Volatilization and Foliar Recapture of Ammonia Following Spring and Fall Application of Nitrogen‐15 Urea to a Douglas‐fir Ecosystem

Abstract

Abstract Seasonal effects on the volatilization and vegetal recapture of NH 3 following application of pelleted 15 N urea at 200 kg N ha −1 to a 40‐yr‐old Douglas‐fir [ Pseudotsuga menziesii (Mirb.) Franco] ecosystem were investigated. Volatilization totalled 14 and 0.7% of applied N in spring and fall, respectively, and the difference appeared to be related to precipitation patterns following fertilization. Volatilization was generally enhanced by small spring rainfalls while depressed by larger fall rains. The isotopic abundance of evolved NH 3 fluctuated from >90 to <10% of that of the fertilizer source. These fluctuations reflected the role of precipitation in the regulation of spatial heterogeneity of urea and urease in soil. Under spring conditions NH 3 was recaptured by potted Douglas‐fir seedlings. Seedlings at 10 cm above the forest floor captured 16 times as much NH 3 as seedlings positioned at the 150‐cm elevation. Labelled N was distributed among seedling tissues in the order: current foliage > 1‐year‐old foliage > roots which is consistent with anabolic incorporation and translocation. This mechanism may have a significant role in plant nutrition when conditions favorable to NH 3 volatilization follow urea fertilization.