  Angular momentum transport in protostellar discs can take place either
radially, through turbulence induced by the magnetorotational instability
(MRI), or vertically, through the torque exerted by a large-scale magnetic
field that threads the disc. Using semi-analytic and numerical results, we
construct a model of steady-state discs that includes vertical transport by a
centrifugally driven wind as well as MRI-induced turbulence. We present
approximate criteria for the occurrence of either one of these mechanisms in an
ambipolar diffusion-dominated disc. We derive ``strong field'' solutions in
which the angular momentum transport is purely vertical and ``weak field''
solutions that are the stratified-disc analogues of the previously studied MRI
channel modes; the latter are transformed into accretion solutions with
predominantly radial angular-momentum transport when we implement a
turbulent-stress prescription based on published results of numerical
simulations. We also analyze ``intermediate field strength'' solutions in which
both modes of transport operate at the same radial location; we conclude,
however, that significant spatial overlap of these two mechanisms is unlikely
to occur in practice. To further advance this study, we have developed a
general scheme that incorporates also the Hall and Ohm conductivity regimes in
discs with a realistic ionization structure.
