16.5.3  Serpentine soils and mine tailings

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Certain rocks, low in silica but rich in ferromagnesium minerals, give rise to soils of typically high pH known generically as ultramafic or, less accurately, serpentine soils (the mineral is mottled green and resembles a snake’s skin). Such soils are rich in Cr, Co, Ni, Fe and Mg, but severely deficient in N, P, K, Ca and possess a high Mg:Ca ratio.

These chemical features select for a highly characteristic flora which often shows stunted growth with chlorotic foliage, and morphologies typical of desert perennials. Such species usually result in clearly defined boundaries between serpentine and adjacent vegetation.

Even more remarkable, serpentine species frequently accumulate prodigious quantities of Cr, Co and Ni, and can even serve as indicator plants for underlying lithology. Serpentine deposits plus associated plant species occur in Tasmania and Queensland, but notable in eastern Australia are the Coolac–Kiandra deposits near Tumut in New South Wales that are dominated by Casuarina stricta, Ricinocarpos bowmanii and Xanthorrhoea australis in place of the Eucalyptus melliodora woodland on adjacent granitic soils. Lyons et al. (1974) provide further details on that deposit, and explain that ‘... X. australis strongly favours soils high in magnesium and low in copper, whereas the distribution of C. stricta appeared to be controlled mainly by the high potassium and nickel values in the soils...’


Table 16.11


Table 16.12

Within Australia the most extensive deposits are adjacent to the eastern goldfields of Western Australia. Vegetation surveys in Western Australia also show plant associations that reflect lithology. For example, a belt of Eucalyptus aff. cylindrocarpus woodland coincided with an Ni anomaly, while soils high in both Ni and Cr supported Melaleuca laterifolia. Certain plant species thus serve as indicators of soil metal content, and Ni deposits lead to sharp discontinuities in species distribution (see Table 16.11 for more details on soil–plant correlations).

Ernst (1974), cited in Brooks (1987), defined the Hybanthion florbundii Alliance to cover the serpentine vegetation of Western Australia. In such communities, the Alliance commonly replaces eucalypt woodland more common from Kalgoorlie–Kambalda to Leonora, and is characterised by the Ni-hyperaccumulator Hybanthus floribundus and its associated species Grevillea acuaria. H. floribundus was only the third such plant to be documented worldwide, and this species is a reliable indicator of nickeliferous soils for two reasons. First, H. floribundus is restricted to ultramafic outcrops and their drainage areas. Moreover, within these shrub communities, a significant positive correlation exists between stand density and soil Ni content, especially on soils where Ni content exceeds 880 ppm. Second, all subspecies and forms of H. floribundus hyperaccumulate Ni, and in one case Co, even when soil levels are low (Table 16.12).

Despite such unusual nutritional physiology where Ni can accumulate to over 5000 ppm dry mass, no selective advantage for plant growth and reproduction from Ni accumulation is obvious, although herbivores might be discouraged by high levels of tissue Ni (see Batianoff et al. 1990 for more details). Mechanisms that permit plant tolerance to such hyperaccumulation have yet to be defined.

Serpentine soils occur naturally, but other metal-containing soils have been formed as a consequence of mining and smelting of ores (e.g. Cu, Pb and Ni), or following coal mining where a highly acidic overburden (pH <4.5) predisposes to Al and Mn toxicity (Section 16.5.2). Plant growth will then be limited by both heavy-metal toxicity and effects of Al or Mn, as well as by deficiency of essential nutrients such as N, P, K, Cu and Mg. Such deficiencies can of course be alleviated by fertiliser applications, which also carry a potential side benefit of heavy-metal attenuation via formation of complexes (e.g. Al–PO4).

Prospects for successful mine site rehabilitation are thus improved on both counts, but increasingly successful colonisers have been shown to have mycorrhizal associations (Khan 1978; Gardner and Malajczuk 1988). Mycorrhizal fungi are widely recognised as enhancing host plant uptake of sparse nutrients (Section 16.4.1), but they can also detoxify heavy metals, and strong selection for tolerance to Ni, Al, Zn and Cu has been reported from Europe and North America (see Jones and Hutchinson 1988). Similarly, selection of metal-tolerant isolates (ecotypes) in the ectomycorrhizal fungus Pisolithus tinctorius has been noted for Al-containing soils in Western Australia, where Al tolerance of isolates was positively correlated with Al concentration in source soil (Egerton-Warburton et al. 1993).

Root anatomy of Eucalyptus rudis seedlings following inoculation with P. tinctorius reveals a well-developed mycorrhizal sheath, a larger root diameter and intact cells compared to smaller root dimensions and necrotic tissue in non-mycorrhizal roots.



Table 16.13


Figure 16.18 Mycorrhizal (M) associations with roots of Eucalyptus rudis enable seedlings to exclude Al more effectively. Tissue Al was significantly lower in all components measured with energy dispersive X-ray analysis (EDS). Levels of elements in those tissues are shown here as peak:background (p:b) ratios. 'Sheath' refers to fungal material external to roots. (Based on Egerton-Warburton et al. 1993; reproduced with permission of Kluwer Academic Publishers)


Figure 16.19 Magnesium (Mg) levels in tissues of Eucalyptus rudis seedlings were significantly increased due to mycorrhizal associations on roots. Leaf veins were not affected, but leaf mesophyll, stele, endodermis, inner root cortex and outer root cortex were all higher for inoculated compared with non-inoculated plants. 'Sheath' refers to fungal material external to roots. Mg levels were determined with energy dispersive X-ray analysis (EDS) and are shown here as peak:background (p:b) ratios. (Based on Egerton-Warburton et al. 1993; reproduced with permission of Kluwer Academic Publishers)

Benefits of mycorrhizal associations in metal-containing soils include improved survival of seedlings, greater growth and higher nutrient content (Table 16.13). One notable difference between inoculated and non-inoculated eucalypt seedlings concerns internal distribution of Al rather than total content per plant. The mycorrhizal sheath of inoculated plants accumulates Al, resulting in lower concentrations in roots and leaves. By contrast, non-mycorrhizal plants show strong accumulation in all tissues (Figure 16.18).

In addition, the higher P content of mycorrhizal seedlings (Table 16.13) also comes from accumulation and storage as polyphosphate granules by the mycorrhizal sheath (see Ashford et al. (1986) and Orlovich et al. (1989) for more information on these structures).

Polyphosphate granules form an effective sink for Al in E. rudis–P. tinctorius associations, and conceivably formation of Al–PO4 complexes in these granules represents a beneficial detoxification site for excess Al in these low pH soils.

K content in mycorrhizal plants was also much higher (Table 16.13), and was associated with a minor reservoir of K in root sheaths, and a major localisation in leaf mesophyll. Xylem-mobile elements such as K would be expected to accumulate at vein endings in this way (Section 5.2).

Mycorrhizal associations have greatest impact on plant Ca and Mg content and distribution (Table 16.13, Mg in Figure 16.19), and such significant increases might affect nutritional physiology in several ways. They could (1) facilitate uptake of Ca and Mg from acidic soils frequently impoverished with respect to Ca and Mg; (2) offset adverse effects of Al on uptake of Ca and Mg by roots; (3) ameliorate Al toxicity through enhanced Ca uptake.

In effect, efficient acquisition and distribution of Ca might be a prerequisite for Al tolerance. Consequently, any selection pressure experienced by mycorrhizal fungi in high-Al soils which has resulted in improved tolerance to heavy metals probably operates via the capacity of mycorrhizal fungi to enhance Ca and Mg uptake into their host plants.