VOLATILIZATION AND FOLIAR RECAPTURE OF AMMONIA FOLLOWING SPRING AND FALL APPLICATION OF NITROGEN-15 UREA TOA DOUGLAS-FIR ECOSYSTEM

Seasonal effects on the volatilization and vegetal recapture of NH//3 following application of pelleted **1**5N urea at 210 kg N ha** minus **1 to a 40-yr-old Douglas-fir left bracket Pseudotsuga menziesii (Mirb. ) Franco right bracket 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 greater than 90 to less than 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.