Root mats

Active zones of Fe oxide reduction in soils can be easily recognized as bleached areas showing the grey colour of the matrix minerals after removal of the staining Fe oxides. Such zones can only form where a microbially metabolizable biomass is available, for example in the lower top soil or along roots. In poorly aerated soils with large structural units (e.g. prisms), root mats often develop only at the surface of these units and bleach their surfaces, whereas the interior is still coloured... 

Plate 16.1 e) Root channel in a gley soil stained by Fe oxide, f) Bleaching of the surface layer of a red soil aggregate by microbial reduction of the hematite. See root mat at the aggregate s surface supplying the biomass. 

Salcedo et al. (1991), working in an Atlantic coastal forest in Recife, Brazil, showed that phosphoms from the litter/fermentation layer is cycled back to the vegetation via mycor-rhizae-mediated mechanisms. However, 6l% of the added moved down to the mineral soil, where P in the soil solution is controlled by microbial biomass activity. In contrast. Stark and Jordan (1978), working on a P deficient upland terra firme forest in San Carlos de Rio Negro, Venezuela (Cuevas and Medina 1988), found that nearly 100% of the added 32p was retained in the root mat associated with the litter layer, with less than 0.1% moving down to the surface of the mineral soil. 

Platy Consists of centimetre-thick plates, up to tens of centimetres in diameter, commonly found above hardpan or chalky layers. Plates may be tabular or wavy in form and may exhibit crude lamination. Some are fragmented calcified root mats. Distinguished from brecciated laminar calcrete by a lack of strong laminations... 

Stringer domal in form. This type of calcrete is strongly polygenetic in nature Closely related to laminar calcretes but not all forms are well laminated. These are sheets of carbonate, usually only a few centimetres thick, of subvertical to sub-horizontal form, that penetrate into carbonate-rich hosts and are related to root mats. They can occur singly or at multiple levels and varied orientations, extending for metres, and can cross-cut each other... 

Figure 2.5 (A) Laminar calcrete (arrowed) overlain by an oolitic—pisolitic layer associated with a calcified root-mat layer, Holocene soil. La Mora area, Tarragona, northeast Spain (see Calvet and Julia, 1983). (B) Stage V-VI profile with hardpan layer overlain by pisolitic and brecciated level with a prominent calcified root-mat layer (arrowed), from San Miguel salinas area, Torrevieja, Alicante, southeast Spain. Such a profile would correspond to a thickened, polyphase profile as shown on the right of Figure 2.8.

Wright et al. (1995), based on studies of rhizogenic palaeosol calcrete profiles from southern Europe, proposed the rhizogenic model (Figure 2.7C). In Mesozoic palaeosols from northern Spain, they identified progressive profile development by calcified root mats from a few isolated laminar levels to thick (>2m) dense carbonate sheets. In early Cenozoic successions in southern France they noted stages of development from scattered Microcodium remains to entire sheets composed almost wholly of this component. 

Wright, V.P., Platt, N.H. Wimbledon, W.A. (1988) Biogenic laminar calcretes evidence of root mats in paleosols. Sedimentology 35, 603-620. 

Generally speaking, most tine roots in tropical forest soils are found in the upper 0.5 m (Kerfoot, 1963), with a marked concentration of roots into a root mat close to the soil surfiice and within the litter layer being especially common on low-fertility soils (Stark and Jordan, 1978 Medina and Cuevas, 1989). It is generally considered that these root mats serve to ensure the maximum retention of nutrients by the vegetation and to minimize any leaching losses. Surveys of tropical forests have indicated almost ubiquitous mycorrhizal associations for such roots (Alexander, 1989 Janos, 1989). 

---