Acidification and weathering associated with deep tongues in sandy Spodosols, Michigan, USA

We studied the effects of preferential flow on weathering of primary minerals in well-drained, sandy Spodosols (Podzols) in northern Michigan, USA. Preferential flow in these soils is manifested as deep eluvial/illuvial tongues. We hypothesized that sands in the best developed (strong) tongues would be more weathered than in tongues that are short and/or thin (weak tongues), and especially so when compared to pedons that lack tongues (off-tongues). Across these three types of morphologies, we sampled at a variety of depths and horizons to establish correspondence between tongue strength and primary mineral weathering, using (Ba+Rb)/Sr data from a portable xrf instrument as a weathering surrogate. Soil acidity and primary mineral weathering vary predictably: strong tongues > weak tongues > off-tongues. This trend continues well below the profile, as BC and C horizons below strong tongues are also weathered, even though morphological indicators of pedogenesis and weathering here are not present. E horizons, which experience the most cumulative percolation of soil water, are strongly weathered, regardless whether they are located above a tongue, or not. Thus, we conclude that the cumulative percolation of acidic water, rich in organo-metallic compounds, drives weathering in these soils, even at depths to which the organics and illuvial materials have not yet reached. That is, the effects of podzolization and weathering extend well below what is morphologically considered the profile. Our data indicate that (a) coatings of organo-metallic substances in B horizons do not “protect” the underlying grains from dissolution and weathering-related reactions, and (b) pedogenesis and weathering can extend far below the profile, beneath areas of preferential flow.