Evidence of active biotic influences in pedogenetic processes. Case studies from semiarid ecosystems of south-west Western Australia

Soil profiles and rooting morphologies were examined under an ecotone where open woodland of multi-stemmed, small, lignotuberous eucalypts (mallee) graded into proteaceous heath. Soils under the mallee showed a Solonetz-type seal which separated, hydrologically, the upper acidic horizon of bleached sand from lower alkaline horizons rich in calcrete, silcrete, finely divided carbonates and clay. Seal composition appeared to vary consistently with overlying species of mallee. The generally acidic lateritic profiles under heath were rich in pisolithic ferricretes and displayed Fe-coated root channels. Both sets of taxa exhibited dimorphic rooting patterns, with ectomycorrhizal roots and seal-penetrating, second-order tap roots developed on the extensive lateral roots of mallee versus a dominance of primary tap roots and cluster root development on laterals of Proteaceae. Overprinting of ferricrete by clays and silicified material was evident where mallee appeared to have invaded areas of heath. Examination of other contemporary lateral facies changes and vertically-stacked paleosol formations in the study region provided corroborating evidence of similar profile attributes, including presence of Fe- or Si-lined root channels, overprinting phenomena and consistency in occurrences of ferricrete and calcrete as expected of each class of vegetation. Observations were related to the concepts of bioengineering of soil profiles through activity of macroflora and associated micro-organisms as set out more generally in our companion review.