Co-occurrence of Proteaceae, laterite and related oligotrophic soils: coincidental associations or causative inter-relationships?

This communication presents the hypothesis that certain Australian lateriti c and related oligotrophic soils may have been partly derived biotically fr om soluble iron-rich complexes generated following secretion of low-molecul ar weight organic acids by phosphate-absorbing specialised proteoid (cluste r) roots of proteaceous plants. Subsequent precipitation of the iron is the n pictured as occurring onto the oxide rinds of developing laterite after c onsumption of the organic components of the complexes by soil bacteria. The hypothesis is first examined in relation to current theories of origins of laterites and the extent of the coincidences worldwide in past and present times between Proteaceae and oligotrophic soil types of lateritic characte r. The paper then provides more definitive lines of evidence supporting the hypothesis, based largely on recent studies by the authors in south-wester n Western Australia. This relates to (a) cases of definitive association in habitats rich in Proteaceae between zones of root proliferation and ferric rete layers in lateritic soils, (b) proximity in soil profiles between ferr ic deposits and current and ancestral root channels, (c) the recovery of ci trate-consuming bacteria from soil profiles and specifically from ferricret e rinds and horizons accumulating sesquioxide organic matter and (d) distri bution of iron and phosphorus within plant and soil profile components cons istent with ferricrete rinds being generated by rhizosphere-mediated intera ctions of plants and microbes under conditions of severely limited availabi lity of phosphorus. The mode of functioning of proteoid root clusters is th en discussed, especially in relation to exudation of organic acid anions, u ptake of phosphorus and the subsequent fate of organic anions and their met al ion complexes in the system. An empirically based scheme is presented in dicating flow profiles for phosphorus and iron between soil, ferricrete rin ds and bacterial and plant components. We then discuss possible carbon cost s to proteaceous plant partners when accessing phosphorus under the nutrien t-impoverished conditions typical of heathlands and open woodlands of Medit erranean-type ecosystems of Western Australia. The paper concludes with a c ritical overview of the hypothesis, particularly its implications regarding possible higher plant : microbial influences shaping soil and landscape ev olution in the regions involved.