DISTRIBUTION AND RETRANSLOCATION OF N-15 IN LODGEPOLE PINE OVER 8 GROWING SEASONS

We studied the distribution and retranslocation of N in 11-year-old Pi nus contorta Dougl. trees following a winter application of N at 100 k g ha-1 as N-15-urea, (NH4NO3)-N-15 or (NH4NO3)-N-15. In all treatments , there was little uptake of N-15 after the first growing season altho ugh labeled N was still present in the soil. In subsequent years, N-15 in the trees was partly retranslocated, and, at the same time, it was diluted by uptake of unlabeled N from the soil. Between Years 1 and 8 after N fertilization, net retranslocation of N-15 from the lower cro wn (branches formed before fertilization) was 14%, and 18-25% of the N -15 in the trees was translocated to the upper and mid-crown. Overall, uptake of N-15 from nitrate was less than from urea or ammonium. Howe ver, when compared with the urea- and ammonium-N sources, N-15 from th e nitrate source initially moved as rapidly into the foliage, but a gr eater proportion of it was retranslocated from the foliage during the second growing season. Nitrogen in foliage and wood formed in the grow ing season following fertilization was more highly labeled (measured a s % N derived from the fertilizer) than in recently formed tissues. La beling was substantially higher in foliage formed before fertilization than in wood of a similar age. In contrast, N in foliage formed after fertilization had only slightly higher labeling than wood of a simila r age, indicating a relatively stable labeling throughout the trees on ce N-15 uptake had ceased. The concentrations of total and labeled N w ere substantially higher in foliage than in either wood or bark. There was evidence of N movement into wood tissues formed before fertilizat ion, presumably along rays, and also of N retranslocation out of xylem cells as they matured. This study of internal N cycles was facilitate d by the use of N-15 labeling because there was little uptake of label ed N after the first growing season, whereas interpretation based on t otal N was obscured by substantial uptake of N from the soil. We concl ude that retranslocation studies based on measurements of total N cont ent should be avoided.