Effects of deep tongues on acidification and weathering in sandy Spodosols, Michigan, USA

Abstract 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 with minimal tonguing (off-tongues). Across these three types of tongue morphologies, we sampled at a variety of depths and horizons to establish correspondence between tongue morphology and primary mineral weathering, using (Ba+Rb)/Sr data 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 strongly weathered. E horizons, which experience the most cumulative percolation of soil water, are also strongly weathered, regardless of location 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 have not yet reached. That is, the effects of podzolization and weathering extend well below what is morphologically considered the profile. Our data therefore indicate that coatings of organo-metallic substances in B horizons do not “protect” the underlying grains from dissolution and weathering-related reactions.