Nh3 Nh2oh No2

Pathway: nitrifier denitrification

Fig. 17.2. Nitrous oxide (N2O) production by nitrifiers and denitrifiers. (From Wrage et al., 2001.)

applications have been shown to double N2O emission rates from forest soils (Regina et al., 1998). Furthermore, an increased input of nitrogen deposition affects the nitrogen leaching or runoff from forests (Dise et al., 1998; Gundersen et al., 1998; De Vries et al., 2003a), causing an elevated indirect N2O emission from surface waters (Fig. 17.1). Atmospheric nitrogen deposition may also increase the emission of the secondary radiatively active trace gas NO from forest soils (Gasche and Papen, 1999; Pilegaard et al., 1999; Van Dijk and Duyzer, 1999).

Nitrification and denitrification can occur simultaneously in soils, though spatially separated. The rates of nitrification and denitrification increase with temperature, soil pH and substrate availability, i.e. with [NH+] and [NO-], respectively. An increased soil aeration and oxygen availability increases nitrification, but decreases denitrification. Variable but small amounts of N2O are released during nitrification and denitrification. The ratio of N2O produced per unit of NH4 consumed (nitrification) tends to increase with decreased substrate pH, which is opposite to the effect on nitrification. An increase in temperature also increases the N2O/NH4 ratio. As with denitrification, the ratio of N2O produced per unit of NO3 consumed tends to increase with substrate (NO3-). However, in contrast to denitrification, the N2O/NO3 ratio decreases with an increase in temperature and soil pH and a decrease in oxygen availability (Granli and B0ckman, 1994). The fraction of N2O released from the nitrification of NH+ is usually much less than 1%, while the fraction of N2O released from denitrification may range from 0.1% to 100%. More information on the role of the various factors is given by Lemke and Janzen (Chapter 5, this volume).

The reduction of N2O to N2 by denitrification is the only natural process by which N2O is removed from the biosphere (see Kroeze et al., Chapter 15, this volume), apart from its destruction in the stratosphere (see Butenhoff and Khalil, Chapter 14, this volume). The rate of N2O consumption in soils, the importance of which is highly uncertain on a global perspective, is related to the diffusivity of the soil, which influences oxygen pressure (pO2) and the residence time of nitrogen gases in the soil matrix. The soil diffusivity depends on the soil moisture and other soil properties such as texture and organic carbon content. Hence, the production and consumption of N2O is controlled by a complex of biotic and abiotic factors (e.g. Brumme et al., 1999; Papen and Butterbach-Bahl, 1999; Butterbach-Bahl et al., 2002c). Increases in carbon sequestration in soils may increase N2O emissions because of an increased denitrification potential (Six et al., 2004; Li et al., 2005). These increases in N2O emissions, converted into CO2 equivalent emissions, may offset the carbon sequestered completely (Li et al., 2005). More information on the impact of soil carbon store on N2O emissions and on GHG emissions is given by Lemke and Janzen (Chapter 5, this volume).

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