High soil acidity under native shrub encroachment in the Cobar Pediplain, south-eastern Australia

This study investigated the chemical characteristics of shallow (0–30 cm) soil profiles under shrubs in areas of dense encroachment and compared them with shallow soil profiles under nearby large trees. Consistent patterns of high soil acidity were found under shrubs, as well as lower litter alkalinity, lower relative concentrations of calcium (Ca²⁺), lower effective cation exchange capacity, and higher aluminium (Al³⁺) and sodium (Na⁺) in the soil profile compared with under trees. Soil pH (CaCl₂) was strongly correlated with the Ca content of surface litter. These findings suggest that shrubs (which at most sites included the shrub form of tree species) cycle alkalinity differently from large and mature trees, resulting in high acidity in the shallow soil profile acidity, and possible loss of alkalinity via surface movement of material from areas of dense encroachment.