  We discuss particle production in the high-energy, small-x limit of QCD where
the gluon density of hadrons is expected to become nonperturbatively large.
Strong modifications of the phase-space distribution of produced particles as
compared to leading-twist models are predicted which reflect in the properties
of cosmic ray induced air showers in the atmosphere. Assuming hadronic
primaries, our results suggest a light composition near GZK cutoff energies. We
also show that cosmic ray data discriminate among various QCD evolution
scenarios for the rate of increase of the gluon density at small x, such as
fixed-coupling and running-coupling BFKL evolution. There are clear indications
for a slower growth of the gluon density as compared to RHIC and HERA, due e.g.
to running-coupling effects.
