Ammonium, nitrate, and urea are the three forms of nitrogen (N) contained in fertilizers. While nitrate (NO3-) and ammonium (NH4+) are immediately available for the crops after application (1), urea needs to be converted (hydrolysis, 7) to NH4+.
Nitrate is the preferred N-form, as it is water-soluble and therefore immediately plant available (2). It enhances the uptake of cations, such as K+, Ca2+, Mg2+. Part of the ammonium can also be directly absorbed by the crops (uptake, 3) and depending on soil characteristics, NH4+ is also converted into NO3- (nitrification, 4).
Denitrification (5) is a process where NO3- is reduced to nitrite (NO2-), nitric oxide (NO), nitrous oxide (N2O) and N2. This reaction is mediated by anaerobic bacteria, and therefore occurs in anoxic environments and is thus scarce in well-aerated agricultural soils. As an ion, NO3- is also quite mobile in the soil and can be leached with excess rainfall (leaching, 9). Therefore, it is important to split high amounts of N fertilization into several smaller application rates and to fertilize at the right time, when the demand of the crop is high.
The soil microorganisms consume mainly NH4+ but also NO3- (immobilization, 6a). The presence of carbon-rich but nitrogen-poor organic matter (for instance straw) enhances immobilization. However, this proportion of N is not lost and becomes plant available later when biomass including the microbial biomass decomposes (mineralization, 6b).
After soil application, urea ((NH2)2CO) breaks down to two molecules of ammonia (NH3) and one molecule of carbon dioxide (CO2). The gaseous NH3 can escape into the atmosphere (volatilization, 8). The reaction of NH3 with water (H20) to form NH4 releases a hydroxide ion (OH-) and thus increases the soil pH. Ammonia volatilization is particularly high in alkaline soils (pH>7). Therefore, temporary pH increases in soil pH facilitates high volatilization losses even on acidic soils.