Abstract
A significant human-driven increment of the available reactive nitrogen (Nr) forms has
occurred during the past century at the global scale, which in turn has increased the amount of Nr deposition.
Grafted apple trees (Gala / M.9 strain T337) were used in a pot experiment conducted in semicontrolled
conditions, where the 15N-labelling technique allowed to trace the fate of N from ammonium
nitrate (15NH4
15NO3, isotopic enrichment: 10.3 atoms %) distributed at three increasing rates (N1, N2,
N4, where N2 is the double of N1 and N4 is the double of N2) either to soil or to canopy (foliar application)
to simulate atmospheric N depositions. At the end of the experiment, plants were destructively
sampled, and N derived from depositions (Ndfd), total N, and biomass of above and belowground organs
were determined. Uptake rates ranged from 21% to 57% and the Ndfd recovery was higher for soil than
for foliar application. Foliar-supplied plants showed a higher Ndfd in leaves and shoots than soil-supplied
ones, while the latter showed a higher Ndfd in roots than the former. Moreover, total N in trunk, shoot
axes and leaves increased with the N rates up to the level N2, with no further increase in N4. Increasing
tree N availability, regardless the supply mode, increased the shoot:root N content. The fact that the N
uptake rate was rather stable at increasing N rates suggests that if N from atmospheric depositions
becomes increasingly available at the canopy or soil level, it will actively contribute to apple tree nutrition
and account for a significant fraction of the apple tree N needs.