Abstract: Two typical broadleaved trees (i.e., Fraxinus rhynchophylla and Acer mono) with winddispersed seeds in Northeast China were selected in this study. A method of ¹⁵N isotope labeling was used to explore the accumulation responses of seeds and seedlings to ¹⁵Nurea soaking concentration (0, 0.05, 0.1 and 0.2 g·L^-1), soaking time (4, 8 and 12 days) and leaf stage (2, 4, 6, and 8 leaves). The results showed that ¹⁵Nurea soaking concentration and soaking time had significantly positive effects on δ¹⁵N values of seeds, i.e., higher ¹⁵Nurea concentration and longer period of soaking (0.2 g·L^-1+12 d) were contributed to more ¹⁵N accumulation of seeds. The maximum multiples of ¹⁵N accumulation in F. rhynchophylla seeds and A. mono seeds were observed in 0.1 g·L^-1+ (4 d, 8 d) and 0.05 g·L^-1+ (4 d, 8 d), respectively. The loss rate of δ¹⁵N values decreased markedly from 2 leaves to 6 leaves and then kept relatively stable with the increasing seedling height, and the total δ¹⁵N values of seedlings started to decline at the stage of 8 leaves. These results suggested that seedlings with 6 leaves were more suitable for tracking seedling source. The δ¹⁵N values in leaves of seedlings were significantly positively correlated with ¹⁵Nurea concentration level, soaking time and δ¹⁵N values of seeds. Overall, the accumulation of ^15Nurea could be found in seeds and seedlings of F. rhynchophylla and A. mono. The combination of ^15Nurea concentration (0.1 g·L^-1), soaking time (8 d) and leaf stage (6 leaves) was the most suitable for tracking the seeds and seedlings of these two broadleaved trees.

Key words: seedling accumulation, stable nitrogen isotope, ^15N labeling method, seed dispersal.