Evidence of Active Biotic Influences in Pedogenetic Processes. Case
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Plant Soil (2006) 289:103–121 DOI 10.1007/s11104-006-9075-6 ORIGINAL PAPER Evidence of active biotic influences in pedogenetic processes. Case studies from semiarid ecosystems of south-west Western Australia W. H. Verboom Æ J. S. Pate Received: 7 February 2006 / Accepted: 22 June 2006 / Published online: 26 October 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Soil profiles and rooting morphologies contemporary lateral facies changes and verti- were examined under an ecotone where open cally-stacked paleosol formations in the study woodland of multi-stemmed, small, lignotuberous region provided corroborating evidence of similar eucalypts (mallee) graded into proteaceous heath. profile attributes, including presence of Fe- or Soils under the mallee showed a Solonetz-type Si-lined root channels, overprinting phenomena seal which separated, hydrologically, the upper and consistency in occurrences of ferricrete and acidic horizon of bleached sand from lower calcrete as expected of each class of vegetation. alkaline horizons rich in calcrete, silcrete, finely Observations were related to the concepts of divided carbonates and clay. Seal composition bioengineering of soil profiles through activity of appeared to vary consistently with overlying macroflora and associated micro-organisms as set species of mallee. The generally acidic lateritic out more generally in our companion review. profiles under heath were rich in pisolithic ferri- cretes and displayed Fe-coated root channels. Keywords Ecotone Æ Lateral facies changes Æ Both sets of taxa exhibited dimorphic rooting Laterite Æ Niche-building Æ Solonetz Æ Woody patterns, with ectomycorrhizal roots and seal-pe- plants netrating, second-order tap roots developed on the extensive lateral roots of mallee versus a dominance of primary tap roots and cluster root Introduction development on laterals of Proteaceae. Over- printing of ferricrete by clays and silicified mate- It is increasingly apparent that organisms of all rial was evident where mallee appeared to have taxonomic groupings engage in niche-building invaded areas of heath. Examination of other activities which not only support their own and each others survival but also determine the trajectory of their evolution and that of the ecosystems in which they occur (see Day et al. W. H. Verboom (&) Department of Agriculture and Food, 10 Doney 2003). In a companion review of the literature Street, Narrogin, WA 6312, Australia (Verboom and Pate, 2006) we support the e-mail: [email protected] hypothesis that higher plants and their associated micro-organisms carry an inherent capacity to J. S. Pate Æ W. H. Verboom School of Plant Biology, The University of Western actively modify the physical and chemical Australia, Crawley, WA 6009, Australia characteristics of their rooting environments. We 123 104 Plant Soil (2006) 289:103–121 use the term ‘bioengineering’ to describe one of the above communities to the other, and such processes and introduce the concept of a relate certain soil profile characteristics to pat- ‘phytotarium’ to connote the major role players terns of root distribution of woody macroflora involved and the collective outcomes of their occupying end members and intervening zones of functioning in an ecosystem. the ecotone. Secondly, we use this background When first pursuing the above line of thinking, and earlier work to describe and interpret we found evidence (Pate et al. 2001; Verboom and selected examples of lateral and vertical facies Galloway 2000) of a role of cluster-root bearing changes in soil profiles encountered in the region. taxa and microbes in the laying down of ferric Some of the facies described embody contempo- layers in oligotrophic soil profiles, and suggested rary differentiation of direct relevance to what that such activities had probably been in place for we describe for our ecotone, others comprise millions of years and been continuously associated paleosols incorporating vertically-stacked chro- with and driven by plant-based acquisition and nosequences representing successive epochs of cycling of limited resources of phosphorous. pedogenetic activity of the same or several dif- Ranging more widely in our companion review ferent types. We discuss the information obtained (Verboom and Pate, 2006), we further fledged this against the broader background of bioengineering hypothesis by using a number of examples of bio- of soil profiles and the ‘phytotarium’ concept. engineering principles at work and their overall effects on contemporary ecosystem functioning. Dominant woody species and associated micro- organisms were then regarded as prime architects Materials and methods for fashioning the autochthonous elements of regoliths, with an end result likely to optimize Location of study areas command and effective utilisation of nutrients and water. Figure 1 provides a map pinpointing the location We recently combined airborne radiometric of the principal study site (E) at which an ecotone and ground-based surveys in a south-west between proteaceous and myrtaceous vegetation Australian setting to uncover complex dynamical was examined. The transect used in this investi- patterns between vegetation types across land- gation is close to Lake King (WGS 84 starting S scapes, with sharp breaks clearly evident between 33°12.12¢ E 119° 48.09¢ and ending S 33°11.81¢ E communities and soils (Verboom and Pate 2003). 