Rabbit Island biosolids programme

Biosolids from the Bells Island treatment Plant have been applied to the 750ha Pinus radiata (D. Don) plantation at Rabbit and Bells Islands since 1996, with repeat applications made approximately every three years.

A long-term research trial was established in 1997 to investigate sustainability of the biosolids application. Biosolids were applied to the trial site in 1997, 2000, 2003, 2006 and 2009 at three application rates: 0 (control), 300 (standard, equivalent to 100 kg N ha-1 year-1 over 3 years, used in the full scale operation) and 600 kg N ha-1 (high). Tree growth response and nutrition were measured along with a number of other environmental variables such as soil and groundwater quality.

Biosolids application significantly increased tree growth since the trial establishment. In 2005 at age 14 years, the average basal area in the standard treatment was 31% greater than in the control, and that in the high rate treatment was 45% greater than in the control; stem volume in the standard treatment was 33% greater than in the control, and that in the high treatment was 46% greater than in the control. This was mainly attributed to improved N supply.

Wood property measurement showed a small reduction of basic wood density and standing-tree sonic velocity. Tests indicated that biosolids treatment may produce trees with slightly lower wood stiffness. However, wood density in the treated trees was still higher than average for P. radiata of this age in New Zealand.

In the high rate biosolids treatment, there was an accumulation of some heavy metals, such as copper and zinc in the surface soil, and copper, chromium, lead and zinc in the litter layer. However, overall concentration of these heavy metals is considered to be low for soils. Biosolids application has resulted in a build-up of soil organic matter, nitrogen and phosphorus, indicating improvement of soil fertility. Groundwater quality, which has been monitored quarterly has not been affected by biosolids application.

Economic conclusions

Repeated application of biosolids to a P. radiata plantation growing on low fertility sandy soil on Rabbit Island have significantly improved tree N nutrition and consequently increased tree growth. Effectively they have transformed it from a relative low productivity to an average or above average productivity forest site.

The increased productivity has also had some negative affects on wood quality attributes with larger branches, and reduced wood density and wood stiffness of the tree crop. However, the increased stem volume and greater average log diameter in the biosolids treatments is predicted to far outweigh any negative effects on log value due to the reduced stiffness. The High and Standard biosolids treatments are predicted to increase the net stumpage value of logs by 41% and 32% respectively at harvesting, providing a large positive impact on the forest owner's economic return.