  We investigate the environmental effect on the metal enrichment of
star-forming galaxies (SFGs) in the farthest spectroscopically confirmed and
X-ray detected cluster, CL~J1449+0856 at $z=1.99$. We combined HST/WFC3~G141
slitless spectroscopic data, our 13-band photometry, and a recent Subaru/MOIRCS
near infrared spectroscopic follow-up to constrain the physical properties of
SFGs in CL~J1449+0856 and in a mass-matched field sample. After a conservative
active galactic nuclei (AGN) removal, stacking individual MOIRCS spectra of $6$
($31$) sources in the cluster (field) in the mass range $10 \leq
\mathrm{log(M/M_{\odot}}) \leq 11$, we find a $\sim4\sigma$ significant lower
[N II]/H$\alpha$ ratio in the cluster than in the field. Stacking a subsample
of $16$ field galaxies with H$\beta$ and [O III] in the observed range, we
measure a [O III]/H$\beta$ ratio fully compatible with the cluster value.
Converting these ratios into metallicities, we find that the cluster SFGs are
up to $0.25\,\mathrm{dex}$ poorer in metals than their field counterparts,
depending on adopted calibrations. The low metallicity in cluster sources is
confirmed using alternative indicators. Furthermore, we observe a significantly
higher H$\alpha$ luminosity and equivalent width in the average cluster
spectrum than in the field. This is likely due to enhanced specific star
formation rate, even if lower dust reddening and/or an uncertain environmental
dependence of the continuum-to-nebular emission differential reddening may play
a role. Our findings might be explained by the accretion of pristine gas around
galaxies at $z=2$ and from cluster-scale reservoirs, possibly connected with a
phase of rapid halo mass assembly at $z>2$ and of high galaxy merging rate.
