Application of biosolids in mineral sands mine rehabilitation: use of stockpiled topsoil decreases trace element uptake by plants
Introduction
Mineral sands mining, along with other forms of surface mining, involves considerable soil disturbance and loss of chemical, biological and physical fertility. A key factor influencing this decline in fertility is a decrease in soil organic matter caused by loss or dilution of topsoil, or mineralization of organic carbon from topsoil stockpiles (Van Aarde et al., 1998). In situations where topsoil has been lost or has become degraded, organic amendments such as sewage sludge (which when treated by anaerobic digestion is commonly called “biosolids”) can supply sufficient organic matter and nutrients to initiate successful soil rehabilitation (Sopper, 1992), given that the proximity of a biosolids source to the mine site makes application of such an amendment economically feasible.
Despite the potential benefits of using biosolids to rehabilitate land disturbed by mineral sands mining, there are some risks involved. Municipal sewage sludges generally contain higher concentrations of heavy metals (derived mainly from industrial sources) such as cadmium, copper or lead than are found in soils (Sommers et al., 1979; Chaney, 1980; Alloway, 1995b). It is well-established that heavy metals in sewage sludges can be taken up by plants following land application, resulting in increases in plant tissue concentrations with increasing rates of sludge application (Bingham et al., 1975; Miller et al., 1995). Uptake of heavy metals by plants grown as crops or pastures presents a risk of potentially toxic elements entering the human food chain, with associated adverse effects (Chaney, 1980).
There are several strategies available to limit heavy metal uptake by plants, including liming soil to which biosolids have been applied (Brallier et al., 1996; Krebs et al., 1998). Increasing soil pH by liming has often been found to decrease plant uptake of metals (Lagerwerff et al., 1977; Kuo et al., 1985), but some authors have found increased metal uptake at high rates of sewage sludge application in limed soil relative to unlimed soil (Pepper et al., 1983). Addition of organic amendments has also been shown to limit plant uptake of metal ions. Soil organic matter was considered to be to be more important than other sorptive phases, such as hydrous oxides of Fe and Mn, in limiting Zn and Cd uptake (White and Chaney, 1980). He and Singh (1993) found that cadmium uptake by ryegrass was decreased by increasing additions of organic matter, particularly in sandy soil.
Contamination of soil and plants is not, however, the only issue involved with application of biosolids and associated heavy metals or other trace elements. In some cases the trace element content of biosolids is appropriate to overcome nutritional deficiencies in growing plants.
We expected that, in agreement with previous research, metal concentrations in plant tissues and metal uptake by plants would both increase with increasing rate of biosolids application to soil being rehabilitated following mineral sands mining. A further assumption was that metal uptake by plants following biosolids application would be decreased by liming the soil materials used. To our knowledge, there are no previous investigations of the combined effects, on metal uptake by plants, of topsoil conservation and biosolids application. We expected, however, that where conserved topsoil was present, metal uptake by plants would be lower than where topsoil was absent, due to the high sorptive capacity of soil organic matter for any heavy metals which might be added in biosolids (White and Chaney, 1980; He and Singh, 1993).
The aims of this research were therefore to measure the growth and metal uptake of the pasture species annual ryegrass (Lolium rigidum) and subterranean clover (Trifolium subterraneum) at different rates of biosolids application, in the presence and absence of both lime and conserved topsoil. This was particularly relevant in that the mined lands in this study were being rehabilitated to pasture (the original land use), and therefore there was potential food chain involvement as a result of ingestion of pasture by grazing animals.
Section snippets
Site details
The Yoganup mineral sands mine is situated near Capel, Western Australia. Target minerals are ilmenite, leucoxene, monazite, rutile and zircon contained in wave-sorted deposits covered by various thicknesses of barren overburden. Mining involves stripping and conserving topsoil and any overburden soil in stockpiles, and the separation of ore containing soils into coarse and fine fractions (>70 and <70 μm fractions, termed tailings sand and clay fines, respectively). Rehabilitation initially
Soil properties and metal concentrations
A summary of the chemical and physical properties of the soil materials and biosolids appears in Table 1. Quartz was the predominant mineral in both conserved topsoil and residue material; the residue also contained significant concentrations of kaolinite and haematite. Application of biosolids to soil increased the concentrations of metals (Table 1). Biosolids had a liming effect on soils, with data for soil pH before and after biosolids application also appearing in Table 1.
Plant productivity
Total dry matter
Discussion
Concentrations of metals, including Cd, in these soils following a single application of biosolids at 25 dry t/ha did not generally exceed Australian guidelines for contaminated sites (ANZECC/NHMRC, 1992). Copper was the only exception; its relatively high concentration in biosolids (1350 mg/kg) resulted in the ANZECC soil guideline of 60 mg/kg being exceeded at a biosolids application rate of 25 dry t/ha (Table 1).
Cadmium concentrations in plants are important because of the known toxicity of
Conclusions
It is clear that biosolids application to these systems has increased metal concentrations in soils and plants, and therefore increased the bioavailable pool of metals over the term of the experiment. In the case of Zn, these increases were sufficient to improve Zn nutrition of clover to above deficiency levels. At the recommended application rate of biosolids of 25 dry t/ha, total concentrations of all metals in soils except copper remained below environmental investigation limits (
Acknowledgements
We acknowledge the financial assistance of Iluka Resources Ltd. and The Western Australian Water Corporation, and the constructive criticism of two anonymous referees.
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