Great question Matthew let me take a stab at some answers.
The 0 horizon in all forests is a detritus layer comprised of all the tree litter, in conifer forests this is mostly needles (leaves) and pine, cedar, juniper needles all leach acids from their surface into the soil, thus acidifying the soil, this builds up over time.
This detritus layer also causes a lack of minerals to be available to the plants since most of the fungi present are in this 0 horizon layer and the acidity tends to kill off most of the bacteria needed for mineral breakdown in the underlying actual soil layer which is normally quite thin.
generally the acids found (in highest to lowest concentration order) are Oxalic, formic, malic and acetic I know of no study that actually quantified the concentrations for any one specific area, but that doesn't mean the figures aren't out there, just that I have never looked hard for them.
A good place to start would be the nearest university's biology department.
In conifer forest it is not minerals leaching out of the soil that creates the lack of minerals, it is the depth of the detritus layer and the composition of this layer.
This layer is where the acids come from that penetrate the actual soil and make this soil acidic, these acids mostly come from needle exteriors, bark exteriors on fallen branches and twigs, rain water that can be acidic from picking up industrial wastes that are airborne are also part of the mix.
Plants rarely pick up in needed or wanted minerals when the microbiome is fully complete, this isn't the case in conifer forests where much of the bacteria is absent, in this type of situation the
roots will pick up aluminum salts and then break these down in situ either in the cambium layer or in the leaves.
Conifer trees that are fortunate
enough to have a good set of mycorrhizae populating their roots have less of a problem with keeping out unwanted/needed minerals such as aluminum, copper, zinc, and other "detrimental metals".
Acidification of soils in conifer forests is an ongoing process where the levels of acids rises then as these acids begin to leach deeper into the horizon layers an equilibrium will begin to establish, at this point the acidification remains more or less stable.
Redhawk