Ammonia volatilization from grassland receiving nitrogen fertilizer and rotationally grazed by dairy cattle

The micrometeorological mass balance method was used to measure ammonia (NH₃) volatilization from rotationally grazed swards throughout the 1987 and 1988 growing seasons. In both years the swards were dressed with calcium ammonium nitrate (CAN) split over 7 dressings. In 1987 the sward received a total of 550 kg N ha^–1, in 1988 a total of 550 or 250 kg N ha^–1. For the 550 kg N ha^–1 treatments there were 8 and 9 grazing cycles, respectively, in 1987 and 1988 and 7 for the 250 kg N ha^–1 treatment. Losses from the 550 N sward were 42.2 and 39.2 kg N ha^–1 in 1987 and 1988, respectively; this was equivalent to 8.5 and 7.7% of the N returned to the sward in the excreta of the grazing cattle. The NH₃ loss from the 250N sward was 8.1 kg N ha^–1 in 1988, which was equivalent to 3.1% of the N returned to the sward in excreta during the growing season. There was a wide variation in NH₃ volatilization between the individual grazing periods. This indicates the necessity of continued measurements throughout the growing season to obtain reliable data on NH₃ volatilization. Soil humidity is suggested to be a key factor, because emissions were high from wet soil, and low from drier soil. Results of a Monte Carlo simulation study showed that the measured NH₃ loss from the 250 and 550 N swards had a standard deviation of 13 and 5% of the mean, respectively.