Non-agricultural ammonia sources include synthetic fertilizers (urban use), oceans, biomass burning, plant decomposition, natural soils, and human excreta (8). Agricultural sources, which are estimated to be the greatest, include livestock production and fertilizer use on crops. Animal agriculture is estimated to contibute approximately 40% of total ammonia emissions (mainly from manure), and crop agriculture is estimated to contribute an additional 20% from synthetic fertilizer application and crop emissions (8).
Ammonia is produced on livestock operations when urea ((NH2)2CO) in urine combines with the urease enzyme present in feces and soil and rapidly hydrolyzes to form ammonia gas and carbamine acid (NH2COOH), which decomposes to release another molecule of ammonia gas and carbon dioxide (see reaction below).
The reaction is quick, volatilizing within minutes and taking anywhere from 2 to 10 hours for ammonia volatilization to peak after mixing of urine and feces (9, 10).
The quantity and rate of ammonia volatilization from manure depends on a variety of factors such as housing and manure management, diet, and meteorological factors. The primary factor affecting the amount of ammonia volatilized from manure is the percentage of dietary crude protein (nitrogen) consumed by animals in feed rations. An increase in crude protein in the diet has been shown to exponentially increase manure nitrogen content and subsequent ammonia emissions (11, 12). This is because the majority of excess dietary nitrogen is excreted in the urine as urea (70%; 14, 15), which readily volatilizes as ammonia in the presence of the urease enzyme, found in abundance in soil and manure and on almost all farmyard surfaces.
Secondary factors that influence ammonia volatilization from manure are manure management techniques, pH, temperature, relative humidity, and wind speed (9, 15, 16, 17). Since there is such a large and constant supply of manure on livestock operations, there are many opportunities for ammonia volatilization to occur.
For dairy operations, model predictions show that of the ammonia emitted from dairy manure, manure application accounts for the greatest portion of volatilization (42 %), followed by housing (30 %), storage (14 %), and animals grazing pasture (14 %) (18).
For feedlots, pen surface volatilization accounts for the largest source of ammonia emissions because manure is not harvested on a daily basis, and is continuously excreated. Surface volatilization accounts for approximately 80% of feedlot ammonia emissions. Manure storage (15%) and land application (5%) account for the additional 20%. Land application is so low because most of the N has already volatilized from the manure prior to land application.
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