FLUXES OF AMMONIA OVER AN ALFALFA FIELD

The cycling of atmospheric ammonia has significance to both agricultural and global N balances. Field data concerning the magnitude of ammonia exchange is not available for many crops and ecosystems. The vertical flux of gaseous plus particulate ammonia above an extensive alfalfa canopy was measured periodically over a 1 yr period. The gradients of mean wind speed, mean temperature and mean ammonia concentration were simultaneously determined during 1 h runs and were combined to estimate average ammonia flux. Total ammonia was determined using a treated filter paper sampling technique. A diurnal pattern of ammonia evolution during the day and deposition at night was observed. At least a portion of the deposition at night was due to the influence of a local ammonia source. The magnitude of the fluxes was small. The nighttime observations yielded an estimated yearly gain of 1.6 kg N ha-1. The daytime observations, excluding periods of 10 days for each of 3 hay harvests yielded an estimated annual loss of 1.2 kg N ha-1. During the 3 periods of hay harvesting an additional 2.3 kg N ha-1 was estimated to have been lost. The data indicate ammonia exchange over the alfalfa field was of little importance to the agricultural N balance, but has relevance to understanding global N cycling.