Abstract: Root observation tubes (minirhizotrons) were used to study the effects of nitrogen addition on the fine root growth of Fraxinus mandshurica and  Larix gmelinii, with the correlations between the fine root lifespan and nitrogen availability analyzed. After the nitrogen addition, the fine root number of
F. mandshurica and L. gmelinii had a decreasing trend, but the fine root diameter became larger and the branching ratio decreased. The survival rate of F. mandshurica fine roots increased, and the median root lifespan prolonged 105 days, compared with the control. No significant responses to the nitrogen addition were observed in the survival rate of L. gmelinii fine roots. The first-order fine roots with smaller diameter, the roots in surface soil (0-15 cm), and the fine roots newly born in spring and summer were vulnerable to extend their lifespan by nitrogen addition, suggesting that the fine r
oots with higher physiological activity were easily to be affected by nitrogen fertilization.

Key words: nitrogen fertilization, fine root lifespan, minirhizotron, Fraxinus mandshurica, Larix gmelinii