  We experimentally and numerically study the temporal dynamics of light
scattered by large clouds of cold atoms after the exciting laser is switched
off in the low intensity (linear optics) regime. Radiation trapping due to
multiple scattering as well as subradiance lead to decay much slower than the
single atom fluorescence decay. These two effects have already been observed
separately, but the interplay between them remained to be understood. Here, we
show that with well chosen parameters of the driving field, the two effects can
occur at the same time, but follow different scaling behaviors. The subradiant
decay is observed at late time and its rate is independent of the detuning,
while the radiation trapping decay is observed at intermediate time and depends
on the detuning through the optical depth of the sample. Numerical simulations
based on random walk process and coupled-dipole equations support our
interpretations. Our study clarifies the different interpretations and physical
mechanisms at the origin of slow temporal dynamics of light in cold atoms.
