Land application of biosolids has attracted renewed interest in recent years because of its beneficial effects and low cost. Biosolids contain relatively high concentration of nitrogen (N). Sometimes biosolids-derived N may cause off-site effects, contributing to groundwater contamination and surface water eutrophication. Nitrogen occurs in a variety of chemical forms and is actively involved in many important biological processes in soil ecosystems, where it is difficult to trace the fate of waste derived N by using conventional chemical methods.

Biological treatment of wastewater can result in enrichment of ¹⁵N in the biosolids. For example, significant ¹⁵N enrichment has been found in municipal biosolids (δ¹⁵N +4 to +13‰). When biosolids with a high δ¹⁵N are applied to an ecosystem where background N has a significantly lower δ¹⁵N, the fate of the waste-derived N can be traced. This provides a good opportunity to trace biosolids-derived N by using its distinctive δ¹⁵N signature to understand and manage the movement of N in the ecosystem. In this study, we will explore the potential of using δ¹⁵N to trace the fate and movement of N derived from land-applied biosolids in a forest ecosystem.

Aerobically digested liquid biosolids from the Nelson regional sewage wastewater treatment plant in New Zealand have an elevated δ¹⁵N. The biosolids have been applied to a Pinus radiata plantation growing on a sandy, low-fertility soil. In a long-term research trial, biosolids were applied to a six-year old P. radiata stand at three rates (0, 300 kg N ha^-1 and 600 kg N ha^-1) in November 1997. Biosolids were applied again at the same rates in November 2000 and October 2003. Foliage N concentrations increased with biosolids application rates, and the δ¹⁵N of current-year needles collected from the upper canopy increased with increasing rate of biosolids application. The elevated δ¹⁵N in biosolids-treated pine foliage indicated that a considerable amount of N was sourced from biosolids. Analysis of δ¹⁵N in understorey species indicated that both legume and non-legume understorey plants took up N from the biosolids and acted as an N sink, reducing N availability for leaching. In addition to research findings, we will also discuss areas that require further study to improve the δ¹⁵N technique.

Acknowledgements: The authors wish to thank PF Olsen & Company Limited, Nelson City Council, Tasman District Council and the Nelson Regional Sewerage Business Unit, NZ Foundation for Science Research and Technology for funding and in-kind support.