Modifications of atmospheric composition and climate have large effects on both the structure and functioning of terrestrial ecosystems. Our understanding of aboveground plant responses to environmental change is becoming clearer [Wand et al., 1999; Rustad et al., 2001; Matson et al., 2002], although their responses to interacting changes are less well characterized and often surprising [Ollinger et al., 2002; Shaw et al., 2002]. The impacts of global environmental change on belowground microbial processes are less well understood [Panikov, 1999; Mikan et al., 2000; Zak et al., 2000b; Asner et al., 2001; Rustad et al., 2001; Matson et al., 2002], especially for key soil N transformations such as nitrification and denitrification.
 Nitrification and denitrification play key roles in regulating the concentration of inorganic N in soil, leaching of nitrate, and the production of N2O, a potent greenhouse gas that also contributes to stratospheric ozone destruction [Smith, 1997; Intergovernmental Panel on Climate Change, 2001]. Thus changes in nitrification and denitrification in response to increasing CO2, increased temperature, and N deposition can directly feed back to atmospheric and climatic change. Furthermore, by mediating N losses from ecosystems, nitrification and denitrification influence ecosystem N stocks over decades to centuries. Because the availability of N in ecosystems may limit C sequestration [Loiseau and Soussana, 1999; Oren et al., 2001], changes in nitrification and denitrification could alter terrestrial C storage and atmospheric CO2 concentrations.
 Nitrification and denitrification are potentially affected by CO2, temperature, and N through a wide variety of complex, interacting mechanisms. Some of the effects are direct (e.g., N addition increases substrate availability for both processes), but many are indirect. For example, nitrification is aerobic and denitrification is anaerobic, so that indirect effects of environmental change on soil O2 concentrations play a key role in controlling these processes. Increased CO2 and temperature have been shown to have strong effects on soil water content and soil biological activity in many field experiments [Rustad et al., 2001; Zak et al., 2000b], thereby exerting strong control over soil O2 concentrations.
 Nitrification is generally favored by increasing the availability of NH4+, the initial substrate for nitrification. It is favored at moderate pH and in well-aerated soils, but declines as soils become very dry. The temperature response of nitrification is approximately bell-shaped with an optimum between ∼20°C and 35°C. The decline at higher temperatures may be partially due to increased biological O2 consumption [Linn and Doran, 1984; Paul and Clark, 1989; Prosser, 1989; Grundmann et al., 1995; Parton et al., 2001; Avrahami et al., 2003]. Denitrification is generally favored by high availability of labile C as a source of energy and of NO3− as an electron acceptor. It is favored in poorly aerated soils, with a pH close to neutrality. The response of denitrification to temperature is similar to that of nitrification, but can have a higher temperature maximum [Tiedje, 1988; Paul and Clark, 1989; Merrill and Zak, 1992; Weier et al., 1993; Strong and Fillery, 2002; Simek and Cooper, 2002].
 Both nitrification and denitrification can produce N2O. During denitrification, NO3− is reduced to NO2− and then to the gases NO, N2O, or N2, the latter being the most reduced form. Increasing soil anoxia, labile C availability, NO3− availability, pH, and temperature shift gaseous emissions toward the more reduced forms [Tiedje, 1988; Paul and Clark, 1989; Weier et al., 1993; Bollmann and Conrad, 1998; Parton et al., 2001; Simek et al., 2002]. During nitrification, some NO, N2O, and N2 can be released through two pathways, the best documented of which is nitrifier denitrification [Webster and Hopkins, 1996; Wrage et al., 2001, 2004]. Nitrification-associated N2O efflux is generally a small fraction of total nitrification N flux, but can often make a major contribution to total soil N2O emissions [Webster and Hopkins, 1996; Kester et al., 1997; Bollmann and Conrad, 1998; Wolf and Brumme, 2002]. There is some preliminary evidence that the fraction of N2O emissions associated with nitrification declines with increasing temperature [Avrahami et al., 2003]. N2O production is therefore a complex process that cannot be easily be related to either total denitrification or nitrification fluxes per se [Webster and Hopkins, 1996; Wolf and Russow, 2000; Wrage et al., 2001, 2004], although some recently developed approaches may provide interesting insights into the metabolic origin of N2O [Yoshida and Toyoda, 2000; Schmidt et al., 2004]. We have examined the responses of nitrification, denitrification, and N2O efflux to elevated CO2, N addition, and warming, based on a review of published experimental results.