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Topsoil Substitutes Experiments

Funding
ACARP (Australian Coal Association Research Program) Project C12033, Thiess Pty Ltd (who also funded the pasture capping extension-experiment mentioned), Xstrata Coal Mount Owen & Coal and Allied.

Background
In previous studies, forest topsoil was identified as the best material for reconstructing a native forest or woodland ecosystems on mine spoil. However, many mines do not have access to forest topsoil, or it is in insufficient supply to cover the spoil areas which need to be rehabilitated. Alternative rehabilitation strategies therefore need to be developed to utilize other readily available bulk materials.

Aim
To find a growth medium, and possible secondary treatments, which can be used as a substitute for forest topsoil, in reconstructing a native forest ecosystem with direct seeding.

Summary of Results
Pasture topsoil, pasture subsoil, woodland topsoil, chitter (coarse coal washery reject), biosolids (sewage sludge) and some combinations of these were trialled as possible topsoil substitutes in both pot trials at the University of Newcastle Plant Growth Facility and in field trials at Mount Owen (Xstrata Coal) and Warkworth Mines (Hunter Valley, NSW, Australia).

Consistently, in both pot and field trials, clayey pasture subsoil yielded good results (below). This is likely to be, in part, due to its ability to trap moisture in the spoil placement area below it and make it available to the plant root system. It is also likely to be assisted by the nutrient content evident in plant growth and a residual capacity to nodulate leguminous plants that form the basis of early colonisation of these areas.

A trial of woodland topsoil showed it to be no more successful than bare spoil in terms of native plant density and species richness, because of high grass and weed competition during the first year.

The addition of biosolids produced the best growth outcomes in shadehouse pot trial conditions by providing both organic matter and nutrients in a range of forms. In the field, however, biosolids produced considerable grass cover that out-competed the native plant species.

Similarly in the pot trial, where major weeds were removed, pasture topsoil produced some of the best outcomes. However, almost no native species survived and grew in pasture topsoil in the field due to excessive grass and weed competition. A trial was set up to test whether pasture topsoil could be capped with a weed-free material to establish native plant species, yet retain the beneficial properties of the topsoil. Shallow ripped chitter or pasture subsoil caps, produced the highest native plant establishment and growth, while using spoil as a capping material was less successful.

Secondary treatments such as fertilizer and worm casts showed the potential to increase plant growth in the secondary treatment pot trial. However no response was observed in the field. Inoculation of pots with rhizobia bacteria significantly increased nodule dry weight in subsoil and biosolids pots, but not in spoil pots which seemed to be too inhospitable to support root nodules, or in pasture topsoil pots which already had good background nodulation capacity. Inoculation in the field (below) produced species-specific responses, with some increases in plant growth.

Conclusions
Wherever available and possible, direct-transfer forest topsoil should be used for rehabilitation, as it produces the highest native plant density and species richness. The best alternative to forest topsoil was found to be clayey subsoil. Subsoil and plastic pre-strip should therefore be stockpiled separately from spoil for future use. Other topsoil such as woodland topsoil needs to be assessed prior to large-scale use. A trial should be set up to test whether capping biosolids with a weed and grass free medium will overcome the negative competitive effects found in biosolids. Pasture topsoil with large weed and grass seed banks should not be used for native forest reconstruction, by direct seeding, without amelioration to combat the weeds and grasses. Capping pasture topsoil with a weed-free material like chitter or subsoil allowed native plant establishment, survival and growth. The report is available from ACARP.