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 o f this study was to evaluate the N dynamics of a corn (Zea mays L., Pi oneer hybrid 3379) cropping system and determine the effect of NH3 tra nsport and crop NH3 compensation point on N relations in the crop. Soi l and plant N measurements were made during the growing season along w ith plant-atmosphere NH3 transport using flux-gradient micrometeorolog ical techniques. The crop generally emitted NH3 throughout the season when the atmospheric NH3 concentrations were near background levels. D uring periods of high atmospheric NH3 concentrations, the crop absorbe d significant amounts of NH3 from the atmosphere. Net seasonal NH3 tra nsport was determined to be a small loss, based on the number of measu rement periods throughout the growing season; however, N isotope studi es in a nearby field estimated significant NH3 losses. Nitrogen isotop e studies cannot account for the substitution of (NH3)-N-14 absorbed f rom the atmosphere for volatilized (NH3)-N-15. These results indicate that care must be taken when using N isotopes to evaluate N losses fro m cropping systems when plants are actively growing. Nitrous oxide flu x was measured late in the cropping season, but showed small denitrifi cation losses during the measurement period. The crop NH3 compensation point varied during the time of day and with respect to soil-plant-at mosphere influence. Net NH3 transport accounted for little of the N lo st from this cropping system.