Proceedings

USE OF BIOSOLIDS IN ESTABLISHING EUCALYPT PLANTATIONS FOR MINE SITE REHABILITATION

Kelly Georgina L.
Department Primary Industries, New South Wales (Australia)

(free)

Since the Kyoto Protocol was first drafted in 1997, many companies have started to investigate the options available to reduce their greenhouse gas emissions and address the threat of climate change. One popular emission reduction strategy involves the establishment of tree plantations to sequester carbon from the atmosphere back into living plant tissue.

Plantation establishment is often favoured because it may also provide financial returns, in the form of saleable timber products and/or tradable carbon credits. In the Hunter Valley (in NSW, Australia) there is also interest in investigating alternative/supplementary industries to coal mining. However, since commercial growth rates had not, until now, been clearly demonstrated in the Hunter Valley, mining companies have been reluctant to embrace timber plantations.

Mines have an obligation to rehabilitate after the mining process. The Upper Hunter Valley has approximately 20,000 ha of land requiring rehabilitation. Due to the nutrient poor nature of the unconsolidated overburden, considerable time and money is spent getting the nutrient cycling process started again. It was thought that recycled organics, such as biosolids, may assist in this process and may make the eucalypt plantations a viable alternative to traditional use of pasture in rehabilitation.

A series of research trials has been established to address key priorities

  • Determining timber yields on a range of sites
  • Determining the most appropriate plantation species
  • Assessing the role of soil ameliorants (such as biosolids and other organic wastes)

The research program (funded by industry and Forests NSW) investigates the role that organic amendments (including Biosolids) have on the successful establishment, survival and growth of trees. This includes assessing the moisture stress of plants, soil moisture and the impact of fertiliser (organic and inorganic) on tree growth. A series of 4 trials over 6 years include the assessment of Biosolids, Green waste, Flyash, Bottom Ash, inorganic fertiliser and various combinations of these residuals. They have been assessed for their usefulness on reshaped overburden and undisturbed buffer land (the mandatory large area of land surrounding mines and power stations). All trials are replicated block designs with split plots for species evaluation. Amendment of the overburden soil with Biosolids elicits a better tree growth response than with standard mineral fertiliser.

On the buffer land the growth response to Biosolids is at least equivalent to inorganic fertiliser, and in some cases better. Growth data from trials where bottom ash is used are very encouraging, with the combination of Biosolids and Bottom Ash resulting in the best early growth and survival. Biosolids in combination with Bottom Ash has significantly better tree growth (diameter and height) than fertiliser and the control. Biosolids alone is statistically equal to fertiliser and significantly better than the control.

Soil chemistry has been analysed throughout the three trials. On the overburden soils treated with Biosolids and Biosolids +Bottom ash have higher levels of total phosphorus and available phosphorus (Bray P). Analyses of foliage samples at 1 and 3 years post-treatment show phosphorus levels are higher in the leaves of trees in the Biosolids and Biosolids+Bottom Ash plots. The trends seen in soil and foliage data for phosphorus mirror the growth trends (diameter and height). Traditional timber species and faster growing clonal material both respond well to amendment of the soil with biosolids. Nutrition from Biosolids (particularly phosphorus), with physical amendment (from Bottom Ash), seems to be the driving force that is generating superior tree growth. The bottom ash provides better soil structure for root and water penetration. Traditionally, rehabilitation has been back to pasture and often with annual applications of fertiliser for 5 years. The rehabilitated land brings little return to the mining companies. Forest NSW has shown, in current trials, that trees can be established with a one-off application of Biosolids (not requiring expensive and repeat applications of fertiliser). With the use of biosolids, good growth can be achieved on the overburden and buffer areas on mine land in the Hunter Valley, NSW, Australia. In addition trees provide a potential income from carbon credits, biomass, pulplogs or sawlogs.

KEY WORDS Biosolids, sewage sludge, nutrition, radiata pine, forestry, leaching, heavy metals, fertiliser

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