Atmospheric ammonia in relation to grassland agriculture and livestock production

Abstract. Farm livestock typically retain 5–23% of dietary nitrogen, and consequently excrete large amounts of nitrogen, mainly in urine. Areas affected by cattle urine may receive the equivalent of several hundred kg nitrogen per hectare. Urea is usually hydrolysed to ammonium carbonate within a few days. This increases the soil pH and thereby assists volatilization of ammonia. Volatilization is also increased by soil warmth and by small soil cation exchange capacities. Over the grazing season in lowland UK about 15% of the nitrogen in urine is likely to be volatilized as ammonia, but only 1–5% of the nitrogen in dung is lost in this way.
Substantial volatilization of ammonia probably occurs from animal houses and after spreading of slurry in the field. About 3–4% of fertilizer nitrogen used in the UK is lost as gaseous ammonia. Cut grass herbage also loses ammonia by volatilization, if allowed to remain in the field in wet conditions. Total annual emissions of nitrogen as ammonia from grassland and livestock in UK are probably 320 000–420 000 tonnes.