  Binary neutron star (NS) mergers have been expected to synthesize r-process
elements and emit radioactively powered radiation, called kilonova. Although
r-process nucleosynthesis was confirmed by the observations of
GW170817/AT2017gfo, no trace of individual elements has been identified except
for strontium. In this paper, we perform systematic calculations of line
strength for bound-bound transitions and radiative transfer simulations in NS
merger ejecta toward element identification in kilonova spectra. We find that
Sr II triplet lines appear in the spectrum of a lanthanide-poor model, which is
consistent with the absorption feature observed in GW170817/AT2017gfo. The
synthetic spectrum also shows the strong Ca II triplet lines. This is natural
because Ca and Sr are co-produced in the material with relatively high electron
fraction and their ions have similar atomic structures with only one s-electron
in the outermost shell. The line strength, however, highly depends on the
abundance distribution and temperature in the ejecta. For our lanthanide-rich
model, the spectra show the features of doubly ionized heavy elements, such as
Ce, Tb and Th. Our results suggest that the line forming region of
GW170817/AT2017gfo was lanthanide-poor. We show that the Sr II and Ca II lines
can be used as a probe of physical conditions in NS merger ejecta. Absence of
the Ca II line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <
0.002 in mass fraction, which is consistent with nucleosynthesis for electron
fraction >= 0.40 and entropy per nucleon (in units of Boltzmann constant) >=
25.
