  Event-by-event fluctuations of the net baryon number and electric charge in
nucleus-nucleus collisions are studied in Pb+Pb at SPS energies within the HSD
transport model. We reveal an important role of the fluctuations in the number
of target nucleon participants. They strongly influence all measured
fluctuations even in the samples of events with rather rigid centrality
trigger. This fact can be used to check different scenarios of nucleus-nucleus
collisions by measuring the multiplicity fluctuations as a function of
collision centrality in fixed kinematical regions of the projectile and target
hemispheres. The HSD results for the event-by-event fluctuations of electric
charge in central Pb+Pb collisions at 20, 30, 40, 80 and 158 A GeV are in a
good agreement with the NA49 experimental data and considerably larger than
expected in a quark-gluon plasma. This demonstrate that the distortions of the
initial fluctuations by the hadronization phase and, in particular, by the
final resonance decays dominate the observable fluctuations.
