NITROGEN DYNAMICS IN IRRIGATED CORN - SOIL-PLANT NITROGEN AND ATMOSPHERIC AMMONIA TRANSPORT

Ammonia transport in cropping systems can affect atmospheric pollution, soil-plant N relationships, and crop N-use efficiency. The purpose of this study was to evaluate the N dynamics of a corn (Zea mays L., Pioneer hybrid 3379) cropping system and determine the effect of NH3 transport and crop NH3 compensation point on N relations in the crop. Soil and plant N measurements were made during the growing season along with plant-atmosphere NH3 transport using flux-gradient micrometeorological techniques. The crop generally emitted NH3 throughout the season when the atmospheric NH3 concentrations were near background levels. During periods of high atmospheric NH3 concentrations, the crop absorbed significant amounts of NH3 from the atmosphere. Net seasonal NH3 transport was determined to be a small loss, based on the number of measurement periods throughout the growing season; however, N isotope studies in a nearby field estimated significant NH3 losses. Nitrogen isotope studies cannot account for the substitution of (NH3)-N-14 absorbed from the atmosphere for volatilized (NH3)-N-15. These results indicate that care must be taken when using N isotopes to evaluate N losses from cropping systems when plants are actively growing. Nitrous oxide flux was measured late in the cropping season, but showed small denitrification losses during the measurement period. The crop NH3 compensation point varied during the time of day and with respect to soil-plant-atmosphere influence. Net NH3 transport accounted for little of the N lost from this cropping system.