Economic analysis of growth response from a pine plantation forest applied with biosolids

Abstract

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.