3. 4. Nitric oxide flux from rewetting

General patterns of responses

Nitric oxide can be produced from: 1) nitrification (Kowalchuk and Stephen, 2001); 2) denitrification (Knowles, 1982); and 3) nitrifier denitrification (Wrage et al., 2001) as described in §3.3. Increases in soil NO flux following rewetting have been reported in various terrestrial ecosystems including cropland (Guenzi et al., 1994), grazing pasture (Hutchinson and Brams, 1992), forest (Wu et al., 2010a), grassland (Hartley and Schlesinger, 2000), savanna (Martin et al., 2003), and desert (McCalley and Sparks, 2008). Laboratory incubations with grassland soil (Yao et al., 2010), grazing pasture soil (Hutchinson et al., 1993), forest soil (Dick et al., 2006) and desert soil (McCalley and Sparks, 2008) have reported similar results of increased NO flux after rewetting. NO rewetting studies have commonly reported short-term (ca., 1−3 d) response following rewetting (Table 2), and the rate of NO flux increase ranged from 40% to more than 800,000% (Table 2, Fig. 2). Some studies indicate that even a single rewetting event could substantially affect annual flux rates of NO (Davidson et al., 1991; Yienger and Levy, 1995; Kitzler et al., 2006), and rewetting events could be important for regional fluxes (Harris et al., 1996; Ghude et al., 2010).



Mechanisms and drivers

The mechanisms responsible for increased NO fluxes following rewetting have been commonly hypothesised as belonging to the two categories: 1) enhanced microbial metabolism by substance supply and 2) physical mechanisms described above (§3.1). Several studies found that nitrification is the dominant source of increased NO flux following wetting of dry soils (Davidson, 1992a; Davidson et al., 1993; Hutchinson et al., 1993). The magnitude of increased NO flux can be influenced by the duration and severity of antecedent dry periods (Butterbach-Bahl et al., 2004; McCalley and Sparks, 2008), change in soil moisture (Yienger and Levy, 1995) and temperature (Smart et al., 1999; McCalley and Sparks, 2008), vegetation type (Barger et al., 2005; McCalley and Sparks, 2008), soil type (Martin et al., 2003), microbial demand for N (Stark et al., 2002), frequency of wetting events (Davidson et al., 1991; Hartley and Schlesinger, 2000), previous disturbances (Levine et al., 1988; Poth et al., 1995), and agricultural management (Hutchinson and Brams, 1992). Interestingly, there are conflicting results concerning the magnitude of increased NO flux after rewetting, which were independent of both the size of rewetting pulse (Davidson, 1992b; Martin et al., 1998) and the periods of antecedent dry days (Martin et al., 1998). Also, other reports have suggested that lower amount of water addition result in higher NO pulses (Hutchinson et al., 1997; Dick et al., 2001). These conflicting results emphasize the uncertainty, and limitations, of predicting the magnitude of NO flux responses based on so few data.