  Because of their low gravitational energies dwarf galaxies are greatly
exposed to energetical influences by the interstellar medium, like e.g. stellar
radiation, winds or explosions, or by their environment. While the metallicity
depletion in dwarf galaxies can be explained in general by supernova-driven
galactic winds, the reason for their low N/O ratios at low O abundance is not
yet completely understood. Stellar yields enrich the different gas phases with
elements that are characteristic for their stellar progenitors. Gas-phase
transitions are necessary for a mixing of elements, but depend sensitively on
the thermal and dynamical state of the interstellar medium. Models of chemical
evolution start usually with a high N/O ratio at low O abundance according to a
metal enrichment by ancient stellar populations with traditional yields, but
can only reproduce the N/O-O peculiarity by the stepwise element release and
mostly by the application of multiple starbursts in order to account also for a
selective element reduction by galactic winds. Chemodynamical models of dwarf
galaxies, however, demonstrate that strong evaporation of clouds by the hot
supernova gas leads to an almost perfect mixing of the interstellar gas and a
large-scale homogenization of abundance ratios. In addition, even with
star-formation self-regulation these models can successfully account for the
observed N/O-O values in a self-consistent way. Although new stellar yields
have been taken into account which provide mainly secondary N production from
massive stars, a significant discrepancy remains between the chemodynamical
dwarf galaxy models and closed-box models that take the chemodynamical
treatment into account. In this paper we therefore discuss to what amount gas
infall is responsible to affect the N/O ratio.