119° 47.77¢) and traverses a gently undulating In particular, our examinations of soil landscapes upland within a large (866 ha), near-pristine and vegetation in semiarid environments of the patch of native vegetation typical of the region. mallee and wheatbelt regions demonstrated par- Figure 1 also gives locations of the two lateral ticularly great contrasts in pedogenetic processes facies changes (Belka (B) and Merredin (M)) and between heathlands dominated by Proteaceae a vertical facies change (Kalannie (K)) which are and Casuarinaceae on acidic lateritic sandplains, described in the second part of the paper. Major and myrtaceous woodlands on alkaline texture- bioclimatic and laterite zones are given for the contrast (duplex-type) soils (see vegetation types study region. defined and listed by Beard 1984, 1990). These contrasting ecosystems feature prominently in Examination of the ecotone this paper. The objectives of this paper are twofold. The ecotone lies in a climatic region classified as Firstly, we set out to examine in detail the Extra Dry Mediterranean (see Beard 1984). The edaphic features and associated formative effects mean annual rainfall at the site is 343 mm and the of functioning of myrtaceous versus proteaceous mean diurnal temperature ranges from 3 to16°C communities in a contemporary setting. We con- in mid-winter and 12 to 30°C in mid-summer. Two centrate particularly on lateral changes displayed thirds of the annual rain is typically received across a soil profile where an ecotone grades from between May and September. 123 Plant Soil (2006) 289:103–121 105 Fig. 1 Large-scale map of the south-west region of Western Australia, indicating locations of study sites. Site E locates the ecotone grading between mallee and proteaceous heath near Lake King examined in detail in the first section of text. Sites B and M locate lateral facies changes examined at Belka and Merredin, respectively. Site K locates a profile at Kalannie where several vertical facies changes were investigated. Major bioclimatic and lateritic zones are delineated An area of 30 km2 surrounding the ecotone Upland soil of the study region consists mostly was examined using aerial photography draped of sandy upper layers overlying either ferricretes over a relatively high-resolution digital elevation set in an acid sandy matrix or calcretes set in an model (DEM) of the same area (see Caccetta alkaline loamy matrix. The former is typically et al. 2000 for DEM specifications). This was vegetated by kwongan-type proteaceous open combined with GPS-based ground verification to woodlands or shrub-heath, the latter by mallee distinguish types of native bush, cleared land and and associated myrtaceous taxa (see Beard 1984, other features. 1990). Other intergrading areas of mixed vegeta- A total magnetic intensity map overlain on the tion and soil type were found to be scattered same DEM provided information on relative size throughout the area. Inventories of species were and positioning of playa lakes, associated hydro- made across the area by comparing tentative field aeolian structures and topographical expression identifications of specimens against voucher of certain bed rock structures and outcroppings in specimens lodged in herbaria. Names of species the region. and authorities were as listed by Green (1985), Complementary information on relative abun- with recent species naming within the Melaleuca dances of elements (thorium (Th), uranium (U) uncinata complex made using the descriptions of and potassium (K)) in surface soils was obtained Craven et al. (2004). Classification of soil types by airborne radiometric survey at a flying height and descriptions of their properties were as given of 60 m and a line spacing of 400 m (UTS Geo- by the FAO (1998), McArthur (1991) and Stace physics Pty Ltd. 2005). Data were interpreted, et al. (1968). essentially as recently described for similar land- Water retention characteristics of samples of scapes of the region by Verboom and Pate (2003). pisolithic, blocky and reticulate ferricrete from 123 106 Plant Soil (2006) 289:103–121 the region were determined by courtesy of Broad scale excavations such as conducted Professor Graham Aylemore at the University of above, deployed the nozzle of the air-spade at an Western Australia. Our study also amplified the angle of 45° and held 10 cm or so away from the earlier work of Nulsen et al. (1986), on the fate of soil surface. However, by directing the air stream rainfall under mallee, by examining the water vertically downwards and closer to the substrate it relations across a texture-contrast profile in early proved possible to slowly pierce specific parts of a summer, using measurements of water contents seal in the upper B horizon and determine whe- and corresponding matric potentials.