MINERAL NITROGEN FERTILIZERS: ENVIRONMENTAL IMPACT OF PRODUCTION AND USE - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 22

methodologies assume that 0.75% of the N that is leached from cropped systems and 1% of
the N that is volatilized and subsequently deposited to downstream ecosystems are emitted
later as N 2 O (De Klein et al., 2006). This topic is so complex, that is credible that the EFs for
leached N and for volatilized and re-deposited N depends on the type of waterway and on the
N status (e.g., limiting or non-limiting) of the receiving ecosystem (Beaulieu et al., 2011).
E FFECT OF N F ERTILIZATION ON NH 3 V OLATILIZATION
Worldwide, agriculture is the biggest source of ammonia (NH 3 ) emission coming from
ammonium (NH 4 + ) contained in fertilizers converted into ammonia and released to the air
(Mosier, 2001; Galloway et al., 2004; Erisman et al., 2008). The emitted NH 3 results in
nitrogen loads to ecosystems through its atmospheric transportation and deposition, which
may impact ecosystems in various ways (Hayashi & Yan, 2010). Ammonia released into the
atmosphere, contributes to acidification and the eutrophication of terrestrial and water
ecosystems. Its impact acts at local and regional scale, depending by the atmosphere
transportation.
Loss of N by ammonia volatilization has ranged from negligible amounts to >50% of the
fertilizer N applied, depending upon fertilizer practice and environmental conditions (Peoples
et al., 1995). In flooded rice fields, ammonia volatilization can account for 20% to >80% of
the total N lost from fertilizer sources (Mosier et al., 1989). The amount and temporal
dynamic of NH 3 emission are affected by type of canopy, climatic and soil conditions,
typology of fertilizer and management of its application (Huijsmans et al., 2003; Sommer et
al., 2004).
Some of the most important factors regulating NH 3 loss are the concentration of
ammoniacal-N, temperature, and pH of the soil solution or irrigation water, since all three
variables markedly affect the partial pressure of NH 3 . The pH in particular affects the
equilibrium between ammonium and ammonia, so that the relative concentration of NH 3
increases from 0.1 to 1, 10 and 50% as the pH changes from 6 to 7, 8 and 9, respectively
(Freney et al., 1983). Soil pH at values in excess of 8.0 are responsible of high ammonia
volatilization (Larney & Olson, 2006; Kim et al., 2008). Urea is rapidly hydrolyzed by urease
to NH 3 within one week in soil (Han et al., 2004). The protonation of NH 3 via NH 3 + H 2 O 
NH 4 + + OH - after urea hydrolysis is the most critical process in the increase of soil pH to
values in excess of 8, thereby resulting in an increase in NH 3 volatilization (Fenn & Hossner,
1985).
On the contrary, NH 3 volatilization appeared inhibited by low soil pH, as well as high
soil cation exchange (ECEC) even at the low soil pH; and the relatively high nitrification
potential (Hayashi et al., 2011). So, the pH-buffer capacity and cation exchange capacity of
the soil are factors able to reduce ammonia volatilization.
Rates of NH 3 emissions are also very sensitive to temperature (Montes et al., 2009). As
temperatures rise, the relative proportion of NH 3 to ammonium present at a given pH
increases, while the solubility of NH 3 in water decreases. This increases the diffusion of NH 3
through the soil, and affects the rate of microbial transformations. Volatilization takes place
mainly during the first days after application, and for a longer time the lower the temperatures