  A small fraction of quasars show an unusually high nitrogen-to-carbon ratio
(N/C) in their spectra. These "nitrogen-rich" (N-rich) quasars are a
long-standing puzzle because their interstellar medium implies stellar
populations with abnormally high metallicities. It has recently been proposed
that N-rich quasars may result from tidal disruption events (TDEs) of stars by
supermassive black holes. The rapid enhancement of nitrogen and the depletion
of carbon due to the carbon--nitrogen--oxygen cycle in supersolar mass stars
could naturally produce high N/C. However, the TDE hypothesis predicts that the
N/C should change with time, which has never hitherto been observed. Here we
report the discovery of the first N-rich quasar with rapid N/C variability that
could be caused by a TDE. Two spectra separated by 1.7 years (rest-frame) show
that the N III]\lambda 1750/C III]\lambda 1909 intensity ratio decayed by
~86%+/-14% (1 \sigma). Optical (rest-frame UV) light-curve and X-ray
observations are qualitatively consistent with the TDE hypothesis; though, the
time baseline falls short of a definitive proof. Putting the single-object
discovery into context, statistical analyses of the ~80 known N-rich quasars
with high-quality archival spectra show evidence (at a 5\sigma\ significance
level) of a decrease in N/C on timescales of >1 year (rest-frame) and a
constant level of ionization (indicated by the C III]\lambda 1909/C IV\lambda
1549 intensity ratio). If confirmed, our results demonstrate the method of
identifying TDE candidates in quasars via abundance ratio variability, opening
a new window of TDE observations at high redshift (z>2) with upcoming
large-scale time-domain spectroscopic surveys.
