  We discuss that hadron-induced atmospheric air showers from ultra-high energy
cosmic rays are sensitive to QCD interactions at very small momentum fractions
x where nonlinear effects should become important. The leading partons from the
projectile acquire large random transverse momenta as they pass through the
strong field of the target nucleus, which breaks up their coherence. This leads
to a steeper x_F-distribution of leading hadrons as compared to low energy
collisions, which in turn reduces the position of the shower maximum Xmax. We
argue that high-energy hadronic interaction models should account for this
effect, caused by the approach to the black-body limit, which may shift fits of
the composition of the cosmic ray spectrum near the GZK cutoff towards lighter
elements. We further show that present data on Xmax(E) exclude that the rapid ~
1/x^0.3 growth of the saturation boundary (which is compatible with RHIC and
HERA data) persists up to GZK cutoff energies. Measurements of pA collisions at
LHC could further test the small-x regime and advance our understanding of high
density QCD significantly.
