APPARENT DEPOSITION VELOCITY AND COMPENSATION POINT OF AMMONIA INFERRED FROM GRADIENT MEASUREMENTS ABOVE AND THROUGH ALFALFA

Understanding the cycling of ammonia between croplands and the atmosphere is of importance to agriculturalists and atmospheric scientists. Flux densities of gaseous ammonia (NH3), particulate ammonium (NH4+), and total ammoniacal nitrogen (AN) were measured using an aerodynamic method above an alfalfa (Medicago sativa, L.) canopy between April and July 1981 at a rural location in central New York State. In air not influenced by local sources, NH3 and NH4+ averaged 1.5 and 3.0 ppb, respectively, at 1 m above the crop. Ambient NH4+ varied consistently with synoptic air masses, being lowest (2.3 ppb) for NW and highest (6.4 ppb) for SW flows. Concentrations and gradients of both species were higher during periods of hay harvest. Gradients of NH3 were much steeper than those of NH4+ within the alfalfa canopy, but NH4+ contributed appreciably (36% on average) to above-canopy AN gradients. Alfalfa's NH3 compensation point was estimated by combining concentration and gradient data with transport resistances. Gaseous gradients indicated a compensation point of 2 ppb, lower than previously published estimates. Conversion of NH3 to NH4+ within the canopy air could have reduced NH3 gradients and caused a low estimate of the compensation point. Acidic aerosols, by keeping NH3 levels low, may compete with plants for NH3. Future studies of ammonia exchange should distinguish between NH3 and NH4+ if flux densities are to be related to ambient conditions. Total AN level is a poor predictor of soil-plant-atmosphere ammonia exchange since high AN was frequently associated with low NH3, and NH3 is more surface reactive than NH4+. © 1990.