Ammonia emissions – what’s the problem?

Ammonia emissions are an environmental burden and a loss of valuable nitrogen for plant growth.

Mitigating ammonia losses from fertilizer application thus provides a double benefit. What exactly are the causes and consequences of ammonia volatilization?

About ammonia

Ammonia is a highly reactive, pungent gas formed of nitrogen and hydrogen. Its chemical formula is NH3. Ammonia occurs in essential biological processes and is not a problem in low concentrations. However, ammonia volatilization into the atmosphere has negative consequences for agriculture, ecosystems and human health:

  • Ammonia volatilization from agricultural land is a loss of nitrogen for plant growth. It therefore comes at a cost for the farmer that needs to be minimized.
  • Ammonia reacts with air humidity to form ammonium (NH4). Ammonium depositions contribute to acidification of land and water.
  • Deposition of ammonium degrades the biochemistry of natural ecosystems and causes eutrophication (i.e. excess nutrient supply leading to e.g. algae proliferation).
  • Ammonia combines with other air pollutants such as sulfuric acid and nitric acid to form secondary particulate matter (PM10). It stays in the air over several days and travels long distances. Particulate matter contributes to respiratory diseases.

Ammonia pollution from agriculture represents a high cost to society. According to the European Nitrogen Assessment, it is estimated at 12 € per kg of emitted nitrogen for health damages and 2 € for ecosystem damages [1].

Where does it come from?

Agriculture

94% of all ammonia emissions in the EU result from agriculture. The remaining 6 % come from waste handling, road transportation and industrial applications.

Animal husbandry and manure

Livestock excreta contain high amounts of ammonia. They are at the origin of 75 % of all ammonia emissions from agriculture in the EU (figure 1). Emissions from livestock can be reduced but are beyond the scope of this Pure Nutrient Fact which is dedicated to mineral fertilizer.

Figure 1: European ammonia emissions in total (left) and from agriculture (right) [4].

Mineral fertilizer

Mineral fertilizer application accounts for 22 % of all ammonia emissions from agriculture in the EU (figure 1). These emissions are due to the transformation of ammonium solved in the soil to gaseous ammonia. The rate of transformation depends on the soil pH level. The higher the soil pH level, the more ammonium is converted to ammonia. The higher the temperature, the more ammonia is then lost to the atmosphere.

Mineral nitrogen fertilizers either directly contain ammonium (ammonium sulfate, ammonium sulfate nitrate, ammoniumnitrate, CAN) or are converted to ammonium inthe soil subsequent to spreading (urea and UAN). Urea andammoniumcontaining fertilizers are therefore subject to potential ammonia losses. Urea, however, is specifically prone to ammonia volatilization (figure 4).

The impact of nitrogen form

Figure 2: Hydrolysis of urea temporarily and locally increases soil pH, resulting in increased ammonia losses.

The case of urea

Hydrolysis of urea to ammonium temporarily increases the pH level in the direct vicinity of the application location. The increased pH level spurs the formation of ammonia, even on acidic soils (figure 2).

With temperatures above 15 °C, urea hydrolysis is fast and local ammonia concentrations in the soil rise, and thus volatilization. Temperatures below 8 °C slow down the transformation of urea to ammonia, but also the subsequent nitrification of ammonia to nitrate, leading again to high ammonia concentrations and volatilization.

Dry conditions reduce diffusion of ammonia in the soil and therefore also increase volatilization. Rainfall after application, in contrast, reduces volatilization.

Field trials conducted in different European regions have demonstrated average ammonia volatilization losses from urea of 13.1 % [2].

Comparing mineral fertilizers

Figure 3 summarizes the estimated ammonia volatilization from different fertilizer types. CAN and AN offer the lowest emission factors of all nitrogen fertilizers.

Table 1 shows the ammonia emissions from main mineral fertilizers according to the emission factors defined by the European Environmental Agency. Urea (53.7%) and UAN (18.4%) together account for 72 % of these emissions, while CAN and AN amount for only 2,9 and 4,6 % respectively.

Figure 3: Ammonia emission factors for different nitrogen fertilizers applied to normal soils (pH ≤ 7) [3][4].

Table 1: Ammonia losses in 2014 from mineral fertilizer in Europe according to standard emission factors. 72 % of overall ammonia losses from fertilizer are caused by urea and UAN. Ammonia emissions from NPK fertilizers (line "Other" in above table) depend on composition (compounds or blends, urea or nitrate based) with nitrate based compounds offering the lowest emissions. [3].

Figure 4: All mineral nitrogen fertilizer is ultimately transformed into nitrate (NO3 - ) before it is taken up by plants. Ammonium (NH4 + ) is an intermediate compound which is either directly applied (as ammonium nitrate NH4NO3) or converted in the soil from urea (CO(NH2) 2). Ammonium is in equilibrium with ammonia (NH3) in the soil solution. The higher the pH, the more the balance shifts in favor of ammonia. UAN is a mix of AN and urea and thus activates all the pathways in the above diagram.