  We analyze a large sample of 885 GCs giants from the APOGEE survey. We used
the Cannon results to separate the red giant branch and the asymptotic giant
branch stars, not only allowing for a refinement of surface gravity from
isochrones, but also providing an independent H-band spectroscopic method to
distinguish stellar evolutionary status in clusters. We then use the BACCHUS
code to derive metallicity, microturbulence, acroturbulence and many
light-element abundances as well as the neutron-capture elements Nd and Ce for
the first time from the APOGEE GCs data. Our independent analysis helped us to
diagnose issues regarding the standard analysis of the APOGEE DR14 for
low-metallicity GC stars. Furthermore, while we confirm most of the known
correlations and anti-correlation trends (Na-O, Mg-Al,C-N), we discover that
some stars within our most metal-poor clusters show an extreme Mg depletion and
some Si enhancement but at the same time show some relative Al depletion,
displaying a turnover in the Mg-Al diagram. These stars suggest that Al has
been partially depleted in their progenitors by very hot proton-capture
nucleosynthetic processes. Furthermore, we attempted to quantitatively
correlate the spread of Al abundances with the global properties of GCs. We
find an anti-correlation of the Al spread against clusters metallicity and
luminosity, but the data do not allow to find clear evidence of a dependence of
N against metallicity in the more metal-poor clusters. Large and homogeneously
analyzed samples from on-going spectroscopic surveys unveil unseen chemical
details for many clusters, including a turnover in the Mg-Al anti-correlation,
thus yielding new constrains for GCs formation evolution models.
