  Observations have revealed that the elemental abundances of carbon and oxygen
in the warm molecular layers of some protoplanetary disks are depleted compared
to those is the interstellar medium by a factor of ~10-100. Meanwhile, little
is known about nitrogen. To investigate the time evolution of nitrogen, carbon,
and oxygen elemental abundances in disks, we develop a one-dimensional model
that incorporates dust settling, turbulent diffusion of dust and ices, as well
as gas-ice chemistry including the chemistry driven by stellar UV/X-rays and
the galactic cosmic rays. We find that gaseous CO in the warm molecular layer
is converted to CO2 ice and locked up near the midplane via the combination of
turbulent mixing (i.e., the vertical cold finger effect) and ice chemistry
driven by stellar UV photons. On the other hand, gaseous N2, the main nitrogen
reservoir in the warm molecular layer, is less processed by ice chemistry, and
exists as it is. Then the nitrogen depletion occurs solely by the vertical cold
finger effect of N2. As the binding energy of N2 is lower than that of CO and
CO2, the degree of nitrogen depletion is smaller than that of carbon and oxygen
depletion, leading to higher elemental abundance of nitrogen than that of
carbon and oxygen. This evolution occurs within 1 Myr and proceeds further,
when the $\alpha$ parameter for the diffusion coefficient is ~0.001.
Consequently, the N2H+/CO column density ratio increases with time. How the
vertical transport affects the midplane ice composition is briefly discussed.
