  Spectroscopic phase curves of hot Jupiters measure their emission spectra at
multiple orbital phases, thus enabling detailed characterisation of their
atmospheres. Precise constraints on the atmospheric composition of these
exoplanets offer insights into their formation and evolution. We analyse four
phase-resolved emission spectra of the hot Jupiter WASP-43b, generated from a
phase curve observed with the MIRI/LRS onboard the JWST, to retrieve its
atmospheric properties. Using a parametric 2D temperature model and assuming a
chemically homogeneous atmosphere within the observed pressure region, we
simultaneously fit the four spectra to constrain the abundances of atmospheric
constituents, thereby yielding more precise constraints than previous work that
analysed each spectrum independently. Our analysis reveals statistically
significant evidence of NH3 (4$\sigma$) in a hot Jupiter's emission spectra for
the first time, along with evidence of H2O (6.5$\sigma$), CO (3.1$\sigma$), and
a non-detection of CH4. With our abundance constraints, we tentatively estimate
the metallicity of WASP-43b at 0.6-6.5$\times$solar and its C/O ratio at
0.6-0.9. Our findings offer vital insights into the atmospheric conditions and
formation history of WASP-43b by simultaneously constraining the abundances of
carbon, oxygen, and nitrogen-bearing species.
