Biosolids use increases pine production

This case study in a pine plantation shows how biosolids significantly improved site productivity without adverse effects on the ecosystem.

APPROXIMATELY 77,000 dry tons of municipal biosolids are produced in New Zealand every year. Traditionally, most solid residuals from wastewater treatment plants are disposed of by landfilling. In recent years, however, land application is becoming a popular option for managing municipal biosolids, due to increasing public, cultural and regulatory pressures to protect the environment. The New Zealand Waste Strategy 2002 suggests that more than 95 percent of municipal biosolids are expected to be beneficially used by December 2007. This policy has enhanced the significance of applying municipal residuals on to land. Application to forested land, in particular, can significantly minimize the chances of contaminants entering the human food chain.

Since the mid-1990s, biosolids have been applied to a nearby 1,000 ha of pine forest plantation growing on a sandy, low fertility soil at Rabbit Island in Nelson, New Zealand. This was the first full-scale biosolids land application operation in New Zealand. The biosolids were aerobically digested liquid (one to three percent solids) and applied using a specially built spreader. The biosolids contained high concentrations of nitrogen (eight to ten percent N) and were low in heavy metals. To investigate the effects of biosolids application on tree growth, nutrition, soil and ground water quality, an experimental research trial was established at the island. Biosolids were applied in 1997 and 2000, at three application rates based on the nitrogen loadings: O (control), 300 (standard, operational rate) and 600 kg N ha^sup -1^ (high), to pine trees planted in 1991.

Biosolids treatment has significantly increased tree growth by increasing tree diameter (and basal area) and volume increment. In August 2002, at age 11, the average tree basal area of the high treatment was 48 percent and the standard treatment was 30 percent greater than the untreated control treatment. Tree volume of the high treatment was 52 percent and the standard treatment was 34 percent greater than the control treatment. These large increases of wood volume in biosolids treatments do not appear to have reduced basic wood density. The significant increase of basal area and live volume of the trees in biosolids treatments were mainly attributed to improved nitrogen supply. Nitrogen concentrations in pine foliage sampled in the summer of 2002 were 1.10 percent, 1.34 percent and 1.49 percent N for the control, standard and high treatments, respectively.

Natural abundance of stable N isotope ^sup 15^N ([delta]^sup 15^N) can potentially be used to identify the source of N in a given ecosystem. Biological treatment of sewage and digestion of sewage sludge resulted in the discrimination against the heavier isotope of N, resulting in the enrichment of ^sup 15^N in the biosolids. For example, [delta]^sup 15^N values in the Nelson biosolids were between 5.00 and 8.71[per thousand]. In contrast, N-limited coniferous forest soils are generally ^sup 15^N depleted and this was reflected in [delta]^sup 15^N in foliage of the control treatment ([delta]^sup 15^N - 1.45 percent). The elevated [delta]^sup 15^N in biosolids treated pine foliage ([delta]^sup 15^N 1.07 percent in the standard and 5.51 percent in in the high treatment) indicated that a considerable amount N was sourced from biosolids. Analysis of [delta]^sup 15^N in representative weeds indicated that both non-legumes and legumes took up N from the biosolids and acted as an N sink, reducing N availability for leaching.

Soil analysis in 2001 indicated that biosolids application did not have any significant effects on many soil chemical properties, for example, no significant changes in soil acidity and concentrations of total organic carbon, nitrogen, heavy metals and exchangeable calcium, magnesium and potassium were observed. The high rate biosolids treatment increased the sodium bicarbonate extractable phosphorus in the soil. Groundwater quality, which is monitored quarterly at the trial site, showed that the concentrations of NO^sub 3^- and heavy metals were well below the Maximum Acceptable Values for inorganic substances of health significance defined by the New Zealand Drinking Water Standard.

In summary, application of biosolids to the pine plantation at Rabbit Island has significantly improved the site productivity and has not caused any measurable adverse effect on the receiving ecosystem.

[Author Affiliation]

Hailong Wang and Gujja N Magesan are environmental soil scientists specializing in beneficial utilization of municipal and industrial wastes, at Forest Research, Rotorua, New Zealand (hailong.wang@forestresearch.co.nz). This research has been funded by PF Olsen Ltd and New Zealand Foundation for Research Science and Technology. The authors are grateful to Peter Wilks, Craig Fisher and Mark Kimberley for their input in this project.