ABSTRACT
In recent years studies of organic impacts on weathering have polarised into two distinct streams, i.e. studies on bare rock surfaces and studies of weather ing within a soil-covered landscape. On bare rock surfaces, there have been many studies of the role of lichens (Jackson and Keller, 1970; Wessels and Schoeman, 1988; Viles and Pentecost, 1994), cyanobacteria (Kauffmann, 1960; Danin, 1983; Viles, 1987), algae (Schaffer, 1932; Wakefield et al., 1996) and fungi (Gorbushina et al., 1993) in weathering. Although many of these studies have been based primarily on microscope observations of field material, some experimental studies have also been carried out (Fry, 1927; Moses and Smith, 1993). There have been several attempts to quantify the rates of weathering (McCarroll and Viles, 1995) and to ascribe the development of pm to cm size
Land Reconstruction and Management Vol. 3, 2004, pp 61-72 ISBN 1-57808-295-1 Science Publishers, Inc., Enfield, USA
topographic features to particular organisms (Jones, 1989; Fiol et al., 1996). Root effects on bare rock surfaces have not been studied recently in any depth, although often large root holes can be seen in many limestone surfaces exhumed from under a soil layer. Studies of the role of organisms in weathering of soil minerals have developed in rather different directions, often attempting to examine mineral nutrition in plants within a context of biogeochemical cycling in ecosystems. When experiments have been carried out, they often utilised root exudates (such as low-molecular weight organic acids) rather than actual roots, as exemplified by the studies of Boyle et al. (1974), Ochs (1996) and Manley and Evans (1986). The features produced on soil mineral grains by biological weathering processes have been less commented on than the overall geochemical effect in these studies, although Jongmans et al. (1997) present some images of the micro-scale weathering features produced by fungi on mineral grains.
