3. 5. Ammonia flux from rewetting

General patterns of responses

Soil NH3 is primarily produced when ammonium ions (NH4+) dissociate into gaseous NH3 under alkaline conditions, and NH3 flux is sensitive to soil conditions that influence NH4+ concentrations (Schlesinger and Peterjohn, 1991; McCalley and Sparks, 2008). Increases in soil NH3 flux following rewetting have been observed in deserts (Schlesinger and Peterjohn, 1991; McCalley and Sparks, 2008). In the Chihuahuan Desert, USA, a simulated rainfall increased NH3 fluxes from 15 μg N m−2 d−1 to 95 μg N m−2 d−1 within 24 hr and the fluxes declined as the soils dried during the next 7 days (Schlesinger and Peterjohn, 1991). Similarly, increased NH3 fluxes following a natural rainfall were 5−10 times higher than pre-rain fluxes in the Mojave Desert, USA (McCalley and Sparks, 2008). Studies examining how rewetting affects NH3 flux have commonly reported 7 d response following rewetting (Table 2), with the rate of NH3 flux increase ranging from 200% to >1,000% (Table 2, Fig. 2).



Mechanisms and drivers

The mechanisms responsible for the response of NH3 to rewetting have not been explored to our knowledge. The magnitude of increased NH3 flux following rewetting of dry soils can be influenced by cover type and soil temperature (Schlesinger and Peterjohn, 1991; McCalley and Sparks, 2008). However, there is a limited literature on NH3 flux and further research is needed to identify the mechanisms controlling the response after rewetting at multiple ecosystems.