Distribution and retranslocation of 15N in lodgepole pine over eight growing seasons

We studied the distribution and retranslocation of N in 11-year-old Pinus contorta Dougl. trees following a winter application of N at 100 kg ha-1 as 15N-urea, 15NH4NO3 or NH4 15NO3. In all treatments, there was little uptake of 15N after the first growing season although labeled N was still present in the soil. In subsequent years, 15N 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 15N from the lower crown (branches formed before fertilization) was 14%, and 18-25% of the 15N in the trees was translocated to the upper and mid-crown. Overall, uptake of 15N from nitrate was less than from urea or ammonium. However, when compared with the urea- and ammonium-N sources, 15N from the nitrate source initially moved as rapidly into the foliage, but a greater proportion of it was retranslocated from the foliage during the second growing season. Nitrogen in foliage and wood formed in the growing season following fertilization was more highly labeled (measured as % N derived from the fertilizer) than in recently formed tissues. Labeling 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 similar age, indicating a relatively stable labeling throughout the trees once 15N uptake had ceased. The concentrations of total and labeled N were substantially higher in foliage than in either wood or bark. There was evidence of N movement into wood tissues formed before fertilization, presumably along rays, and also of N retranslocation out of xylem cells as they matured. This study of internal N cycles was facilitated by the use of 15N labeling because there was little uptake of labeled N after the first growing season, whereas interpretation based on total N was obscured by substantial uptake of N from the soil. We conclude that retranslocation studies based on measurements of total N content should be avoided.