Over the last 15 years a series of 9 research and 11 demonstration trials (funded by Sydney Water and Forests NSW) has been established to investigate the effects of biosolids on plantation pine. Highlights from each project, together with overall recommendations will be discussed.
The work began in the early 1990’s with liquid biosolids (5% solids) (the most common product at that stage). The product was surface applied to older stands (post-thinning). Surface application was necessary to prevent damage to tree roots. Improved growth of 30% (over the control) was achieved with a rate of 30 dry t/ha. The research then developed to investigate the expanding dewatered biosolids market. Again with surface application significant improvement in growth was achieved.
The older aged stands were targeted to maximise the benefit on merchantable trees. While silvicultural benefits were encouraging, environmental issues were also addressed. A large scale catchment study was established looking at water quality from catchments treated with varying rates of dewatered biosolids (up to 40 dry t/ha). Rates of up to 150 dry t/ha were also monitored in smaller experimental sites where environmental parameters such as chemistry of the soil, soil water and surface water were studied extensively. At rates of 80- 150 dry t ha-1 there were occasional statistically significant impacts on N and P concentrations, but these levels did not exceed guidelines for receiving waters . At application rates of up to 80dry t/ha on plantation pine, no deleterious effects on the environment have been observed. Whilst for operational reasons rates of 30-80 dry t/ha are appropriate, 30 dry t/ha is the recommended rate. This rate gives effective response whilst minimising application costs.
Having established the effectiveness of surface applied biosolids to older age stands, incorporating biosolids prior to establishment was then investigated. There was some concern that the biosolids would provide an environment that was too chemically rich for young seedlings. However, the results of incorporating biosolids (20 dry t/ha) were outstanding; growth in the areas treated with biosolids increased as much as 100% over the control.
Biosolids proved an effective fertiliser in pine (both environmentally and siviculturally) but it was important to consider its effectiveness compared to conventional fertiliser protocols.
Plots with fertiliser and biosolids (together with a non-fertilised control) were established after first-thinning of a 23 year old radiata pine stand. Growth and nutrition were assessed annually to determine how an application of 30 dry t/ha biosolids compared to conventional fertiliser regimes (200kg/ha Nitrogen and 75kg/ha Phosphorus; roughly equivalent to 30 dry t/ha in terms of total N and P). After five years, the trees in the biosolids plots have significantly greater height, diameter, basal area and volume increment than those in the mineral fertiliser and control plots.
Whilst data from tree cores treated with liquid biosolids (up to 30 dry t/ha) show that there is some reduction in density, analysis of timber disks shows it does not affect the wood density on the tree scale. In addition the density returns to post application state after 3-5 years and does not vary significantly form. When wood quality was assessed for trees treated with 30 dry t/ha dewatered biosolids, there was some loss in core density. Mineral fertiliser treatments also decreased in density, but not to the same extent. However the biosolids trees grew faster. It would seem that it is not the type of fertiliser used (biosolids or mineral fertiliser), but rather the rate of nutrient application, and its resulting impact on growth rate, that impacts core density.
Wood strength was not affected by either biosolids or mineral fertiliser. It can be concluded that the benefits of an additional growth are not outweighed by loss of timber quality.
The database generated by 15 years of research and demonstration shows that biosolids is a very effective and environmentally appropriate fertiliser for plantation pine. Increases in basal area of up to 30% are not uncommon. Incorporating biosolids prior to establishment gives a greater growth response, but also increases branch thickness, which impacts the tree form and merchantability. Surface application to post thinned stands increases growth and does not affect tree form or wood quality, and is therefore more profitable as most of the treated trees will remain until harvest.
KEY WORDS Biosolids, sewage sludge, nutrition, radiata pine, forestry, leaching, heavy metals, fertiliser
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