  The inclusive spectra of preequilibrium nucleons produced in low-energy
antiproton-nucleus collisions are thoroughly investigated within the framework
of the Lanzhou quantum molecular dynamics model for the first time. All
possible reaction channels such as elastic scattering, annihilation, charge
exchange and inelastic scattering in antibaryon-baryon, baryon-baryon and
meson-baryon collisions have been included in the transport model. The
unexpected neutron/proton ratio in comparison to the pion and proton induced
reactions is caused from the isospin effects of pion-nucleon collisions and the
density dependence of symmetry energy. It is found that the $\pi^{-}$-neutron
collisions enhance the neutron emission in the antiproton annihilation in a
nucleus. Different to heavy-ion collisions, the isospin effects are pronounced
at the low kinetic energies. A soft symmetry energy with the stiffness of
$\gamma_{s}=$0.5 at subsaturation densities is constrained from the available
data of the neutron/proton spectra.
