Nitrogen resorption and fractionation during leaf senescence in typical tree species in Japan

In northeastern Japan,an area of high precipitation and mountains,beech(Fagus creanata Blume),larch(Larix kaempferi Lamb.),cedar(Cryptomeria japonica D.Don) and black locust(Robinia pseudoacacia L.) were evaluated for N resorption and N isotope fractionation in preand post-abscission leaves in comparison to green leaves.The highest leaf N concentration in summer corresponded to the N-fixing black locust,followed in decreasing order by the deciduous beech and larch and evergreen cedar.On the other hand,the lowest N resorption efficiency corresponded to black locust and the highest to beech,in increasing order by larch and cedar.All tree species returned significant amounts of N before leaf abscission;however,N isotope fractionation during leaf N resorption was only found for beech,with a depleted N isotope value from green to pre-abscission leaf.The most N,however,was resorbed from pre-abscission to post-abscission.This result may indicate that δ15N fractionation took place during N transformation processes,such as protein hydrolysis,when the concentration of free amino acids increased sharply.The difference in the type of amino acid produced by each species could have influenced the N isotope ratio in beech but not in the other tree species.The results of this study showed that it is possible to infer the type and timing of processes relevant to N resorption by analyzing leaf δ15N variation during senescence.