  A major uncertainty in accretion disk theory is the nature and properties of
gas turbulence, which drives transport in protoplanetary disks. The commonly
used viscous prescription for the Maxwell-Reynolds stress tensor gives rise to
a meridional circulation where flow is outward near the midplane and inward
away from it. This meridional circulation has been proposed as an explanation
for the presence of high-temperature minerals (believed to be of inner solar
system provenance) in comets. However, it has not been observed in simulations
of magnetohydrodynamical (MHD) turbulence so far. In this study, we evaluate
the extent to which the net transport of solids can be diagnostic of the
existence of meridional circulation. To that end, we propose and motivate a
prescription for MHD turbulence which has the same free parameters as the
viscous one. We compare the effects of both prescriptions on the radial
transport of small solid particles and find that their net, vertically
integrated radial flux is actually quite insensitive to the flow structure for
a given vertical average of the turbulence parameter $\alpha$, which we
explain. Given current uncertainties on disk turbulence, one-dimensional models
are thus most appropriate to investigate radial transport of solids. A
corollary is that the presence of high-temperature material in comets cannot be
considered an unequivocal diagnostic of meridional circulation. In fact, we
argue that outward transport in viscous disk models is more properly attributed
to turbulent diffusion rather than to the mean flows of the gas.
