N2O exchange

The production of N2O in (forest) soils is predominantly due to the microbial processes of nitrification and denitrification (Granli and B0ckman, 1994; Conrad, 1996). During nitrification, ammonia oxidizers convert NH3 to nitrite (NO2-) and nitrite oxidizers (NO2-) to nitrate (NO3-). N2O develops as a by-product of ammonia oxidation or via a pathway called nitrifier denitrification, i.e. the reduction of NO- via NO to N2O and N2 by nitrifiers (Poth and Focht, 1985; Poth, 1986; Wrage et al., 2001). Denitrification is a reductive process by which denitrifiers reduce NO3-

sequentially to NO, N2O and finally to N2 (Conrad, 1996), as illustrated in Fig. 17.2. Understanding the relative importance of nitrifiers and denitrifiers in producing N2O is the key to understanding the mechanism of N2O production and for accurately upscal-ing and quantifying the N2O source strength of European forests.

The positive relationship between nitrogen deposition and N2O emissions from forest soils has mainly been attributed to the increased availability of nitrogen (NH+ and NO3-) for the microbial processes of nitrification and denitrification (Rennenberg et al., 1998; Corre et al., 1999), as illustrated in Fig. 17.1. Various studies (e.g. Brumme and Beese, 1992; Zechmeister-Boltenstern and Meger, 1997; Brumme et al., 1999; Papen and Butterbach-Bahl, 1999; Butterbach-Bahl et al., 2002a) showed that atmospheric nitrogen deposition is a key site parameter influencing N2O emissions. These studies have shown that temperate forests can function as significant sources for N2O, especially if these forests are affected over decades by high rates of atmospheric nitrogen deposition. In addition to atmospheric nitrogen deposition, many commercial forests receive applications of fertilizer nitrogen in the form of urea or ammonium nitrate. Such

Nitrification

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