  We elaborate on the radiative behaviour of tree-level scattering amplitudes
in the soft regime. We show that the sub-leading soft term in single-gluon
emission of quark-gluon amplitudes in QCD is controlled by differential
operators, whose universal form can be derived from both on-shell recursion
relation and gauge invariance, as it was shown to hold for graviton- and
gluon-scattering.
