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Biosolids have been applied to a 1000 ha Pinus radiata D. Don plantation in Nelson, New Zealand, since 1996. A research trial was established in a 6-year-old stand in 1997 with three stocking rates (300, 450, and 600 stems·ha–1), and biosolids were applied at ages 6, 9, and 12 years at three application rates based on total nitrogen loading at each application of 0 (control), 300 (standard), and 600 kg N·ha–1 (high). The aim of this study was to evaluate the responses of tree growth rate and wood properties to biosolids application at midrotation. Since the trial was established, tree diameter and volume increment in the biosolids-treated plots have increased markedly. This increase in growth is being maintained at midrotation, with the year 13–14 volume increment being 40% greater in the standard treatment than the control, and 46% greater in the high treatment than the control. The response in growth has occurred across all stockings in the trial, although there has been a tendency for the difference in growth between the standard and high rates to be greater at higher stockings. A small but significant reduction of basic wood density and standing-tree sonic velocity in P. radiata was found in the biosolids treatments. Although the reductions in wood density and stiffness may slightly decrease the average log value in biosolids treated trees, this will be greatly outweighed by the large increase in wood volume.

Since the mid-1990s, a Pinus radiata (D. Don) plantation growing on a sandy, low fertility soil at Rabbit Island near Nelson, New Zealand received aerobically digested liquid biosolids. An experimental research trial was established on the site to investigate the effects of biosolids applications on tree growth, nutrition, soil and ground water quality. Biosolids were applied to the trial site in 1997 and 2000, at three application rates: 0 (control), 300 (standard) and 600 kg N ha−1 (high). Biosolids application significantly increased tree growth. This was mainly attributed to improved N supply, demonstrated by the enhanced N concentration in the tree foliage. Soil analysis indicated that biosolids application have not caused significant changes in concentrations of most nutrients. However, biosolids treatments significantly increased the available P (Olsen P). Of the heavy metals only total Cu concentrations in the soil increased after biosolids application. Groundwater quality, which was monitored quarterly, has not been affected by biosolids application. The concentrations of nitrate and heavy metals in groundwater were well below the maximum acceptable values in drinking water standards. Biological treatment of sewage and digestion of sewage sludge resulted in the enrichment of 15N in the biosolids (δ15N values between 5.0 and 8.7‰). Such enrichment was used as a tracer to study the fate of biosolids derived N. The elevated δ15N in biosolids treated pine foliage indicated that a considerable amount N was sourced from biosolids. Analysis of δ15N in understorey plants showed that both non-legume and legume understorey plants took up N from the biosolids, and acted as a N sink, reducing N availability for leaching. Our study showed that application of biosolids to a plantation forest can significantly improve tree nutrition and site productivity without resulting in any measurable adverse effect on the receiving environment.

Since the mid-1990s, treated biosolids from a municipal sewage scheme in Nelson, New Zealand, have been applied to a nearby Pinus radiata forest plantation growing on a sandy, low fertility soil. The objective of this study was to determine the impact of biosolids application on tree growth, and to analyse the economic implications of any increase in growth. Data were obtained from a trial established in 1997 in a stand of 6-year-old trees. Treatments included a control, and two rates of biosolids, applied in 1997 and again in 2000 with targeted rates of 300 and 600 kg N ha � 1 . Biosolids significantly increased tree growth in terms of stem diameter, volume and, to a lesser extent, height. After 5 years, trees receiving the lower rate had effectively gained 0.74 years volume growth over the controls, while those receiving the higher rate had gained 1.10 years. However, diameters of largest branches of treated trees had increased by almost 1 cm over the controls, somewhat reducing the potential value of the increased growth. Ignoring application costs but accounting for the increased branch size, the extra growth achieved by age 11 years was estimated to have increased the net present value at time of application by New Zealand dollars ($) 217 and 411 ha � 1 for the lower and higher rates, respectively, using a 7% discount rate. If increased growth rate is maintained for the remainder of the rotation by repeated biosolids applications, much greater increases in value can be expected. Indications from this study are that land application of biosolids can significantly increase the economic returns from a plantation forest on such sites, potentially offsetting the costs of biosolids transport and application. # 2003 Elsevier B.V. All rights reserved.

Treated biosolids have been applied to 750-ha of a Pinus radiata forest plantation on Rabbit Island near Nelson City in New Zealand since 1996. A long-term research trial was established in 1997 to investigate the effects of the biosolids applications on the receiving environment and tree growth. An analysis of the likely economic impact of biosolids application shows that biosolids application has been beneficial. Stem volume of the high treatment (biosolids applied at 600 kg N ha(-1) every three years) was 36% greater than the control treatment (no biosolids applied), and stem volume of the standard treatment (300 kg N ha(-1)) was 27% greater than the control treatment at age 18 years of age. Biosolids treatments have effectively transformed a low productivity forest site to a medium productivity site. Although this increased productivity has been accompanied by some negative influences on wood quality attributes with reduced wood stiffness, wood density, and larger branches, an economic analysis shows that the increased stem volume and greater average log diameter in the biosolids treatments outweighs these negative effects. The high and standard biosolids treatments are predicted to increase the net stumpage value of logs by 24% and 14% respectively at harvesting, providing a large positive impact on the forest owner's economic return.