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 250 N 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.