Saturday, 15 July 2006

Using δ15N to trace biosolids-derived nitrogen in a forest ecosystem.

Hailong Wang and Guna Magesan. Ensis, 49 Sala Street, Rotorua, New Zealand

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.

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