Using 15N to determine a budget for effluent-derived nitrogen applied to forest

W. C. Tozer*, K. J. Wilkins, H. Wang, M. Van Den Heuvel, T. Charleson, W. B. Silvester

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Citations (Scopus)


Using stable N isotopes, the fate of effluent-derived N has been determined within a land based municipal effluent irrigation scheme. Over 900 metric tonnes (t) of effluent-derived N have been applied to 192 ha of production conifer forest near Rotorua (NZ) over the past 11 years. The effluent N has a natural isotopic signal, generated by the treatment process, allowing it to be traced into various components of the system. Using this isotopic signal, a realistic approximation of storage capacity of various components of the system has been generated, including a calculation of the contribution of effluent N exiting the catchment via stream flow. Forest storage accounts for 50% of the applied N with a considerable proportion of that immobilized in wood and soil. The wetland, although not intensively sampled, retains 115 t, (13%) of the applied N. Denitrification, including that occurring within the wetland, accounts for 23 t (3%). Nitrogen isotope data confirm that the rise in NO 3 concentrations is directly attributable to effluent N. Currently 88% of NO3-N in the stream is effluent-derived. Using current N isotope values for the stream and extrapolating over the discharge period, export of effluent N via the stream is estimated as 263 t (29%) of the applied N. Overall the forest and wetland ecosystem has intercepted or denitrified 65% of applied N, with 29% lost to the stream, and 50 t (5%) unaccounted for. The forest ecosystem is currently over-supplied with N and a number of management implications flows from these findings. In the long term the continued application of effluent N to the current irrigation area is not sustainable.

Original languageEnglish
Pages (from-to)13-30
Number of pages18
JournalIsotopes in Environmental and Health Studies
Issue number1
Publication statusPublished - Mar 2005
Externally publishedYes


  • EFFLUENT irrigation
  • Ground water NO
  • Land treatment
  • N budget
  • Natural abundances
  • Nitrogen allocation
  • Nitrogen-15
  • Stream NO


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