Bioengineering of soil profiles in semiarid ecosystems: the 'phytotarium' concept. A review

This review draws attention to information from the literature and our own observations supporting the view that higher plants and micro-organisms display an intrinsic capacity to be proactively involved in pedogenetic processes. 'Bioengineering' of this kind is deemed to be spearheaded by principal deep-rooted tree and shrub species and to result in optimisation of command and conservation of water and nutrients within an ecosystem. Specific examples discussed in the paper include, the formation of silicon- or iron-based linings of vertical channels and pores, binding of sand on roots, generation of organically derived hydrophobicity, development of clay-based hardpans and texture-contrast seals, precipitation of silcrete, calcrete and ferricrete pavements, effective accessing and conservation of P resources, including mining by microbes and the biological cycling of Si and Al via plants and micro-organisms. In each case, definitive roles and mechanisms are suggested for the organisms involved, particularly in relation to formative effects relating to secretion of organic acids, dispersing agents and other classes of exudate. We introduce the term 'phytotarium' to connote the collective outcomes of the above biotic influences in construction and maintenance of niches peculiar to specific vegetation types and then review the evidence of imprinting of soil profiles due to operation of phytotaria. Examples given relate to the lateral and vertical facies encountered in certain contemporary soil profiles and paleosols with which we are familiar and are described in a companion paper.