monooxygenase of nitrifying bacteria; CH4 oxidation by unknown methanotrophic bacteria that utilize CH4 for growth; OCS hydrolysis by bacteria containing carbonic anhydrase; N2O reduction to N2 by denitrifying bacteria; NO consumption by either reduction to N2O in denitrifiers or oxidation to nitrate in heterotrophic bacteria. Wetland soils, in contrast to upland soils are generally anoxic and thus support the production of trace gases (H2, CO, CH4, N2O, and NO) by anaerobic bacteria such as fermenters, methanogens, acetogens, sulfate reducers, and denitrifiers. Methane is the dominant gaseous product of anaerobic degradation of organic matter and is released into the atmosphere, whereas the other trace gases are only intermediates, which are mostly cycled within the anoxic habitat. A significant percentage of the produced methane is oxidized by methanotrophic bacteria at anoxic-oxic interfaces such as the soil surface and the root surface of aquatic plants that serve as conduits for O2 transport into and CH4 transport out of the wetland soils. The dominant production processes in upland soils are different from those in wetland soils and include H2 production by biological N2 fixation, CO production by chemical decomposition of soil organic matter, and NO and N2O production by nitrification and denitrification. The processes responsible for CH4 production in upland soils are completely unclear, as are the OCS production processes in general. A problem for future research is the attribution of trace gas metabolic processes not only to functional groups of microorganisms but also to particular taxa. Thus, it is completely unclear how important microbial diversity is for the control of trace gas flux at the ecosystem level. However, different microbial communities may be part of the reason for differences in trace gas metabolism, e.g., effects of nitrogen fertilizers on CH4 uptake by soil; decrease of CH4 production with decreasing temperature; or different rates and modes of NO and N2O production in different soils and under different conditions.
