  We study the production of strange hadrons in nucleus-nucleus collisions from
4 to 160 A GeV within the Parton-Hadron-String Dynamics (PHSD) transport
approach that is extended to incorporate essentials aspects of chiral symmetry
restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top
of the deconfinement phase transition as implemented in PHSD. Especially the
$K^+/\pi^+$ and the $(\Lambda+\Sigma^0)/\pi^-$ ratios in central Au+Au
collisions are found to provide information on the relative importance of both
transitions. The modelling of chiral symmetry restoration is driven by the
pion-nucleon $\Sigma$-term in the computation of the quark scalar condensate
$<q {\bar q}>$ that serves as an order parameter for CSR and also scales
approximately with the effective quark masses $m_s$ and $m_q$. Furthermore, the
nucleon scalar density $\rho_s$, which also enters the computation of $<q {\bar
q}>$, is evaluated within the nonlinear $\sigma-\omega$ model which is
constraint by Dirac-Brueckner calculations and low energy heavy-ion reactions.
The Schwinger mechanism (for string decay) fixes the ratio of strange to light
quark production in the hadronic medium. We find that above $\sim$80 A GeV the
reaction dynamics of heavy nuclei is dominantly driven by partonic
degrees-of-freedom such that traces of the chiral symmetry restoration are hard
to identify. Our studies support the conjecture of 'quarkyonic matter' in
heavy-ion collisions from about 5 to 40 A GeV and provide a microscopic
explanation for the maximum in the $K^+/\pi^+$ ratio at about 30 A GeV which
only shows up if a transition to partonic degrees-of-freedom is incorporated in
the reaction dynamics and is discarded in the traditional hadron-string models.
