Agricultural sources of ammonia

The three main sources of NH₃ on farms are animal wastes, fertilizers, and crop residues. The first of these accounts for about 80% of agricultural emissions.

Of the N consumed by farm animals in feed, only a small proportion (roughly one-fifth) is retained by the animal; the rest is excreted in feces and urine. Some of this N (especially in urine) occurs as urea, a form easily converted to NH₃ and CO₂. As a result, a large proportion of the N in manure can be lost as NH₃ soon after excretion. On pig farms, for example, 40­95% of the nitrogen excreted may be lost before the manure is applied to the field. Much of that, perhaps 10­40% of the N lost, may occur from the barn even before storage. Ammonia losses from cattle manure are often less than from pig manure, probably amounting to less than 50% of the total N content.

Losses of N during storage of manure can also be high, depending on method of storage. In a US study, about 60­80% of N was lost from pig manure in lagoons exposed to air, compared to losses of only 30­65% from that stored in underground pits and later spread as liquid. Another estimate suggests that the proportion of pig manure N lost as NH₃ is less than 10% for anaerobic storage, 10­25% for semi-aerobic systems, and 25­85% during composting. The differences reflect the degree of exposure to air and the amount of water and acid present.

Some NH₃ is also released when manure is applied to land, particularly if a slurry is sprayed into the air. Most loss occurs shortly after application. For example, a study of NH₃ loss from cattle manure showed that about half of the total emission occurred within 1 day (Fig. 33).

Another potential source of NH₃ is fertilizer. Two forms, both widely used in Canada, are especially important: anhydrous ammonia (pure NH₃) and urea. When anhydrous ammonia is injected into soil, it normally converts immediately to NH₄+ in soil water and then is held tightly by the soil. If the soil is extremely dry, however, as much as 20% of the NH₃ can escape. On the other hand, if it is so wet that the soil does not close up after injection, as much as 50% can be lost. Urea fertilizer, like the urea in livestock manure, quickly converts to NH₃ and CO₂ after it is applied. If the fertilizer is not mixed into the soil, large amounts of NH₃ can be released to the atmosphere.

A third possible source of NH₃ from farms is crop residues. Appreciable amounts of NH₄+ can be produced during the decay of N-rich residues like legume green manures. If the residues are allowed to decay on the soil surface, some of this NH₄+ may convert to NH₃ and be lost to the atmosphere.

Based on data from 1990, NH₃ emissions from all sources in Canada amount to about 520 Gg (thousand tonnes) of N per year. Of this, about 90% comes from agriculture, largely from livestock production (Table 17). These estimates, however, are still preliminary.