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.